$ Retinal vascular disease $
Zones of ROP ?
-three zones -The centre of the retinal map for ROP is the optic disc
dimeter of foueola ?
0.35 mm diameter
WHAT ARE THE TYPES OF RETINOPATHY OF PREMATURITY ?
1 Active disease 2 Cicatricial disease
WHAT ARE RETINAL ARTERIAL OCCLUSIVE DISEASE ?
1 Branch retinal artery occlusion 2 Central retinal artery occlusion 3 Cilioretinal artery occlusion 4 Asymptomatic retinal embolus
WHAT ARE THE TYPES OF PRIMARY RETINAL TELANGIECTASIA ?
1 Idiopathic macular telangiectasia 2 Coats disease
WHAT ARE RETINOPATHY IN BLOOD DISORDERS ?
1 Leukaemia 2 Anaemia 3 Hyperviscosity
E.G OF PHACOMATOSES ?
1. ANGIOMATOSIS RETINAE (VON HIPPEL LINDAU SYNDROME) 2. TUBEROUS SCLEROSIS (BOURNEVILLE DISEASE) 3. NEUROFIBROMATOSIS (VON RECKLINGHAUSEN'S DISEASE) 4. ENCEPHALOFACIAL ANGIOMATOSIS (STURGE-WEBER SYNDROME)
Congenital remnants of the hyaloid arterial system persist in what forms ?
1. Bergmeister's papilla 2. Vascular loop 3. Mittendorf dot
E.G. OF RETINAL VASCULAR TUMOURS ?
1. CAPILLARY HAEMANGIOMA 2. CAVERNOUS HAEMANGIOMA OF RETINA
Risk factors of DR ?
1. Duration of diabetes 2. Types of diabetes= type 1 > type 2 3. Age of onset of diabetes also act as a risk factor= after puberty risk factor 4.Sex= females than males (4:3). 5. Poor metabolic controle 6. Hereditary= AR 7. Pregnancy= accelerate 8. HTN 9. Otherriskfactors include smoking, obesity, anaemia and hyperlipidaemia.
what are Fundus changes of chronic hypertensive retinopathy ?
1. Generalized arterial narrowing 2. Focal arteriolar narrowing 3. Arteriovenou.s nickiJ1g 4. Arteriolar reflex changes. 5. Superficial retinal haemorrhages (flame-shaped) 6. Hard exudates 7. Cotton wool spots
acute retinal necrosis (ARN) caused by ?
1. HSV 2 ( < 15 yr ) & 1 2. VZ
D/D of Leucocoria ?
1. congenital cataract, 2. inflammatory deposits in vitreous following a plastic cyclitis or choroiditis, 3. coloboma of the choroid, 4. the retrolental fibroplasia (retinopathy of prematurity), 5. persistent hyperplastic primary vitreous, 6. toxocara endophthalmitis and 7. exudative retinopathy of Coats.
dimeter of Fovea centralis ?
1.5 mm in diameter
what is the horizontal & vertical dimeter of optic disc ?
1.76 mm horizontally and 1.88 mm vertically.
Clinically, the hypertensive fundus changes can be described 2 ways which are ?
1• Benign or chronic hypertensive retinopathy, and 2• Malignant hypertensive retinopathy.
RETINAL DEGENERATIONS CLASSIFIED AS ?
1• Peripheral retinal degenerations, 2•VitreoreLinal degenerations, and 3• Macular degenerations, • Age-related degeneration • Myopic macular degeneration
dimeter of Macula lutea ?
5.5 mm in diameter
Effect ofOzurdex lasts for ?
6 months.
ANAEMIC RETINOPATHY DEVELOPS WHEN HB IS LESS BY ? HB FALLS BY ?
< 5 gm% ,50%
what are Visual field changes in RP ?
Annular or ring-shaped scotoma is a typical feature which corresponds to the degenerated equatorial zone of retina. As the disease progresses, scotoma increases anteriorly and posteriorly and ultimately only central vision is left (tubular vision).
what is the hallmark of hypertensive retinopathy ?
Arteriovenous nickiing
what is another name for Bevacizumab ? dose ?
Avastin , 1.25 mg
why retinal oedema, haemorrhages, and leakage oflipids (hard exudates) occures in case of DR ?
B/O Breakdown of blood-retinal barrier
why microaneurysms, and haemorrhages occures in case of DR ?
B/O Weakened capillary wall
T/T OF RD ?
Basic principles and steps of RD surgery are sealing of retinal breaks, reducing the virreous traction on the retina, and flattening of retina by draining of subretinaL fluid and external or internal tamponade. 1. Sealing of relinal breaks. 2. Drainage of SRP. 3. Mainte nance of chorioretinal apposition i . Sciera{ buckling, ii. Pneumatic relinopexy ill. Pars plan,, vitrectomy, endolaser plwlocoagulation a11d internal tamponade.
what is Bergmeister's papilla ?
Bergmeister's papilla refers to the flake of glial tissue projecting from the optic disc. It is the commonest congenital anomaly of the hyaloid system.
WHAT IS CAPILLARY HAEMANGIOMA ?
Capillary haemangioma of retina is rare sight threatening benign acquired tumor. It may present as a solita ry or multiple !es.ions. Almost 50% of solitary tumors a nd almost a ll multiple tumours are a part ofVon-1 lippel Lindau (VH L) disease
CAUSE OF OCULAR ISCHAEMIC SYNDROME ?
Carotid artery stenosis
C/F OF PRIMARY VITREORETINAL LYMPHOMA ?
Clinical features. PVRL presents as masquerade syndrome as it is misdiagnosed as uveitis, because of following clinical feamres.
DX OF PRIMARY VITREORETINAL LYMPHOMA ?
Diagnosis in a suspected case on OCT, FA, USG and MRI is confirmed by cytology of vitreous samples or subretinal nodule.
when examination done in case of in PDR with no high-risk characteristics ?
Every 2 months
RETINA : ENTOPTIC AND ALLIED PHENOMENA : GENERAL CONSIDERATIONS Entoptic phenomenon Halos Phosphenes ENTOPTIC VISUALIZATION OF OPACITIES IN THE OCULAR MEDIA Basic facts Entoptic phenomenon related with the tear film and cornea Entoptic phenomenon related to the lens Entoptic phenomenon related to vitreous ENTOPTIC VISUALISATION OF RETINAL AND CHOROIDAL VESSELS Retinal blood vessels Retinal capillary circulation Entoptic visualization of choroido-capillary circulation HALOS AND PHOSPHENES Halos Physiological Pathological • • • • • • • • • • Phosphenes Pressure phosphenes Self-illumination of retina Pressure phosphenes Electrical phosphenes Radiation phosphenes Light energy-induced phosphenes AFTER IMAGES Types Visualization Factors affecting Physiological basis
GENERAL CONSIDERATIONS Delibrations in this chapter will be on: Entoptic phenomena, Halos, Phosphenes, and After images Entoptic phenomena refer to visualization of reproducible visual perceptions arising from within the eye. These may be sensations arising from the normal intraocular structures (such as retinal blood vessels) or sensations arising from opacities in the ocular media, i.e. cornea, aqueous, lens or vitreous. Other forms of entoptic sensations include halos and Halos refer to entoptic visualization of coloured ring around small white light viewed from a distance. Phosphenes are the glowing sensations which result from inadequate retinal stimuli. In addition to these entoptic phenomena, this chapter also includes after images which, though related to but strictly speaking are not entoptic phenomena. In fact, these are visual sensations experienced following cessation of stimulation of eye. • • • • • ENTOPTIC VISUALIZATION OF OPACITIES IN THE OCULAR MEDIA BASIC FACTS Normally in day-to-day life, one is not aivare of the imperfections in the ocular media in spite of the fact that with the exception of aqueous humour none of the ocular media are perfectly transparent since they are composed of cells with nuclei. As a matter of fact even large opacities within the anterior segment of the eye involving the cornea and lens may not be recognized subjectively as imperfections in the retinal image. This is due to the fact that these opacities lie so far in front of the retina that the umbral portions of their shadows may not fall on the retina. However, opacities closer to the retina in the vitreous do tend to cast umbral shadows on the retina and are subjectively perceived even when quite small.1 Factors ivhich influence the entoptic visualization of opacities in the ocular media include: The vergence of light within the eye, The location of opacities within the eye relative to photoreceptor layer, The optical density of the opacities, and The refractive nature of the opacities.1 In general, any object situated anterior to the photoreceptor layer of the retina can be seen entoptically, provided its optical density differs from that of the surrounding media. Any opacity ivithin the entoptic field throivs a shadoiv—umbral, penumbral or both—depending on its translucency and location on the retina. Opacities of ocular media in general can be divided into translucent and non-translucent. Non-translucent opacities include opaque foreign bodies of the cornea, pigment clumps on the anterior surface of lens capsule, • lenticular opacity, opaque intraocular foreign bodies, asteroid bodies, and some vitreous floaters. The non-translucent media opacities absorb light (do not transmit or scatter light), cast dense shadows and reduce the amount of light reaching the retina. These opacities when located anteriorly cause little disturbances in vision, even if they are considerably large in size. However, when situated posteriorly, may cause a positive scotoma (Fig. 6.11.1). Translucent opacities of ocular media include tear droplets and mucus threads on the corneal surface, irregularities of the corneal surface, breaks in Descemet's membrane, corneal scars, early lenticular changes, and perhaps the majority of vitreous floaters. These opacities scatter light and are often damaging to vision even when they are relatively small in size (Fig. 6.11.2). ENTOPTIC PHENOMENA RELATED WITH THE TEAR FILM AND CORNEA Tear droplets and mucus on the corneal surface Droplets of tear fluid and mucus on the corneal surface act as convex lens and when viewed by the pinhole method are seen as bright spots or bright lines surrounded by dark margins. These move up and down with the movements of the lids when palpebral fissure is widened or narrowed. The lacrimal fluid that adheres to the upper eyelid margin produces a longitudinal stripe which becomes more evident when the palpebral fissure is narrowed.2 Irregularities of the corneal surface and corneal scar Again, with the pinhole method of viewing, superficial horizontal bands caused by wrinkling and folding of the corneal epithelium may be seen in the entoptic field as bright horizontal striae with dark edges. Following rubbing of the eyes, these horizontal striae convert into irregular trelliswork pattern, probably from the irregular wrinkling of corneal epithelium. Corneal scars, particularly those located near the visual axis can likewise be perceived in the entoptic field.1 ENTOPTIC PHENOMENA RELATED TO THE LENS Most persons have small opacities in the lens, but these do not interfere in any way with the vision. Under normal conditions of illumination, one is not aware of the lenticular imperfections, particularly if they are dense and they block rather than scatter light. Dense localized opacities in the lens being so far anterior to the retina, that they do not cause shadows on the retina (Fig. 6.11.1A). In general, opacities in the lens are observed very easily by the pinhole method (Fig. 6.11.2), and the vacuoles, clefts and star figures may be readily perceived (Fig. 6.11.3). Posterior subcapsular irregular opacities usually scatter and diffract light and produce great visual disability and annoyance. Depending upon the refractive index of the opacities vis-a-vis surrounding lens fibres, these may appear dark (when of less refractive index) and light (when of greater refractive index).1 Fig. 6.11.1. Effect of intraocular opacities on the vision: A, small anterior opacity (e.g. in posterior part of lens) may not cause sharp shadow (umbra) on the retina; B, shadows from the opacities located near the retina are sharply seen. Fig. 6.11.2. Pinhole method of entoptically visualizing the opacities in ocular media. The illuminated pinhole is placed at anterior focus of the eye. Fig. 6.11.3. Entoptic image from a lens with early cuneiform cortical cataract. ENTOPTIC PHENOMENA RELATED TO VITREOUS Muscae volitantes (flying flies) or the vitreous floaters resulting from small vitreous opacities are undoubtedly the most common of entoptic visualization. There is a long list of causes of vitreous opacities but the common ones include senile vitreous degeneration and posterior vitreous detachment, vitreous haemorrhage, inflammatory cells and exudates, and myopic vitreous degeneration.3 As described earlier, when the opacity lies well forward in the vitreous, it will have to be relatively large to disturb the retina by its umbra. A small opacity present in the anteriormost part of vitreous will cast its umbral cone far in front of the retina and may cause little or no annoyance. However, opacities located in the posterior vitreous are very annoying since they have larger and more sharply defined umbrae, with a less evident penumbra (Fig. 6.11.1).2 Vitreous floaters may be described as hazy spots or specks, hairlike objects, single or chained globules, dark lines or floating cob-web meshwork. They characteristically drift away with the movements of the eyes. The floaters become more conspicuous while staring at a bright uniform background such as the sky, a white wall, a white page of notebook or while looking through a microscope. The vitreous floaters situated anterior to the centre of rotation of the eyeball appear entoptically to move against the movement of the eyeball while those situated posterior to the centre of rotation entoptically appear to move with the movement of the eyeball. Thus, entoptical movement of the floaters is just reverse the movement of ENTOPTIC VISUALIZATION OF RETINAL AND CHOROIDAL VESSELS RETINAL BLOOD VESSELS Since, the retinal blood vessels lie anterior to the photoreceptors, they should cast their shadow which should be perceived. However, under ordinary conditions of illumination, this is not so; because the visual elements underlying the vessels become adapted to this pattern of illumination. Purkinje for the first time noted that if light is thrown into the eye at an unusual angle, either obliquely through the cornea or through the sclera, so that the shadows of the vessels fall upon a different set of photoreceptors (which are not adapted to such illumination), the vessels become entoptically visible. The magnified shadows of the vascular network seen surrounding the foveal avascular region are called as 'Purkinje figures'. a. b. Methods of eliciting entoptic visualization of retinal vessels 1. Tmnsillumination technique of visualizing the vascular tree of retina is a commonly used clinical test of retinal function. In this method, a small flashlight (from a transilluminator, a penlight or a fibreoptic transilluminator) is placed over the closed eyelids near the outer canthus or directly on the conjunctiva and is oscillated in position to prevent local adaptation to the shadows of retinal vessels. The vascular tree is readily visualized by most subjects. In the presence of dense opacities in the ocular media, entoptic visualization of the retinal vessels by the patient serves as a retinal function test. However, this test has two severe limitations: Some persons with normal retinal function cannot be made to understand the test, so a negative test is not conclusive evidence of lack of function. The avascular foveal zone is not tested. 2. Pinhole disc method. It is a better method of entoptically visualizing the retinal vessels. In this method, the subject is asked to look at a bright background through a pinhole disc kept in motion in front of the eye. The blood vessels are then seen as dark branching lines on a bright background. The constant motion of the pinhole disc avoids the retinal image stabilization and thus the vessels are seen continuously. Further, the pinhole mechanism produces parallel light rays within the eye and enhances contrast, allowing even visualization of the retinal capillaries. 1. 2. 3. Clinical applications of entoptic visualization of retinal vessels It can be performed as a retinal function test, as described earlier; Entoptic visualization of retinal vessels has also been of great value in the study of stabilized retinal image; and In measuring the foveal avascular zone. Pulsation of retinal blood vessels Pulsation of the retinal vessels can also be visualized entoptically, especially after physical exercise.4 Two phases of pulsation are clearly visualized. The first is of rapid expansion of the arterial tree which is synchronous with the cardiac systole. This is followed immediately by a second phase which is a slower contractile motion along the same path and corresponds to the flatter and more slowly moving descending limbs of the pulse wave. • • • • RETINAL CAPILLARY CIRCULATION It has now been established that the movements of red blood cells and white blood cells across the retinal capillaries can be perceived entoptically as "luminous darting points', and this phenomenon has been used clinically as another method of measuring the central avascular zone of the fovea.2 If one looks with relaxed accommodation at a brightly illuminated surface, such as a white sheet of paper or the clean blue sky, one readily perceives small luminous dancing spots. These usually appear as bright circles against a somewhat darker background. They frequently have short light tails similar to comets, which most probably represent after images. The fact that these dancing luminous spots represent retinal capillary circulation is evidenced by following observations: The dancing spots are fairly rapid in their movement and follow sinusoidal paths, which in their shape resemble retinal capillary loops. These spots cannot be seen within the area of foveal avascular zone. Pressure on the eyes slows the movement of these spots. Ophthalmodynamometry has revealed that their movement ceases with pressures over 50 mm Hg.2 By viewing the luminous spots by one eye and the Purkinje figure by the other eye, Marshall (1935) has reported that these spots represent cells within the deep retinal capillary bed, which lies at the level of the inner plexiform and inner nuclear layers.5 ENTOPTIC VISUALIZATION OF CHORIOCAPILLARY CIRCULATION Marshall (1935) reported that he could visualize the choriocapillary circulation entoptically in his own eyes.5 He described that while gazing intently with relaxed accommodation at any brightly lit uniform field, first one will see the luminous darting points representing the retinal capillary circulation. On continuing the gaze, these points may disappear and be replaced by a darkened field in which choriocapillary circulation may be observed. One can observe surging circulation in irregular sinuses, somewhat fan-like in appearance of a dark reddish grey colour, bounded by a black ground meshwork and covering the whole field. This effect lasts only a few seconds but it may usually be repeated several times. HALOS AND PHOSPHENES HALOS Halos or the coloured halos refer to entoptic visualization of coloured rings around small white lights viewed from a distance. These coloured rings result from breaking of the white light into seven colours by the various layers of cells of the ocular media through which the light must pass on its way to retina. Halos may be either physiological or pathological. Physiological halos Four distinct physiological halos as seen entoptically around the bright light by normal individuals have been described.6 These are as follows: ▪ 3° diameter halo. It is the least distinct of the four physiological halos and is probably formed by the corneal epithelial cells. ▪ 4.5° diameter halo. It is one of the more distinct halos and is most likely formed by the corneal endothelial cells and/or lens epithelial cells. ▪ 6° diameter halo. It is the most distinct and the largest described halo. Emsley and Fincham (1922) suggested that it is formed by diffraction of the light by the radially arranged lens fibres.7 Simpson (1953) described that this lenticular halo is not a homogenous band of colour, but is composed of innumerable radial rays of varying lengths and brightness;8 each ray being a complete six-coloured spectrum. All the rays combine to form a halo composed of concentric coloured rings. In the centre, there is a disc of white called the ciliary corona (Fig. 6.11.4). Fig. 6.11.4. A physiological lenticular halo, After Simpson.8 ▪ 9° diameter halo. This is very indistinct halo and is believed to be a second order of diffraction from the structures that produce the 4.5° ological halos Three types of pathological coloured halos known are as follows: 1. Coloured halos of corneal oedema Though commonly known by the name coloured halo of glaucoma, basically it is caused by oedema of the deeper layers of the corneal epithelium. Vivid halos are produced either from the droplets of fluid between the cells or from the cells swollen with fluid. As the name indicates, such oedema is commonly associated with raised intraocular pressure (IOP) and halo formation may wax and wane with elevation and lowering of IOP. Other causes of halos due to corneal oedema are bullous keratopathy and ultraviolet keratopathy.9 Their diameter is about 7° to 12°.10 2. Coloured halos of corneal mucus These halos, basically seen in patients with conjunctivitis, are produced by mucus, pus or particulate matter on the corneal surface. These halos disappear immediately on winking or cleaning the corneal surface. Their diameter is usually large—12° to 14°.10,11 3. Coloured halos of immature cataract Lenticular halos are distinct but somewhat smaller than those produced by corneal oedema; and can be differentiated from each other by following tests: Emsley-Fincham stenopaeic-slit test In this test, an open slit about 1 mm wide is passed in front of the pupil from right to left. During this test, the halos due to corneal oedema remain intact while the lenticular halo is broken into segments. The lenticular halo is visualized as depicted in the centre of each diagram, portion of the halo being eclipsed (Figs 6.11.5A to E). When the horizontal lens fibres are exposed, as in 'A' and 'E', the horizontal portions only of the halo are seen. When the vertical lens fibres are exposed, as in 'C', the vertical portions only of the halo are seen. Intermediate regions produce intermediate effects ('B' and 'D'). By placing slit over the centre of the eye and moving it back and forth, the arm of the halo widens and narrows.7 Fig. 6.11.5. Emsley-Fincham stenopaeic-slit test. For description see text. Simpson pinhole test Simpson modified the Emsley-Fincham test by replacing stenopaeic slit with a 2 mm pinhole. An effect very similar to Emsley-Fincham stenopaeic-slit test can be produced by using 2 mm pinhole. As the pinhole cannot illuminate more than a small section of the grating at any time, the complete halo can never be seen, regardless of the position of the pinhole over the pupil. Fig. 6.11.6 depicts movement of the pinhole within the pupil margin from right to left ('A' through 'E'). The partial lenticular halo rotates much as does a Catherine wheel.8 Fig. 6.11.6. Simpson pinhole test.8 For description see text. PHOSPHENES Phosphenes are glowing sensations produced by nonphotic or the so-called inadequate stimuli. The retinal phosphenes include: selfillumination, pressure or mechanical phosphenes, electrical phosphenes, radiation phosphenes and light energy-induced phosphenes. Self-illumination or self-light of the eye Self-illumination of retina refers to the sensation of greyness or of light experienced by an individual, when the eyes are kept in dark and the dark adaptation is complete. This sensation probably arises from both the retina and cortex and is also called the self-light of the eye. Mechanical or pressure phosphenes Tire luminous sensations produced by pressure on the eyeball (pressure phosphenes) include the following conditions: Purkinje blur ring of gentle pressure It refers to a very bright, well-circumscribed ring of blue-white caused by a direct pressure effect on the sensory retinal elements in the dark-adapted eye.4 This sensation is perceived when a gentle but forceful pressure is applied to either the nasal or temporal portion of the eyeball. This sensation appears in opposite periphery, immediately following pressure on the eyeball. Friedman's blue ring of prolonged pressure It is broad circular band (ring) of blue colour produced by a prolonged digital pressure over the eyes. The centre of this phosphene is oval and is devoid of colour. Measured from the fixation point it extends about 2° above and below and about 3° on each side. Friedman believed the effect may be due to ischaemia and cautioned against experimentation.4 Fery rings of Purkinje It refers to a dark-grey or pale-blue oval area ringed by a bright bluewhite border which appears in the region of blind spot when the eyeball is turned extreme nasally or temporally, while staring at an evenly illuminated bright surface. It is supposed to be produced by the stimulation of retina adjacent to the optic disc, through traction by the optic nerve.4,12 Similar phosphenes in the peripheral retina may be produced by more extensive eye movements. These are caused by retinal stimulation due to scleral traction at the site of muscle insertion (internally near to ora) and are known as movement phosphenes. Accommodate phosphenes of Czemark This phosphene is produced by peripheral retinal stimulation by pull on the ora serrata by the contracting ciliary muscle during a sudden movement of accommodation.13 A similar phosphene may also be produced during strenuous exercise and mechanical movements of a detached retina. Flick phosphene of Nebei (phosphene of quick eye movement) It is a short-lived (0.3 sec), bright, sheaf-like patterned phosphene which occurs when the fully dark-adapted eyes are rapidly rotated (flicked). The phosphenes are seen in the two eyes simultaneously and are two separate images. The apex or narrow ends of the sheaflike patterns are truncated and point in the direction of rapid eye movement and localize, in each eye, the optic disc in the subjective visual field (Fig. 6.11.7). This rapid eye movement phosphene, which generally occurs in individuals over 40 years of age, is mechanically related to, but definitely distinct from Moore's lightning streaks. Fig. 6.11.7. Flick phosphene of Nebel,14 It is proposed that this phosphene originates from an instantaneous transient deformation of the posterior surface of the vitreous close to the optic disc. Occurrence of this phosphene thus represents an early stage of posterior vitreous degeneration.14 Moore's lightning streaks • • • • Moore in 1935 noted an entoptic phenomenon the subjective lightning streak.15 In 1940, he gave the following description of the lightning streak:16 These flashes of light seen on the temporal field of vision (never nasal) are vertical in direction and can be likened with lightning. The flashes of light are accompanied by the simultaneous development of a crop of opacities in the vitreous. They seldom occur before middle age and are more frequent in female sex. They do not imply any serious disease of the eye, either at the time or subsequently. Later it has been established that though these lightning streaks are not serious, but at the same time these should not be taken lightly. Patients complaining of such sensations should be subjected to a thorough vitreal and retinal examination up to ora serrata. It is now believed that this phosphene is probably the result of vitreous detachment, which occurs in many persons of middle age. As the vitreous shrinks forward towards the base, traction on the seeing nasal periphery can produce this phosphene seen in the temporal field of vision. However, similar traction on the temporal retina near the ora serrata may not be noted subjectively (in the nasal field) since the periphery of temporal retina is non-seeing (blind).17-19 Non-specific light flashes Non-specific light flashes though similar to Moore's lightning streaks have no specific pattern like it. However, in general, these are related to vitreous detachment, vitreoretinal traction, retinal hole formation and retinal detachment. Therefore, all such patients should be subjected to thorough vitreoretinal examination with direct and indirect ophthalmoscopy and biomicroscopy.19-24 Patterned electrical phosphenes (Lohmann patterns) Electrical phosphenes are observed with the passage of weak electric currents through the eye. Lohmann (1940) described these phosphenes as beautiful, coloured pattern that could be seen when all or most of the retina was uniformly illuminated and at the same time subjected to alternating electric current.25 Potts and coworkers (1968) have shown that the electrical phosphenes arise proximally to the receptors and may be used to study the integrity of the conduction pathway.26 Radiation phosphenes These appear with the passage of X-ray or other ionizing radiations through the retina. The phosphene of X-irradiation has been described as a homogenous, luminous, blue-green or yellow-green glow filling the entire visual field. It has been likened to an atmospheric electrical discharge behind clouds on the horizon. These phosphenes can only be perceived, if the eye is capable of perceiving light and has intact central visual pathway.27,28 Light energy-induced phosphenes Blue arcs of the retina This is an entoptic phenomenon perceived as two blue-coloured arcs which radiate from a fixation point and converge to the blind spot (Fig. 6.11.8). These areas correspond to the parafoveal arcuate nerve fibre pathway. Fig. 6.11.8. Blue arc of the retina. This entoptic phenomenon is perceived by an observer when in a dark room he fixates a point slightly to the temporal side of small source of light with one eye. It is best demonstrated by using an extended rectangle of red or orange light oriented paraxially with the local nerve fibre pattern. Visualization of the blue arcs may be reinforced following moderate light adaptation, whereas the arcs may degenerate following prolonged dark adaptation.29-33 When, instead of temporal edge, only the upper edge of stimulating light is fixated, only the lower arc appears; and conversely when only the lower edge is fixated, only the upper arc appears. Fixation on the nasal side of the source produces a blue haze between the area where the upper and lower arcs appeared. This has been referred to as the blue spike (Fig. 6.11.9). Fig. 6.11.9. Blue spike. Various theories have been put forward to explain the occurrence of this phosphene. Till date the most accepted view states that perhaps the blue arcs as well as the blue spike result from a secondary electrical excitation of neighbouring retinal nerve fibres or neurons by active nerve fibres.34,35 Haidinger's brushes Haidinger's brushes refer to an entoptic phenomenon perceived by a normal eye while observing a surface illuminated by plane-polarized white light. It appears as faint yellow and blue brush-like pattern radiating from the fixation point with their long axis oriented at right angles to the transmission plane of polarization (Fig. 6.11.10). The best method of observing Haidinger brushes is to look at a background of diffuse blue light through a sheet of Polaroid. Fig. 6.11.10. Haldinger brushes, as visualized through Polaroid against a blue sky. The brushes appear dark against a brighter background. It has been agreed by many workers that the Haidinger brushes are caused by variations in absorption by oriented macular pigment in the foveal region.36-38 It has been assumed that the yellow pigment overlying the photoreceptors in macular region absorbs blue light and results in Haidinger's brushes. Therefore, any process that upsets the orientation of this pigment, even though it does not disturb the photoreceptors themselves, may lead to disappearance of the brushes. Hence, the test for Haidinger brushes may diagnose early changes in the macula such as oedema before occurrence of visual disturbances or being visible ophthalmo-scopically.39 Haidinger brushes have also been used in the diagnosis and treatment of binocular suppression scotoma in some forms of esotropia.40 As a research tool, the Haidinger brushes have been used in the study of stabilized retinal images41 and of the birefringency of the eye in general and of the cornea in particular (since the birefringence of the collagen fibrils within the cornea also influences the orientation of the brushes).42,43 Maxwell Spot It is an entoptic phenomenon in which an observer sees a circle surrounded by a blue halo in intermittent diffuse blue light. The Maxwell spot appears at the point of fixation. It might represent the entoptic visualization of the pigment xantlrophyll within central fovea, but this is not certain.44 The phenomenon may also be related to the fact that the central fovea is devoid of blue colour receptors. Stiles-Crawford effect Stiles and Crawford have shown that pencils of light entering the eye obliquely are less effective as stimuli than those entering the pupil centrally.45 They pointed out that this effect is not due to aberrations in the optical system but is most likely related to the orientation of the receptors in the retina. This directional sensitivity of the retina is referred to as the Stiles-Craivford effect. It has been demonstrated that entoptic visualization of the Stiles- Crawford effect provides interesting evidence of retinal contours in myopia.46 It has shown that in some cases of myopia, the macula stops in a posteronasal direction. Under such circumstances, the cones within temporal fovea are stimulated head on, whereas those within nasal fovea are stimulated obliquely (Figs 6.11.11 to 6.11.13). Fig. 6.11.11. Normal retinal directional sensitivity effect. Light in the bundle entering through the centre of the pupil (A) is more effective in stimulating the retinal cones than that in the bundle coming into the eye near the edge of the dilated pupil, and hence reaching retinal cone obliquely (B) (From Westheimer G: Arch Ophthalmol 79 : 584, 1968). Fig. 6.11.12. A: Appearance of foveal blur patch of a bright star against a dark sky seen under artificially hyperopic condition with large pupil in a right eye with asymmetric Stiles-Crawford effect. B: Schematic diagram illustrating path of rays making up the blur patch. Right edge of visualized pattern (A) corresponds to nasal edge of retinal blur patch and the fact that it is brightest implies that receptors in foveal region are pointing to nasal edge of dilated pupil. (From Westheimer G: Arch Ophthalmol 79 : 587, 1968). Fig. 6.11.13. A: Appearance of foveal blur patch of a bright star against a dark sky seen with the uncorrected myopic right eye with large pupil. The Stiles-Crawford pattern of this eye is asymmetric. B: Schematic diagram illustrating path of rays making up the blur patch. The fact that the visualized pattern is brightest near its left border (A) implies that the receptors in the region of the fovea point in the direction of the nasal edge of the dilated pupil. (From Westheimer G: Arch Ophthalmol 79 : 584, 1968). AFTER IMAGES After images refer to the visual sensations experienced following cessation of stimulation of the eye by light or patterns of light and dark. Though related to, but strictly speaking after images are not entoptic phenomena. After images arise from outside, in contrast to the entoptic phenomenon which arise from within the eye. After images were described hundreds of years ago and were studied comprehensively by Purkinje.47 • • TYPES OF AFTER IMAGES Positive versus negative after images Positive after image is visualized similar to the original stimulus, i.e. the dark areas of the stimulus appear dark and light areas of the stimulus appear light. Negative after image is reversed, i.e. the light areas of the stimulus appear dark and dark areas appear light. Homochromatic versus complementary after images Homochromatic after image is of the same colour as the stimulus. Complementary after image is of the colour complementary to the colour of the stimulus. VISUALIZATION OF AFTER IMAGES ▪ Negative after images. These can be observed after staring for a few moments at a bright source of light and then transferring the gaze to a dimly illuminated even background. A dark image of the light will be seen surrounded by a light illuminated field. A coloured stimulus will be seen in its complementary colour. Negative after image fades in about 15 sec but can usually be brought back ultimately prolonged for about a minute, if the eyes are repeatedly closed for a few seconds and then reopened. ▪ Positive after images. These can be visualized by staring at a dark pattern in very bright light for only 2 to 3 sec and then occluding the eyes. A faint positive pattern similar to the original can be seen with closed eyes for about 5-10 seconds, since positive after images are more transient. Classical method for study of after images is with the help of disc of Bidwell.47,48 It produces afterimagery known as Bidzvell's ghost. This method may be used to demonstrate formation of after images. Details of it are beyond the scope of this book. • • • • FACTORS AFFECTING NATURE OF AFTER IMAGES A few important factors on which the nature of after imagery depends are as follows: Nature and intensity of stimulus Occurrence of succeeding stimuli Region of the retina stimulated. State of retinal adaptation. PHYSIOLOGICAL BASIS OF AFTER IMAGES It has been suggested that, perhaps the after images arise from photochemical reactions in the retina.49-53 However, the possibility that some phases of afterimagery have a central origin cannot as yet be ruled out. hr general, after image phenomena are not so simple and clear cut as appears from their definition. Further, unfortunately relatively little has been learned of the visual system through the study of after images. CLINICAL APPLICATIONS Bielschowsky's after image test and the after image transfer test are
to differentiate between ischaemic and non-ischaemic CRVO which ocular investigation should be done ?
Goldmann perimetry and ERG
S/N on Generalized arterial narrowing or attenuation ?
HTN cuses > 2 phases 1• Viasoconstrictiue phase 2• Sclerotic phase 1• Viasoconstrictiue phase= occurs due to diffuse vasospasm which manifests when a significant elevation of blood pressure has persisted for an appreciable period and is characterised by an increase in retinal arteriolar tone. 2• Sclerotic phase= occurs due to intimal thickening, hypoplasia of tunica media, and hyaline degeneration; and is characterised by arteriolar narrowing associated with tortuosity
what are types according to OCT-based classification of diabetic macular edema ?
I. Non-tractional DME. 2. Tractlonal DME.
Outer plexiform layer digram ?
It consists of connections of rod spherules and cone pedicles with the dendrites of bipolar cells and horizontal cells.
what is Internal limiting membrane ?diagram ?
It is me innermost layer and separates the retina from vitreous. It is formed by the union of terminal expansions of the Muller's fibres, and is essentially a basement membrane.
what is another name for Ranibizumab ? dose ?
Lucentis , 0.3 mg
RETINA : PHYSIOLOGY OF RETINA METABOLISM AND PHYSIOLOGICAL ACTIVITIES OF RETINA Retinal metabolism Disc morphogenesis Shedding and phagocytosis of photoreceptor outer segments
METABOLISM AND PHYSIOLOGICAL ACTIVITIES OF RETINA RETINAL METABOLISM The respiratory rate of the retina is twice that of the brain. Half of the respiratory rate is accounted for by the ellipsoid regions of the photoreceptors, which are rich in mitochondria. Unlike the brain, the retina does not require insulin for glucose to enter the cells. Muller cells possess glucose-6-phosphatase activity, which enables them to release glucose from their stores into the neuro-retina. ▪ Nourishment and oxygen required by the retina are supplied to its tissues from the bloodstream. It is well known that the retina ceases to function a few minutes after its blood supply is stopped. This shows how essential are the vegetative processes taking place in the retinal cells, if vision is to operate. Glucose, lipids, amino acids and other provisions such as vitamins and minerals, are all needed by the retina. An adequate oxygen supply is essential. These items come from the capillaries in the choroid and via the central retinal artery and also, to a small extent, through the circle of Zinn and (probably in trifling amounts) from the vitreous. Carbonic acid and other catabolites return via such routes. Carbohydrates are essential for the production of energy and the retina is sensitive to any fall in concentration within its tissues. It can tolerate a fall in concentration as low as 30 mg/100 ml without any disturbance of activity, but if the concentration becomes any lower, vision suffers. Deprivation of glucose for 8 to 10 minutes results in irreversible cellular damage. Since carbohydrates and oxygen have a close relationship, a similar situation is very likely to occur with a shortage of oxygen. ▪ Production of energy in the retina proceeds in the same way as in other tissues. The most important method of carbohydrate breakdown is via glycolysis (the Embeden-Meyerhof process) to pyruvate and lactate, after that, via the Krebs cycle, to carbonic acid and water. In retina, glycolysis occurs even if there is a sufficient supply of oxygen, this is unlike other tissues where glycolysis occurs, only if there is no oxygen present. Glycogen is found stored in retina, essentially in the glial cells such as Muller's fibres. Such a store serves as a buffer against changes in the concentration of glucose in the tissues. Lipids make up an insignificant source of energy for the retina. Some intermediate building blocks are needed in the process of anabolism. Amino acids are also essential for the cells. Above all, proteins are needed for enzymes and for anabolism and catabolism. In the elderly, retinal metabolic processes wane. In addition, external influences, such as ionizing radiation, may lower metabolism. DISC MORPHOGENESIS Disc morphogenesis has been studied mainly in rod-dominated retinas. This highly specialized process takes place in several stages: ▪ Plate formation. Cytoplasm filled plates are formed by the accumulation of lipid and proteins within a budding of the outer segment membrane. Opsins are present in the membrane of these plates from the onset of disc morphogenesis. ▪ Expansion. The plates expand to reach the width of the rod or cone outer segments. As each plate enlarges, a further plate develops proximally, so that a stack of expanding plates is formed. ▪ Rim proteins are added at the cilium and loop outwards into the plasma membrane. The transmembrane proteins peripherin/rds and rom-I have been localized to the margins of photoreceptor discs. They have no enzymatic properties and are thought to act structural proteins. ▪ Zipping up of upper and lower membranes is seen only in rods, and is followed by the internalization of the newly formed discs. ▪ Maturation and stabilization of the discs—interactive homodimers between peripherin/rds molecules, and between rom-I molecules at the hairpin/disc lamellae junction are thought to stabilize this region of the disc. Linkage of rhodopsin molecules in adjacent lamellae also contributes to disc stability. SHEDDING AND PHAGOCYTOSIS OF PHOTORECEPTOR OUTER SEGMENTS Shedding describes the sloughing of the apical sections of photoreceptor outer segments. In rods, it is maximal 1 hour after light exposure, and in cones shedding is maximal 2-3 hours after the onset of darkness. Mammalian rod outer segment disc shedding follows alight entrained, free running circadian rhythm. The signal to shed is not disrupted by optic nerve transection, so it is thought to originate within the retina and not within the central nervous system. The exact nature of the signal has yet to be determined. It could be transmitted via a neural pathway or be a paracrine effect of a diffusible substance. The leukotrienes LTC4, and a GABA-induced reduction in Melatonin production are thought to participate in these effector pathways. The retinal pigment epithelial cells have specific receptors for rod outer segments at their apices, and the oligosaccharides on the N-terminal at these sites. The retinal pigment epithelium is able to phagocytose many different materials at different rates, suggesting that there are several phagocytic mechanisms in these cells. The ingestion of rod outer segments is thought to involve secondary messengers (possibly cAMP). This is followed by digestion, which is aided by the numerous lysosomal acid hydrolases found in the retinal pigment epithelium.
S/N on Maintenance of chorioretinal apposition ?
Ma inte nance of choriore tinal apposition is required for at least a couple of weeks. This can be accomplished by either of the following procedures depending upon the clinical condition of the eye: i . Sciera{ buckling, ii. Pneumatic relinopexy ill. Pars plan,, vitrectomy, endolaser plwlocoagulation a11d internal tamponade.
what is third order neurons in visual pathway ?
Neurons of geniculate body
What is the ora serrata?
Oraserrata. lt is the serrated peripheral margin where the retina ends. Here the retina is firmly attached both to the vitreous and the choroid. The pars plana extends anteriorly from the ora serrata.
Annular or ring-shaped scotoma charesticaly seen in which disese ?
RP
what is Stage of active inflammation ( active retinal vasculitis) ?
The affected peripheral veins are congested and perivascular exudates and sheathing are seen along their surface. Superficial haemorrhages ranging from flame-shaped to sheets of haemorrhages may be present near the affected veins.
ozurdex Common Side Effects in Retinal Vein Occlusion and Uveitis ?
The most common side effects reported in patients for retinal vein occlusion and uveitis include: increased eye pressure, conjunctival blood spot, eye pain, eye redness, ocular hypertension, cataract, vitreous detachment, and headache.
Management of rop ?
Treatment of well-established disease is unsatisfactory. • Prophylaxis is thus very important To reduce high risk ROP, the premature newborns should not be placed in incubator with an 0 2 concentration of more than 30% and efforts should be made to avoid infections and attacks of apnoea. • Early diagnosis and treatment is essential to prevent blindness in high-risk cases. Therefore, a regular screening and timely intervention is recommended.
RETINA : VISUAL PATHWAY Anatomy of different components of visual pathway Arrangement of nerve fibres in different parts of the visual pathway Blood supply of visual pathway Lesions of the visual pathway
VISUAL PATHWAY Each eyeball acts as a camera; it perceives the images and relays the sensations to the brain (occipital cortex) via the visual pathway which comprises optic nerve, optic chiasma, optic tract, geniculate body and optic radiations. Anatomy of visual sensory system can be discussed under two main heads: Anatomy of different components of visual pathway and Arrangement of visual fibres. • • • ANATOMY OF DIFFERENT COMPONENTS OF VISUAL PATHWAY OPTIC NERVE Each optic nerve (second cranial nerve) starts from the optic disc and extends up to the optic chiasma, where the two nerves meet (Fig. 6.1.12). It is the backward continuation of the nerve fibre layer of retina which consists of about 1-2 million axons originating from the ganglion cells. It also contains the afferent fibres of light reflex and some centrifugal fibres. Fig. 6.1.12. Visual pathways. Morphologically andembryologically, the optic nerve is comparable to a sensory tract (white matter) of the brain, because of the following points: Optic nerve is an outgrowth of the brain. Unlike peripheral nerves, it is not covered by neurilemma (so it does not regenerate when cut). Fibres of optic nerve, numbering about a million, are very fine, 2-10 μm in diameter as compared to 20 μm in sensory nerves. • • Optic nerve is surrounded by meninges. Both the primary and secondary sensory neurons are in the retina. Parts of the optic nerve Optic nerve is about 47-50 mm in length and can be divided into 4 parts: intraocular (1 mm), intraorbital (30 mm), intracanalicular (6-9 mm) and intracranial (10 mm). Intraocular part The intraocular part of optic nerve, also known as optic nerve head, about 1 mm in length starts at the optic disc where the axons forming nerve fibre layer of retina make an orthogonal turn and pass through the lamina cribrosa to appear at the back of eye as intraorbital part of the optic nerve (Fig. 6.1.13). Fig. 6.1.13. Schematic drawing of optic nerve head. The intraocular portion of the optic nerve head has an average diameter of 1.5 mm, which expands to approximately 3 mm just behind the sclera, where the neurons acquire a myelin sheath. Structure of optic nerve head. The optic nerve head may be arbitrarily divided into the following four portions from anterior to posterior: 1. Surface nerve fibre layer. This part is essentially composed of axonal bundles, i.e. nerve fibres of the retina (94%) which converge on the optic disc and astrocytes (5%). The optic disc is covered by a thin layer of astrocytes, the internal limiting membrane of Elschnig, which separates it from the vitreous and is continuous with the internal limiting membrane of the retina. When the central portion of this membrane is thickened, it is referred to as the central meniscus of Kuhnt. All the layers of retina, apart from nerve fibre layer, near the optic nerve, are separated from it by a partial rim of glial tissue called the intermediary tissue of Kuhnt. 2. Prelaminar region. The predominant structures at this level are neurons and a significantly increased quantity of astroglial tissue. The border tissue of Jcoby (a cuff of astrocytes) separates the nerve form the choroid. 3. Lamina cribrosa. It is a fibrillar sieve-like structure made up of fenestrated sheets of scleral connective tissue lined by glial tissue. It bridges the posterior scleral foramina or the scleral canal. The bundles of optic nerve fibres leave the eye through these fenestrations. A rim of collagenous connective tissue with some admixture of glial cells which intervenes between the choroid and sclera and optic nerve fibres is called the border tissue of Elschnig. The lamina cribrosa gets its rich blood supply from the circle of Zinn. 4. Retrolaminar region. This area is characterised by a decrease in astrocytes and the acquisition of myelin that is supplied by oligodendrocytes. The addition of myelin sheath nearly doubles the diameter of the optic nerve (from 1.5 to 3.0 mm) as it passes through the sclera. The axonal bundles are surrounded by connective tissue septa. The posterior extent of the retrolaminar region is not clearly defined. Infraorbital part Intraorbital part of the optic nerve, about 25 mm in length, extends from back of the eyeball to the optic foramina and exceeds the anteroposterior distance from the globe to the optic foramina by 8 • • • • mm. So, this part is slightly sinuous to give play for the eye movements. Important relations of this part are as follows (Fig. 6.1.14): Fig. 6.1.14. Relations of intraorbital part of optic nerve. Optic nerve in the orbit is covered by dura, arachnoid and pia. The pial sheath contains capillaries and sends septa to divide the nerve into fasciculi. The subarachnoid space containing cerebrospinal fluid ends blindly at the sclera but continues intracranially. Central retinal artery along with the accompanying vein crosses the subarachnoid space to enter the nerve on its inferomedial aspect about 10 mm from the eyeball. Anteriorly, the nerve is separated from the extraocular muscles by the orbital fat. Posteriorly, near the optic foramina, the optic nerve is closely surrounded by the annulus of Zinn and the origin of the four rectus muscles (Fig. 6.1.15). Some fibres of the superior rectus muscle and medial rectus muscle are adherent to its sheath here and account for the painful ocular movements seen in retrobulbar neuritis. • • • • • Fig. 6.1.15. Relations of intracanalicular part of optic nerve. Long and short ciliary nerves and arteries surround the optic nerve before these enter the eyeball. Between the optic nerve and lateral rectus muscle are situated the ciliary ganglion, divisions of the oculomotor nerve, the nasociliary nerve, the sympathetic and the abducent nerve (Fig. 6.1.14). Ophthalmic artery, superior ophthalmic vein and the nasociliary nerve cross the optic nerve superiorly from the lateral to medial side. Intracanalicular part This part (Fig. 6.1.15) is closely related to the ophthalmic artery which crosses the nerve interiorly from medial to lateral side in the dural sheath and then leaves the sheath at the orbital end of the canal. Sphenoid and posterior ethmoidal sinuses lie medial to it and are separated by a thin bony lamina. This relation accounts for retrobulbar neuritis following infection of the sinuses. Intracranial part • • • • • • • • • • • This part of the optic nerve, about 1 cm in length, lies above the cavernous sinus and converges with its fellow (over the diaphragma sellae) to form the chiasma. Intracranial part of optic nerve is about 4.5 mm in diameter. It is ensheathed in pia mater only, but receives arachnoid and dural sheaths at the point of its entry into the optic canal. The internal carotid artery runs, at first below and then lateral to it. The ophthalmic artery arises from the internal carotid artery below the optic nerve at about its middle. The anterior perforated substance, the medial root of the olfactory tract and the anterior cerebral artery lie above this part of the optic nerve. Meningeal sheaths of optic nerve Intracranial part of the optic nerve is covered by pia only, while the intracanalicular and intraorbital parts of the nerve have three coverings: the pia, arachnoid and dura. Meningeal sheaths and the subarachnoid and the subdural spaces around the optic nerve are continuous with those of the brain. Anteriorly, all the three meningeal sheaths terminate by becoming continuous with the sclera. At the apex of the orbit, the dura splits into two layers, the outer is continuous with the periosteum of the orbit while the inner forms the dural sheath of the optic nerve. Arachnoid layer is connected to pia with numerous trabeculae. These connections have led to the concept of a compound piaarachnoid meninx, containing cerebrospinal fluid. Pia mater sends numerous septa into the optic nerve, dividing its fibres into fascicles. In fact, these septa are admixture of pia and glial tissue, leading to the term 'pia-glia'. • • • • • • 1. 2. 3. OPTIC CHIASMA It is a flattened structure measuring about 12 mm horizontally and 8 mm antero-posteriorly (Fig. 6.1.12). It is ensheathed by the pia and surrounded by cerebrospinal fluid. It lies over the diaphragma sellae and, therefore, presence of a visual field defect in a patient with a pituitary tumour indicates suprasellar extension. Posteriorly, the chiasma is continuous with the optic tracts and forms the anterior wall of the third ventricle. Nerve fibres arising from the nasal halves of the two retinae (about 53% fibres) decussate at the chiasma. Variations in the location of the chiasma may have important clinical significance (Fig. 6.1.16) as follows: Central chiasma is present in about 80% of normal cases. It lies directly above the sella, so the expanding pituitary tumours involve the chiasma first. Prefixed chiasma is present in about 10% of normal cases. It is located more anteriorly over the tuberculum sellae. In such a situation, the pituitary tumour may involve the optic tracts first. Postfixed chiasma is present in the remaining 10% of normal cases. It is located more posteriorly over the dorsum sellae so that pituitary tumours are apt to damage Relations of the chiasma Relations of chiasma (Fig. 6.1.17) are: Anterior: Anterior cerebral arteries and their communicating arteries. Posterior: Tuber cinereum, infundibulum (hypophyseal stalk), pituitary body, mamillary body and posterior perforated substance. Superior (above): Third ventricle. Inferior (below): Hypophysis. Lateral: Extracavernous part of the internal carotid artery and the anterior perforated substance. Fig. 6.1.17. Relations of optic chiasma. • • • OPTIC TRACTS These are cylindrical bundles of nerve fibres running outwards and backwards from the posterolateral aspect of the optic chiasma, between the tuber cinereum and anterior perforated substance to unite with cerebral peduncles. Each optic tract consists of fibres from the temporal half of the retina of the same eye and the nasal half of the opposite eye. Posteriorly, each optic tract ends in the lateral geniculate body. The pupillary reflex fibres pass on to superior colliculi through the superior brachium. • • LATERAL GENICULATE BODY Lateral geniculate bodys (LGBs) are oval structures situated at the optic tracts. Each geniculate body consists of six layers of neurons (grey matter) alternating with white matter (formed by optic fibres). Fibres of second order neuron coming via optic tracts relay in these neurous. • • • OPTIC RADIATIONS The optic radiations or geniculocalcarine pathway extend from the lateral geniculate body to the visual cortex (Fig. 6.1.18). They pass forwards and then laterally through the area of Wernicke as optic peduncles, anterior to lateral ventricle and traversing the retrolenticular part of internal capsule, behind the sensory fibres and medial to auditory tract. The fibres of optic radiations then spread out fanwise to form a medullary optic lamina. This is at first vertical but becomes horizontal near the visual cortex. The inferior fibres of the optic radiations, which subserve the upper visual fields, first sweep anteroinferiorly in Meyer's loop around the anterior tip of the temporal horn of the lateral ventricle, and into the temporal lobe. The superior fibres of the radiations, which subserve the inferior visual fields, proceed directly posteriorly through the parietal lobe to the visual cortex. Fig. 6.1.18. Optic radiations: A, lateral view; B, transverse section. • • • • • VISUAL CORTEX It is located on the medial aspect of the occipital lobe in and near the calcarine fissure. It may extend on to lateral aspect of the occipital lobe, but limited by a semilumar sulcus, the sulcus lumatus. The visual cotex is subdivided into the visuosensory area (striate area 17) that receives the fibres of the optic radiations, and the surrounding visuopsychic area (peristriate area 18 and parastriate area 19) (Fig. 6.1.19). Modified nomenclature recongnizingfive vistml areas has been described recently is as follows: First visual area (VI) in area 17. Second visual area (V2) occupying the greater part of area 18, but not the whole of it. Third visual area (V3) occupying a narrow strip over the anterior part of area 18. Fourth visual area (V4) within area 19, and Fifth visual area (V5) at the posterior end of the superior temporal gyrus. Fig. 6.1.19. Location of visual cortex on superolateral A, and medial B surfaces of the cerebral hemisphere. A section through the vistml cortex of a fresh brain can be identified by naked eye, by the presence of a white line, the stria of Gennari (which is a layer of medullated fibres disposed in different structures). Microscopic structure of the vistml cortex is similar to other parts of cerebral cortex and consists of 6 layers, which from without inward are: plexiform layer, external granular layer, external pyramidal layer, internal granular layer, ganglionic layer and multiform layer (Fig. 6.1.20). Fig. 6.1.20. Microscopic structure of five different types of cerebral cortex. • • • • ARRANGEMENT OF NERVE FIBRES IN DIFFERENT PARTS OF THE VISUAL PATHWAY IN THE RETINA Arrangement of nerve fibres of the retina is described at page 175. There occurs no overlap between the lower and upper halves of fibres of peripheral part of retina. Line dividing nasal and temporal fibres passes through the centre of fovea. Hence, the temporal macular fibres remain on the same side, while the nasal ones cross. Upper temporal retinal fibres are separated from the lower by the macular fibres; an arrangement which holds throughout the central visual pathway. IN THE OPTIC NERVE a. In the optic nerve head Arrangement of nerve fibres in the optic nerve head is exactly same as in the retina (Fig. 6.1.21). b. In the distal region (behind the eye) In the distal region of optic nerve, the nerve fibres are distributed exactly as in the retina, i.e. the upper temporal and lower temporal fibres are situated on the temporal half of the optic nerve and are separated from each other by a wedge-shaped area occupied by the papillomacular bundle. The upper nasal and lower nasal fibres are situated on the nasal side (Fig. 6.1.21). Fig. 6.1.21. Arrangement of fibres in the distal region (behind the eyeball) of optic nerve. c. In the proximal region (near the chiasma) In the proximal region of optic nerve, the macular fibres are centrally placed (Fig. 6.1.22). Fig. 6.1.22. Arrangement of fibres in the proximal region of optic nerve. Note central position of papillomacular bundle. • • IN THE CHIASMA In the chiasma, the visual fibres are arranged as below (Fig. 6.1.23): Fig. 6.1.23. Decussation of fibres in the chiasma. 1. Nasal peripheral fibres constitute about three-quarters of all the fibres and cross over to enter the medial part of the opposite optic tract in the following manner: The lower nasal fibres in the optic nerve traverse the chiasma low and anteriorly (so are first affected in the tumours of pituitary body producing upper temporal quadrantic field defects). These fibres form convex loops in terminal part of the opposite optic nerve (therefore ipsilateral blindness due to lesions of the proximal most part of the optic nerve is associated with contralateral field defects) and then cross to the opposite tract and occupy its lower quadrant. The upper nasal fibres of the optic nerve traverse the chiasma high and posteriorly (therefore are involved first by lesions coming from above the chiasma, e.g. craniopharyngiomas) and after crossing occupy the upper nasal quadrant of the opposite optic tract. Some of these fibres make a loop in the ipsilateral optic tract before crossing. 2. Temporal fibres from retina which occupy the temporal half of the optic nerves, remain uncrossed and run backwards in the lateral part of the optic chiasma to reach the dorsolateral part of the optic tract. 3. Macular fibres which occupy the central part at the proximal end of optic nerve, keep this position in the anterior part of the chiasma. Then the crossing (nasal) macular fibres get separated from the uncrossed fibres and pass as a bundle obliquely backwards and upwards to decussate in the posterior most part of the chiasma, which is related to the supraoptic recess. Lesions here may produce central temporal hemianopic scotoma. 4. Other fibres. In addition to the visual fibres, the optic chiasma also contains commissural fibres, pupillary fibres and fibres connecting the two medial geniculate bodies, the globus pallidus and the hypothalamus. • • • • IN THE OPTIC TRACT In the optic tract, the visual fibres are rearranged as follows (Fig. 6.1.24): Macular fibres, both uncrossed (temporal macular fibres from the ipsilateral retina) and crossed (nasal macular fibres from the opposite retina) occupy the dorsolateral aspect of the optic tract. Upper peripheral fibres both uncrossed (from upper temporal part of lateral retina) and crossed (from upper nasal part of opposite retina) are situated medially in the optic tract. Lower peripheral fibres both uncrossed (from lower temporal quadrant of ipsilateral retina) and crossed (from lower nasal quadrant of opposite retina) are situated laterally in the optic tracts. Other fibres: Besides the visual fibres, the optic tracts also contain the other fibres as described in optic chiasma. Fig. 6.1.24. Arrangement of fibres in the optic tract. • • • IN THE LATERAL GENICULATE BODY Position of visual fibres Position of visual fibres in lateral geniculate body (LGB) is as below (Fig. 6.1.25): Macular fibres coming in the optic tract occupy the posterior two-thirds of the lateral geniculate body (LGB). Upper retinal fibres occupy the medial half of the anterior onethird of the LGB. Lower retinal fibres occupy the lateral half of the anterior onethird of the LGB. Fig. 6.1.25. Arrangement of fibres in the lateral geniculate body. Relay of second order neurons in LGB The neurons of LGB form the third order neurons of the vision. The axons of the second order neurons (ganglion cells) synapse with the dendrites of the neurons of LGB (Fig. 6.1.26). There is a regular point to point localization of the retina in the lateral geniculate body nucleus which is also carried from the latter to the visual cortex. LGB has got 6 laminae 1 to 6. The crossed fibres end in the laminae 1, 4 and 6 while the uncrossed fibres end in the laminae 2,3 and 5 in such a way that those from the corresponding parts of two retinae end in neighbouring parts of the adjacent laminae. Therefore, the smallest lesion of the retina results in degeneration of three laminae of the LGB in which the retinal fibres end. Hence, the conducting unit in optic nerve fibres is a 3 laminae unit. Since the optic radiations commence from all the six laminae (6 laminae unit), so a lesion in the visual cortex results in degeneration of all the 6 laminae of LGB. Fig. 6.1.26. Arrangement of termination of axons of ganglion cells (second order neurons of vision) of the two eyes in the lateral geniculate body. For explanation see text. • • • • IN THE OPTIC RADIATIONS In the optic radiations, there occurs a temporal rotation of the fibres, thereby (Fig. 6.1.27): Upper retinal fibres occupy the upper part of the optic radiations. Lower retinal fibres occupy the lower part of the optic radiations. Macular fibres lie in the central part of optic radiations separating the upper retinal from the lower retinal fibres. Other fibres: Besides the visual fibres, the optic radiations also contain the fibres that pass from the cerebral cortex to LGB, to the superior colliculus and to the oculomotor nuclei. Fig. 6.1.27. Arrangement of fibres in the optic radiations. IN THE VISUAL CORTEX The visual cortex is also called the cortical retina, since a true copy of the retinal image is formed here. It is only in the visual cortex that the impulses originating from corresponding points of two retinae meet. There is a point to point projection of the retina in the visual cortex in such a way that the right visual cortex is concerned with perception of objects situated to the left of the vertical median line in the visual fields and left visual cortex with the objects situated to the right half. In other words, the right visual cortex receives impulses arising from the temporal half of right retina and nasal half of left retina; and the left visual cortex receives those arising from the temporal half of the left retina and nasal half of the right retina. The visual fibres contained in the optic radiations are relayed in the visual cortex in the following manner (Fig. 6.1.28): Fig. 6.1.28. Arrangement of fibres in visual cortex. Fibres from the macular area relay in an extensive area placed posteriorly in the visual cortex. Fibres from the peripheral retinae end anterior to the macular fibres; those from the upper retinae above the calcarine sulcus. • The visual areas give off efferent fibres also. These reach various parts of the cerebral cortex in both hemispheres. In particular, they reach the frontal eye field, which is concerned with eye movements. Like other 'sensory' areas, the visual areas are, therefore, to be regarded as partly motor in function. This view is substantiated by the fact that movements of the eyeballs and head can be produced by stimulation of the areas 17 and 18, which constitute an occipital eye field. Efferents from the visual areas also reach the superior colliculus, the pretectal region, and the nuclei of the cranial nerves supplying muscles that move the eyeballs. There is physiological evidence of a corticogeniculate projection. Fibres also reach the thalamus (pulvinar). • • • • BLOOD SUPPLY OF VISUAL PATHWAY The visual pathway receives its blood supply from the two arterial systems, the carotid and the vertebral, connected to each other at the base of brain by the arterial circle of Willis (see page 514). The branches of the carotid system which contribute to the blood supply of visual pathway are ophthalmic artery, small branches of internal carotid artery, posterior communicating artery, anterior cerebral artery and middle cerebral artery. The arteries of vertebral systems are cortical, central and choroidal branches from the posterior cerebral arteries. Similar to the brain, the visual pathway is mainly supplied by the pial network of vessels except the orbital part of optic nerve which is also supplied by an axial system derived from the central retinal artery. The pial plexus around different parts of the visual pathway gets contribution from different arteries described infra. BLOOD SUPPLY OF RETINA See page 177. • • • • BLOOD SUPPLY OF THE OPTIC NERVE 1. Intraocular part of optic nerve (optic nerve head) is supplied as below (Fig. 6.1.29): Surface nerve fibre layer is mainly supplied by the capillaries derived from the retinal arterioles, which anastomose with vessels of the prelaminar region. Occasionally, a ciliary-derived vessel from the prelaminar region may enlarge to form the cilioretinal artery. Prelaminar region is supplied by vessels of ciliary region. There is lack of agreement as to whether these vessels are derived primarily from the peripapillary choroidal system or from separate branches of the short posterior ciliary arteries. Lamina cribrosa region is also supplied by the ciliary vessels which are derived from the short posterior ciliary arteries and arterial circle of Zinn-Haller. Retrolaminar region is supplied by both the ciliary and retinal circulation with the former coming from recurrent pial vessels. The central retinal artery provides centripetal branches from the pial plexus and also centrifugal branches. • • Fig. 6.1.29. Blood supply of the optic nerve head. 2. Intraorbital part of optic nerve is supplied by two systems of vessels—a periaxial and an axial (Fig. 6.1.30): Periaxial system of vessels supplying this part of optic nerve is derived from the six branches of internal carotid artery namely: ophthalmic artery, long posterior ciliary arteries, short posterior ciliary arteries, lacrimal artery and central artery of retina before it enters the optic nerve and circle of Zinn. Axial system of vessels supplying the axial part of the optic nerve is derived from (i) the intraneural branches of the central retinal artery, (ii) central collateral arteries which come off from the central retinal artery before it pierces the nerve, and (iii) central artery of optic nerve. Fig. 6.1.30. Blood supply of the optic nerve. The capillary network for the optic nerve is derived from both the systems. Anastomosis between the two systems has also been demonstrated. • • • • 3. Intracanalicular part of optic nerve. The nerve within the optic canal is supplied only by the periaxial system of vessels. The pial plexus in this part is fed mainly by branches from the ophthalmic artery. 4. Intracranial part of optic nerve. This part of the optic nerve is exclusively supplied from the periaxial system of vessels. The pial plexus here is contributed by 4 sources: Branches from the internal carotid artery either directly or through the recurrent branch of anterior superior hypophyseal artery (supply the inferior aspect of the optic nerve containing lower retinal fibres) Branches from anterior cerebral artery (supply the superior aspect of the optic nerve containing upper retinal fibres) Small recurrent branches from the ophthalmic artery and Twigs from the anterior communicating artery. • • • The venous drainage The venous return in the optic nerve head is primarily by the central retinal vein. The orbital part is drained by peripheral pial plexus and also by central retinal vein in the distal part. The intracranial part is drained by the pial plexus which ends in anterior cerebral and basal vein. • • BLOOD SUPPLY OF THE OPTIC CHIASMA The vessels may enter the chiasma directly or through the pial plexus (Fig. 6.1.31). The main blood supply of the chiasma is derived from the branches of the anterior cerebral and internal carotid arteries with some contribution from other arteries as follows: The superior aspect of chiasma is supplied by branches from the anterior cerebral and anterior communicating arteries. The inferior aspect of the chiasma is supplied by branches from the internal carotid artery, anterior superior hypophyseal artery and posterior communicating artery. A branch from the ophthalmic artery supplies the anteroinferior margins of the chiasma. Fig. 6.1.31. Blood supply of the optic chiasma.Venous drainage The superior aspect of the chiasma is drained by the superior chiasmal vein which ends in the anterior cerebral vein. The inferior aspect of the chiasma is drained by the preinfundibular vein which drams into the basal vein. BLOOD SUPPLY OF THE OPTIC TRACT The pial plexus supplying the optic tract receives contribution from the posterior communicating artery, anterior choroidal artery and branches from the middle cerebral artery (Fig. 6.1.32). Though there is no anastomosis, but there is considerable overlapping in the blood supply of the optic tract by the anterior choroidal artery and by the branches of middle cerebral artery. Therefore, the occlusion of the anterior choroidal artery does not result in hemianopia. Venous drainage from the superior and inferior aspects of the optic tract is by the anterior cerebral vein and the basal vein, respectively. • • • • BLOOD SUPPLY OF LATERAL GENICULATE BODY Posterior cerebral artery supplies the posteromedial aspect of LGB and thus nourishes the fibres coming from the superior homonymous quadrants of the retinae (Fig. 6.1.32). Anterior choroidal artery almost solely nourishes the anterolateral aspect of LGB, and thus supplies the fibres coming from the inferior homonymous quadrants of the retinae. The region of the hilum which contains the macular fibres is supplied by a rich anastomosis from both the posterior cerebral and the anterior choroidal arteries. Venous drainage of LGB is by the basal vein. Fig. 6.1.32. Blood supply of posterior visual pathway. • • • • BLOOD SUPPLY OF THE OPTIC RADIATIONS Anterior choroidal artery through the perforating branches supplies the optic radiations anteriorly over the roof of the inferior horn of lateral ventricle (Fig. 6.1.32). Deep optic artery, a branch of middle cerebral artery, supplies the middle part of optic radiations when they lie lateral to the descending horn of the lateral ventricle. Calcarine branches of the posterior cerebral artery and perforating branches from the middle cerebral artery supply the posterior part of the optic radiations as the fibres spread out to reach the visual cortex. Venous drainage from the optic radiations is mainly by the basal vein and in some part by the middle cerebral vein. • • • BLOOD SUPPLY OF VISUAL CORTEX The posterior cerebral artery supplies the visual cortex mainly through the calcarine artery supplemented by its other two branches—the posterotemporal and parietooccipital arteries (Fig. 6.1.32). Hence, in the event of calcarine artery occlusion, the macular area is spared. The terminal branches of middle cerebral artery supply the anterior end of the calcarine sulcus and the lateral aspect of the occipital pole. At the posterior pole, there exists a rich anastomosis between the posterior and middle cerebral arteries. Venous drainage from the medial aspect of the occipital cortex is by the internal occipital vein which ends into the great cerebral vein of Galen and straight sinus; and from its superolateral aspect by the inferior cerebral vein which ends in the cavernous sinus. LESIONS OF THE VISUAL PATHWAY Salient features and important causes of lesions of the visual pathway at different levels (Fig. 6.1.33) are as follows: nerve; 2. Proximal part of optic nerve; 3. Central chiasma; 4. Lateral chiasma (both sides); 5. Optic tract; 6. Geniculate body; 7. Part of optic radiations in temporal lobe; 8. Part of optic radiations in parietal lobe; 9. Optic radiations; 10. Visual cortex sparing the macula; 11. Visual cortex, only macula. 1. Lesions of the optic nerve These are characterized by marked loss of vision or complete blindness on the affected side associated with abolition of the direct light reflex on the ipsilateral side and consensual on the contralateral side. Near (accommodation) reflex is present. Common causes of optic nerve lesions are: Optic atrophy, traumatic avulsion of the optic nerve, indirect optic neuropathy and acute optic neuritis. 2. Lesions through proximal part of the optic nerve Salient features of such lesions are: Ipsilateral blindness, contralateral hemianopia and abolition of direct light reflex on the affected side and consensual on the contralateral side. Near reflex is intact. 3. Sagittal (central) lesions of the chiasma These are characterized by bitemporal hemianopia and bitemporal hemianopic paralysis of pupillary reflexes. These usually lead to partial descending optic atrophy. Common causes of central chiasmal lesions are: suprasellar aneurysms, tumours of pituitary gland, craniopharyngioma, suprasellar meningioma and glioma of third ventricle; third ventricular dilatation due to obstructive hydrocephalus and chronic chiasmal arachnoiditis. 4. Lateral chiasmal lesions Salient features of such lesions are binasal hemianopia, associated with binasal hemianopic paralysis of pupillary reflexes. These usually lead to partial descending optic atrophy. Common causes of such lesions are distension of third ventricle causing pressure on each side of the chiasma and atheroma of the carotids or posterior communicating arteries. 5. Lesions of the optic tract These are characterized by incongruous homonymous hemianopia associated with contralateral hemianopic pupillary reaction (Wernicke's reaction). These lesions usually lead to partial descending optic atrophy and may be associated with contralateral third nerve paralysis and ipsilateral hemiplegia. Common causes of optic tract lesions are syphilitic meningitis or gumma, tuberculosis and tumours of optic thalamus and aneurysms of superior cerebellar or posterior cerebral arteries. 6. Lesions of lateral geniculate bodies These produce homonymous hemianopia with sparing of pupillary reflexes, and may end in partial optic atrophy. 7. Lesions of optic radiations Their features vary depending on the site of lesion. Involvement of total optic radiations produces complete homonymous hemianopia (sometimes sparing the macula). Inferior quadrantic hemianopia (pie on the floor) occurs in lesions of parietal lobe (containing superior fibres of optic radiations). Pupillary reactions are normal as fibres of the light reflex leave the optic tracts to synapse in the superior colliculi. Lesions of optic radiations do not produce optic atrophy, as the first order neurons (optic nerve fibres) synapse in the lateral geniculate body. Common causes of lesions of optic radiations include vascular occlusions, primary and secondary tumours and trauma. 8. Lesions of the visual cortex Congruous homonymous hemianopia (usually sparing the macula) is a feature of occlusion of posterior cerebral artery supplying the anterior part of occipital cortex. Congruous homonymous macular defect occurs in lesions of the tip of the occipital cortex following head injury or gunshot injuries. Pupillary light reflexes are normal and optic atrophy does not occur following visual cortex lesions.
in CRAO obstruction occeres at what level ?
at the level of lamina cribrosa.
what is foueal auascular zone (FAZ) ?
area about 0.8 mm in diameter (including foveola and some surrounding area) does not contain any retinal capillaries and is calledfoueal auascular zone (FAZ).
in ERG which wave affected first ?
b-wave(scotopic) affected before ;a wave(photopic)
Exophyt,ic reli11oblastoma differentiated from ?
causes of exudativc retinal detachment in children such as: Coats' disease
what is Nervefibrelnyer (stratum opticum) diagram ?
consists of axons of me ganglion cells, which pass through me lamina cribrosa to form me optic nerve. For distribution and arrangement of retinal nerve fibres
In older patients, arteriosclerotic changes may preexist due to ?
involutional sclerosis.
what is long form of FIPTs?
focal intraretinal periarterial transudates
OCT is useful in CRVO for ?
for evaluation of 1. macular oedema, 2. sub-retinal fluid accumulation and 3, development of epiretinal membrane (ERM).
copper wirin is seen in ?
grade 3k/w/c c=3 ( copper kami kimtich mhanun 3)
what are the stages of Clinical course of the Eales' disease ?
l . Stage of active inflammation ( active retinal vasculitis) 2. Stage of ischaemia or vascular occlusion 3. Stage of retinal neovascularization 4.. Stage of sequelae or advance stage of disease
what is Inner plexiform layer ?
lt essentially consists of connections between the axons of bipolar cells and dendrites of the ganglion cells, and processes of amacrine cells.
what is Inner nuclear layer digram ?
lt mainly consists of cell bodies of bipolar cells. It also contains cell bodies of horizontal, amacrine and Muller's cells and capillaries of central artery of retina. The bipolar cells constitute the.first order neurons.
what are the Anomalies of the nerve fibres ?
medullated (opaque) nerve fibres.
what is thinest part of retina ? dimeter ?
ora serrate (0.1 mm)
what is thickest part of retina ? dimeter ?
peripapillary region (0.56 mm)
arteriolar reflex and A-V nipping result from ?
thickening of the vessel wall and are a reflection of the duration of hypertension (chronic hypertension).
The risk of proliferative diabetic retinopathy (PDR) is higher in which type ?
type 1 DR
diabetic macular edema (DME) is more common in wich type ?
type 2 DR
what is Arteriovenous nicking ?
where arteriole crosses and compresses the vein, as the vessels share a common adventitious sheath.
Staging of ROP ?
(ICROP) 5 stages= ( D R E T T ) • Stage I=Demarcation line • Stage 2=ridge • Stage 3=extraretinal fibrovascular proliferation • Stage 4a=Subtotal retinal detachment (- macula) • Stage 4b=Subtotal retinal detachment (+ macula) • Stage 5=Total retinal detachment
what is Mizuo-Nakamura phenomena ? cause ?
- prolonged dark adaptation of three hours or more, leads to disappearance of this unusual discoloration and the appearance of a normal reddish appearance in oguchi disese. - caused by the overstimulation of rod cells.
bells palsy ? It is inflamation of facial n within which foramen ?
- stylomastoid foramen - m/c LMN facial palsy - increse incidence in DM,HTN - reactivation HSV in geniculate ganglion
retinal changes start occuring in PIHTN at what bp ? & marked changes ate what bp ?
1. 160/ 100 mm of Hg 2. 200/ 130 mm of Hg.
Medical therapy in case of retinal venous oclussion ?
1• Intravitreal anti-VEGFs= e.g. 1.25 mg Bevacizumab (Avastin), or 0.3 mg Ranibizumab (Lucentis) are useful for the associated CME and neovascularization Anti-VEGF have to be repeated every 1- 2 months. 2• lntravitreal triamcinolone (1 mg/0.1 mL) may be given for the associated CME. Repeated injections of triamcinolone may be required. 3• Tntravitreal dexamethasone slow release implant (Ozurdex) may be used as alternative to triamcinolone, which needs to be repeated every 2 months. Effect of Ozurdex lasts for about 3-6 months.
RETINAL ASTROCYTOMA CAN BE A/W ?
1• Tuberous sclerosis 2• Neurofibromatosis rype I 3• Retinitis pigrnentosa
what is the distance between o.disc & fovea ?
2 disc diameters (3-4 mm) away from the temporal margin of the disc and about l mm below the horizontal meridian.
surface area of all retina ?
266 mm2
C/F OF RETINAL ARTERY OCCLUSIONS ?
Clinically retinal artery occlusion may present as 1. central retinal artery occlusion ( 60%) or 2. branch artery occlusion (35%) or 3. cilioretinal artery occlusion (5%). It is more common in males than females. It is usually unilateral but rarely may be bilateral (1 to 2% cases).
what is significant macular oedema (CSME) ?
Clinically significant macular oedema (CSME) CSME is the term coined during early treatment diabetic retinopathy study (ETDRS}. It is diagnosed if one of the fllowing three criteria are present on slit-lamp examination with 90D lens : • Thickening of the retina at or within 500 micron of the centre of the fovea. • Hard exudates at or within 500 micron of the centre of fovea associated with adjacent retinal thickening. • Development of a zone of retinal thickening one disc diameter or larger in size, at least a part of which is within one disc diameter of the foveal centre.
fundus findings on early cases ischemic CRVO ?
Early cases on fundus examination reveal 1 • Retinal veins show massive engorgement, congestion and tortuosity 2 • Retinal haemorrhages are massive (almost whole fundus is full of haemorrhages giving a 'splashedtomato' appearance), 3• Cotton wool spots Numerous {usually more than 6 to 10), 4 • Disc shows oedema and hyperaemia, 5 • Maculararea is full of haemorrhages and is severely oedematous and 6 • Break through vitreous haemorrhage may be seen in some cases.
RETINA : CONTRAST SENSITIVITY : CONTRAST SENSITIVITY GENERAL CONSIDERATIONS Introduction Types of contrast sensitivity Spatial contrast sensitivity Temporal contrast sensitivity MEASUREMENT OF CONTRAST SENSITIVITY Arden gratings Cambridge low contrast gratings Pelli-Robson contrast sensitivity chart Visitech charts Vector vision charts Fact CS charts NEURAL MECHANISMS AND FACTORS AFFECTING CONTRAST SENSITIVITY Neural mechanisms Factors affecting Refractive errors Age Lenticular changes Ocular and systemic diseases DIAGNOSTIC APPLICATIONS • • • • • Before LASIK surgery Age-related macular degeneration Glaucoma Cataract Diabetic retinopathy
GENERAL CONSIDERATIONS INTRODUCTION Contrast sensitivity is the ability to perceive slight changes in luminance between regions which are not separated by definite borders and is just as important as the ability to perceive sharp outlines of relatively small objects. It is only the latter ability which is tested by means of the Snellen's test types. In many diseases, loss of contrast sensitivity is more important and disturbing to the patient than the loss of visual acuity. Further, contrast sensitivity may be impaired even in the presence of normal visual acuity. The first measurement of contrast sensitivity function of the human visual system was reported by Schade, in forms of modulation transfer function (MTF). Campbell and Green in 1968 first measured contrast sensitivity using sinusoidal gratings and concluded that measurement of contrast sensitivity gives a more complete description of the function of retina. TYPES OF CONTRAST SENSITIVITY 1. Spatial contrast sensitivity Spatial contrast sensitivity refers to detection of striped pattern at various levels of contrast and spatial frequencies. In its measurement, patient is presented with sine wave grating of parallel light and dark bands (Arden gratings) and is asked to tell the minimum contrast at which the bars can be seen at each frequency. The width of the bars is defined as spatial frequency, which expresses the number of pairs of dark and light bars subtending an angle of 1 degree at the eye. A high spatial frequency implies narrow bars, whereas a low spatial frequency indicates wide bars. 2. Temporal contrast sensitivity Here the contrast sensitivity function is generated for time-related (temporal) processing in the visual system by presenting a uniform target field modulated sinusoidal in time rather than as a function of spatial position. Both temporal and spatial contrast sensitivity testing yield significantly more complete and systematic data on the status of visual performance than the conventional tests. 1. 2. 3. 1. 2. 3. MEASUREMENT OF CONTRAST SENSITIVITY When a subject is presented with the grating frequencies and contrast below which resolution is impossible indicates the threshold level; and the reciprocal of this contrast threshold gives the contrast sensitivity. Contrast sensitivity is measured as (Lmax- Lmin/Lmax + Lniin); where L is the luminance recorded by photocells scanning across the gratings. There are three variables in the measurement of contrast sensitivity: Average amount of light reflected depends on illumination of paper and darkness of ink. Degree of blackness in relation to the white background, i.e. contrast. The distance between the grating periods or cycles per degree of visual angle. Various methods have been developed to measure contrast sensitivity. Bodis Wollner introducing contrast sensitivity measurement in clinical practice, suggested the name 'visuo-gram' analogue to an 'audiogram' to describe a patient's 'contrast sensitivity curve'. The deficits were expressed in terms of decibels; and three types of deficits were described: High frequency type characterized by increasing loss at high frequency. A level loss type characterized by a similar loss for all spatial frequencies. A selective loss type characterized by deficits in a narrow band of spatial frequencies. SINE WAVE VERSUS SQUARE WAVE GRATING ▪ Sine ivave patterns. Visual scientist will describe contrast in terms of alternating bars of light and dark in terms of spatial frequency, the units are described as cycles per degree (cpd). One cycle consists of a black bar and white space next to it (Fig. 6.8.1). Fig. 6.8.1. Sine wave patterns of different contrast used for testing contrast sensitivity. ▪ Square ivave or foucalt gratings are also used to describe contrast sensitivity. However, in optics, very few images can be described as perfect square wave gratings with perfect sharp edges. Sine wave patterns are considered essential element from which any pattern can be constructed. Combination of different sine waves can add up to produce a square pattern (Fig. 6.8.2). This trick of breakdown of any alternating pattern into unique sum of sine waves is known as Fourier transformation (Fig. 6.8.3). Fig. 6.8.2. Square wave and sine wave grating patterns measured as cycles per degree. Visual system operates by breaking down observed patterns and scenes into sine waves of different frequencies. The brain then adds them up again to produce mental impression of a complete picture. Fourier transformations are the ways that the visual system encodes and records retinal images. Fig. 6.8.3. Fourier transformation; summation of sine waves gives square wave pattern. RECORDING OF CONTRAST SENSITIVITY Recording of contrast sensitivity for a person is called as contrast sensitivity testing function (CSTF). A good optical system has high contrast sensitivity for low frequencies. It gradually decreases for the higher spatial frequencies as diffraction and other aberrasions make detection of finer details more difficult. Retina-brain tend to enhance the contrast of spatial frequencies of 2-6 cpd. CS also decreases with increasing age because of more diffraction by lens and decreased ability of retina-brain processing to enhance the contrast with increasing age. CS also decreases with decrease in retinal luminance. As mentioned above, CS was first used by neurologist Dr. Ivan Bodis-Wollner 40 years back in patients of occipital lesions, who had disturbing visual symptoms but retained good visual acuity. In 1977, a British ophthalmologist Dr GB Arden introduced first commercially available contrast sensitivity product. Later he and his colleagues published a number of papers in patients of glaucoma, cataract, optic neuritis, amblyopia, etc. FDA developed interest in CS in late 1980 and early 1990 when multifocal lenses became available and laser vision correction was introduced. Now testing of CS has been incorporated in routine clinical practice and FDA has approved its use in clinical trials. Now the standard of FDA trials is testing of four different frequencies under two different levels of lighting conditions; photopic and mesopic (85 cd/m2 and 3 cd/m2). CS testing has also been found to be useful in patients who retain good Snellen acuity but suffer from disturbing visual symptoms. Snellen acuity is also insensitive tool in patients who are developing cataract and have normal Snellen acuity. Its usefulness has been found in many other ocular diseases like refractive surgery, multifocal intraocular lens implants, glaucoma, diabetic retinopathy, amblyopia, optic neuritis and age-related macular degeneration. In general, the methods recommended to measure contrast sensitivity include: simple plates, cathode ray tube display on a screen letter acuity charts, laser interferometer which produces grating on the retina, visual field testing using low contrast rings on stimuli, pattern discrimination test, prototype for forced choice printed test, visually evoked cortical potentials to checker board pattern reversal dependent contrast threshold measurement, two alternative forced choice test, and many more. Some of the simple, inexpensive but reliable methods of measuring contrast sensitivity are described in brief. 1. Arden gratings Arden in 1978, introduced a booklet containing seven plates—one screening plate (No. 1) and six diagnostic plates (No. 2-7). The contrast changes from top to bottom and covers a range of approximately 1.76 log units. The plates are studied at 57 cm, with spatial frequency increasing from 0.2 cycles/degree to 6.4 cycles/degree, each being double the frequency of the previous one. A score of 1-20 is assigned to each plate, depending upon the amount of plate uncovered. Sum of six plates with an upper limit of 82 was established for normal subjects together with an interocular difference of less than 12. 2. Cambridge low contrast gratings Cambridge low contrast gratings consist of a 'set often plates' containing gratings in a spiral bound booklet. To perform the test, the booklet is hung on a wall at a distance of 6 metres. The pages are presented in pairs one above the other. One page in each pair contains gratings and the other is blank (Fig. 6.8.4), but have the same mean reflectance. The subject is simply required to choose which page, top or bottom, contains the gratings. The pages are shown in order of descending contrast and told to stop when the first error is made. Four descending series are shown separately to each eye. When no error is made at plate 10, then a score of 11 is given. Depending upon the total score of the patient from four series, the contrast sensitivity is noted from the conversion table (Fig. 6.8.5). Fig. 6.8.4. Cambridge low contrast gratings. From Wilkins etal. Fig. 6.8.5. Cambridge low contrast gratings score sheet and conversion table. 3. Pelli-Robson contrast sensitivity chart This chart consists of letters which subtend an angle of 3 degrees at a distance of 1 meter. The chart is printed on both the sides. The two sides have different letter sequence but are otherwise identical. The letters on chart are organized as triplets, there being two triplets in each line (Fig. 6.8.6A and B). The contrast decreases from one triplet to the next. The log contrast sensitivity varies from 0.00 to 2.25. Fig. 6.8.6. Pelli-Robson contrast sensitivity chart: (A) Photograph; and (B) Log contrast sensitivity score of each triplet. To perform the test, the chart is hung on the wall, so that its centre is approximately at the level of subject's eye. The chart is illuminated as uniformly as possible so that the luminance of the white areas is between the acceptable range of 60 to 120 cd/m, which corresponds to a photographic exposure between 1/15 and 1/30 second at f/5.6 with an ASA of 100. The luminance is determined with the help of a light meter. While recording, the subject sits directly in front of the chart at a distance of one metre (with the best distance correction) (Fig. 6.8.7). The subject is made to name or outline each letter on the chart, starting from the upper left corner and reading horizontally across the line. Subject is made to guess, even when concluded, when the subject guesses two of the three letters of the triplet incorrectly. The subject's sensitivity is indicated by finest triplet from which two of the three letters are named correctly. Fig. 6.8.7. Measurement of contrast sensitivity with Pelli-Robson chart. 4. The visitech chart This chart consists of sine wave gratings and is used at a distance of 3 m from the subject. In this test, contrast is assessed at several spatial frequencies (distance of separation of the grating bars) and the subject has to identify the orientation of the gratings, i.e. w5. Vector vision charts Vector vision CSV 1000 (USA) charts test frequency of 3, 6,12 and 18 cpd. 6. Fact CS charts The fact CS chart test for 1.5,3,6,12 and 18 cpd. NEURAL MECHANISMS AND FACTORS AFFECTING CONTRAST SENSITIVITY NEURAL MECHANISMS OF CONTRAST SENSITIVITY Campbell and Green gave the concepts of different visual channels for handling information about different bands of spatial frequencies. This concept indicates that retina is non-uniform. Fovea is specialized for high acuity and is responsible for high spatial frequencies. In the retinal periphery, only low frequency channels are represented. For coarse grating, central and peripheral retina have equal contrast sensitivity per unit area of retina, but larger the retinal area stimulated greater is the sensitivity. Thus, contrast sensitivity will be reduced in peripheral retinal diseases, and the use of low frequency grating would provide rapid check of peripheral retinal function. Further, Campbell and Robson proposed the existence within nervous system of linearly operating independent mechanisms selectively sensitive to limited range of spatial frequencies. The orientation to limited range of spatial frequencies, the orientation selectivity and the interocular transfer of the adaptation effect implicated the visual cortex as the site of these neurons. They attempted to explain the preliminary and essential role of such interactions in the recognition of complex images and generalization for magnification. FACTORS AFFECTING CONTRAST SENSITIVITY 1. Refractive errors. Visibility of low spatial frequencies is not limited by the refractive property of the eye; the refractive errors affect only the higher frequencies. 2. Age. There occurs a definite decrease in contrast sensitivity with increasing age. It has been reported that from twenties onwards, contrast sensitivity scores for normal population decline with age by about 10% each decade of life. The average decline over the lifespan is similar to the range of sensitivity within the normal population at any given age. 3. Lenticular changes. Early lens changes can reduce contrast sensitivity essentially for low spatial frequencies. This decrease in contrast sensitivity is not related to the visual acuity. 4. Ocular and systemic diseases. Contrast sensitivity is also found to be affected by various ophthalmic as well as systemic diseases. It is decreased in cases with retinal, optic nerve and visual pathway diseases, glaucoma, ocular hypertension, retrobulbar neuritis, multiple sclerosis, amblyopia, diabetes mellitus, and pituitary adenoma, etc. DIAGNOSTIC APPLICATIONS The contrast sensitivity function in recent years has become of interest as a possible diagnostic indicator of visual function. Deviations from normal standards have been reported in a number of conditions; some of which are listed above. It has been reported that contrast sensitivity (modulation transfer functions) may provide a fairly complete statement of the relations among spatial frequency or the fineness of visual details, the contrast required for resolution of detail, and the luminance of the stimulus. Visual acuity is the least indicative of visual function. It is contrast sensitivity that is the most important thing to measure in terms of visual function. Contrast sensitivity testing provides a more comprehensive assessment of vision than visual acuity testing. This testing is especially important when assessing, treating, and following patients who have undergone LASIK or those with age-related macular degeneration (AMD), glaucoma, or cataracts. 1. Before LASIK surgery Contrast sensitivity testing is an important preoperative measurement for all patients who are to have LASIK. It is important to do contrast sensitivity testing prior to LASIK to establish a baseline for postoperative comparison. Such a finding could make the surgeon cautious about doing the surgical procedure without some other testing to explain reduced contrast results prior to LASIK. If the patient comes back after LASIK with reduced contrast, there would be no explanation as to why this would happen after LASIK. One should know this beforehand in any patient. After LASIK, contrast sensitivity is usually reduced for 2 to 3 months. Although immediate contrast sensitivity testing can be done, it is best to wait a few months, with a follow-up contrast sensitivity test completed 3 months after LASIK as it returns to normal after 3 months of LASIK surgery. 2. Age-related macular degeneration Patients with treatable macular disease should have contrast sensitivity testing as part of the preoperative evaluation to assess the effectiveness of treatment. CS testing can indicate how far AMD has progressed in terms of how much the contrast has been damaged by this macular process. Hence CS testing is imperative as a guideline and as a follow-up test in these patients. This is particularly true in patients who are to undergo photodynamic therapy or intraocular steroid treatment. The Snellen visual acuity measurement does not provide the whole story. It is recommended that CS testing is done immediately after treatment and again at 6 weeks and 3 months in patients with AMD. If these patients have fairly good contrast, they will do better with low vision aids also. 3. Glaucoma Standard visual field testing does not reveal early progression of glaucoma. Patients with glaucoma have a specific response to contrast sensitivity testing. Everything else can seem to be normal in these patients. The acuity is stable, pressures are normal, the fields seem to be stable, however, the contrast can gradually decrease. Contrast tests that provide midfrequency information are the most useful in glaucoma management because they test the frequency most at risk from cell damage. Patients with glaucoma who are at risk 4. Cataract Typical high-contrast visual acuity testing is not accurate in patients with cataracts. A patient can have a cataract and yet see dark letters on the Snellen chart, which gives a false impression. Different types of cataract may respond differently to contrast sensitivity testing. Depending upon the type of cataract, one will want to do a contrast test as part of the initial examination as it gives an edge on when to order surgery. A reduced contrast sensitivity test usually indicates the need for surgery before a visual acuity test will. 5. Diabetic retinopathy Visual acuity testing does not provide sufficient information towards retinal function in patients of diabetic eye disease. CS has been widely studied in patients of diabetic eye disease. Patients of diabetes show lowering of CS before they actually develop diabetic retinopathy and have normal visual acuity. Dissociation in visual acuity and CS occurs early in the diabetic eye disease. Disturbance in foveal avascular zone has been found to be associated with lowering of CS for 6 and 12 cpd. Panretinal photocoagulation in proliferative diabetic retinopathy and macular treatment in macular oedema can reduce the CS, hence it is important that patients
what is Refsum's syndrome ?
RP + peripheral neuropathy and + cerebellar ataxia.
explain treatment for macular oedema and retinal neovascularisation ?
T/T is required in patients presenting with marked visual loss (usually ischaemic CRVO) and those with progressive visual loss on observation l. Medical therapy, 2. Laser therapy, 3. Surgical therapy
ozurdex concentration ?
dexamethasone 0.7 mg
what is second order neuron of visual pathway ?
ganglion cell
silver wiring is seen in ?
grade 4 k/w/c ( silver jast kimtich copper peksha mhanun 4 )
what is another name for lschaemic CRVO ?
haemorrhagic retinopathy
what is HETINOPATHY IN PREGNANCY-INDUCED HYPERTENSION ?
htn + proteinuria +generalised oedema
what are types of retinoblastoma ?
i. Endophytic retinoblastoma ii. Exophytic retinoblastoma iii. Mixed endophylic and iv. Diffuse infiltrating tumours
what are the various Classifications of retinoblastoma ?
i. Reese-Ellsworth classification, iL International classification ofretinoblastoma (JCRB),
S/N on Scleral buckling ?
inward indentation of sclera to provide external tamponade is still widely used to achieve the above mentioned goal successfully in simple cases of primary RD. Sciera! buckling is achieved by inserting a n explant (silicone sponge or solid silicone band) with the help of mattress type sutures applie d in the sclera Radially-oriented explant is most effective in sealing an isolated hole, and circumferential explant (encirclagie) is indicated in breaks involving three or more quadrants.
CHARACTERISTIC FEATURE OF PRIMARY RETINAL TELANGIECTASIA ?
irregular dilation of capillary bed and segmental dilation of neighbouring venules
what is lnterphotoreceptor matrix (1PM) ?
is present in the potential space between pigment epithelium and the neurosensory retina and constitutes a strong binding mechanism between the two (by binding pigment epithelium to the photoreceptor).
what is Ganglion cell layer diagram ?
it mainly contains me cell bodies of ganglion cells (the second order neurons of visual pamway). There are two types of ganglion cells. 1. The midget ganglion cells= are present in the macular region and the dendrite of each such cell synapses with the axon of single bipolar cell. 2.Polysynaptic ganglion cells= lie predominantly in peripheral retina and each such cell may synapse with up to a hundred bipolar cells.
what is Exophytic retinoblastoma ?
lt grows outwards and separates the retina from the choroid. On fundus examination il gives appearance of exudative retinal detachment.
posterior pole and peripheral retina separated by ?
retinal equator
what is retinal equator ?
retinal equator an imaginary line which is considered to lie in line with the exit of the four vena verticose.
At the optic disc, all the retinal layers terminate except ?
the nerve fibres {1-1.2 million), which pass through the lamina cribrosa to run into the optic nerve.
what is Keith and Wagner classification ?
• Grade I. Mild generalized arteriolar attenuation, particularly of small branches, with broadening of the arteriolar light reflex and vein concealment • Grade II. Marked generalized narrowing and focal attenuation of arterioles associated with deflection of veins at arteriovenous crossings (Salus' sign) • Grade Ill. Grade II changes plus copper-wiring of arterioles, banking of veins distal to arteriovenous crossings (Bonnet sign), tapering of vein s on either side of the crossings (Gunn sign) and right-angle deflection of veins (Salu's sign). Flame-shaped haemorrhages, cotton-wool spots an d hard exudates are also present (Fig. 12.12C). • Grade Iv. All changes of grade Ill plus silver-vviring of arterioles and papilloedema (Fig. 12.12D).
Bell's phenomenon ? become noticable in which dis (2) ? inverse bells phenomenon seen in (3) ?
- The facial nerve carries the afferent fibers for this reflex, while the efferent fibers travel via the oculomotor nerve to the superior rectus muscle that controls upper eyelid movement. In Bell's palsy, this movement is seen because the eyelids fail to close properly. Bell's phenomenon (also known as the palpebral oculogyric reflex[1]) is a medical sign that allows observers to notice an upward and outward movement of the eye, when an attempt is made to close the eyes. The upward movement of the eye is present in the majority of the population, and is a defensive mechanism.[2] The phenomenon is named after the Scottish anatomist, surgeon, and physiologist Charles Bell. Bell's phenomenon is a normal defense reflex present in about 75% of the population, resulting in elevation of the globes when blinking or when threatened (e.g. when an attempt is made to touch a patient's cornea). - It becomes noticeable only when the orbicularis muscle becomes weak as in, for example, bilateral facial palsy associated with the Guillain-Barré syndrome - Due to the bells palsy, Bell's phenomenon is elicited when the patient is asked to close his eyelids - Preoperative and postoperative examination of eye ball for bells eye phenomenon is necessary. Patients with 1. congenital ptosis, 2. residual ptosis and 3. requiring repeat surgery for ptosis correction are more prone to development of inverse bell phenomenon. Post-operative soft tissue oedema or aberrant connections of nervous system is the possible mechanism for it. Once inverse bell phenomenon develops copious lubricating eye drops and frequent examination of eye is required until it get resolved.
Von Hippel-Lindau Disease ocular findings ?
- vhl affects cns,retina,visceral organs - AD, arise from VHL gene ch 3p - hemangioblastomas ( retinal + cns ) - opth.manifestsioons r charecteristic & dx of vhl dis - mutation in vhl gene > disruption of vhl protein > unregulated level of HIF ( hypoxia inducible factors ) > elevated level of VEGF,EPO > tumor - earliest lesion identified is retinal hemangioblastoma 45.65% prevelance, 50 % B/L,loc= temporal, periferal retina. exudative rd, maculae edema, uveitis,catarct - other= cerebellar hemangioblastoma,pheochromocytoma,RCC - dx with FA,IO, - t/t is photocoagulation,cryotherapy,radiation,sx.
WHAT ARE RETINAL VENOUS OCCLUSIVE DISEASE ?
1 Impending central retinal vein occlusion 2 Non-ischaemic central retinal vein occlusion 3 Ischaemic central retinal vein occlusion 4 Hemiretinal vein occlusion 5 Papillophlebitis
WHAT ARE HYPERTENSIVE EYE DISEASE ?
1 Retinopathy 2 Choroidopathy
rop divided in to ?
1 active rop 2 cicatrical rop
what is dose for lntravitreal triamcinolone ?
1 mg/0.1 ml
WHAT ARE PERIPHERAL RETINAL DEGENERATIONS ?
1. Lattice degeneration 2. Snail tract degeneration 3. Degenerative retinoschisis 4. White-with-pressure and white-without pressure 5. Focal pigment clumps 6. Peripheral chorioretinal atrophy (paving stone degenerntion) 7. Microcystoid retinal degeneration
what are visual symptoms of RP ?
1. Night blindness. It is the characteristic and earliest feature and may present several years before the visible changes in the retina appear. it occurs due to degeneration of the rods. 2. Dark adaptation. Light threshold of the peripheral retina is increased; though the process of dark adaptation itself is not affected unlil very late. 3. Tubular vision. i.e. loss of peripheral vision with preservation of central vision occurs in advanced cases. 4. Central vision is also lost ultimately after many years.
what is Blood supply of retina ?
1. Outer four layers of the retina= choroidal and vascular system formed by contribution from anterior cilia1y arteries and posterior ciliary arteries. 2. inner six layers= central retinal arte1y In some individuals, cilioretinal artery(branch from posterior ciliary arteries) is present as a congenital variation and supplies the rnaculararea. In such cases central vision is retained in the eventuality of central retinal artery occlusion ( CRAO ).
what are Atypical forms of retinitis pigmentosa ?
1. Retinitis pigmentosa sine plgmento 2. Sectorial RP 3. Pericentric RP 4. Retir1itis punctata albescens.
steps in Surgical technique for Enucleation ?
1. Separation of conjunctiva and Tenon's capsule 2. Separation ojextraocular muscles 3. Cutting of optic nerve 4. Removal of eyeball 5. inserting an orbital implant 6. Closure of conj unctiva and Tenon's capsule
what are 2 types of ganglion cells ?
1. The midget ganglion cells= are present in the macular region and the dendrite of each such cell synapses with the axon of single bipolar cell. 2.Polysynaptic ganglion cells= lie predominantly in peripheral retina and each such cell may synapse with up to a hundred bipolar cells.
what are signs of CRAO ?
1. Visual acuity= reduced ( <3/60 in 90% cases) except in a few cases with cilioretinal artery supplying the macula 2. Direct pupilary reflex is absent and relative afferent pupillaiy defect (RAPD) is positive. 3. Fundus examination= - Marked narrowing of retinal arteries - Retina becomes milky white due to ischaemic oedema. - In eyes with cilioretinal artery part of macula remains normal in colour - Cherry-red spot is seen in the center of macula due to vascular choroid shining through the thin retina in foveal region, in contrast to the surrounding pale retina - Cattle trucking, i.e. segmentation of blood colunm is seen in the retinal veins. - Atrophic changes. In most cases retinal oedema resolves over a period of 4- 6 weeks and atrophic changes in the form of grossly attenuated thread like arteries atrophic appearing retina and consecutive optic atrophy occur in long standing cases. 4. Fundus imaging studies include= • OCT shows hyper reflectivity with thickening of inferior retina. • Autofluorescence imaging shows reduced autofluorescence. • Fundus fluorescein angiography (FFA) shows delay in arterial filing (cilioretinal artery when present will fill in early phase) and masking of choroidal vasculature due to retinal oedema.
Approved Uses of ozurdex ?
1. adults with diabetic macular edema 2. To treat adults with macular edema following branch retinal vein occlusion (BRVO) or central retinal vein occlusion (CRVO) 3. adults with noninfectious inflammation of the uvea (uveitis) affecting the back segment of the eye
Anomalies of the retina proper ?
1. albinism, 2. congenital night blindness, 3. congenital day blindness, 5. Oguchi's disease, 6. congenital retinal cyst, 7. congenital retinoschisis, 8. congenital retinal detachment and 9. coloboma of the fundus.
what are Congenital anomalies of the macula ?
1. aplasia, 2. hypoplasia and 3. coloboma.
what are Anomalies of the optic disc ?
1. crescents, 2. sites inverses, 3. congenital pigmentation, 4. coloboma, 5. drusen and 6. hypoplasia of the optic disc.
Viral retinitis caused by ?
1. cytomegalic inclusion disease, 2. rubella, and 3. herpes zoster.
what are Constituent molecules of IPM ?
1. lnterphotoreceptor retinal binding protein (IRBP), 2. proteoglycan glycos amino glycans (PAG) (sulphated and nonsulphated chondroitin and hyaluronic acid), 3.fibronectin, 4.sialoprotein associated with rods and cones (SPARC), 5. intercellular adhesion molecules, 6. hyaluronic acid receptor (CD44 antigen), and 7. lysosomal enzymes 8. (matrix metaJloproteinases and tissue inhibitors of metalloproteinases, i.e. TIMP).
Function of RPE ?
1. metabolic support 2. acts as an antireflective layer.
what are Ocular Associations of retinitis pigmentosa ?
1. myopia, 2. primary open angle glaucoma, 3. microphthalmos, 4. conical cornea {keratoconus) and 5. posterior subcapsular cataract.
what are Anomalies of vascular elements in retina ?
1. persistent hyaloid artery and 2. congenital tortuosity of retinal vessels.
COMPLICATIONS OF RD ?
1. proliferative vitreoretinopathy (PVR), 2. complicated cataract, 3. uveitis and 4. phthisis bulbi.
Manifestations of lntraocular stage of retinoblastoma divided in which subgroup ?
1. quiescent presentations and 2. painful red eye presentations
what is Functions of IPM ?
1. retina attachment and adhesions, 2. molecular trafficking, 3. facilitation of phagocytosis 4. photoreceptor outer segment alignment.
what are Common vascular disorders of retina ?
1. retinal artery occlusions, 2. retinal vein occlusions, 3. diabetic retinopathy, 4. hypertensive retinopathy, 5. sickle cell retinopathy, 6. retinopathy of prematurity and 7. retinal telangiectasia.
WHAT ARE THE INFLAMMATORY DISORDERS OF RETINA ?
1. retinitis 2. chorioretinitis 3. neuroretinitis 4. retinal vasculitis
Hence, anri- VEGFs are preferred over IVTA these days B/O ?
1. risk of glaucoma, 2. steroid induced cataract, and 3. increased vulnerability to endophthalmitis
what include in mx of DR ?
1. screening, 2. investigations and 3. treatment
RETINOPATHIES OF BLOOD DISORDERS SEEN IN WHICH DISEASES ?
1. sickel cell anaemias, 2. leukaemias 3. polycythemias.
of how many lschaemic CRVO develops rubeosis iridis within 3 month ?
20 %
what are the fundus findings in CRVO ?
3. Fundus examination= - Marked narrowing of retinal arteries - Retina becomes milky white due to ischaemic oedema. - In eyes with cilioretinal artery part of macula remains normal in colour - Cherry-red spot is seen in the center of macula due to vascular choroid shining through the thin retina in foveal region, in contrast to the surrounding pale retina - Cattle trucking, i.e. segmentation of blood colunm is seen in the retinal veins. - Atrophic changes. In most cases retinal oedema resolves over a period of 4- 6 weeks and atrophic changes in the form of grossly attenuated thread like arteries atrophic appearing retina and consecutive optic atrophy occur in long standing cases.
MX OF RETINAL VEIN OCCLUSIONS ?
A. Clinical evaluation and investigations I. Ocular examination and Investigations II. Systemic examination and investigations B. Differential diagnosis C. Treatment I. Treatment of systemic and ocular assocjations ll. Observation and monitoring Ill . Treatment for macular oedema and retinal neovascularisation,
T/T of RB ?
A. Conservative tumour destructive therapy to salvage eyeball 8. Enucleation C. Palliative therapy
S/N on Sealing of relina l breaks ?
All the retinal breaks should be de tected, accu rately localised and sealed by producing aseptic chorioretinitis, with cryocoagulation, or photocoagularion or diathermy. CryocoaguJation is utilised, with scleral buckling and pneumoretinopexy while endo-laser photocoagulation is used during V-R surgery.
Increased vascular pemieability in HTN retionopathy due to ?
Arteriolar narrowing due to vasospasm is the primary response to raised blood pressure >>> vasospasm >>> arterio narrowing >>>hypoxia >>> breakdown of inner blood retinal barrier >>> Increased vascular pemieability >>> leads to= 1. haemorrhages, 2. exudates, 3. focal retinal oedema, 4. macular oedema, 5. focal intraretinal periarterial transudates (FlPTs), and 6. disc oedema.
what is physiological blind spot. ?
Because of absence of photoreceptors (rods and cones), the optic disc produces an absolute scotoma in the visual field called! as physiological blind spot.
C/F OF COATS' DISEASE ?
C/F= 1• Typically affects one eye of boys in their first decade of life. 2• In early stages, it is characterised by large areas of intra and subretinal yellowish exudates and haemorrhages associated with overlying dilated and tortuous retinal blood vessels and a number of small aneurysms near the posterior pole and around the disc. 3• It may present wilh visual loss, strabismus or leucocoria (whitish pupillary reflex) and thus needs to be differentiated from retinoblastoma. 4• Condition usually progresses to produce exudative retinal detachment and a retrolental mass. ln late stages complicated cataract, uveilis and secondary glaucoma occur, which eventually end in phthisis bulbi. 5• FFA highlights abnormal vessels, leakage and areas of capillary drop out.
DESCRIBE CAVERNOUS HAEMANGIOMA OF RETINA ?
Cavernous haemangioma of retina and optic disc head is a rare unilateral congenital hamartoma. C/F= Cavernous haemangioma may present as loss o f vision due to vitreous haemorrhage or mostly is discovered on routine fundus examination. It may involve any part of retina, occasionally optic nerve. Ophthalmoscopic features= Retinal cavernous haemangioma is a cluster of aneurysms with associa ted greyish tissue, resembling a bunch of grapes in appearance. Fluid levels in the form menisci are also seen in the lesion. Complications= in the form of haemorrhages and epiretinal membrane (ERM) formation can occur.
what is Acute hypertensive optic neuropathy ?
Changes include: • Disc oedema and haemorrhages on the disc and peripapillary retina which occur due to vasoconstriction of peripapillaiy choroidal vessels supplying the optic nerve head. The ischemia of the optic nerve head leads to stasis of axoplasmic flow, thus the lesion is a form of anterior ischaemic optic neuropathy. • Disc pallor, of variable degree, may occur late in the cource of disease
CLASSIFICATION OF TUMOURS OF RETINA ?
Classlficaticn of tumors of retina = A. Primary tumours 1. Neuroretlnal tumors • Retinobla:sroma • Astrocytoma 2. Retinal pigment epithelial(RPE) tumours • Congenital hypertrophy ofRPE • Congenital simple hamartoma of RPE • Combined hamartoma of retina and RPE • Adenoma and adenocarcinoma ofRPE • Hyperplasia and migration ofRPE simulating uveal melanoma 3. Retinal vascular tumours • Capillary haemangioma • Cavernous haemangioma • Racemose haemangioma • Vasoproliferative tumor 4. Primary vilreorelinal lymphoma. B. Secondary tumours l. Direct extension, e.g. from malignan t melanoma of the choroid. 2. Metastatic carcinomas from the gastrointestinal n·act, genitourinary Lract, lungs, and pancreas. 3. Metastatic sarcomas. 4. Metastatic malignant melanoma from the skin. C. Paraneoplastic syndromes L. Cance r-associated retinopathy 2. Melanoma-associated retinopathy.
RETINA : CRITICAL FLICKER FUSION FREQUENCY : DEFINITION AND FACTORS AFFECTING Definition Factors affecting CCF frequency Test field-related factors Background-related factors Observer-related factors TALBOT-PLATEAU LAW OF CCF Practical applications FLICKER AND ELECTRORETINOGRAM MECHANISM OF FLICKER AND FUSION
DEFINITION AND FACTORS AFFECTING DEFINITION When intermittent light stimuli are presented to the eye, a sensation of 'flicker' is evoked. As the frequency of presentation of the stimuli is increased, a point is reached at which flicker sensation fuses to form the sensation of continuous stimulation. This frequency is known as the critical flicker fusion (CFF) frequency. The CFF frequency serves as a measure of the temporal resolving power of the visual system under the particular condition of stimulation. FACTORS AFFECTING CFF FREQUENCY I. Test field-related factors 1. Luminance. In general, the CFF frequency in-creases with increase in the luminous intensity of the flickering light.1,2 For a foveal test field, this relationship is linear over a wide range, between 0.5 and 10,000 trolands (photons), and this is the basis of the so-called Ferry-Porter law which states that the critical fusion frequency is proportional to the logarithm of the luminance of the flickering patch (Fig. 6.10.1).3 At a very high luminance of fusion, frequency passes through a maximum in the region of 50 to 60 cycles per sec. At very low luminance, in the scotopic range, the fusion frequency is remarkably low, of the order of 5/sec, so that under these conditions, the temporal resolution of the fovea is extremely small, a separation of 200 msec between successive stimuli not being discriminated. Fig. 6.10.1. Relation to critical flicker fusion frequency with luminance of the test field. 2. Spectral composition. As we know, different wavelengths of light stimulate the rods and cones differently; so the equal energy spectral lights have different luminances which according to Ferry-Porter law should result in different CFF values. Figure 6.10.2 shows the effects of different spectral distributions of the illuminant on the relation between CFF and retinal illuminance.4 Except for small differences of maximal CFF at high brightness, the upper (cone) branch of the flicker function is the same for all seven wavelengths. The major effect of changing the spectral distribution of the test stimulation is to vary the accentuation of the low-luminance rod branch of the function. The shorter the wavelength of the test light, the more prominent is the rod branch. Fig. 6.10.2. Relation of critical flicker fusion frequency with logarithm of retinal illuminance for each of seven spectral regions. From Hecht and Shaer,4 3. Size of the stimulus. As stimulus size is increased over a range of l°-5°, in diameter, there is nearly linear relation between CFF and logarithm of area.5 This relation is maintained for luminances varying over a range of 3 log units and for 9 retinal locations as far as 10° out in the periphery. 4. Retinal position of the stimulus. Since the CFF function for rods differs from that of cones, the CFF for a test stimulus confined to a limited area of the retina depends on the relative number of rods and cones stimulated in the area. Therefore, if the size of test field is held constant, but it is located at increasing distances out in the periphery, the influence of rods is enhanced and that of cones is decreased. As shown in Fig. 6.10.3 for a test stimulus presented 15° away from the point of fixation (0°), the high luminance cone branch of the function is much reduced and reaches a much lower maximum frequency.3 This change in flicker fusion frequency with increasing eccentricity is not a simple function of eccentricity but varies in different retinal locations.6 Fig. 6.10.3. Relation of critical flicker fusion frequency with log retinal illuminance for three different retinal locations (fovea, 5° above fovea and 15° above fovea) of the test stimulus. From Hecht and Verijp.3 5. Duty cycle. It refers to proportion of light to darkness during a flicker cycle. It has a complex effect on CFF, that varies with stimulus luminance and area. It is reported that CFF varies in proportion to the logarithm of the dark fraction of the cycle, provided that flash trains of different duty cycles are held at the same time average luminance.1 6. Duration of flashes. The average retinal illuminance rises when the duration of flashes is held constant. This changes the ocular adaptation level, which in turn influences the CFF level. 7. Number of flashes in the train. During the first several flashes of a flickering stimulus, an initially dark-adapted eye becomes partially light-adapted. Further, the response of the eye becomes increasingly well synchronized.8,9 With these changes, there occurs an increase of CFR. This increase of CFF might be related to the cone late receptor potential.10 8. Monocular versus binocular presentation. Following facts have been reported regarding monocular versus binocular presentation of • • • • • • test flashes vis-à-vis CFF: Synchronised flash trains of two eyes fuse at a higher frequency than the alternating flash trains, which stimulate each eye during the dark intervals at the other.11 Steady light presented to one eye reduces CFF measured for the other eye.12-14 Intermittent light having a frequency above CFF at one eye reduced CFF for the other eye more than did a steady light of equal brightness. This effect implies a cortical, rather than retinal limit on CFF.12 9. Figtiration (shape, striation, pattern) of test stimulus. Change in figuration of test stimulus may affect the CFF (because of the border effects) as below: CFF for a striated test field is reported to be lower than that of a uniform test field of equal overall size.15 CFF is reported to decrease to a minimum and then increase again as stripe width of test field is varied.15 The minimum CFF with striped field is reported to be below that of an unpatterned field equated in area to the lighted part of the striped field.15 II. Background-related factors 1. Luminance. The effects of background luminance that surrounds the test field are as follows: The highest CFF is obtained with a surround matched to the Talbot brightness of the test area, a condition that minimizes border contrast. When the test field is surrounded by darkness, the CFF (in addition to luminance and other features of test field) also depends upon stray light, scattered within the ocular media or that reflected from the illuminated area on to other parts of the retina. 2. Area of the background. CFF is reported to increase in proportion to the logarithm of the area for surround up to 4 degrees.16 Further, large surround light adapts a large part of the retina. III. Observer-related factors 1. Adaptation. In general, the higher the level of light adaptation, the shorter is its critical duration and thus higher is the CFF for a given test stimulus. The state of adaptation affects the CFF since it governs the relative sensitivity of rod and cone mechanisms.17 In other words, with increasing dark adaptation, the flicker fusion frequency decreases at the fovea, but this relation is non-monotonic at 10° and 50° from the fovea. 2. Pupil size. The pupillary constriction evoked by the light lowers the retinal illuminance and thus reduces CFF. Therefore, to prevent interaction between pupil size and CFF, an artificial pupil is often placed before the observer's eye. Alternatively, the pupil size may be kept constant by use of mydriatic-cycloplegic agent. 3. Age of the observer. It has been established that CFF decreases with the age. The factors such as changes of the ocular media, macular pigment, retinal and central nervous mechanisms may be responsible for this age-related decline in CFF. 4. General health. In general, the reduced body efficiency, fatigue, decreased vitality and general debility are associated with a decline in CFF. 5. Effects of drug. Hypoxia and inhalation of carbon dioxide are reported to lower the CFF, whereas hyperventilation increases CFF. Physostigmine, when applied topically, is reported to decrease CFF. THE TALBOT-PLATEAU LAW OF CFF This law makes a general statement which implies that, when CFF has been reached, the intensity of the resultant sensation, i.e. the brightness of the intermittently illuminated, but non-flickering patch, is the mean of the brightness during a cycle. According to this law, for example, if the subject's view of an illuminated patch of luminance of 5 milli lamberts, is interrupted by rotating an opaque disc infront of it from which a sector has been removed, the resultant sensation will be equal to that given by a continuous stimulus of luminance equal to 5' Area of sector/Total area of the disc. Practical applications This law finds a useful application in many experimental studies in which it is desired to cut down the illumination by accurately determined amounts. FLICKER AND ELECTRORETINOGRAM As discussed earlier, the electroretinogram (ERG) is a characteristic sequence of potential changes which occur in response to a flash of light. These changes outlast the duration of stimulus flash. A second flash falling on the eye during these changes starts off a new series of events, but the extent to which it will modify the existing state of retinal potential will depend upon how soon it follows. With rapidly repeated stimuli, a condition may be reached that the potential never falls to its baseline; and at one level the frequency may be high enough that the records appear unbroken and smooth. From the point of view of ERG, this frequency may be called as critical flicker fusion (CFF) frequency. However, this does not necessarily mean that at this frequency even subjective sensation is likewise smooth. MECHANISM OF FLICKER AND FUSION The mechanism of flicker and fusion has been studied in detail by Enroth18 and many other researchers.19,20 It has been reported that the mechanism of flicker and fusion is related to the pre-excitatory inhibition of the succeeding stimulus on the excitatory effect of the preceding In other words, the actual period between flashes that permits fusion is determined by the latency of this pre-excitatory inhibition and the latency of the excitatory response. Further, the ganglion cells respond to a flash of light in different manners and are described as Pure ON-, Pure OFF-, and ON-OFF-ganglion cells accordingly since they give a burst of spikes at ON, OFF or both at ON and OFF. It has been reported that with repeated stimuli, usually fusion of one response occurs before that of the other, so that if, for example, ONresponse fuses first, the record will be the same or that for a flickering OFF-response. Thus, if we consider an OFF-element at the end of the first flash, it discharges after a latency of a few msec. The second flash will inhibit this discharge provided the latency of its inhibitory effect is not too long; and as shown in Fig. 6.10.4, it will occur when the dark period is equal to the difference between the two latencies. These latencies are variable, so that simple relationships between them and flicker-rate are not to be expected, although in general there is, indeed, a strong correlation between the length of the dark period and the difference of latencies.20 Fig. 6.10.4. Diagrammatic representation of OFF-latency and preexcitatory inhibition latency during flicker of an OFF-element. Flashes are indicated by black rectangles. A, the dark interval is too long for the inhibition of the light flash between the OFF-iatency and the inhibitory latency. From Enroth, C.18 In contrast to OFF-elements, with ON-elements, the significant factor is probably the postexcitatory inhibition, that brings to an end the discharge in response to the onset of the flash. This inhibition is seen when the element is stimulated with a single flash of light; the response is a primary activation lasting about 20-70 msec and consisting of a burst of spikes; this is followed by a discharge pause of 80-250 msec, and then a secondary activation. The discharge pause represents postexcitatory inhibition, and fusion will presumably occur when the flashes are so timed that the primary activation of one flash is due to fall in the discharge pause of the preceding one. It has been concluded by some workers that the latency of onset of
what id Diffuse infiltrating tumours ?
Diffuse infiltrating tumours show just a placoid thickness of retina and not a mass. Such cases are usually diagnosed late.
what are the Manifestations of extraocular extension of retinoblastoma ?
Diue to progressive enlargement of rumour, the globe bursts through the sci era, usually near the limbus or near the optic disc. It is followed by rapid fu ngation and involvement of extraocular tissues resulting in marked proptosis
choroid rupture mechanism of injury ?
During a closed globe injury, the eyeball is first mechanically compressed and then rapidly hyperextended. The sclera's tensile strength resists this compression. The retina is elastic and stretches during such an injury. However, Bruch membrane breaks because it does not have sufficient tensile strength or elasticity. The choriocapillaris is injured and bleeds into the subRPE and/or subretinal space. Such hemorrhage may hide the choroidal rupture initially. Over days, the blood clears and a whitish/yellowish, curvilinear, crescent-shaped subretinal streak is visible, usually concentric to the optic disc. Over time, choroidal neovascularization (CNV) can develop. In most, the CNV involutes over time. In about 30%, the CNV may recur, with a serous or hemorrhagic pigment epithelial detachment, anytime following formation of the choroidal rupture. If the rupture or CNV does not involve the foveal center, vision may not be affected.
optic nerve avulsion ? parhogenesis of traum ?
During isolated blunt trauma to the orbital framework, the globe continues to move anteriorly without any active resistance, in contrast to the globe, the optic nerve with more delicate bony and soft tissue relations, likely to remain relatively static. Thus the junction between the optic nerve and ocular coat suffers the maximum distractive injury due to anteroposterior tractional forces. In addition to this, physiological Bell's phenomenon may induce torsional tension at this junction leading to further worsening of distractive forces and violent separation of optic nerve from the globe.
what is finfing seen in EOG of RP ?
Electro-oculogram {EOG) is subnormal with an absence of light peak.
what are Elschnig's spots ?
Elschnig's spots are small black spots surrounded by yellow halos, these are formed due to clumping and atrophy of the infarcted pigment epithelium (focal white spots) described above.
when examination done in case of severe NPDR ?
Every 3 months,
when examination done in casse of of moderate NPDR ?
Every 6 months,
when examination done in casse of no diabetic retinopathy or there is mild NPDR ?
Every year
double ring sign seen in ?
HYPOPLASIA OF OPTIC DISC
WHAT ARE RETINAL DYSTROPHIES ?
Hereditary retinal dystrophies primarily affect the outer retina (photoreceptors and RPE). Common retinal dystrophies can be classified as below: A. Generalised photoreceptor dystrophies B. Macular dystrophies
HISTOPATHALOGY OF RB ?
Histopathalogy= The tumour chiefly consists of small rounnd cells with large nuclei, resembling the cells of the nuclear layer of retina. These cells may present as a highly undifferentiated or well-different.iated tumour. • Microscopic features of a well-differentiated tumour include Flexner-Wintersteiner rosetes, (highly specific of retinoblastoma), Homer-Wright rosettes, pseudo rosettes and tleurenes formation • Other histologic features are presence of areas of necrosis and calcification.
what is arterial circle of Zinn or Haller ?
However, anastomosis between the retinal vessels and ciliary system of vessels (short posterior ciliary arteries) does exist with the vessels which enter the optic nerve head from the arterial circle of zinn or Haller. Branches of this circle invade lamina cribrosa and also send branches to Lhe optic nerve head ( optic disc) and the surrounding retina.
SIGNS OF RRD ?
I . External examination, eye is usually normal. 2. lntraocular pressure is usually slightly lower or may be normal. 3. Marcus Gwm pupil (relative afferent pupillary de.feet) is present in eyes with extensive RD. 4 . Plane mirror examination or Distant Direct ophthalmoscopy reveals an altered red reflex in the pupillary area (i.e. greyish reflex in the quadrant of detached retina). 5. Ophthamoscopy should be carried out both direct and indirect techniques. Retinal detachment, is best examined by indirect ophthalmoscopy using scleral indentation ( to enhance visualization of the peripheral retina anterior to equator). • Freshly-detached retina gives grey reflex instead of normal pink reflex and is raised anteriorly (convex configuration). It is thrown into folds which oscillate with the movements of the eye. These may be small or may assume the shape of balloons in large bullous retinal detachment. In total detachment retina becomes funnel-shaped, being attached only at the disc and ora serrata. ??? Retinal vessels appear as dark tortuous ??? cords oscillating with the movement of detached retina. • Retinal breaks associated with rhegmatogenous dletachment are located with difficulty. These look reddish in colour and vary in shape. These may be round, horse-shoe shaped, slit-like or in the form of a large anterior dialysis (Fig. 12.29). Retinal breaks are most frequently found in the periphery (commonest in the upper temporal quadrant). Associated retinal degenerations, pigmentation amd haemorrhages may be discovered. • Vitreous pigments may be seen in the anterior vitreous (tobacco dusting or Shaffer sign). with posterior vitreous detachment. • Old retinal detachment is characterized by retinal thinning (due to atrophy), formation of subretinal demarcation line (high water marks) clue to proliferation of RPE cells at the junction of flat detachment and formation of secondary intraretinaJ cysts (in very old RD). 6. Visual field charting reveals scotomas corresponding to the area of detached retina, which are relative to begin with but become absolute in longstanding cases. 7. Electroretinography (ERG) is subnormal or absent. 8. Ultrasonography confirms the diagnosis. It is of particular value in patients with hazy media especially in the presence of dense cataracts and vitreous haemorrhage.
TYPES OF RETINITIS ?
I. Infectious retinitis a. Bacterial retinitis -Acute purulent retinitis -Subacu te retinitis of Roth. b. Viral retinitis. -Cytomegalovirus (CMV) retinitis C. Mycotic, rickettsial or parasitic retinitis II. Non-infectious retinitis
S/N on OCT-based classifica tion of diabetic macular edema ?
I. Non-tractiottal DME. It may be of following types: a. Spongy thickening of macula (>290 μ), b. Cystoid macular oedema (CME}, and c. Neurosensory detachment with or without (a) or (b) above. Patients with above changes, seen on OCT, are further divided into two types: • Centre involving diabetic macular edema with loss of foveal contour • Non-centre involving diabetic macular edema, with thickening within 3000 um of the centre of the macula, but not involving the centre. 2. Tracional DME. It may be of following types: a. Vitreo-foveal traction (VFT}, and b. Taut/thickened posterior hyaloid membrane. Note. The OCT classification has a bearing on the management since non-tractional DME is treated conservalively whereas tractional DME is purely treated by pars-plana vitrectomy (PPV} with removal of posterior hyaloid.
RETINA : PHOTOCHEMISTRY OF VISION Introduction Vitamin A and visual pigments Dietary source of retinol Absorption and storage Transport from liver to the eye Utilization of vitamin A for synthesis of rhodopsin Visual pigments Rhodopsin Cone pigments Light-induced changes Rhodopsin bleaching Rhodopsin regeneration Visual cycle Photochemistry of photopic vision
INTRODUCTION As mentioned earlier, the light falling upon the retina is absorbed by the photosensitive pigments in the rods and cones and initiates photochemical changes which in turn initiate electrical changes and in this way the process of vision begins. To understand the process of photochemistry, it is mandatory to know in detail about the photosensitive compounds, i.e. the rod I pigments and the cone pigments. Further, pivotal to all photoreactions in animal tissue is the presence of fat-soluble compound, vitamin A. Therefore, the subject of photochemistry will be discussed under following heads: Vitamin A and visual pigments Visual pigments Light-induced changes VITAMIN A AND VISUAL PIGMENTS The vitamin A cycle from its dietary intake to its transport to the eye is depicted in Fig. 6.3.1 and described below. Fig. 6.3.1. Vitamin A cycle from its dietary intake to its transport to the eyes. Dietary sources of retinol Dietary sources of retinol for humans include animal foods and plant foods. ▪ Animal foods contain vitamin A as such (i.e. retinol); some foods are much richer in retinol than others. The liver, which stores retinol, is the best source. Milk products are also very rich in retinol. ▪ Plant foods do not contain vitamin A as such but in the form of precursors the carotenoid pigments (carotenes). The carotenes cannot be used directly in the photochemical process but must be converted into vitamin A (retinol) by metabolic activity in the wall of the small intestine. Three types of carotenes—the alpha, beta and gamma—are present in plant food. The beta carotenes yield 2 molecules of vitamin A, while the alpha and gamma yield one molecule each.1 Absorption and storage In the intestine, the vitamin A is esterified and reaches the bloodstream through the intestinal lymphatics. Most of this retinol from the bloodstream is transported to the liver, where it is stored. In the liver, retinol becomes bound with the retinol-binding protein (RBP). It is quite stable in this combination.2 Transport from liver to the eye The retinol-protein complex enters the circulation and reaches the target tissues where it is utilized. In the retina, it becomes attached to the specific receptors present on the basal surfaces of the retinal pigment epithelial (RPE) cells. Then, it is assumed that the RBP is left outside and the retinol is transported by a specific transport protein inside the RPE cells (Fig. 6.3.2). Fig. 6.3.2. Utilization of vitamin A for synthesis ofrhodopsin. Utilization of vitamin A for synthesis of rhodopsin Figure 6.3.2 summarizes the process of utilization of vitamin A for synthesis of rhodopsin. As shown in Fig. 6.3.2, inside the RPE cells, there occurs no change in the retinol (vitamin A). Thus the retinol passes through the RPE cells (unchanged) into the outer segments of the photoreceptor's. Inside the photoreceptor's outer segment, the retinol is oxidized to retinene by the enzyme retinene reductase. The retinene then combines immediately with the protein opsin to form the rhodopsin. The NAD oxidative system (present in the RPE) supports the reaction of rhodopsin formation by removing hydrogen. Therefore, for the formation of rhodopsin, it is essential that the RPE and photoreceptor outer segment must be closely opposed to each other. The freshly formed rhodopsin molecule is then incorporated into the newly forming double discs, which then assume their place in the innermost portion of outer segment of photoreceptors. VISUAL PIGMENTS Visual pigments are those substances which have the property of absorbing light (the visible portion of electromagnetic spectrum). When a substance absorbs all wavelengths of light equally, it appears grey or black. A green pigment absorbs light of all wavelengths except green and thus appears green. However, most of the pigments in the visual cells are not limited in their absorption to one small band of wavelengths but rather absorb, to a greater or lesser extent, over a broad range of the spectrum. The peak of each pigment's absorption curve is called its absorption maximum. The visual pigments in the eyes of humans and most other mammals are made up of a protein called opsin and retinene, the aldehyde of vitamin A. The term retinene is used to distinguish this compound from retinene two which is found in eyes of some animal species. Since the retinene are aldehydes, they are called retinals. The vitamin A as such is alcohol and thus called retinol. RHODOPSIN (VISUAL PURPLE) ▪ Rhodopsin is the photosensitive visual pigment present in the discs of the rod outer segments. It consists of a protein opsin (called as scotopsin) and a carotenoid called retinal (the aldehyde of vitamin A). Rhodopsin is thus a membrane-bound glycolipid which is held in a rigid, highly organized arrangement, partially by the action of phospholipid present in the plasma membrane of the photoreceptor discs. Human rhodopsin has a molecular weight of 40,000.3 It is one of the many serpentine receptors coupled to G proteins (Fig. 6.3.3A). Rhodopsin protein is insoluble in water but can be taken into solution, if detergent is added. The rhodopsin is essentially a solute in a twodimensional solution. It is sensitive to heat and chemical agents (ethanol, strong acids or alkalies) that denature the protein. ▪ Protein opsin of the human rhodopsin is a 348 amino acid protein that crosses the disc membrane seven times (Fig. 6.3.3B). Two palmitate molecules are linked with cysteines via thioester linkages at the intracellular C-terminal. These fatty acids are anchored into the lipid bilayer forming a fourth intracellular loop. Oligosaccharide residues are located on the extracellular N-terminal; there is some evidence that these may help to maintain the structural integrity of the disc. The amino acid sequences for all human cone opsins are almost identical, the difference in spectral absorbance being determined by a few different amino acids. The light absorbing form of vitamin A is retinal, which binds to opsin at a Schiff base linkage site to form rhodopsin. Fig. 6.3.3. Schematic diagram of rhodopsin depicting: A, Serpentine receptor coupled with G protein; B, Protein opsin crosses the disc membrane seven times. ▪ Absorption spectrum of rhodopsin as shown in Fig. 6.3.4 depicts that its peak sensitivity to light lies within the narrow limits of 493-505 nm. It absorbs primarily yellow wavelength of light, transmitting violet and red to appear purple by transmitted light; it is, therefore, also called visual purple. Fig. 6.3.4. Absorption spectrum of rhodopsin. • • • • • CONE PIGMENTS Visual pigments present in the cones have not been so intensively studied as the rhodopsin. There are three kinds of cone in primates. Cone pigments are somewhat different from the rhodopsin, in that they respond to specific wavelengths of light, giving rise to colour vision. These differences are present in the opsin portion of the molecule, whereas the chromophore 11-cis-retinal remains the same. Peak absorbance wavelengths of the 'blue', 'green' and 'red' sensitive cones lie at about 435, 535 and 580 nm, respectively. Relative sizes of the population of the three types of cone are not well established, though several lines of evidence indicate that blue-sensitive cones are the least prevalent. Molecular details of the cone pigment protein remain obscure LIGHT-INDUCED CHANGES As also mentioned earlier, the light falling upon the retina is absorbed by the photosensitive pigments in the rods and cones and initiates photochemical changes which in turn initiate electrical changes and in this way the process of vision sets in.4 The photochemical changes occur in the outer segments of both the rods and the cones. These changes have been intensively studies in the rod's outer segments. However, similar reactions probably also apply to the cones. The photochemical reactions studied in the rod outer segments can be described under three headings: Rhodopsin bleaching, Rhodopsin regeneration, and Visual cycle. • RHODOPSIN BLEACHING As discussed earlier, the rhodopsin consists of a protein called opsin and a carotenoid called retinene (vitamin A aldehyde or 11-cisretinal). The light absorbed by the rhodopsin converts its 11-cis-retinal into all-trans-retinal. These are isomers having same chemical composition but different shapes (Fig. 6.3.5). This light-induced isomerization of 11-cis-retinal into all-trans-retinal occurs through formation of many intermediates which exist for a transient period (Fig. 6.3.6). Fig. 6.3.5. Isomerization of 11-cis-retinal to all-trans-retinal. • Fig. 6.3.6. The scheme for the reactions set into motion by light falling on the rhodopsin. One of the intermediate compounds (metarhodopsin II, also called as activated rhodopsin) of the above isomerization chain reaction acts as an enzyme to activate many molecules of transducin. The transducin is a GTP/GDP exchange protein present in an inactive form bound to GDP in the membranes of discs and cell membrane of the rods. The activated transducin (bound to GTP) in turn activates many more molecules of phosphodiesterase (PDE) which catalyses conversion of cyclic guanosine monophosphate (cGMP) to GMP, leading to a reduction in concentration of cyclic-GMP (cGMP) within the photoreceptor (Fig. 6.3.7). The reduction in cyclic-GMP is responsible for producing the electrical response (receptor potential), which marks the beginning of the nerve impulse. • Further details are described in the section on electroneurophysiology of vision. Fig. 6.3.7. The scheme for reactions triggered by rhodopsin bleaching which affect cGMP: A, light-induced conversion of rhodopsin (R) into the active form (R*); B, activation of G-protein (G), GTP/GDP exchange and activation of cGMP phosphodiestrase (PDE) protein; C, phosphorylation of photolysed rhodopsin (R'). The all-trans-retinal (produced from light-induced isomerization of 11-cis-retinal) can no longer remain in combination with the opsin and thus there occurs separation of opsin and all-transretinal. 5,6 This process of separation is called photodecomposition and the rhodopsin is said to be bleached by the action of light. RHODOPSIN REGENERATION The all-trans-retinal separated from the opsin (as above), subsequently enters into the chromophore pool existing in the photoreceptor outer segment and the pigment epithelial cells (for this, close approximation of RPE and photoreceptor is must). The alltrans- retinal may be further reduced to retinol by alcohol dehydrogenase, then esterified to re-enter the systemic circulation. The first stage in the reformation of rhodopsin, as shown in Fig. 6.3.6, is isomerization of all-trans-retinal back to 11-cis-retinal. The process is catalyzed by the enzyme retinal isomerase. Energy for the regeneration process is supplied by the overall metabolic pool of the photoreceptor outer segment.7 The 11-cis-retinal in the outer segments of photoreceptors reunites with the opsin to form rhodopsin. This whole process is called regeneration of the rhodopsin. Thus the bleaching of the retinal photopigments occurs under the influence of light, whereas the regeneration process is independent of light, proceeding equally well in light or darkness. The amount of rhodopsin in the rods, therefore, varies inversely with the incident light. VISUAL CYCLE In the retina of living animals, under constant light stimulation, a steady state must exist under which the rate at which the photochemicals are bleached is equal to the rate at which they are regenerated.8 This equilibrium between the photodecomposition and regeneration of visual pigments is referred to as visual cycle (Fig. 6.3.8). The bleaching regeneration equilibrium which is reached in either the rods or the cones during various conditions of incident light accounts for only a portion of the sensitivity change that is observed. The remaining portion of the adaptive mechanism occurs under the influence of neural elements within the retina and does not involve a photopigment mechanism.9 The exact level within the retina, at which this visual mechanism operates is unclear, although a gathering amount of evidence indicates that it is at the level of synapses of the photoreceptors with the dendrites of the bipolar cells and may involve activity of horizontal cells. Fig. 6.3.8. Visual cycle, showing rhodopsin bleaching and regeneration. PHOTOCHEMISTRY OF PHOTOPIC VISION Like rhodopsin, cone pigments also consist of the protein opsin (called photopsin) and the retinene (11-cis-retinal). Photopsin differs slightly from the scotopsin (rhodopsin). As mentioned earlier, there are three classes of cone pigments: red-sensitive (erythrolabe), green-sensitive (chlorolabe) and blue-sensitive (cyanolabe), which have different absorption spectra. It has been assumed that when light strikes the cones, the photochemical changes occur in the cone pigments which are very similar to those of rhodopsin. However, it has been noted that, nearly total rod bleaching occurs before significant bleaching can be observed in the cones. This differential bleaching quality sets aside the scotopic rod portion of the visual system from the photopic portion which functions during brightly lighted conditions.
RETINA : COLOUR VISION : 6.9 COLOUR VISION INTRODUCTION MECHANISM OF COLOUR VISION Theory of colour vision Photochemistry of colour vision NEUROPHYSIOLOGY OF COLOUR VISION Genesis of visual signals in photoreceptors Processing and transmission of colour vision signals in the retina Processing of colour vision in LGB Analysis of colour vision in visual cortex PHENOMENA ASSOCIATED WITH COLOUR VISION Simultaneous colour contrast Successive colour contrast Phenomenon of colour constancy Hierarchy of colour-coded cells COLOUR TRIANGLE BASED ON PHOTOPIGMENTS THE COLOUR-METRIC CIE colour space system • • • • • • • • Munsell colour system NORMAL COLOUR ATTRIBUTES Hue Lightness Saturation COLOUR BLINDNESS Introduction Congenital colour blindness Acquired colour blindness Tests for colour vision
INTRODUCTION Colour sense is the ability of the eye to discriminate between colours excited by light of different wavelengths. Some broad facts about colour vision are as follows: Colour vision is a function of cones and thus better appreciated in photopic vision. Colour is a perceptual phenomenon, not just a physical property of an object. Many factors determine the colour perceived: the spectral composition of light from the object is important, but the spectral composition of light from the visual surroundings and the state of light adaptation of the eye also contributes. Sensation of colour is subjective. Individuals are taught names for their colour sensations and subsequently use these names whenever the same sensation is obtained. There are three different types of cones, viz. red-sensitive, green-sensitive and blue-sensitive, which combinedly perform the function of colour vision. All colours are a residt of admixture in different proportion of three primary colours: the red (723-647 nm), green (575-492 nm) and blue (492-450 nm). Colours have three attributes: Hue, intensity and saturation. For any colour, there is a complementary colour that, when properly mixed with it, produces a sensation of white. The colour perceived depends in part on the colour of other objects in the visual field. Thus, for example, a red object is seen red, if the field is illuminated with green or blue light but as pale pink or white if the field is illuminated with red light. A normal person can see all zvavelengths between violet to red. If the wavelength is shorter than that of violet, the light becomes ultraviolet (UV) and is beyond visibility. If the wavelength is greater than 750 nm, the light is infrared and is again beyond visibility. Human beings could have seen even • UV light as blue cones retain some sensitivity at around 10 nm, but crystalline lens blocks all UV rays. Consequently, after cataract operation, one can see the UV rays to some extent. In dim light, all the colours are seen as grey-, this is called Purkinje shift phenomenon. MECHANISM OF COLOUR VISION THEORIES OF COLOUR VISION Many theories have been put forward to explain the properties of human colour vision, but two have been particularly influential. • • 1. Trichomatic theory The trichromacy of colour vision was originally suggested by Young1 and subsequently modified by Helmholtz.2 Hence, it is called Young- Helmholtz theory. It postulates the existence of three kinds of cones, each containing a different photopigment and maximally sensitive to one of three primary colours, viz. red, green and blue. The sensation of any given colour is determined by the relative frequency of the impulse from each of the three cone systems. In other words, a given colour consists of admixture of the three primary colours in different proportion. The correctness of the Young-Helmholtz's trichomacy theory of colour vision has now been demonstrated by the identification and chemical characterization of each of the three pigments by recombinant DNA technique, each having different absorption spectrum as below (Fig. 6.9.1).3,5,6 Red-sensitive cone pigment, also known as erythrolabe or long wavelength-sensitive (LWS) cone pigment, absorbs maximally in a yellow position with a peak at 565 nm. But its spectrum extends far enough into the long wavelength to sense rod. Fig. 6.9.1. Absorption sectrum of three cone pigments. Green-sensitive cone pigment, also known as chlorolabe or medium wavelength-sensitive (MWS) cone pigment, absorbs maximally in the green portion with a peak at 535 nm. • Bine-sensitive cone pigment, also known as cyanolabe or short wavelength-sensitive (SWS) cone pigment, absorbs maximally in the blue-violet portion of the spectrum with a peak at 440 nm. Thus, the Young-Helmholtz theory concludes that blue, green and red are primary colours, but the cones with their maximal sensitivity in the yellow portion of the spectrum are light at a lower threshold than green. It has been studied that the gene for human rhodopsin is located on chromosome 3, and the gene for the blue-sensitive cone is located on chromosome 7. The genes for the red- and green-sensitive cones are arranged in tandem array on the q arm of the X chromosomes. 2. Opponent colour theory The opponent colour theory of Hering points out that some colours appear to be 'mutually exclusive'. There is no such colour as 'reddish green', and such phenomenon can be difficult to explain on the basis of Trichromatic theory alone. In fact, it seems that both theories are useful in that: The colour vision is trichromatic at the level of photoreceptors, and Colour opponency is explained by subsequent neural processing as discussed in neurophysiology of colour vision. PHOTOCHEMISTRY OF COLOUR VISION The cone pigments like rhodopsin also consist of 11-cis-retinal and an opsin part. While the 11-cis-retinal is similar to that of rhodopsin, the opsin part known as photopsin is different than the opsin part of the rhodopsin. The green-sensitive and red-sensitive cone pigments are very similar in structure, their opsins show 96% homology of amino acid sequence; whereas, each of these pigments has only about 43% homology with the opsin of blue-sensitive cone pigment. All the three cone pigments have about 41% homology with the rod pigment rhodopsin. Though the photochemistry of the cones has not been studied in detail, however, all evidences point to the fact that the principles of photochemistry of rhodopsin can be applied to the cone pigments. The only difference being that the three different types of cones are bleached by light of different wavelength. NEUROPHYSIOLOGY OF COLOUR VISION Similar to photochemical changes, the physiological processes concerned with colour vision are also the same as for the vision in general. So all the processes are just mentioned here with a specific note wherever indicated. GENESIS OF VISUAL SIGNALS IN PHOTORECEPTORS The photochemical changes in the cone pigments followed by a cascade of biochemical changes produce visual signal in the form of cone receptor potential. As already discussed, the cone receptor potential has a sharp onset and offset; whereas the rod receptor potential has a sharp onset but slow offset. However, as yet there is no direct electrical evidence from primate receptors for the three-receptor spectral sensitivity responses. PROCESSING AND TRANSMISSION OF COLOUR VISIONS SIGNALS IN THE RETINA The action potential generated in the photoreceptors is transmitted by electronic conduction to the other cells of the retina across the synapses of photoreceptors, bipolar cells and horizontal cells and then across the synapses of bipolar cells, ganglion cells and amacrine cells. The neurophysiological activities (concerned with the processing and transmission of colour vision signals) occurring in the different retinal cells can be summarized as below. Horizontal cells Svaetichin7 studied the response in fish horizontal cells and reported that slow graded potential noted in these cells was similar to receptor responses, in that the amplitude of the response increased steadily with increasing illumination. But, unlike the receptors, horizontal cells showed two completely different kinds of response. First, there was a hyperpolarizing response with a broad spectral function which Svaetichin termed as luminosity (I) response; and second, a chromatic (c) response which was hyperpolarizing for part of the spectrum and depolarizing for the remainder of the spectrum. This observation caused a considerable interest since it provided the first physiologic evidence for opponent colour coding in the vertebrate retina. It also represents the first stage in the visual system where evidence of chromatic interactions has been found and where wavelength discrimination can occur. Bipolar Cells Recordings made from goldfish bipolar cells showed a 'centresurround' spatial pattern.8 Red light striking in the centre of these cells caused hyperpolarization and green light in the surroundings caused depolarization. However, still it has not been possible to record from the very slender bipolar cells of primates. Amacrine Cells • • • The exact role of these cells in colour vision is not clear. However, some workers have speculated that they may act as an 'automatic colour control'.9 Ganglion Cells It is at this level that we see first direct evidence in the visual system for colour coding in primate eyes. As already discussed, there are three distinct groups of ganglion cells designated as W, X and Y cells. Each of these serve a different function. It has been observed that colour sensation is mediated by the 'X' ganglion cells. A single ganglion cell may be stimulated by a number of cones or by a few cones. When all the three types of cones (red-, green- and bluesensitive) stimulate the same ganglion cell the resultant signal is white. Opponent colour cell: Some of the ganglion cells are excited by one colour type cone (e.g. red) and are inhibited by other (i.e. green) or vice versa. This system is called 'opponent colour cell' system and is concerned in the 'successive colour contrast'. Double opponent colour cell: These ganglion cells have a system which is opponent for both colour and space. This system is called 'double opponent cell' system and is concernd with the 'simultaneous colour contrast'. The double opponent cells have a receptive field with a centre and surround. The response may be 'on' to one colour (e.g. red) in the centre and 'off' to it in the surround; while the response may be 'off' to green in the centre and 'on' to it in the surround. These systems indicate that the process of colour analysis begins in the retina and is not entirely a function of the brain. Distribution of colour vision in the retina Trichromatic colour vision mechanism extends 20-30 degrees from the point of fixation. Peripheral to this red and green become indistinguishable, and in the far periphery all colour sense is lost, although cones are still found in this region of retina10 (Fig. 6.9.2). Fig. 6.9.2. Map of distribution of colour vision in retina. From Wald, G.10 The very centre of fovea (1/8 degree) is blue blind. When a red test object is brought from the periphery in the field of vision, the individual first becomes aware of a colourless object in the periphery. Then as the object is advanced, it is seen successively as salmon pink or yellow and eventually as red. • • • • - - - - PROCESSING OF COLOUR SIGNALS IN LATERAL GENICULATE BODY (LGB) Following observations have been made:11-13 All lateral geniculate body neurons carry information from more than one cone cell. Colour information carried by the ganglion cells is relayed to the parvocellular portion of the LGB. Spectrally nonopponent cells which give the same type of response to any monochromatic light constitute about 30% of all the LGB neurons. Spectrally opponent cells make 60% of LGB neurons. These cells are excited by some wavelengths and inhibited by others and thus appear to carry colour information. These have been classified into 4 types: Cells having red and green antagonism with +R/-G Cells having red and green antagonism with +G/-R. Cells with blue and yellow antagonism with +B/-Y. Cells with blue and yellow antagonism with +Y/-B. ANALYSIS OF COLOUR SIGNALS IN THE VISUAL CORTEX Colour information from the parvocellular portion of the LGB is relayed to the layer IVc of the striate cortex (area 17). From there, the information passes to the blobs in layers II and III. The neurons in the blobs lack orientation-specificity but respond to colours. Like the ganglion cells and LGB cells, they are centre-surround cells. Many are double-opponent cells, which, for example, are stimulated by green centre and inhibited by green surround and are inhibited by red centre and stimulated by red surround.14-17 From the blobs, colour information is relayed to thin strips in the visual association area and from there to a specialized area concerned with colour, which in human is in the lingual and fusiform gyri of occipital lobe. SUMMARY OF NEURAL ENCODING OF COLOUR From the above description, it is clear that the process of colour analysis begins in the retina and is not entirely a function of the brain. In the retina, neural processing of colour probably begins at the level of the ganglion cells, although horizontal and amacrine cells may modulate the properties of ganglion cells receptive fields. There are two main types of colour opponent ganglion cells: Red-green opponent colour cells use signals from red and green cones to detect red/green contrast within their receptive field. Blue-yellow opponent colour cells obtain a yellow signal from the summated output of red and green cones, which is contrasted with the output from blue cones within the receptive field. Colour information follows the parvocellular pathway to visual cortex. PHENOMENA ASSOCIATED WITH COLOUR VISION SIMULTANEOUS COLOUR CONTRAST This phenomenon refers to perception of particular coloured spot against the coloured background. For example, a grey spot appears greenish in a red surround and reddish in a green surround. In general, the colour of the spot tends to be towards the complementary of the colour of the surround. It is a function of double opponent cells of the visual system. SUCCESSIVE COLOUR CONTRAST It is a phenomenon of coloured after images, and as a general rule the colour of the after image tends to be near the complementary of the primary image. For example, when one sees at a green spot for several seconds and then looks at a grey card, one sees a red spot on the card. Successive colour contrast is a function of the opponent cells of the visual system. PHENOMENON OF COLOUR CONSTANCY It refers to a phenomenon in which the human eye continues to perceive the colour of a particular object unchanged even after the spectral composition of the light falling on it is markedly altered. This phenomenon was observed by Edvrin Land (while developing the Polaroid colour camera), who noted that changing the colour of a light illuminating a scene altered the hue of a colour picture taken by the camera, but did not significantly alter the hue of the scene as observed by human eye. As an example, consider the way we think of new-fallen snow as white, whether looked at by moonlight, at highnoon or in the blush of the setting sun. The phenomenon of colour constancy has been explained as follows: First, the brain computes from all the colours in the scene the overall hue of the entire vision. This computation is helped when some area in the picture is known by the person to be white. Using this information of overall hue, the brain then adjusts mathematically for the changed colours of the illuminating light, though the exact neural mechanism for doing this has not been explained. It has also been observed that the 'blobs' (irregular peg-shaped blobs of cells) present in the primary visual cortex demonstrate colour constancy when the illuminating light changes its wavelength. Therefore, it is believed that somewhere in the neighbourhood of these blobs is located the computational machinery that allows for this phenomenon of colour constancy. HIERARCHY OF COLOUR-CODED CELLS The phenomenon of hierarchy of colour-coded cells suggests a system of serial analysis of colour sense. The colour-coded cells have been reported to be arranged in a hierarchy manner as follows: The opponent colour cells being located in ganglion cells18 and lateral geniculate neurons12,13 and the double opponent cells with either 'centre-surround' or bar-flank receptive fields in the layer IV of striate cortex.19,20 Complex and hypercomplex colour-coded cells have been described in the layers II, III, V and VI of the striate cortex in the form of 'blobs'.21,22 This sequential arrangement suggests that perhaps the cells at one level of the hierarchy converge to form the receptive field of the cells at the next higher level. COLOUR TRIANGLE BASED ON PHOTOPIGMENTS This is a three-dimensional graphic representation of all the colours drawn on the basis of trichromacy of colour mixture (Fig. 6.9.3). The three axes of this colour space, mutually at right angles are scaled to represent various amounts of pigment absorption by each of the three cone pigments. Various interpretations made from this colour triangle are as follows: A particular colour is represented by a line or vector which has its origin at zero absorption for the three photopigments and for its entire length represents a specific ratio of absorption of the three photopigments. Length of the line/vector identifies brightness of the given colour. The angle or direction of the vector in the colour space represents the hue. The colours that result primarily from absorption by greenabsorbing and red-absorbing photopigments lie close to the floor of the colour space. All the colours seen by human eye fall inside a 'solid' which intercepts the triangular plane in the model in the form of a colour triangle. Bichromates have only two cone pigments, and consequently all colours for them can be represented as vectors in a twodimensional plane of this colour space. Fig. 6.9.3. Colour triangle based on photopigments. THE COLOUR-METRIC Various colour-metric systems have been developed to get around the practical everyday problem of making sense that two individuals mean the same colour when naming it. They have been evolved on a psychological basis and have no connection whatsoever with the physiology of eye. The two important colour-metric systems used internationally in industry, in printing and in the graphic arts, are described below in brief. 1. The CIE colour space system This system was developed by the International Commission of Illumination (CIE—Commission Internal de Eclairage), primarily for a precise identification of colours for such items as textiles, paints, food colouring products and soil types. CIE colour space system is based on the amounts of three primary colours necessary to match a specified colour. The CIE chromaticity diagram (Fig. 6.9.4) is constructed by placing the three reference wavelengths chosen say 450 nm, 520 nm and 650 nm at the X, Y and Z of a triangle. In this model, point E represents a source radiating equal amounts of energy in equal intervals of wavelength throughout the spectrum. Standard C is a substitute for daylight and consists of a tungsten lamp with a defined blue filter in front of it and standard A is a defined tungsten lamp seen at a defined current and voltage. Fig. 6.9.4. CIE chromaticity diagram. For description see text. The relative amount of each primary (X, Y, Z) required to match or specify a given wavelength results in a set of "tristimulus values" across the spectrum (Fig. 6.9.5) and are expressed as chromaticity co-ordinates (x, y, z). Because the co-ordinates represent relative amounts of primary (e.g. × = X/x + y + z), only two of them are required to specify the chromaticity of a colour. The luminance quality is expressed separately on the Y value. Two chromaticity (X, Y) coordinates allow this colour specification in a two-dimensional space. The Z axis is in the plane of observer. Fig. 6.9.5. CIE tristimulus values for spectral colour of equal energy. 2. The Munsell colour system In the Munsell colour-metric system, all the colours are represented in a cylinder in terms of hue, value and chroma (HVC). This system covers a wide range of colours and is thus widely used in medicine and industry. The colour attributes in the cylinder are represented as below: Hue dimension, i.e. dominant spectral colour is located on the circumference of the cylinder. Value dimension, i.e. lightness is indicated by moving up or down the cylinder. NORMAL COLOUR ATTRIBUTES As mentioned earlier, the colour has three attributes: hue, lightness and saturation. HUE Hue, i.e. dominant spectral colour is determined by the wavelength of the particular colour. Fig. 6.9.6 shows the wavelength discrimination function throughout the spectrum for the normal human eye. Fig. 6.9.6 clearly depicts that there are two regions (approximately at 490 nm and 590 nm) of the visible spectrum where the ability to discriminate between adjacent wavelengths is at a maximum. Here discrimination is less acute in the middle of the spectrum and deteriorates rapidly at the ends of the spectrum.23 Fig. 6.9.6. Wavelength discrimination function throughout the spectrum for the normal human eye. From Le Grand Y.23 LIGHTNESS The lightness or brightness of a colour depends upon the luminosity of the component wavelength. In photopic vision (daylight), normal eye has a peak luminosity function at approximately 555 nm and in scotopic vision (dim light) at about 507 nm. The wavelength shift of maximum luminosity from photopic to scotopic viewing is called Purkinje shift (Fig. 6.9.7). Fig. 6.9.7. Photopic (solid lines) and scotopic (dotted lines) luminosity curves. SATURATION Saturation refers to degree of freedom from dilution with white. It can be estimated by measuring how much of a particular wavelength must be added to white before it is distinguishable from white. The more the wavelength required to be added to make the discrimination the lesser the saturation and vice versa. COLOUR BLINDNESS INTRODUCTION An individual with normal colour vision is known as 'trichomate'. In colour blindness, faculty to appreciate one or more primary colours is either defective (anomalous) or absent (anopia). It may be congenital or acquired. • • • • • • • CONGENITAL VERSUS ACQUIRED COLOUR BLINDNESS Acquired colour vision defects differ from congenital defects in fundamental ways, although it is not always easy to distinguish the two clinically. Acquired defects begin after birth. Unlike congenital defects, they may fluctuate in severity and type. Individuals with acquired defects often have an associated decrease in visual acuity and visual field loss. In congenital deficiency, except for the monochromatic variety, it is not common to find these symptoms. Congenital defects are easier to classify by type into precise categories of protan, deutan, or tritan. Acquired defects are often hard to classify and may be a combination of types. Congenital defects usually affect both eyes equally. Acquired defects are frequently monocular. Congenital defects are most commonly protan or deutan and rarely tritan. In contrast, acquired defects most commonly affect the short-wave spectrum (blue-yellow defect; Kollner's rule). Congenital defects are predominantly X-linked, and therefore, usually found in men. Acquired defects result from disease, aging, or drugs, processes that affect men and women equally. Clinical evaluation It is preferable that both an anomaloscope and a test of chromatic discrimination such as the FM 100-hue test are used. Tests for bluecolor defects are also necessary. Each eye should be examined separately and testing must take into account factors such as reduced visual acuity and visual field defects. In older individuals, short-wavelength defects may be a part of the aging process and it may be difficult to sort out the effects of disease pathology. Pseudoisochromatic tests, especially tritan plates, can be an important screening tool. However, individuals with acquired defects may not give the expected readings. The lines of color confusion are often different for individuals with acquired defects than those expected for congenital defects. Thus, for example, a patient with an acquired defect may miss both classification numerals meant to distinguish protan from deutan defects. Before using isochromatic plates, the visual acuity of the patient should be checked. • • I. CONGENITAL COLOUR BLINDNESS INTRODUCTION Colour vision deficiency (CVD) is a common functional disorder of vision affecting 8% males and 0.4% females in Caucasian societies. It is an X-linked recessive inherited condition. The classification system for congenital colour vision dates back to the nomenclature introduced by von Kries in 1875. This system describes congenital defects by the Greek terms protan (Ist), deutan (second), and tritan (third), referring to red, green, and blue deficiencies, respectively. Table 6.9.1 summarizes the various forms of abnormal congenital colour vision defects. Broadly, it may be of the following types: Dyschromatopsia Achromatopsia Table 6.9.1. Types of congenital defective colour vision Classification Mechanism Characteristics Monochromasy Typical monochromasy Once thought to be due to an absence of cones or to cones being filled with rhodopsin but now proposed to be the result of mutation of genes encoding the cone-specific alpha and beta subunits of the cation channel Colour blind. No perception of colours Colours distinguished by brightness differences only. Very insensitive to red light Nystagmus. Low visua l acuity 6/36 to 6/60. Painless photophobia Classification Mechanism Characteristics Blue cone monochromasy S (Blue) cone pigment only. No L (red) or M (green) cone pigment Nystagmus. Low visua l acuity 6/12 to 6/24. Painless photophobia Colour blind. Colours distinguished by brightness differences only. Rudimentary colour vision in mesopic vision from red and blue cone activation. Very insensitive to red light Dichromasy Protanopia Absence of L (red) cone pigment Very reduced ability to identify colours. Confuse red, yellow and green, white and green, and blue and purple. Reduced sensitivity to red light Deuteranopia Absence of M (green) cone pigment Very reduced ability to identify colours. Confuse red, yellow and green, and white and green Tritanopia Absence of S (blue) cone pigment Very reduced ability to identify colours. Confuse blue with blue-green and green, and white with yellow Anomalous trichromasy Classification Mechanism Characteristics Protanomaly L (red) cone pigment absorption spectrum shifted to shorter wavelengths of light May confuse white with green and confuse reds, yellows and greens but loss of colour discrimination varies greatly between individuals. Reduced sensitivity to red light. Make abnormal colour matches: For example will add excess red in the colour match R + G = Y Deuteranomaly M (green) cone pigment absorption spectrum shifted to longer wavelengths of light May confuse white with green and confuse reds, yellows and greens but loss of colour discrimination varies greatly between individuals. Make abnormal colour matches: For example will add excess green in the colour match R + G = Y Tritanomaly Partial loss of S (blue) cone pigment Loss of colour discrimination for blues, blue-greens, and greens This table is adapted from International Recommendations for Colour Vision Requirements for Transport. CIE Technical Report 143. Vienna; CIE: 2001. • • • • • • • • 1. Dyschromatopsia Dyschromatopsia, literally means colour confusion due to deficiency of mechanism to perceive colours. It can be classified into: Anomalous trichromatism Dichromatism a. Anomalous trichromatic colour vision. Here the mechanism to appreciate all the three primary colours is present but is defective for one or two of them. It may be of following types: P rot anomalous: It refers to defective red colour appreciation. Deuter-anomalous: It means defective green colour appreciation. Tritanomalous: It implies defective blue colour appreciation. b. Dichromatic colour vision: In this condition, faculty to perceive one of the three primary colours is completely absent. Such individuals are called dichromates and may have one of the following types of defects: Protanopia, i.e. complete red colour defect. Deuteranopia, i.e. complete defect for green colour. Tritanopia, i.e. absence of blue colour appreciation. Red-green deficiency (protanomalous, protanopia, deuter anomalous and deuteranopia) is more common. Such a defect is a source of danger in certain occupations such as drivers, sailors and traffic police. Blue deficiency (tritanomalous and tritanopia) is comparatively rare. • • • • 2. Achromatopsia It is an extremely rare condition presenting as cone monochromatism or rod monochromatism. ▪ Cone monochromatism is characterised by presence of only one primary colour and thus the person is truely colour blind. Such patients usually have a visual acuity of 6/12 or better. ▪ Rod monochromatism may be complete or incomplete. It is inherited as an autosomal recessive trait. It is characterized by: Total colour blindness, Day blindness (visual acuity is about 6/60), Nystagmus, Fundus is usually normal. MOLICULAR GENETICS AND INHERITANCE OF COLOUR BLINDNESS Molecular genetics The molecular genetics of human colour vision is complex because one of three distinct genes has to be expressed in a particular cone cell that is otherwise identical to its neighboring cones. The complexity arises in part because M-and L-sensitive genes, which are approximately 98% identical, are juxtaposed on the X chromosome. Recent advances in molecular biology have allowed remarkable parallel advances in understanding the molecular basis for colour perception. The most reasonable hypothesis that links genotype to clinical phenotype is the spectral proximity hypothesis, which states that red-green colour discrimination is a function of the difference between the wavelengths of maximal absorption of the M-and Lsensitive pigments. An optimal separation might be approximately 30 nm. Inheritence of colour blindness Colour blindness is present as an inherited abnormality in Caucasian populations in about 8% of the males and 0.4% of the females. Tritanomaly and tritanopia are rare and show no sexual selectivity. However, about 2% of the colour blind males are dichromates who have protanopia or deuteranopia, and about 6% are anomalous trichromates in whom the red-sensitive or the green-sensitive pigment is shifted in its spectral sensitivity. These abnormalities are inherited as recessive and X-linked characteristics, i.e. they are due to an abnormal gene on the X-chromosome. Since all each parent and since these abnormalities are recessive, females show a defect only when both X chromosomes contain the abnormal gene. However, female children of a man with X-linked colour blindness are carriers of the colour blindness and pass the defect on to half of their sons. Therefore, X-linked colour blindness skips generations and appears in males of every second generation. Hemophilia, Duchenne's muscular dystrophy, and many other inherited disorders are caused by mutant genes on the X chromosome. The common occurrence of deuteranomaly and protanomaly is probably due to the arrangement of the genes for the green-sensitive and red-sensitive cone pigments. They are located near each other in a head-to-tail tandem array on the q arm of the X chromosome and are prone to recombination (unequal crossing over) during development of the germ cells. Different combinations of introns may also occur, with both processes rendering one pigment inactive or producing opsins that have shifted spectral sensitivities. • • II. ACQUIRED COLOUR BLINDNESS Etiological profile Special consideration should be given to the study of acquired colour vision defects. These defects are the secondary features of a variety of pathological states. The category of acquired defects includes changes in pre-receptor, receptor, and post-receptor mechanisms that affect the perception of light stimuli. It may follow damage to macula or optic nerv, Usually, it is associated with a central scotoma or decreased visual acuity. Blue-yellow impairment is seen in retinal lesions such as CSR, macular oedema and shallow retinal detachment. • Red-green deficiency is seen in optic nerve lesions such as optic neuritis, Leber's optic atrophy and compression of the optic nerve. Acquired blue colour defect (blue blindness) may occur in old age due to increased sclerosis of the crystalline lens. It is owing to the physical absorption of the blue rays by the increased amber-coloured pigment in the nucleus. TESTS FOR COLOUR VISION It is useful to group these tests into several different categories based on design. Pseudoisochromatic plates, arrangement tests, anomaloscopes, and lanterns represent the most widely used designs. Different tests are appropriate for different circumstances (Table 6.9.2). Table 6.9.2. Categories of colour vision tests Test type Function Example Screening tests Quick diagnosis Pseudoisochromati c plates, e.g. Ishihara test Grading tests Assess severity of the defect FM 100-hue test Diagnostic tests Precisely classify a defect Anomaloscope Vocational tests Simulate environment encountered on the job Lantern test PLATE TESTS J. Stilling first introduced pseudoisochromatic designs in 1873. Today, they are the most commonly used screening tests for colour deficiency in clinical practice. These tests are inexpensive, durable, and readily available. Most tests provide very efficient (90-95%) screening of congenital red-green defects. On the other hand, the tests have distinct limitations. They must be administered under the standard viewing conditions for which they were designed. They are not effective in grading the severity of the colour defect and thus tell us limited information about the extent or type of deficiency. In short, plate tests are best used as screening tools. Plate tests come in several forms; however, the principle of construction is the same. Ishihara test is the prototype of this category. The test consists of a series of cards on which a figure is printed in multiple colours against a multicoloured background. The figure is recognizable to normal trichromats, but camouaged to those with colour defects. The figure used in the test is usually an easily identifiable number, letter, or shape. Figures and background are drawn in dots. The size of the dots used in plate design can be varied or uniform; however, the only difference between the figure and the background is the colour. Saturation and lightness are accounted for, such that detection of the figure in ways other than hue is unlikely. • Ishihara test The Ishihara test is published in a full 38-plate edition, an abridged 24-plate edition and a 14-plate edition for quick screening. Scoring instructions for the Ishihara plates accompany each test. In the 38- plate edition, for example, a normal score permits four or fewer errors. In the 24-plate edition, two errors or less are considered normal. The 16-plate edition also puts two errors or less in the normal range. It does not matter which edition is used because the fail criterion is three or five errors and total number of errors has no diagnostic significance. The Ishihara can be used in children as young as 5 years. The colour differences between the figure and the background are chosen to separate normal trichromats from mild anomalous trichromats. If the differences in colour are too large, anomalous trichromats will be able to identify the hidden figure. Small differences may cause normal trichromats to fail some screening plates. How to test colour vision by Ishihara chart Room should be adequately lit by daylight. Interesting facts The Ishihara test was designed by Shinobu Ishihara (1879-1963), who was a surgeon in the Japanese army before specializing in ophthalmology. Some of his research was on the selection of soldiers and he was asked to devise a test to screen military recruits for colour vision deficiency. He was helped by a colour-blind physician who tested the plates, which were originally painted in water colours and used hirangana symbols. The Ist edition using Arabic numerals was published in 1917 but few were sold • • • • • • until 1929 when the International Congress of Ophthalmology in Holland recommended its use for testing military personnel. In 1958, it was adopted in Japan as the official test for school children when a law was introduced to require colour vision testing in schools as part of a general medical assessment. The Ishihara test is not the first pseudoisochromatic test of colour vision. The German ophthalmologist Jakob Stilling had devised his test in 1878 but the Ishihara test has proved to have the best sensitivity and specificity of all the pseudoisochromatic tests. Nature of the test should be explained to the patient. Full refractive correction is worn. It is preferable to do the test before pupillary dilatation. One eye is first occluded and the other eye is tested. The plates are kept at a distance of 75 cm from the subject with the plane of the paper at right angle to the line of vision. The standard time taken to answer each plate is 3-5 seconds. ARRANGEMENT TESTS Modern arrangement tests were first put into popular use in the 1940s and 1950s by Dean Farnsworth. Farnsworth originally designed the Farnsworth-Munsell 100-hue test (FM 100-hue test) and the Farnsworth Dichotomous test for colour blindness (Farnsworth Panel D-15). The principle of design for these tests is coloured caps of fixed chroma and value selected from the hue circle. The patient is asked to arrange randomly placed caps in what he/she perceives to be a natural order. The colour differences between adjacent caps on the FM 100-hue test were designed to be very small. Good colour vision as well as chromatic discrimination ability is needed to perform well on this test. Farnsworth Panel D-15 was designed to have larger colour differences, but, unlike the FM 100-hue test, only one box containing 15 caps is presented to the patient. ANOMALOSCOPES An anomaloscope is an instrument that uses the principle of colour matching to test colour vision. It serves as a clinical standard for diagnosing and classifying congenital colour deficiency. The use and maintenance of an anomaloscope can be challenging and requires the expertise of a trained technician. For this reason, anomaloscopes are not used as frequently as other clinical tests. The Nagel (model I) anomaloscope is the most common and accurate of these and uses the Rayleigh equation to diagnose redgreen colour vision defects. The subject views a circular field through a telescopic barrel. This field is divided into two parts, each of which is filled with light of different spectral wavelengths. The lower half of the field is filled with spectral yellow at 589 nm. According to the Rayleigh match, the subject should be able to match the 'colour' of the upper to the lower field by adjusting the mixture of red and green light. The luminance knob is particularly useful in distinguishing deutan from protan defects. The subject is asked to comment on whether the upper and lower fields are the same colour. Normal and dichromatic individuals will accept this as a normal or near-normal match. Deuteranomalous trichromats will state that the upper field is too red and protanomalous trichromats will state that the upper field is too green. Scoring of an anomaloscope examination begins by LANTERN TESTS Lantern tests are simple devices geared towards measuring a subject's competency to perform a specific task, namely recognize coloured light signals. Lanterns are used in the maritime, air, and railway industries to screen employees. The subject is asked to name the colours of the light signals that are presented to him. Two types of lanterns exist: those that present single colours and those that present pairs of lights together. The speed and sequence of colour presentation is important to the efficacy of the test. Individuals with defective colour vision make characteristic mistakes in naming the colours presented to them; however, most lantern tests are not meantto screen or categorize colour defectives for clinical purposes
RETINA : ELECTROPHYSIOLOGY OF RETINA AND VISUAL PATHWAY : ELECTRORETINOGRAM Components Physiological basis Measurement Technique Factors influencing Cone and rod function Clinical application ELECTROOCULOGRAM Technique Measurement and interpretation of results Components Clinical applications VISUALLY EVOKED POTENTIAL Technique Types Factors affecting Clinical applications EARLY RECEPTOR POTENTIAL • • • • • • • Components Physiological basis Technique Clinical applications ELECTOPHYSIOLOGICAL TESTS: ESSENTIAL APPLIED ASPECTS Time taken Potential indications Practical status
INTRODUCTION Electrical activity in the retina and visual pathway is the inherent property of the nervous tissues—which remain electrically active at all times and the degree of activity alters with stimulation. Presence of an active membrane, which provides the source of electricity, is a prerequisite for the electrical responses to be evoked in the nervous tissue. In the retina, electrical power is generated at the junction between the photoreceptors and pigment epithelium which forms the functional membrane. The electrical potential thus generated is known as retinal resting potential. As discussed in the photochemistry and neurophysiology of vision, when light excites the retina, a series of rapid and well-defined electrical responses can be evoked from each layer of the retina. The receptor potential generated in the photoreceptors is transmitted by electrotonic conduction (i.e. direct flow of electric current, not action potential) to the other cells of the retina, viz. horizontal cells, bipolar cells, amacrine cells and the ganglion cells. However, at the level of ganglion cells, these responses are converted into action potential, i.e. the responses are transmitted as nerve impulses or spikes. The visual signals then leaving the eye along the optic nerve fibres are coded as nerve impulses and the rate of firing is related to the strength of stimuli. The conduction velocity of optic nerve fibres is proportional to the fibre diameter. The smaller diameter retinal ganglion cells which are densely packed at the fovea and its neighbourhood usually have fine fibres and slow conduction velocities; while the larger retinal ganglion cells in the peripheral retina have large fast conducting fibres. The electrophysiological tests based on the electrical phenomena in the retina and visual pathway have become very important tools in the diagnosis of retinal diseases. These allow an objective evaluation of the retinal functions. The electrophysiological tests include: Electroretinography (ERG) Electrooculography (EOG) Visually evoked response (VER) and • Early receptive potential (ERP). ELECTRORETINOGRAM Electroretinogram (ERG) is the record of changes in the resting potential of the eye induced by a flash of light. It was Holmgren1 who in 1865 for the first time showed that an alteration in the electrical potential occurs when light is thrown on the retina. The credit for the first such recording from humans in 1877 goes to Dewar.2 Granit's (1933) analysis of the retinal electrical potential has provided a frame of reference for further recent advances.3 Riggs' (1941) introduction of practical recording contact lens electrode for use in humans was an important contribution in making electroretinography a routine clinical investigation.4 Modern adaptations to ERG testing include Ganzfeild or full feild diffuse illumination to elicit a response from the entire retina. Computerized averaging and narrow bandpass filters have enabled the detection of ERG responses as low as 0.1 pv. In 1992, Sutter and Tran developed the multifocal ERG, which allows topographical maping of many isolated focal ERG responses. COMPONENTS AND SITE OF ORIGIN OF THE ELECTRORETINOGRAM There exists a potential difference of about 1 mV between the cornea and the posterior pole of the eye. This is known as the corneoretinal potential. This potential is modified by the action of light on the retina. The resultant waveform from modification in the corneoretinal potential in response to a brief flash of light is known as the electroretinogram. The electroretinogram (ERG) is a graphical tracing of the summated action potentials generated in the retina in response to changes in retinal illumination. It is to be noted that the ERG (full field) does not have any representation of horizontal amacrine and ganglion cell function. This is probably due to the fact that the neuronal processes of these cells are tangential. This is in contrast to the processes of photoreceptors, Muller's cells and bipolar cells, which are, arranged radially due to which the current flows out. The electroretinogram is thus the composite of electrical activity from the photoreceptors, Muller cells and the retinal pigment epithelium. It consists of following components (Figs 6.5.1 and 6.5.2). Fig. 6.5.1. Components of electroretinogram (A) and oscillatory potential (B). a-wave It is the initial cornea-negative wave. It arises from the photoreceptors (rods and cones). When recorded in isolation from the other neurons, it is known as the late receptor potential (LRP) or Granit's PIII.5,6 b-wave It is a large cornea-positive wave. It arises from the Muller cells, representing the activity of bipolar cell layer (Fig. 6.5.2).7 The b-wave is disturbed by a ripple of three or four small wavelets known as the oscillatory potential (Fig. 6.5.1B). a-wave It is the initial cornea-negative wave. It arises from the photoreceptors (rods and cones). When recorded in isolation from the other neurons, it is known as the late receptor potential (LRP) or Granit's PIII.5,6 b-wave It is a large cornea-positive wave. It arises from the Muller cells, representing the activity of bipolar cell layer (Fig. 6.5.2).7 The b-wave is disturbed by a ripple of three or four small wavelets known as the oscillatory potential (Fig. 6.5.1B). c-wave It is a prolonged positive wave with a lower amplitude. Being considerably slower, it is not used clinically. It represents the metabolic activity of the pigment epithelium (Fig. 6.5.2).8 Fig. 6.5.2. Origin of components of electroretinogram. PHYSIOLOGIC BASIS OF THE ORIGIN OF ELECTRORETINOGRAM Origin of a-wave As discussed earlier, light falling on photoreceptors produced hyperpolarization; as a result the outermost portion of the photoreceptor becomes positive whereas the innermost portion is negative (Fig 6.5.2); therefore, the cornea is negative and the a-wave is generally recorded as downward deflection. As shown in Fig 6.5.3, a-wave comes about by the summation of potential change along the length of the photoreceptors. Both the rods and cones contribute to the a-wave, but their contributions can be separated by appropriate stimulus condition. A blue dim flash striking a dark-adapted eye produces a rod ERG, while a bright red flash striking a light-adapted eye produces a cone ERG.9 Fig. 6.5.3. Origin of a-wave Origin of b-wave This positive wave arises from the Muller cells in the inner retinal layers (Fig. 6.5.2). The Muller cell is a glial cell, a modified astrocyte, and therefore, has no synaptic connection. The Muller cells of retina like the astrocytes in the optic nerve or central nervous system respond to potassium concentration in the extracellular space. When light strikes a photoreceptor, it releases potassium in amounts related to the light intensity. The Muller cells respond to this potassium by changing their membrane potential in linear relationship to this extracellular potassium (Fig. 6.5.4). Thus, although originating from the Muller cells present within the bipolar cell layer, the b-wave is dependent on electrical activity originating within the photoreceptor layer. In this way, the Muller cells can provide a b-wave from either cone or rod receptors.7 Fig. 6.5.4. Origin of b-wave. Therefore, factors that affect the generation of a-wave will have an obligatory effect on the b-wave production. Cone signals are faster and, if intense enough, can render the Muller cell insensitive to an upcoming rod signal. Further, only the cones respond to light flickered at approximately 20 Hz or faster. This flicker ERG, as it is known, is made up only of the on-and-off effect of the a-wave mechanism. The positive going waves are then 'off effects, from the cone a-wave rather than b-wave as many observers have reported. The wavelets on the b-wave (oscillatory potential) appear to have a separate origin from the b-wave. It has been suggested that these wavelets reflect feedback circuits in the inner retina. These wavelets are seen on the photopic (light-adapted) b-wave. They require a bright stimulus, and the significance of each individual wavelet is unknown. These disappear in cone dysfunction syndromes. These can be selectively abolished in animals by clamping the retinal circulation and a similar effect is seen in humans after central retinal artery occlusion and in severe diabetic retinopathy.10 Origin of c-wave This positive wave is generated from the retinal pigment epithelial (RPE) cells in response to rod signals only.8 This may be because the rod cells are in direct contact with the apical ends of the RPE cells, while the cones do not appear to make such direct contact (Figs 6.5.2 and 6.5.5). This component is not traditionally emphasized or measured in clinical practice. Fig. 6.5.5. Origin of c-wave. Note. From the above description, it is evident that the ERG arises from parts of the retina distal to the ganglion cells. Thus, a normal ERG may be recorded from an eye with advanced optic atrophy, provided that the outer layers of retina are intact. MEASUREMENT OF COMPONENTS AND TYPES OF ERG Measurement of the ERG components includes determination of time relationship and amplitude of deflection. The amplitudes of the waves of ERG normally vary depending upon factors such as duration and intensity of preadaptation, state of retinal stimulation, the characteristics of the stimulus applied, the recording equipment and age and sex of the individual. Because of these variables, absolute values of a-wave and b-wave amplitudes cannot be fixed; but rather must be determined independently by the individual laboratory. Measurement of ERG component Measurement of ERG components is shown in Fig. 6.5.6. It takes the following into consideration. • • 1. Amplitude a-wave amplitude. It is measured from the baseline to the trough of the a-wave (Fig. 6.5.6A). b-wave amplitude. It is traditionally measured from the trough of the a-wave to the peak of the b-wave (Fig. 6.5.6B). • • 2. Time sequences Latency. It is the time interval between the onset of stimulus and beginning of the a-wave response. Normally, it is about 2 msec (Fig. 6.5.6B). Implicit time. It is the time from the onset of the light stimulus until the maximum a-wave or b-wave response. Considering only a-wave and b-wave, the duration of ERG response is less than one-fourth second. Fig. 6.5.6. Measurement of ERG components: A, Amplitude; B, Time sequence. • • • • • • • • • • • Types of ERG The ERG can be classified variously: 1. Depending upon the stimulus zone, i.e. the area of retina being stimulated (and hence contributing to ERG), the ERG is of two types: Full field ERG. A Granezfeld field allows a full field uniform stimulation of retina. It enables a direct correlation between the ERG amplitude and the area of functioning retina that is stimulated. Focal ERG. Focal ERG is useful to record macular action potential but is yet to gain widespread clinical application. Multifocal ERG (mfERG) records multiple (61 to 103 hexagones) local responses elicited from the central 40-50 degree of retina under light adopted conditions. The mfERG has supplated focal ERG. 2. Depending upon the type of stimulus, ERG can be: Single flash ERG. A bright flash ERG is useful in the presence of media opacities particularly in vitreous haemorrhage. Flicker fusion ERG Red flash ERG Blue filter ERG; are useful in differentiating rod responses from cone responses. Pattern ERG is used as an indicator of health of ganglion cell layer. 3. Depending upon the state of retinal adaptation, ERG can be: Scotopic, ERG, Mesopic ERG, or Photopic ERG. Note. Therefore, before interpreting an ERG, it is always necessary to determine the state of retinal adaptation under which the curve was recorded. This is essential because the latency, amplitude and waveform pattern are largely dependent on the state of retinal adaptation. TECHNIQUE OF ERG RECORDING Technique of ERG recording is as below (Fig. 6.5.7). • • Fig. 6.5.7. Technique of ERG recording. Application of electrodes ▪ Active electrode (main electrode) is often placed on the cornea, embedded in a contact lens after topical anaesthesia. Recently, the contact lens electrode is beginning to be replaced by 'wick electrodes' in the conjunctival sac or skin electrodes of gold foil on the eyelids. They give smaller responses than contact lens electrode (still most commonly used electrodes). ▪ Reference electrode (silver chloride electrode) is placed on the patient's forehead. It serves as the negative pole, since it is placed closer to the electrically negative posterior pole of the eye. ▪ Ground electrode is placed on the ear lobe. Recording protocols Several recording protocols have been suggested. Worldwide standardized protocols for full-field ERG testing are routinely updated and set forth by the International Society for Clinical Electrophysiology of Vision (ISCEV). One of the simplest recording protocols suggested involves the five basic retinal responses (3 darkadapted and 2 light-adapted) and is called the standardized ISCEV full-field ERG (Fig. 6.5.8): ▪ Scotopic ERG is begun after the pupils have been dilated and the retina dark adapted for 20 mins: Step 1: Dark-adapted 0.01ERG (rod response) is obtained using a dim white flash stimulus of 0.01 cd s m-2 with a minimum interval of 2 s between flashes (Fig. 6.5.8A). Step 2: Dark-adapted 3.0 ERG( maximal combined rod-cone response) is obtained with a white flash stimulus of 3.0 cd s m- 2 with a minimal interval of 10 s between flashes (Fig. 6.5.8B). • • • Fig. 6.5.8. Exemplary waveforms of the six basic ERG responses. Step 3: Dark-adapted 3.0 oscillatory potentials are obtained with a white flash stimulus of 3 cd s m-2 using high and low pass filters (Fig. 6.5.8C). ▪ Photopic ERG is begun after the step 3. For these, the room is gradually brightened. First the dark lamp, then the room light and finally the Granezfeld bowl 13 is put on. After a minimum light adaptation time of 10 minutes, the light-adapted or photopic responses are obtained. Step 4: Light-adapted 3.0 ERG (single flash cone response) is obtained with a white flash stimulus of 3.0 cd s m-2 with a minimum interval of 0.5 s between flashes (Fig. 6.5.8D). Step 5: Light-adapted 3.0 flicker ERG (30 Hz flicker) is obtained with a white stimulus of 3.0 cd s m-2 with a flash rate of 30 stimuli per second (Fig. 6.5.8E). The stimulus The stimulus is an illuminated bowl which projects diffuse light into all parts of the fundus. The flash strength of ERG testing is described in units of candala seconds per meter squared (cd s m2). The intensity of the light and the frequency and duration of flashes varied as per protocol. Usually, the pupils are dilated with a mydriatic and this certainly gives more controlled results in young normal subjects. In an elderly subject, the pupil response to mydriatics may be variable and the useful results can be obtained without resorting to mydriatics. Recroding and amplification of the response The elicited response is then recorded from the anterior corneal surface by the contact lens electrode which picks up the electrical potential that exists between the negative posterior pole and the positive cornea following light stimulation of the retina. After arising at the contact lens, the signal is then channeled through consecutive devices for preamplification, amplification and finally display. Either an oscilloscope or a polygraph can be employed in the recording of these responses. Oscilloscope is more sensitive for recording of the various ERG subcomponents. • - - • • • • • • Factors influencing the ERG response 1. Stimulus With increase in the intensity of the stimulus following successive changes occur in ERG response: a-wave continues to increase in size, whereas the b-wave reaches a maximum. The latency of the peaks shortens and the position of the wavelets on the b-wave changes. When a flickering light stimulus is used with a frequency of 30 Hz, then the resulting ERG represents the cone response only, because the response frequency of the scotopic neural mechanism does not exceed 20-21 Hz. At higher frequencies, ERG loses its characteristic waveform and becomes sinusoidal and eventually disappears as the electrical flicker fusion frequency is reached. 2. Recording equipment Inadequately positioned electrodes or faulty connections can distort or abolish the response. 3. Dark adaptation The ERG increases in size during dark adaptation. The b-wave becomes slower and more rounded in the darkadapted state. Stimulation with red light under light-adapted condition favours the photopic ERG and stimulation with a subcone threshold blue light under dark-adapted condition favours the scotopic response. 4. Age and sex A small ERG can be elicited within hours of birth, if a strong stimulus is used. • • • ERG reaches its adult value after the age of 2 years. In adult life, the size of ERG gradually declines. ERG size is slightly larger in women than men. Cone and rod functions as determined by ERG Photopic versus scotopic ERG ▪ Photopic ERG refers to testing the patient with a bright flash of light or with red light under fully light-adapted condition of sufficient illumination to suppress rod responses. A typical cone response elicited by photopic ERG shows a (Fig. 6.5.9A) lower amplitude and shorter implicit time; because only approximately five to eight million cones are contributing under light-adapted stage. Scotopic ERG recorded after 20 minutes of dark adaptation as shown in Fig. 6.5.9B shows a large amplitude with longer implicit time; because under dark-adapted conditions in addition to 6-8 million cones, 125 million rods also contribute. Thus, it should be noted that scotopic response is not equivalent to a selective measurement of rod function since it measures both cone and rod activity. Fig. 6.5.9. Photopic (A) (lower amplitude, shorter implicit time) versus scotopic (B) (large amplitude, with longer implicit time) ERG response. ▪ Rod response can be isolated by stimulating the fully dark-adapted eye with a flash of very dim light or with blue light. Such a response is characterized by reduced amplitude and longer implicit time (Fig. 6.5.10). Fig. 6.5.10. Isolated rod response ERG, elicited by low intensity blue stimulus. Note, smaller amplitude and longer implicit time. ERG with orange-red stimulus Both cone and rod responses can be isolated on the same recording by a orange-red stimulus using Wratten 23-A filter. A typical orangered response shows an initial hump or x-wave which represents cone activity and the final peak hump that represents rod activity (Fig. 6.5.11). Fig. 6.5.11. ERG recording with orange-red stimulus; x-wave represents cone response andb-wave represents rod activity. Flicker ERG Cone response can also be isolated by using a flicker light stimulus of 30-40 Hz to which rod cannot respond. Cone response can be elicited in normal eyes up to 50 Hz, and beyond this the individual responses are no longer recordable (critical flicker fusion). In patients with diffuse cone disease, responses often will not be elicited beyond 20-30 Hz. Foveal ERG It is recorded by special type of stimuli to get specific information from the foveal region. PATTERN ELECTRORETINOGRAM Multifocal ERG The multifocal ERG (mfERG) records multiple (61-103 hexagonals) local responses elicited from the central 40-50 degree of retina under light-adapted conditions. The mfERG has supplanted focal ERG. Thus the multifocal electroretinogram (mfERG) measures the spatial distribution of central cone function and is a valuble adjunct to full-field ERG. Typically, this lasts 4-8 minutes and is obtained in 15- 50 second segments to make it easier for the subject to suppress eye movements and blinks. The subject views a black and white pattern of hexagonal elements each of which flashes on and off with its own pseudo-random binary sequence, known as an M-sequence. Crosscorrelations are then used to calculate the individual responses to each of the stimulus hexagons, and these resemble real ERGs. Because of the high density of cone receptors in the fovea the response to the central hexagon in a normal subject has the largest response density. mfERG waveform Each focal trace response consists of an initial negative wave (N1) followed by a positive peak (P1) and a later negative wave(N2), similar to a conventional photopic ERG. CLINICAL APPLICATIONS OF ERG ERG is very useful in detecting functional abnormalities of the outer retina (up to bipolar cell layer), much before the ophthalmoscopic signs appear. However, ERG is normal in diseases involving ganglion cells and the higher visual pathways such as optic atrophy. Further, since the ERG measures diffuse responses of the retina, so the isolated diseases, including localized macular haemorrhage, macular holes, exudates, chorioretinitis lesions and localized areas of detachment will not be detectable by ERG amplitude changes. Clinical applications of ERG are: • • Diagnosis and prognosis of retinal disorders 1. Retinitis pigmentosa (IRP) and other inherited retinal dystrophies. Patients with retinitis pigmentosa (RP) show a marked reduction in the amplitude of ERG at an early stage in the disease process. The reduction in amplitude is out of proportion to the ophthalmoscopic changes and in some instances the ERG may be abnormal when the fundus appears normal. There is a relationship between the area of functioning retina and the ERG. So far the ERG cannot indicate the progress or the inheritance pattern in an individual with RP. In Leber's amaurosis, the ERG plays an important primary diagnostic role. Therefore, when the child is suspected of having Leber's amaurosis, ERG should be included in the examination under anaesthesia. Other retinal degenerations where ERG plays a diagnostic role include choroideremia, gyrate atrophy, pathological myopia and other variants of retinitis pigmentosa. 2. Diabetic retinopathy. Oscillatory potential of the ERG is selectively abolished in patients with diabetic retinopathy. The disappearance of these wavelets seems to reflect retinal ischaemia because a similar effect is seen in central retinal artery occlusion. The recording of ERG in juvenile diabetics with a disease duration longer than 5 years has been shown to be valuable for the identification of those at risk for the development of proliferative retinopathy.10 3. Retinal detachment. In a case with detached retina, there occurs an immediate reduction in the size of the b-wave coincidental with the loss of vision. The amount of reduction of b-wave depends on the extent of the detachment. ERG may also give some guide to surgical prognosis. 4. Vascular occlusions of the retina. In cases with central retinal artery occlusion, the typical change seen is disappearance of oscillatory potential with reduction in b-wave. Similar but less marked changes are seen in cases of central retinal vein occlusion. There is some hope that the ERG may help to predict those cases liable to develop neovascularization. 5. Toxic and deficiency states. Certain ocular toxic drugs particularly those stored in the pigment epithelium may cause alteration in the response, sometimes before there is any clinical evidence of toxicity. Chloroquine toxicity in particular may be detected in this way. Other drugs include quinine, lead, phenothiazines, indomethacin, etc. to assess reginal function when fundus examiantion is not possible. It is important to remember that the ERG can be recorded even in the presence of dense opacities in the media such as corneal opacity, dense cataract and vitreous haemorrhage; and provided the stimulus is sufficiently bright, the response should be normal in the absence of disease. Therefore, it is useful to perform ERG when a detachment or other retinal disorders are suspected behind the opacities of the ocular media. • • • • • • • • • • • • • Abnormal ERG response The abnormal ERG response to a moderate intensity full field Granezfeld stimulus can be graded as below. 1. Supernormal response is characterized by a potential above the normal upper limit, i.e. when the amplitude is greater than two standard deviations above the mean for both 'a' and 'b' waves or only the 'S' wave. Such a response is seen in: Subtotal circulatory disturbances of retina Early siderosis bulbi, and Albinism 2. Subnormal response is characterized by a potential less than 2 standard deviations beneath the mean normal. A subnormal ERG indicates that a large area of retina is not functioning. Such a response is seen in: Early cases of retinitis pigmentosa, before the appearance of ophthalmoscopic signs. Chloroquine and quinine toxicity. Retinal detachment. Systemic diseases like vitamin A deficiency, hypothyroidism, mucopolysaccharidosis and anaemia. 3. Extinguished response is characterized by complete absence of response in conditions of total failure of rod and cone function. Such a response is seen in: Advanced cases of retinitis pigmentosa. Complete retinal detachment. Advanced siderosis bulbi. Choroideremia. Leber's congenital amaurosis. Luetic chorioretinitis. 1. • • • 2. • • • 3. 4. Negative response is characterized by a large a-wave. Such a response indicates gross disturbances of the retinal circulation as seen in arteriosclerosis, giant cell arteritis and central retinal artery and vein occlusions. Other abnormalities of ERG ▪ The latency period and implicit time are prolonged in retinal degenerations. ▪ Oscillating potential abnormalities. ERG elicited with high intensity light stimulus shows a series of rhythmic oscillations in the form of small wavelets superimposed on b-wave. These are known as oscillatory potential (Fig. 6.57B). Thus, the oscillatory potential is recorded in a photopic or light adapted ERG. These wavelets reflect feedback inhibitory circuits in the inner nuclear layer. The oscillatory potential is extremely vulnerable to interference from retinal circulation and synaptic transmission within the retina. Abnormalities that can occur in oscillatory potential are: Disappearance of oscillatory potential can occur in: Diabetic retinopathy Ischaemic disorders of retina Central retinal artery occlusion (CRAO). Changes in oscillatory potential can occur in: Retinitis pigmentosa Behcet's disease, and Siderosis bulbi. Rapid decline in amplitude of oscillatory potential is an important sign of impending progression to proliferative state in diabetic retinopathy. 1. 2. 3. Limitations of ERG Since the ERG measures only the mass response of the retina, isolated lesions like hole, haemorrhage, a small patch of chorioretinitis or localized area of retinal detachment cannot be detected by the amplitude changes. The ERG responses (subnormal, extinguished or negative) though representing the mass activity of rods and cones, are not diagnostic of any particular condition, i.e. it is not possible to differentiate the various conditions on the basis of ERG findings alone. Disorders involving ganglion cells (e.g. Tay-Sachs' disease), optic nerve or striate cortex do not produce any ERG abnormality. ELECTROOCULOGRAM Electrooculography (EOG) is based on the measurement of resting potential of the eye which exists between the cornea (+ve) and back of the retina (-ve) during fully dark-adapted and fully light-adapted (following long-duration exposure to bright light) conditions. EOG was discovered by DuBois-Reymond (1849) and popularized by Riggs (1954) and Francaid and coworkers (1956).11,12 Arden's (1962) contribution towards interpretation of the results made the EOG a clinically relevant investigation.13 ELECTROPHYSIOLOGICAL BASIS OF EOG The electrophysiological basis of EOG is explained by the retinal electric field. The human retina consists of an electrically charged nervous membrane, the outer layers of which are electronegative and inner layers, electropositive. The internal electropositivity is transmitted to the cornea while the outer electronegativity is transmitted to the periocular tissues. A potential difference of 2-10 mV is recorded by a millivoltmeter connected to two electrodes, one of which is placed on the cornea and the other in the posterior part of exposed eyeball or optic nerve. This is absolutely constant in the resting state and has been known by several names such as: retinal resting potential, steady potential, static potential, dark potential or the corneofundal potential. This resting potential forms a vast electric TECHNIQUE OF RECORDING ▪ Prerequisites to record an EOG are: Good fixation Normal extraocular motility, and Visual field of at least 60° ▪ Electrodes are placed over the orbital margin near the medial and lateral canthi serve as active electrodes (E1-E4 in Fig. 6.5.12A). A forehead electrode serves as a ground electrode or indifferent electrode (E5 in Fig. 6.5.12A). ▪ Patient's position and fixation. Patient sits erect in a room with the head position controlled at a certain fixed distance from three fixation lights (dimly lit, usually red), which are placed in patient's line of vision. The central light serves for central fixation, and the two side lights which can be fixed after an excursion of anywhere from 30° to 60° serve as the right and left fixation lights (Fig. 6.5.12B). Fig. 6.5.12. Technique of recording electro-oculogram (EOG): A, Position of electrodes (A and change in polarity of the electrodes in three gaze positions (A2); B, Ocular movements during recording; C, record of EOG. ▪ Stimulus light. An arrangement is made to make the eyes lightadapted with the help of a bright, long-duration stimulus. The pupil size is controlled by instillation of mydriatics. Ordinarily, a pupil diameter of any value greater than 3 mm allows little variation in the EOG response. ▪ Recording. The patient is asked to move the eye sideways (medially and laterally) by fixating right and left fixation lights alternately and keep there for few seconds, during which recording is done. In this procedure, the electrode near the cornea (e.g. electrode placed near lateral canthus, when the eye is rotated laterally) becomes positive. Fig. 6.5.12A depicts the change in polarity of the electrodes in three gaze positions. The recording is done every one minute. To begin with to have baseline amplitude, the recording procedure is started with stimulus lights on. After a standardized period of light adaptation, all lights are extinguished (except for fixation lights) and responses are recorded for 15 minutes under dark-adapted condition. The stimulus lights are then turned on again and responses recorded for 15 minutes under light-adapted conditions. • • • MEASUREMENT AND INTERPRETATION OF RESULTS Normally, the resting potential of the eye progressively decreases during dark adaptation reaching to dark trough (level of minimal height of the potential in dark) in approximately 8-12 minutes. With subsequent light adaptation, the amplitude starts rising and reaches to light peak (level of maximal height of the potential in light) in approximately 6-9 minutes (Fig. 6.5.12C). Results of EOG are then interpreted by finding out the Arden ratio (light size ratio) as follows: Normal light rise ratio values are 185 or above Subnormal or borderline values are between 185 and 165. Abnormal values are below 165. COMPONENTS OF EOG As described above, the EOG response has two components: ▪ Dark trough (DT). This is light-insensitive component of the EOG. Although its exact origin is not clearly understood. Contribution probably occurs from the retinal pigment epithelium (RPE), photoreceptors, and inner nuclear layer.14 ▪ Light peak (LP). This light rise component of the EOG is lightsensitive. Both the rod and cone systems contribute to the EOG light rise.15 However, since normal EOG values are generally obtained in congenital achromats and in patients with recessive congenital stationary nyctalopia (night blindness), the EOG light rise is probably not mediated exclusively by the rod or cone system elements. The fact that the light rise can be abolished by central retinal artery occlusions provides an evidence that the EOG light rise resides, at least in part, at or just distal to the bipolar cell layer, which is supplied by the retinal circulation. Further, since EOG like ERG is normal in diseases involving ganglion cells and higher visual pathway such as optic atrophy, confirms that the light rise is not contributed by these layers of the retina. The fact that EOG is more markedly abnormal than the ERG in diffuse or widespread lesion involving RPE, indicates that RPE function is very important for the light rise of EOG. Thus light rise appears to be a measure of RPE and photoreceptor integrity. It results from a slow depolarization of the basal (choroidal) membrane of the RPE.16 CLINICAL APPLICATIONS Similar to ERG, the EOG is an overall mass response and, therefore, is not affected by localized lesions of the sensory retina or retinal pigment epithelium. Since the EOG reflects the presynaptic functions of the retina, any disease that interferes with functional interplay between the retinal pigment epithelium (RPE) and the photoreceptors will produce an abnormal or absent light rise. Thus EOG is affected in diseases such as retinitis pigmentosa and other hereditary degenerations, vitamin A deficiency, retinal detachment, toxic retinopathies and retinal vascular occlusions. As a general rule, those conditions which cause a reduction in the size of b-wave in ERG also produce a reduction in the value of Arden ratio. Hence, EOG serves as a test that is supplementary and complementary to ERG and in certain conditions is more sensitive than the ERG, e.g. patients with vitelliform macular degeneration often show a striking reduction of the EOG in the presence of a normal ERG. Therefore, the only clinically important role of EOG is in the diagnosis of Best's vitelliform macular dystrophy and to differentiate it from pseudovitelliform dystrophies.17 The EOG may also enable detection of carriers of this form of macular dystrophies.18,19 In other retinal diseases, EOG does not add information over ERG. However, in case of chloroquine toxicity or siderosis bulbi, EOG may reveal abnormality in the presence of a normal ERG tracing. VISUALLY EVOKED POTENTIAL We know that when light falls on the retina, a series of nerve impulses in the form of electrical changes are generated and passed on to the visual cortex via the visual pathway. The electrical potential changes produced in the visual cortex by these impulses can be recorded by electroencephalography (EEG) technique; and is known as visually evoked potential (VEP). Thus VEP is nothing but the EEG records taken from the occipital lobe (visual cortex). In other words, VEP is an averaged and amplified record of action potentials in the visual cortex. It is important to note that VEP is the only objective technique available to assess clinically the functional state of the visual system beyond the retinal ganglion cells. Since there is disproportionately large projection of the macular area in the occipital cortex, the VEP represents the macula dominated response.20,21 Thus, VER is an indicator of the integrity of the visual conduction pathway, which includes the optic nerve, optic chiasma, optic tract, lateral geniculate body, optic radiations and visual cortex. VER, however, does not help to localize the site of the abnormality. TECHNIQUE OF RECORDING VEP (Fig. 6.5.13) Fig. 6.5.13. Technique of recording visually evoked response (VER) and record of normal VEP pattern. Pre-requisites for recording VEP An important necessity to be able to record the VEP is to average out the EEG potentials. EEG voltages are of about 50 mV, occur continuously and are easily detected by the scalp electrodes. The random EEG activity at any point between the stimuli will be positive or negative to an equal frequency and so its average will be zero. This is known as averaging out. The VEP voltages, unlike the random EEG response, are about 5-10 mV amplitude and appears only in response to a visual stimulus, i.e. it is time locked to the stimulus. During the test, there should be no distracting light or sound waves. Mathematically, the test has to be repeated 50100 times to enable the computer to discriminate the VEP waves from the noise waves. 1. 2. 3. 4. Equipment for recording VEP In order to record VEP, four groups of equipment are needed: Visual stimulus producing device, Suitable scalp electrodes, Amplifier, and Computer and read out systems. These four systems combindly form the so-called electro encephalographic (EEG) machine. • • • • Types of VEP recording Depending upon the type of stimulus used, VEP is of two types: 1. Flash VEP. It is recorded by using a calibrated intense diffuse light or stimulus that releases flashes from a shutter 1-5 times/sec. It merely indicates that light has been perceived by the visual cortex. It is not affected by the opacities in the ocular media such as corneal opacity, cataract and vitreous haemorrhage. 2. Pattern VEP. It is recorded by using some patterned stimulus displayed on a TV screen. The pattern stimulus is usually in the form of black and white chess board squares (checker board) whose size can be adjusted. Pattern VEP is further of two types: Pattern appearance VEP. In the case of pattern appearance, a black and white checker board is presented in an on-off sequence. Pattern reversal VEP. In this case, the pattern of the stimulus is changed (i.e. black squares go white and the white squares become black) alternately. The pattern VEP depends on the form sense and thus gives a rough estimate of the visual acuity. It is important to note that both these types of stimuli (flash as well as patterned) can also be varied in terms of luminance and frequency of presentation and thus produce different responses: The rapidly repeated stimulus produces a sinusoidal type of response referred to as the 'steady state' response. Stimuli repeated less than two or three times a second produce a characteristic waveform known as the 'transient' response. Recording protocol for VEP Recording protocol for VEP consists of first recording pattern VER followed by Ganzfeld flash VER and when indicated bright flash VER. ▪ For pattern VER recording, the patient wears the spectacle correction and is seated one metre from the screen. The need for him to cooperate is emphasized. One eye is tested at a time so the other eye is patched. The patient is instructed to focus on the central red dot and the pattern is attenuated on the screen. ▪ For flash VER test, after recording the pattern VER, the patient is moved over the Ganzfeld hemisphere bowl. The flash is repeated 75 times for two runs before the eye being tested, while the other eye is patched. ▪ Bright flash VER may be performed with the patient seated or lying, awake or asleep or even under anaesthesia intraoperatively. In the awake state, the patient is asked to look at a photostimulator 5 cm from the open eye. About 50 stimuli are usually presented. NORMAL VERSUS ABNORMAL RECORD OF VEP ▪ Pattern-reversal VEP. It consists mainly of a positive wave (P100) and two negative waves (N75 and N135) (Fig. 6.5.13B). The commonest wave used for clinical cases is P100, since it is very robust measure with minimal interocular and intrasubject measurement variation. The amplitude can be measured as N75-P100; or as P100-N135; or as a mean of these two measures. Normally, the response amplitude is of the order of 10-25 μV and is fully established by the age of six months. The amplitude is considered abnormal, if it is below 10 μV and abolished, if less than 3 μV. ▪ Flash VEP. The flash VEP tracing is more complex as compared to pattern VER curve which is simpler and less variable. Its primary response is an M-shaped multiphasic curve with a series of negative and positive peaks designated in numerical sequence as N1, P1, N2, P2, N3 and P3 (Fig. 6.5.13C). The significant components include N2 and P2 recoreded at 90 and 120 msec, respectively. Factors influencing VEP 1. Stimulus. The character of the normal VEP depends upon the type of stimulus used as described earlier. In patterned stimulus, the transient response increases in amplitude with the decrease in the size of checks, reaching a peak when the check subtends about 15° arc at the eye. 2. Position of electrodes on the scalp also influences the character of normal VEP response. 3. Age and sex. The child's VEP is characteristically large and it has been shown that the amplitude reaches a peak in 5-8 years old. After a slight decline, a further increase occurs in the early teens and after this a gradual decline is seen over the years. Sex differences in the responses are less evident, although in childhood, males have slightly larger responses and in adulthood, females have slightly larger responses. 4. Attention of the patient to stimulus. Because of the relatively large macular representation on the occipital cortex, the response is mainly derived from the central few degrees of the retina. Therefore, if the subject looks to the side of the stimulus, there is a rapid fall off in the size of response. 5. Effect of diseases on VEP. See Clinical applications describe CLINICAL APPLICATIONS Pattern versus flash VEP in clinical practice ▪ Advantages of pattern VEP in clinical practice are its: Increased sensitivity in detecting axonal conduction defects, and A decreased variability in response ▪ Indications for patterns VEP include: Pre-chiasmal lesions Patients with nystagmus Pattern onset/offset VEP for malingerers ▪ Advantages of flash VEP in clinical practice are: It is easy to record. Therefore, it is the test of choice for evaluating visual function in infants. This test can be undertaken even with the child asleep, as the stimulus is bright enough to bypass the eyelids. It can be used in the presence of vitreous haemorrhage and other dense media opacities. It can be used in patients with very poor vision. • • • • • • General comments about clinical applications of VEP Presence of reduced amplitude is very nonspecific, as it is dependant on patient's co-operation. It gains significance only if it becomes reduced on serial testing. Reduced amplitude on a pattern VER can be seen in refractive errors, poor co-operation, optic nerve hypoplasia, amblyopia, purposeful defocusing, optic nerve compression and anterior ischaemic optic neuropathy. Changes in latency is a useful clinical sign as it is less easily influenced by volition. Bilateral symmetry is seen with both flash and pattern VER. Therefore, an asymmetrical response is more indicative of an abnormality. Grossly abnormal flash VER suggests a poor visual prognosis, however, a normal record does not imply good visual capacity. Therefore, its chief role could be in excluding certain patients from undergoing extensive surgical procedures. High inter-individual variability is seen with flow VER; however, the variability is less than 10% when response from the two eyes of the individual are compared. • • • Common Clinical Conditions in which VER has clinical applications 1. Optic nerve diseases. VEP is very useful in detecting the afferent pathway defects, especially, those involving the optic nerve. Optic neuritis. In this condition, the VEP recorded after stimulation of the involved eye shows a reduced amplitude and delay in the transmission, i.e. increased latency (more important) as compared to the VEP recorded after stimulation of the normal eye. These changes occur even when there is no defect in the visual acuity, colour vision or field of vision. Following resolution of the optic neuritis the amplitude of VEP waveform may become normal, but the latency is almost always prolonged and is a permanent change. About 96% patients with multiple sclerosis are reported to have a delayed latency. Compressive optic nerve lesions. These are usually associated with a reduction in the amplitude of the VEP without much change in the latency. During orbital or neurosurgical procedures. A continuous record of optic nerve function in the form of VEP is helpful in preventing inadvertent damage to the nerve during surgical manipulation. 2. Measurement of visual acuity in infants, mentally retarded and aphasic patients. VEP is useful in assessing the integrity of macula and visual pathway in infants, mentally retarded and aphasic patients. Pattern VEP gives a rough estimate of visual acuity objectively. Peak VEP amplitude in adults occurs for checks between 10 and 20' of arc and this corresponds to a visual acuity of 6/5. 3. Malingering and hysterical blindness. VEP is quite useful in distinguishing between cases of organic blindness and malingering or hysterical blindness by confirming the fact that the visual pathway is intact even in patients claiming no perception of light. Pattern-evoked VEP amplitude and latency can be altered by voluntary changes in fixation pattern or accommodation. However, the presence of a repeatable response from an eye in which only light perception is claimed indicates that pattern information is reaching the visual cortex and thus strongly suggests a functional component to the visual loss. A characteristic of hysterical response seems to be large variations in the response from moment to moment. The first half of test may produce an absent VEP and second half a normal VEP. 4. Assessment of vistial potential in patients ivith opaque media. VEP is quite useful in assessing the visual potentials in eyes with opaque ocular media, e.g. in the presence of corneal opacities, dense cataract and vitreous haemorrhage. Pattern VER for obvious reasons is not of any use in patients with media opacities. 5. Lateralizations of defects in the vistial pathivay. VEP provides a useful information for localizing defects in the visual pathway in difficult cases such as in children and non-cooperative elderly patients. Asymmetry of the amplitudes of VEP recorded over each hemisphere imply a hemianopic visual pattern. However, the differentiation of tract lesion from that of optic radiations lesion is difficult. Decreased amplitude of VEP recorded over the contralateral hemisphere, when each eye is stimulated separately indicates a bitemporal visual deficiency and may localize the site of chiasmal pathology. 6. Role of VEP in unexplained vistial loss. VEP is quite useful in elucidation of unexplained visual loss in general and in patients with orbital and/or head injury in particular. 7. Amblyopia. Response to flash VEP is normal but in pattern VEP, there occurs decrease in the amplitude with relative sparing of latency. So pattern VEP can be used in the detection of amblyopia and in monitoring the effect of occlusion on the normal as well as the amblyopic eye, especially in small children. 8. Glaucoma. VEP may help in detecting central field defects. 9. Refraction. While testing visual acuity in infants, mentally retarded and aphasic patients with the help of VEP, an approximate assessment of degree of refraction can also be made. In pattern VEP, amplitude depends on whether the stimulus is in focus on the retina. The greatest amplitude is generated by a pattern in exact focus on the retina. In this way, the refractive error can be determined by measuring VEP amplitude with changes in the power of trial lenses. 10. Other conditions affecting VEP. Other conditions such as retinal dysfunction, miotic pupil, media opacities, accommodative error can also cause an increase in latency and reduction in amplitude of VEP. EARLY RECEPTOR POTENTIAL The light-induced changes in ocular potential which begin with the early receptor potential (ERP) are followed by electroretinogram (ERG) and are completed by the much slower electrooculogram (EOG) (Fig. 6.5.14). The ERP is a rapid response that can be detected when the retina is stimulated with an intense flash of light.22,23 Stimulus intensity required to generate the ERP is approximately 106 times brighter than that required to elicit the ERG.24 In humans, ERP is completed within 1.5 msec and is followed by the leading edge of the a-wave of the ERG.25-28 Fig. 6.5.14. The light-induced changes in ocular potential. Components The human ERP recorded at the cornea consists of two waves R1 and R2 (Fig. 6.5.14). Rl. This is the initial cornea-positive wave. It has been associated with the conversion of lumirhodopsin to metarhodopsin I.29 R2. It is the later cornea-negative wave. It has been associated with the conversion of metarhodopsin I to metarhodopsin II.30 Physiological basis The ERP is generated in the outer segment of photoreceptors by the spatial change of the charged envelops of the photoreceptor molecule as it undergoes isomerization.30 Its amplitude depends upon the concentration and orientation of the unbleached visual pigment in the outer segment of photoreceptors.31,32 When the retina is heated, the ERP disappears just at the temperature at which the visual pigment molecules lose their regular orientation.32 In humans, Technique of recording ERP A special scleral lens with the active electrode in a side arm is used to record the ERP. The lens and the side arm are filled with saline solution to ensure a saline bridge between the cornea and the recording electrode. The reference electrode is taped on the forehead and the ground electrode behind the ear. Patient sits in front of a flash gun (shielded) and the stimulus flash is presented in maxwellian view to maximize stimulus intensity. Clinical applications The ERP obtained under conditions of dark adaptation has been found to be subnormal in amplitude in young patients with dominant forms of retinitis pigmentosal as well as sex-linked and autosomal recessive retinitis pigmentosa.25,36 Although the ERP has provided a valuable new dimension for clinical research, the ERG remains at this time the preferred electrophysiologic test for routine diagnostic evaluation of retinal function. • • • • • ELECTROPHYSIOLOGICAL TESTS: ESSENTIAL APPLIED ASPECTS TIME TAKEN FOR VARIOUS ELECTROPHYSIOLOGICAL TESTS Time taken for various electrophysiological tests is as follows: ERG: 60 minutes (40 min DA + drops) EOG: 45 minutes Pattern ERG: 30 minutes Flash or pattern VEP: 30-45 minutes Special VEPs: 30-60 minutes In case of paediatric assessment (under 7 years), multiply time by 1.5. POTENTIAL INDICATIONS OF VARIOUS ELECTROPHYSIOLOGICAL TESTS Outline of potential indications of specific electrophysiological tests as summarized in Table 6.5.1. Table 6.5.1 Outline of potential indications for specific electrophi/siological tests • • • PRACTICAL STATUS OF ELECTROPHYSIOLOGY ERG is an extremely helpful tool in detecting or confirming retinitis pigmentosa, even in the absence of typical bony corpuscles. EOG is useful in differential diagnosis of abnormalities of the retinal pigment epithelium such as Best's disease. VEP is a useful tool along with ERG and other clinical assessments to differentiate various conditions such as cortical visual impairment and delayed visual maturation.
RETINA : NEUROPHYSIOLOGY OF VISION : NEUROPHYSIOLOGY OF VISION INTRODUCTION GENESIS OF VISUAL IMPULSE IN THE PHOTORECEPTOR Phototransduction Cone versus rod receptor potential PROCESSING AND TRANSMISSION OF VISUAL IMPULSE IN THE RETINA Neurotransmitters in the retina Physiological activities in the retinal cells PROCESSING AND TRANSMISSION OF VISUAL IMPULSE IN THE VISUAL PATHWAY Optic nerve, chiasma and optic tract Lateral geniculate body Optic radiations ANALYSIS OF VISUAL IMPULSE IN THE VISUAL CORTEX Retinotopic organization Functional anatomy and organization of the visual cortex Types of visual cortex Layers of primary visual cortex • • • • • • • • • Connections of primary visual cortex Physiology of visual cortex Concept of receptive field of striate cortex Columnar organization of striate cortex Serial versus parallel analysis of visual image Role of extrastriate cortex (visual association cortex) in visual functions Psychophysiological aspect of visual functions 'THREE-PART-SYSTEM' HYPOTHESIS OF VISUAL PERCEPTIONS First system Second system Third system
INTRODUCTION Neurophysiological processes concerned with vision can be discussed as under: Genesis of visual impulse in the photoreceptors. Processing and transmission of visual impulse in the retina. Processing and transmission of visual impulse in the visual pathway. Analysis of visual impulse in the visual cortex. A three-part system hypothesis of visual perception. GENESIS OF VISUAL IMPULSE IN THE PHOTORECEPTOR PHOTOTRANSDUCTION As discussed earlier, the formation of metarhodopsin-II (also known as activated rhodopsin) triggers a cascade of biochemical reactions which results in reduction in the concentration of cyclic-GMP in the photoreceptor; which in turn triggers the genesis of visual impulse by producing a generator potential in the photoreceptors.1 This process of translation of the information content of a light stimulus into electrical signals is known as transduction. The transduction process is highly quantum efficient, in that a single photon causes a response in the rod's membrane potential.2 The sequence of events in photoreceptors by which incident light leads to production of a nerve impulse (phototransduction) are explained below (Figs 6.4.1 and 6.4.2). Fig. 6.4.1. Basis of genesis of photoreceptor hyperpolarization. Fig. 6.4.2. Sequence of events involved in phototransduction process in the photoreceptor. Standing potential or dark current Normally, the inner segment of the photoreceptor continually pumps Na+ from inside to outside, thereby creating a negative potential on the inside of entire cell. However, the Na+ channels present in the cell membrane of the outer segment of photoreceptor are kept open by the cyclic-GMP, in the dark. So, Na+, from the extracellular fluid, flows inside the outer segment, i.e. in dark. As a result, the cell membrane in the outer segment is hypopolarized with respect to the inner segment, i.e. the current flows from the inner to the outer segment (Fig. 6.4.1). Current also flows to the synaptic ending of the photoreceptor. This is called standing potential or dark current.3 Hyperpolarising receptor potential When light strikes the photoreceptor, the amount of cyclic-GMP in the photoreceptor is reduced (as discussed in photochemistry of vision), so some of the Na+ channels (which were kept open by cyclic-GMP in dark) are closed, and the result is a hyperpolarizing receptor potential.4,6 Thus the photoreceptor potential is different from the receptor potentials in almost all other sensory receptors in that the excitation of photoreceptor causes increased negativity of the membrane potential (hyperpolarization), rather than decreased negativity (depolarization) which is characteristic of all other receptors. Normally, in dark, the electro-negativity inside the rod membrane is about 40 millivolts and after excitation it approaches about 70 to 80 millivolts.10 Further, the eye is unique in that the receptor potential of the photoreceptors is local graded potential, i.e. it does not propagate and does not follow the 'all or none law'. CONE VERSUS ROD RECEPTOR POTENTIAL The cone receptor potential has a sharp onset and offset, whereas the rod receptor potential has a sharp onset and slow offset. The curve relating the amplitude of receptor potentials to stimulus intensity have similar shapes in rods and cones, but the rods are much more sensitive. Therefore, rod responses are proportionate to stimulus intensity at levels of illumination that are below the threshold for cones. On the other hand, cone responses are proportionate to stimulus intensity at high levels of illumination when the rod responses are maximal and cannot change. That is why cones generate good response to change in light intensity above background but do not represent absolute illumination well, whereas rods detect absolute illumination. PROCESSING AND TRANSMISSION OF VISUAL IMPULSE IN THE RETINA The receptor potential generated in the photoreceptors (as discussed above) is transmitted by electronic conduction (i.e. direct flow of electric current, not action potential) to the other cells of the retina, viz. horizontal cells, bipolar cells, amacrine cells and ganglion cells. However, the ganglion cells transmit the visual signal by means of action potential. • • • • • NEUROTRANSMITTERS IN THE RETINA Role of neurotransmitters employed for synaptic transmission in the retina still have not all been delineated clearly. However, a great variety of different synaptic transmitters are found in the retina. A few assumptions are as follows: Glutamine, an excitatory transmitter, is released by rods and cones at their synapses with bipolar and horizontal cells. Amacrine cells produce five different types of inhibitory transmitters. They include: gamma aminobutyric acid (GABA), glycine, dopamine, acetylcholine and indolamine. Transmitters of the bipolar cells and horizontal cells have still not been isolated. Cholinesterase has been found in the processes of Muller, horizontal, amacrine, and ganglion cells. In the human retina, only the true acetylcholinesterase has been found, suggesting that acetylcholine may be the dominant synaptic neurotransmitter in the human. Carbonic anhydrase has also been isolated from cones an PHYSIOLOGICAL ACTIVITIES IN THE RETINAL CELLS The neurophysiological activities (concerned with the processing and transmission of visual signal) occurring in the different retinal cells are summarized below. Horizontal cells ▪ Phenomenon of lateral inhibition. Horizontal cells transmit signals horizontally in the outer plexiform layer from rods and cones to the bipolar cells.11 Their main function is to enhance the visual contrast by causing lateral inhibitions. This phenomenon of lateral inhibition has been observed by record of electrical activities occurring in the retina (Fig. 6.4.3), which shows that when a minute spot of light strikes the retina, the central most area is excited but the area around (called as surround) is inhibited. Thus, instead of the excitatory signal spreading widely in the retina because of the spreading dendritic and axonal trees in the plexiform layers, transmission through the horizontal cells puts a stop to this by providing lateral inhibition in the surrounding area. It has been found to be an essential mechanism which allows high visual accuracy in transmitting contrast borders in the visual image. Thus, the principal purpose of the microcircuitary of the outer plexiform layer seems to be the processing of spatial information. Fig. 6.4.3. Showing phenomenon of lateral inhibition in the surround receptive plexiform layer. The central photoreceptor has been stimulated with light and the inner portion of the cell membrane has become more negative. The signal is transmitted upwards to the bipolar cell and also horizontally via the horizontal cells. This horizontal transmission results in inhibition of the photoreceptorbipolar cell synapse of the neighbouring photoreceptor element. The stimulated bipolar cell may be hyperpolarized or depolarized. ▪ Concept of receptive field has been evolved to explain the processing of visual signal. In general sense, the receptive field is defined as the influence area of a sensory neuron. It is circular in configuration. It has been observed that receptive field of the horizontal cells is very large in contrast to the photoreceptor cell. i. ii. Bipolar cells The bipolar cells are neurons of the first order of visual pathway. Their dendrites are stimulated by the light-induced hyperpolarization of the photoreceptors. The important points delineated regarding the physiological activities concerning the bipolar cells are as follows: ▪ Some bipolar cells depolarize while others hyperpolarize (Figs 6.4.3 and 6.4.4) when the photoreceptors are excited; i.e. the two different types of bipolar cells provide opposing excitatory and inhibitory signals in the visual pathway. Two possible hypotheses put forward to explain this differential response are as follows: Perhaps the depolarizing bipolar cells respond to the excitatory neurotransmitter, glutamate and the hyperpolarizing bipolar cells do not. Perhaps some bipolar cells receive direct excitation from the photoreceptors, whereas the others receive the signal indirectly through the horizontal cells. Because horizontal cell is an inhibitory cell, this would reverse the polarity of the electrical response. ▪ Receptive field of the bipolar cell is also circular in configuration but has got a centre-surround antagonism.7 As shown in Fig. 6.4.4, in case of centre depolarizing cells (also called as 'on' cell), the light striking the centre of receptive field activates and the light striking the 'surround' inhibits bipolar cell output. The reverse occurs in the centre hyperpolarizing cell (also called as 'off cell'), i.e. the light striking the 'centre' is inhibitory and the light striking the 'surround' is excitatory to bipolar cell output. The size of the centre of the bipolar cell receptive field is determined by the reach of its dendrites and that of the much larger 'surround' is determined by the spread of interconnected horizontal cells. Fig. 6.4.4. The centre-surround response to light in 'on' or centre depolarizing bipolar cell (left) and 'off' or centre-hyperpolarizing bipolarcell(right). Plus signs indicate regions giving a depolarizing response, minus signs, a hyperpolarizing one. ▪ Spatial information processing. The importance of the above described reciprocal relationship between the depolarizing and hyperpolarizing bipolar cells is that it provides a second mechanism for lateral inhibition (spatial information processing) in addition to horizontal cell mechanism. Further, this reciprocal relationship allows half of the bipolar cells to transmit positive signals and the other half to transmit negative signals; both of these have a useful role in transmitting visual information to the brain. Amacrine cells ▪ Amacrine cells receive information at the synapse of the bipolar cell axon with ganglion cell dendrites (Fig. 6.4.5) and use this information for temporal processing at the other end of the bipolar cell.12-14 As shown in Fig. 6.4.5, at the synapse, the bipolar cells project on to both ganglion and amacrine cells. The amacrine cell then adjusts the bipolar cell in a negative feedback arrangement as to the subsequent response that will be projected on to the ganglion cell. Fig. 6.4.5. The bipolar-amacrine-ganglion cell Interaction. For explanation see text. ▪ Electrically, the amacrine cells produce depolarizing potentials and spikes that may act as generator potentials for the propagated spikes produced in the ganglion cells. ▪ Functions of amacrine cells. Various types of amacrine cells have been identified by morphological or histochemical means. Functions • • • • • of some of the types of amacrine cells have been characterized as below: Some amacrine cells are part of the direct pathway for rod vision, i.e. the impulse travels from rod to bipolar cells to amacrine cell to ganglion cells. Some amacrine cells respond very strongly at the onset of a visual signal, but the response dies out rapidly. Other amacrine cells respond very strongly at the offset of the visual signal but again the response dies quickly. One type of amacrine cells responds both when a light is turned on or off signalling simply a change in illumination irrespective of direction. Another type of amacrine cells is direction sensitive and respond to movement of a spot across the retina in a specific direction. Summary. Thus the amacrine cells help in temporal summation and in the initial analysis of visual signals before they even leave the The ganglion cells ▪ Electrical response of bipolar cells (local graded potential) after modification by the amacrine cells is transmitted to the ganglion cells which in turn transmit their signals by means of action potential to the brain. Thus, the ganglion cell action potentials are similar to digital or frequency modulation (FM), while the slow graded potentials of the rest of retina are analogous to analog or amplitude modulation (AM) (Fig. 6.4.6). Fig. 6.4.6. The amplitude modulation (AM) and frequency modulation (FM), electrical responses from the retina. For explanation see text. ▪ The ganglion cells which produce propagated spikes are of two types in terms of their centre response: "on-centre" cells that increase their discharge and "off-centre" cells that decrease their discharge upon illumination of the centre of their receptive fields. • • • ▪ Three distinct groups of ganglion cells (W, X and Y) have been described depending upon the function they serve and are as follows:15,16 W-ganglion cells are small (diameter >10 micrometre) and constitute about 40% of all the ganglion cells.17 Their dendrites spread widely in the inner plexiform layer and thus they have broad fields in the retina. These cells receive most of their excitation from rods, transmitted by way of small bipolar cells and amacrine cells and are thus important for much of our rod vision under dark conditions. These cells are also especially sensitive for detecting directional movements anywhere in the field of vision. X-ganglion cells: Most numerous (55% of total cells), are of medium diameter (between 10 and 15 micrometres). They have very small fields, because their dendrites do not spread widely. Thus, their signals represent discrete retinal locations and so the visual image is mainly transmitted through these cells. Further, since every X-ganglion cell receives input from at least one cone cell, so probably they are responsible for the colour vision as well. Y-ganglion cells: These are the fewest (5% of total) and largest (up to 35 micrometres in diameter) of all the ganglion cells. However, they have a very broad dendritic field and are thus able to pick up signals from a widespread retinal area. They respond to rapid changes in visual image, either rapid movement or rapid change in the light intensity. Cleland et al18 described X-cells as sustained cells and Y-cells as transient cells. The studies performed while recording from optic nerve fibres identify transient units as type I and sustained units as type II.19-21 The response of Y-cells (type I fibres) is phasic, with a transient excitation to a spot stimulus that decays quickly (Fig. 6.4.7). The X-cell (type II fibres) response has a tonic sustained component to a spot stimulus. • • Fig. 6.4.7. Diagrammatic illustration of four types of responses to light and dark spot stimuli in the cat's optic nerve. From Satio et al.19 ▪ Ganglion cells affect the relative sensitivity of different parts of the retina as follows: Degree of visual acuity in the central retina is high in comparison with poorer acuity peripherally. This is because of the fact that the number of ganglion cells in the centre of fovea (about 35,000) is equal to the number of cones. Peripheral retina has much greater sensitivity to weak light than the central retina. This results partly from the fact that rods are about 300 times more sensitive to light than are cones, and it is further magnified by the fact that as many as 200 rods converge on the same optic nerve fibre in the peripheral retina. PROCESSING AND TRANSMISSION OF VISUAL IMPULSE IN THE VISUAL PATHWAY OPTIC NERVE, CHIASMA AND OPTIC TRACT Optic nerve fibres are the axons of retinal ganglion cells and carry the total output of retina. Single optic nerve fibre can be excited only by a specific stimulus falling on a restricted area of the retina. This area is defined as the receptive field and is usually circular or elliptical in configuration.22 The optic nerve fibres decussate partially at the optic chiasma. The fibres from the nasal half of the retina cross in the midline while the fibres from the temporal half of the retina remain uncrossed. This implies that the visual input from the temporal halves of the visual field goes to the opposite side while the input from the nasal halves of the visual field goes to the same side. Fibres from the fovea pass both ipsilaterally and contralaterally, leading to bilateral representation of the foveal visual field. The crossed and uncrossed fibres beyond the optic chiasma constitute the optic tract of either side. The optic tracts end in the neurons of the lateral geniculate body (LGB). LATERAL GENICULATE BODY Functions Lateral geniculate body (LGB) serves two principal functions: 1. Relay station. LGB serves as a relay station to relay visual information from the optic tract to the visual cortex by way of the geniculo-hypercalcarine tract. The relay function is very accurate, so much so that there is exact point-to-point transmission with a high degree of spatial fidelity all the way from the retina to visual cortex. The signals from the two eyes are kept apart in lateral geniculate body. 2. To "gate" the transmission of signals. The second major function of the LGB is to "gate" the transmission of signals to the visual cortex, that is, to control how much of the signals be allowed to pass to the cortex. LGB receives gating control signals from two major sources: (a) corticofugal fibres from the primary visual cortex, and (b) the reticular area of the mesencephalon. Both of these are inhibitory and thus control the visual information that is allowed to pass. • Retinotopic projection The ganglion cell axons project a detailed spatial representation of the retina on the lateral geniculate body, with precise point-to-point localization. Fig. 6.1.25 shows the retinotopic projection on the LGB. As depicted in the anatomical description, the LGB contains 6 well-defined layers. On each side, layers 1, 4, 6 receive input from the contralateral eye, whereas layers 2, 3, and 5 receive input from the ipsilateral eye (Fig. 6.1.26). In each layer also there is a point-to-point representation of the retinal and all the six layers are in register, so that along a line perpendicular to the layer, the receptive fields of the cells in each layer are almost identical.23,24 The layers 1 and 2 of the LGB have large cells and are called maganocellular, whereas layers 3-6 have small cells and are called parvocellular (Fig. 6.4.8). Maganocellular layers receive their visual input almost entirely from the large Y-ganglion cells of the retina. The magano-cellular system provides a very rapidly conducting pathway to the visual cortex, i.e. carry signals for detecion of movement and flicker. However, this system is 'colour blind', i. e. transmits only black and white information, and also its point-to-point transmission is poor (because Y-ganglion cells are few in number and their dendrites spread widely in the retina). • Fig. 6.4.8. Cross-section of the lateral geniculate body showing maganocellular (1 and 2) andparvocellular (3, 4 5, 6) areas. Parvocellular layers receive their input almost entirely from the X-ganglion cells and thus transmit colour vision and also convey accurate point-to-point spatial information, for texture, shape and fine depth vision; but at only a moderate velocity of conduction rather than high velocity. • • • • • Electrophysiological properties of the LGB In many respects, the electrophysiological properties including receptive fields of both P and I cells of LGB are similar to retinal ganglion cells and the optic nerve axons.28-33 Without exception, all geniculate receptive fields possess the familiar on-centre/off-centre configuration. The classification of receptive fields as sustained (X) and transient (Y) is maintained in the LGB. The receptive field organization of the geniculate cell differs from that of ganglion cell in the organization of its periphery. Hubel and Wiesel identified a high degree of peripheral suppression in geniculate receptive field.28,29 The larger 'off' periphery cancels the effects of an 'on' centre. Thus, they are a little more sensitive in responding to spatial differences in retinal illumination rather than to the illumination itself. Thus the disparity between responses to small spot illumination of the entire retina is accentuated by the LGB neurons. The majority of geniculate relay cells have binocular receptive OPTIC RADIATIONS The optic radiations are composed of axons of lateral geniculate relay cells which project to the visual cortex on the same side. The optic radiations maintain a retinotopic organization in their passage to the visual cortex. The central portion contains macular fibres, while the dorsal fibres carry information from the upper parts of the retinae. The lower or ventral fibres represent lower retinal quadrants (Fig. 6.1.27). ANALYSIS OF VISUAL IMPULSE IN THE VISUAL CORTEX A. RETINOTOPIC ORGANIZATION Visual cortex lies in the occipital lobe near the posterior pole. Just as the ganglion cell axons project a detailed spatial representation of the retina on the lateral geniculate body (LGB), the LGB projects a similar point-to-point representation on the visual cortex. Details of the anatomy of visual cortex and retinotopic organization of the visual cortex have been described on pages 184 and 188, respectively. B. FUNCTIONAL ANATOMY AND ORGANIZATION OF THE VISUAL CORTEX TYPES OF VISUAL CORTEX It can be divided into a primary visual cortex and a secondary visual cortex. ▪ Primary visual cortex. It is also known as Brodmann's cortical area 17 or visual area I or simply V-l. Still another name for primary visual cortex is the striate cortex, because this area has a grossly striated appearance. The axons from the lateral geniculate body end in primary visual cortex. ▪ Secondary visual cortex. It includes visual association areas which lie anterior, superior and inferior to the primary visual cortex. Secondary signals are transmitted to these areas for further analysis of the visual meaning. The visual association areas include the Brodmann's area 18 or visual area II or simply V-2; area 19 (visual area III or V 3) and so on. LAYERS OF PRIMARY VISUAL CORTEX Primary visual cortex, like other portions of cerebral cortex, has six distinct layers (Fig. 6.4.9). Layers I, II and III are thin and contain pyramidal cells. Layer IV is thickest and contains stellate cells. Layer IV may be further subdivided into layers, a, b, ca and cb; layers IV a + b contain the white stripe of Gennari. Layer V and VI are again relatively thin. Fig. 6.4.9. Layers of visual cortex vis-a-vis analysis of visual impulse in the visual cortex. • • CONNECTIONS OF PRIMARY VISUAL CORTEX 1. Geniculate afferents The axons from the lateral geniculate nucleus (the geniculocalcarine fibres) terminate mainly in layer IV.37 The rapidly conducted signals from the Y retinal ganglion cells terminate in layer IV ca and from here are relayed vertically both outwards toward the cortical surface and inwards toward deeper levels.38 The visual signals from the medium-sized optic nerve fibres derived from the X ganglion cells in the retina terminate in layers IVa and IVc, (the shallowest and deepest portions of layer IV). From here these signals again are transmitted vertically both towards the surface of the cortex and to deeper layers. The X ganglion pathway transmits accurate point-topoint type of vision and also the colour vision.38 • • 2. Subcortical connections The reciprocal connections returning from striate to LGB arise from the pyramidal cells of layer VI. Cells in the upper part of layer VI project primarily to maganocellular layers, while the projection to parvocellular layer arises mainly cells in the lower aspect of layer VI.39 Axons from pulvinar to striate cortex terminate among the dendrites of layers I and V whose axons project back to pulvinar and to superior colliculus.39 • • • 3. Corticocortical connections Striate cortex is extensively interconnected with other cortical regions. A few important ones are as follows: Fibres to extrastriate visual regions arise from the pyramidal cells of layers II and III of the striate cortex. Fibres to contralateral striate cortex, via corpus callosum also arise in layer III. Reciprocal connections from the extrastriate visual region and from contralateral striate cortex are made by the fibres that terminate predominantly in layers II and III of striate cortex. • • • • • C. PHYSIOLOGY OF VISUAL CORTEX The present information available on physiology of the visual cortex is just the tip of iceberg. Thus, it is just not possible to construct a circuit diagram for the visual cortex with this little information about the enormous complexity of the interactions between the highly branched processes of cortical neurons. However, microelectrode recording and receptive field mapping by various workers have led to a number of significant observations and to an interesting model for how cortical cells interact. Hubel and Wiesel40-46 showed that there are some peculiarities associated with the functions of striate cortex. They observed that unlike retinal ganglion cells and lateral geniculate neurons (which respond to both diffuse retinal stimulation and spot stimulus) the cortical neurons prefer stimuli in the form of straight line, bar or edge presented in the proper spatial orientation. Thus, in visual cortex, the orientation and configuration receptive field differ from those earlier points in the visual pathway. Some of the aspects of physiology of visual cortex have been delineated, though incompletely, can be discussed under following headings: Concept of receptive field of striate cortex. Columnar organization of striate cortex. Serial versus parallel analysis of visual image. Role of extra-striate cortex in visual functions. Psychophysiological aspects of visual functions. CONCEPT OF RECEPTIVE FIELD OF STRIATE CORTEX Hubel and Wiesel (Nobel prize winners) were pioneers in this field.40- 46 They identified and classified the cortical receptive fields of the cat and monkey. They named the cortical cells as three receptive field types—the simple, complex and hypercomplex. These are classical discoveries and form the basis on which a large portion of the visual processing theory is based. Simple cells Simple cells are found mainly in layer IV of the primary visual cortex (area l 7) and form the first relay station within the visual cortex. These respond to bars of light, lines or edges, but only when they have a particular orientation.40-42,45 When, for example, a bar of light is rotated as little as 10 degrees from the preferred orientation, the firing rate of simple cells is decreased, and if stimulus is rotated much more, the response disappears. The orientation of a stimulus that is most effective in evoking a response is called the receptive field axis orientation. The receptive field of simple cells can be mapped with small spots of light into 'on-areas' and 'off-areas', which like those of cells in the lateral geniculate body, are mutually antagonistic. However, unlike lateral geniculate neurons, the receptive fields of simple cells are arranged in parallel bands of 'on-areas' and 'off regions, rather than concentric centre-surround arrangement of geniculate body (Fig. 6.4.10). Receptive fields of simple cells often have a central band that is either an 'on-region' or an 'off-region', with parallel flanking region on two sides that are opposite (Fig. 6.4.10C to G). Thus, the simple cell receptive fields play an important role not only in the detection of lines and borders in the different areas of retinal image, but also detects the orientation of each line or border— that is whether it is vertical, or horizontal or lies at some degree of inclination. It is assumed that for each such orientation of a line a specific neuronal simple cell is stimulated. Fig. 6.4.10. Arrangement of receptive fields of lateral geniculate body and primary visual cortex (area). A 'on-centre geniculate receptive field; B, 'off-centre geniculate receptive field; C-G, arrangement of receptive field of simple cortical cells. X, areas give excitatory responses (on-responses) and D areas give inhibitory responses (offresponses) Receptive field axes are shown by continuous lines through field centres. After Hubal and Wiesel.42 Complex cells These cells are found in the cortical layers above and below layer IV of areas 17,18, and 19 of visual cortex, and only rarely in layer IV itself. They resemble simple cells in requiring a preferred orientation of a linear stimulus but are less dependent upon the location of a stimulus in the visual field than the simple cells. They often respond maximally when a linear stimulus is moved laterally without a change in its orientation (Fig. 6.4.11). Unlike simple cells, it is not possible to map out distinct antagonistic 'on' and 'off' region in the receptive fields of complex cells. Complex cells often receive input from both eyes and are thus called binocular. The receptive fields of a given binocular complex cell are on corresponding parts of the two retinae and have identical receptive field properties. Fig. 6.4.11. Receptive fields and stimuli. A, for ganglion cells and lateral geniculate cells (receptive field is circular with an excitatory centre and an inhibitory surround or an inhibitory centre and an excitatory surround). There is no preferred orientation of a linear stimulus). B, for simple cortical cells which respond best to a linear stimulus with a particular orientation in a particular part of the cell's receptive field. C, for complex cortical cells which respond to linear stimuli with a particular orientation, but they are less selective in terms of location in the receptive field and often respond maximally when the stimulus is moved laterally, as indicated by the arrow. Modified from Hubel.47 • • • • Four types of complex cell receptive field are described according to their preferred stimulus: Activated by a slit-nonuniform field, Activated by a slit-uniform field, Activated by an edge, and Activated by a dark bar. Thus, the complex cell receptive fields play an important role in the detection of lines, bars and edges, especially so, when they are moving. In other words, by means of simple and complex cells, the person perceives the features, orientation, and movements of objects. Therefore, simple and complex cells together are known as 'feature detectors'. Hypercomplex cells These are found in cortical layers II and III of the areas 17, 18, 19. These cells retain all the properties of complex cells but also have the added feature of requiring the line stimulus to be of a specific length. The stimuli to which they respond vary greatly, as does their complexity. Four types of 'lower' hypercomplex cell and two categories of 'higher' hypercomplex cell were described by Hubel and Wiesel.44 Dreher has classified hypercomplex cells into two types: I and II.48 Thus, the hypercomplex cells play a role in the detection of lines of specific length, angles or other shapes. COLUMNAR ORGANIZATION OF STRIATE CORTEX Orientation columns Hubel and Wiesel43,44 identified the orientation column as the unit of organization in the visual cortex, which can be defined as 'vertical grouping of cells with identical orientation specificity'. The visual cortex is thus organized structurally into several million vertical columns of neuronal cells, each column having a diameter of 25-50 micrometres.49 Roughly, the number of neurons in each of the vertical column is around 100,000. It has been observed that the orientation preferences of neighbouring column differ in a systematic way; as one moves from column to column across the cortex, there are sequential changes in orientation preference of 5-10 degrees (Fig. 6.4.12).50 Thus, it is possible to speculate that for each ganglion cell receptive field in the visual field, there is collection of column in a small area of visual cortex representing the possible preferred orientation at small intervals throughout the full 360 degrees. Fig. 6.4.12. A, Orientation preferences of 15 neurons encountered as a microelectrode penetrated the visual cortex obliquely. The preferred orientation changed steadily in a counter-clockwise direction. B, Results of a similar experiment plotted against distance the electrode travelled. In this case, there were a number of reversals in the direction of rotation (Modified from Hubel and Wiesel).50 The orientation columns can be mapped with the aid of radioactive 2-deoxyglucose. The uptake of this glucose derivative is proportionate to the activity of neurons. Balkemore51 described two separate column systems for processing depth-perception information. One column system contains binocular units with exactly the same retinal disparity for properly oriented stimuli (constant depth column). The second type is constant direction columns which view points perpendicular to the centre of the contralateral eye. Together they localize points in threedimensional space. Ocular dominance columns Ocular dominance columns refer to an independent system of columns which exist in the visual cortex with respect to the binocular input to cortical cells. As discussed earlier, the simple cells in layer IV of striate cortex receive input from one eye only, whereas most complex and hypercomplex cells in layers above and below the layer IV receive input from both eyes. Although most cortical neurons are binocularly activated, there remains a strong monocular dominance. Neurons with receptive fields dominated by one eye are grouped alternately into left eye and right eye columns that are 0.25-0.5 mm in width. The ocular dominance columns can be mapped by injecting a large amount of a radioactive amino acid into one eye. The amino acid is incorporated into protein and transported by axoplasmic flow to the ganglion cell terminals, across the geniculate synapses and along the geniculocalcarine fibres to the visual cortex. Layer IV becomes evenly labelled, but above and below this layer in the cortex, labelled columns alternate with unlabelled columns receiving input from the uninjected eye. The result is a vivid pattern of strips that covers much of the visual cortex (Fig. 6.4.13) and is separated from and independent of the grid of orientation columns.52,53 Fig. 6.4.13. Representation of ocular dominance columns in a relatively large segment of monkey striate cortex of right occipital lobe. View is of layer IV C seen from above; ocular dominance columns for one eye are in black and those for the other eye in white. The foveal representation is to the right. From Levay et al52 and Hubel and Wiesel.53 Thus, a group of binocular complex and hypercomplex cells in layers II, III, V and VI that receive a stronger input from one of the two eyes, along with the cells in layer IV that receive input from the same eye, is called an ocular dominance column.45 There are other methods also to demonstrate ocular dominance columns. The reason for the existence of rigorously ordered complex binocular input to some complex cells is unknown but may have something to do with binocular stereoscopic vision. The colour blobs Interspersed among the primary visual columns are special columnlike areas called colour blobs (Fig. 6.4.9). These receive lateral signals from the adjacent visual column and respond specifically to colour signals.45,54 Therefore, it is presumed that these blobs are the primary areas for deciphering colour. Also in certain secondary visual areas, additional colour blobs are found, which presumably perform still higher levels of colour deciphering. SERIAL VERSUS PARALLEL ANALYSIS OF VISUAL IMAGE Hubel and Wiesel,40-46 who first studied receptive fields of visual cortex and named those receptive field types as simple, complex and hypercomplex, have suggested a hierarchial model for cell interconnection. The sequence for simple to complex to hypercomplex is serial analysis with more and more details being deciphered. In the Hubel-Wiesel40-46 model (Fig. 6.4.14), a complex cell is thought of as receiving input from several simple cells of the same orientation whose receptive fields are overlapping to produce the complex cell receptive field. This suggestion is strengthened by the observation that cells of the same orientation are located in the same cortical column and that a major feature of cortical anatomy is the wealth of vertical interconnections between cells. Since the complex cells are binocular and simple cells are mainly monocular, this adds support to the idea that complex cells are at a more advanced stage of processing. Fig. 6.4.14. Hubel and Wiesel's (42) model of serial pathway explaining the organization ot complex receptive fields. A number of cells with simple fields, of which three are shown schematically, are imagined to project to a single cortical cells of higher order. Areas marked with * represent the excitatory regions and those marked with represent the inhibitory region. Thus, an orderly process (serial system) for synthesizing increasing complex receptive fields has been clearly established, and there is no disagreement about the columnar organization of the cortex. There has been some controversy as to whether this serial mechanism of visual processing is the only process or whether information is also handled by means of parallel inputs. However, Hoffman and Stone55 propose that the transmission of different types of visual information into different brain locations represents parallel processing systems. From the foregoing discussion, it should now be clear that the visual image is deciphered and analyzed by both serial and parallel pathways. It is the combination of both types of these analyses that gives one full interpretation of a visual scene. However, highest levels of analysis are still beyond the present physiological understanding. ROLE OF EXTRASTRIATE CORTEX (VISUAL ASSOCIATION CORTEX) IN VISUAL FUNCTIONS For many years, the traditional view of the organization of the visual system held that the striate cortex (area 17) and two "djacent cortical zones in the occipital lobe (areas 18 and 19) were the only higher visual centres. However, recent advances in the technique of neuroanatomy and neurophysiology have led to many alterations and revisions in the old concepts and brought forward few new concepts. It is now believed that from the neurons (simple, complex and hypercomplex) of striate cortex (area 17, or VI) information goes to neurons in area 18 (V2), area 19 (V2) and many more like V3, V4 and MT (Fig. 6.4.15). Fibres from these extrastriate areas go back to striate cortex, they are connected to each other and also receive visual input from the pulvinar.56-58 Such cells, which receive information from the feature detectors (simple and complex cells) and hypercomplex cells, are sometimes called 'pontifical cells' (pontiff— Pope or Bishop, i.e. the highest master). It would appear from the present evidence that some of these regions are specialized for processing particular aspects of visual information. Some of the examples of functionally specialized areas are as follows: Colour processing area V4 area in rhesus monkey shows group of clusters of opponent colour.59,60 These are much less prevalent in the other extrastriate regions indicating that V4 area forms a colour detecting specialized area. This concept has been supplemented by the fact that a few patients have been reported with cortical lesions (at the level of extrastriate visual cortex at the occipital temporal junction) that produced deficient colour vision; other aspects of visual function such as acuity and stereopsis were preserved.61,62 Motion (movement processing area) The extrastriate area called MT area (located in middle portion of temporal lobe) may be specialized in some way for the analysis of motion in the visual scene. This concept is based on the observation that a striking number of cells in this region show a strong preference Stereoscopic depth perception area The extrastriate cortex area V2 and probably also V3 may be processing information for stereoscopic depth perception,65,66 since, these cells (called as retinal disparity cells) respond poorly or not at all when the receptive field in one eye is stimulated alone, but respond quite well when both receptive fields are stimulated simultaneously. Cells having uncrossed retinal disparity and those having crossed retinal disparity are noted. Such units with binocular disparity in the receptive fields could thus be functioning in stereoscopic depth perception.67 It is quite likely that eventually additional cortical areas will be discovered. PSYCHOPHYSIOLOGICAL ASPECT OF VISUAL FUNCTIONS The complex psychophysiological aspect of visual function can be assigned to specific areas of the brain since vision is intimately related in the verbal language and reading. The visual cortex connects, by way of its associational cortex, with tactile sensory motor auditory, olfactory and speech centres (Fig. 6.4.15). The corpus callosum connects the two hemispheres while other bundles of axons connect the cortical area of same hemisphere (Fig. 6.4.15). It is these interconnections that allow us to perceive several qualities simultaneously and to synthesize a unified picture for our minds' eye. For example, while walking into a room we do not identify each object separately, but form an impression from its appearance, sound or orders. The brain's response to stimuli is not that of excitation to individual stimuli but is in the form of an overall gestalt, i.e. overall picture. Fig. 6.4.15. Visual pathway, visual cortex and connections with some of the visual association areas. It has been observed that the angular gyrus (area 40) of the parietal lobe functions as the visual memory centre for words by forming associations between visual and auditory centres. • 'THREE-PART-SYSTEM' HYPOTHESIS OF VISUAL PERCEPTIONS From the foregoing discussion on physiology of vision, a hypothesis that a '3-part-system' is responsible for visual sensations can be well evolved from the data available till date. Although much is still unknown about the physiology of vision. The pathways involved in this 3-part-system are summarized in Fig. 6.4.16. Components of this 3-part-system are as follows: First system is concerned with the perception of movement, location and spatial organization (Fig. 6.4.16A). Fig. 6.4.16. Organization of visual pathway into a three-part-system that processes information in parallel. Information from the system that detects movement, location and spatial organization (A); the • • system that detects colour (B); and the system that detects shapes (C), is then integrated into single visual perception. Second system is concerned with the perception of colour (Fig. 6.4.16B), and Third system is concerned with the perception of shapes (Fig. 6.4.16C). These three systems process the information as parallel systems. Each of these systems processes the information in a serial system. Ultimately, information from all the three systems is then integrated into a single visual perception.
RETINA : VISUAL ACUITY : INTRODUCTION Visual angle COMPONENTS OF VISUAL ACUITY Minimum visible Resolution Recognition Hyperacuity FACTORS AFFECTING VISUAL ACUITY Stimulus related factors Observer related factors MEASUREMENT OF VISUAL ACUITY In school children and adults Snellen's test types Landolt's test types In preschool children Vision tests in 3-5 years Vision tests in 2-3 years Vision tests in 1-2 years In infants • • • Optokinetic nystagmus Preferential looking test Visually evoked response
INTRODUCTION Vision or visual perception is a complex integration of light sense, form sense, sense of contrast and colour sense. Visual acuity is considered a measure of form sense; so it refers to the spatial limit of visual discrimination. Clinically, visual acuity is determined by discriminating letters on a chart, which requires discrimination at a retinal level accompanied by an ability to interpret the form and shape of the letters at a higher level. In a situation where higher centres are not yet fully developed or are abnormal, contrast sensitivity can be used to give a prediction of visual acuity. Technically speaking, a visual acuity measurement involves the determination of a threshold. In terms of visual angle, the visual acuity is defined as the reciprocal of the minimum resolvable visual angle measured in minutes of arc for a standard test pattern. Therefore, to understand visual acuity, a knowledge about visual angle is essential. VISUAL ANGLE Visual angle is the angle subtended at the nodal point of the eye by the physical dimensions of an object in the visual field (Fig. 6.7.1). Visual angle is a useful and convenient mode of specifying the spatial extent of objects or elements in the visual field. It has been observed that the two adjacent points can be seen clearly and discretely only when these two points (say A and B in Fig. 6.7.1) produce a visual angle not less than 1 min. Fig. 6.7.1. Visual angle (ANB) subtended at the nodal point by the physical dimensions (AB) of the object. The dimensions of the visual angle depend upon the size of the object as well as its distance from the eye. Therefore, to be seen clearly, either the object should be large enough or should be placed near (at an appropriate distance). In terms of the length of the retinal image, it has been seen that the two points (A and B) will be seen clearly when their image size (A'B') is more than 4.5 m. It is so, because the diameter of individual cone stimulated by the image points A' and B' is 1.5 m each and at least one cone in between (of 1.5 m diameter) must be unstimulated. The retinal image size for a given visual angle may vary slightly with changes in viewing distance and associated changes in accommodation of the lens, but this effect is relatively small.1 • • • • COMPONENTS OF VISUAL ACUITY In clinical practice, measurement of the threshold of discrimination of two spatially separated targets (a function of the fovea centralis) is termed visual acuity. However, in theory, visual acuity is a highly complex function that consists of the following components: Minimum visible, Resolution, Recognition, and Minimum discriminable. • • • • MINIMUM VISIBLE The ability to determine whether or not an object is present in an otherwise empty visual field is termed visibility or detection. This kind of task is referred to as the minimum visible or minimum detectable function. The limit of visibility reflects the absolute threshold of vision. The minimum visible spatial threshold level will depend upon the specification of stimulus such as size, shape, illumination and so on. A few observations made about the minimal visible threshold are as follows: A black dot against a white background can be detected, if its diameter is of the order of 30 seconds of arc or more.2 A black square can be discriminated against a light background when the length of a diagonal is 30 seconds or slightly less.3 An extended line (e.g. visualization of a thin telegraph wire against a uniform sky) with a thickness of as little as one-half second of arc may be discriminable.4 The ability to discriminate such a fine line when its image is of sufficient extent implies depending upon some kind of process that involves the convergence of subthreshold signals from a number of individual retinal elements along the extent of retinal image at a common point. The addition of these subthreshold signals yields a discriminable suprathreshold level of activity. Detection of an illuminated object against a dark background solely depends upon their intensity and not on their size. • • • • RESOLUTION (ORDINARY VISUAL ACUITY) Discrimination of two spatially separated targets is termed resolution. The minimum separation between the two points, which can be discriminated as two, is known as minimum resolvable. Measurement of the threshold of discrimination is essentially an assessment of the function of the fovea centralis and is termed ordinary visual acuity. The distance between the two targets is specified by the angle subtended at the nodal point of the eye. The normal angular threshold of discrimination for resolution measures approximately 30 to 60 sec. arc; it is usually called the minimum angle of resolution (MAR). If the minimum separation between two light bars is considered, the threshold value becomes increasingly smaller as the width of the bars increases, reaching a limiting condition of approximately one-half second of arc when the light bars have become so broad that the overall presentation is indiscriminable from a dark line against a large homogenous light background.3,4 The minimum separation, that can be discriminated between two dark bars, will become infinitesimal as the bars become wider and the stimulus is seen as a light line against a dark background.5 The clinical tests determining visual acuity measure the form sense or reading ability of the eye. Thus, broadly, resolution refers to the ability to identify the spatial characteristics of a test figure. The test targets in these tests may either consist of letters (Snellen's chart) or broken circle (Landolt's ring). More complex targets include gratings and checker board patterns. RECOGNITION It is that faculty by virtue of which an individual not only discriminates the spatial characteristics of the test pattern but also identifies the patterns with which he has had some experience. Recognition is thus a task involving cognitive components in addition to spatial resolution. For recognition, the individual should be familiar with the set of test figures employed in addition to being able to resolve them. The most common example of recognition phenomenon is identification of faces. The average adult can recognize thousands of faces.6 MINIMUM DISCRIMINABLE OR HYPERACUITY The human eye is capable of seeing more than the ability of the retinal cones to resolve. This ability is called hyperacuity. It is due to the involvement of higher cortical centres in the parietal cortex. Thus, minimum discriminable refers to spatial distinction by an observer when the threshold is much lower than the ordinary acuity. The best example of minimum discriminable is vernier acuity, which refers to the ability to determine whether or not two parallel and straight lines are aligned in the frontal plane.7 It is the smallest offset of a line which can be detected (Fig. 6.7.2). It is measured using a square wave grating. The threshold values of vernier acuity are in the range of only a few seconds (2-10) of arc. It is lens than limit of Snellen visual acuity and is, therefore, called hyperacuity. Hyperacuity should not be confused with the threshold for the minimum visible, where merely the presence or absence of a target is being judged. The mechanism subserving hyperacuity is not clearly known but so much is clear; no contradiction is involved with the optical and receptor mosaic factors that limit ordinary visual acuity.8 Fig. 6.7.2. Typical target configuration for detecting vernier acuity. FACTORS AFFECTING VISUAL ACUITY As discussed earlier, the resolution part of spatial discrimination is considered synonymous with ordinary visual acuity. And we know that where an observer exhibits the so-called normal visual acuity, all the elements (optical, anatomic and physiologic) concerned with the vision are at or near their peak performance. In general, the factors that influence the spatial resolution can be classified into the physical and physiological. ▪ Physical factors include those which influence the light characteristics of the distribution and hence influence the nature of retinal image. ▪ Physiological factors are those which influence the processing of the stimulus and are thus mainly observer related. However, there is some overlap between the physical and physiological groups. For example, the lens is a physical factor but the related accommodation process is physiological. Similarly, the size of pupil which controls the amount of light entering the eye is a physical factor but the reflexes controlling its size are physiological processes. Therefore, in the ensuing discussion, these factors have been classified into stimulus-related and the observer-related factors. Further, the list of such factors is exhSTIMULUS-RELATED FACTORS 1. Luminance As shown in Fig. 6.7.3, in general visual acuity increases with an increase in the luminance of test object, within certain limits.9 However, it also depends upon the adaptation stage of the eye. Lythgoe10 in his experiments (Fig. 6.7.4) has shown the following effect of the brightness of the surroundings on the visual acuity vis-àvis luminance of test object: Fig. 6.7.3. Relation between visual acuity and logarithm of luminance. The lower curve represents rod function; the upper curve represents cone function. From Hecht, S.C. If the subject is placed in a dark box and is allowed to see the test object through a small window, he can be considered to be in a state of dark-adaptation even though the luminance of test object is high enough to stimulate cone vision. In this case, the acuity increases up to a maximum at approximately 10 e.f.c. and then begins to fall (Fig. 6.7.4A). • • • When the walls of the box are given a luminance of 0.11 e.f.c., i.e. if the subject is only partly dark-adapted, the acuity continues to increase to a maximum at approximately 50 e.f.c., but then falls off (Fig. 6.7.4B). When the luminance of the box is continuously adjusted so that it is equal to that of the test object, it is found that visual acuity increases progressively with the luminance of the test object (Fig. 6.7.4C). Fig. 6.7.4. Effect of brightness of the surroundings (i.e. state of adaptation) on visual acuity. For explanation see text. From Lythgole.10 If the luminance of the box is made greater than that of the test object, generally the acuity is lower than when the luminances are equal. So, from the above observation, it is clear that retinal interaction plays an important role in so far as the acuity of vision is influenced by the luminance of the test object vis-à-vis general illumination of the eye. This has been explained by Pirenne and his colleagues11 who put forward the following hypothesis: ▪ Multiple unit hypothesis giving an explanation for the phenomenon of the progressive increase in visual acuity with the increase in luminance of the test object is best illustrated by a diagram (Fig. 6.7.5). It is a fact that the requirement for maximal visual acuity is the effective one-to-one relationship between cones and optic nerve fibres. Any deviation from this relationship must reduce the resolving power. Thus, as shown in Fig. 6.7.5, a perfect three-to-one relationship would give a visual acuity of one-third the maximum, since the effective size of the receptor is three times as large. The visual acuity, other things being equal, must, therefore, depend upon the size of effective retinal units that can be brought into operation. Pirenne et cil11 in their multiple unit hypothesis have suggested that it is mainly because increasing the luminance of the test-object brings into operation a smaller unit, i.e. smaller groups of receptors converging on a single optic nerve fibre, that the basic relationship between visual acuity and luminance pertains. Fig. 6.7.5. Concept of retinal units in determining visual acuity: A, The cones beha ve in a perfect one-to-one manner, so that the effective retinal mosaic for discriminating a grating is constituted by rows of single cones. B, Because of convergence in the visual pathway, three cones behave as units, hence the grating at limiting resolution should be three times as wide. 2. Stimulus geometry Geometrical configuration of the stimulus also influences the visual acuity. Fig. 6.7.6 shows the relationship of visual acuity with log retinal illumination with the stimuli having different geometry, i.e. Landolt's ring (Fig. 6.7.6A) and gratings (Fig. 6.7.6B). As shown in Fig. 6.7.6, the general shape of the two curves is very similar, but numerical values of visual acuity for a given luminance do differ by a substantial amount. At lower luminance, visual acuity is higher for grating than for Landolt's ring and reverse is true at higher luminances.1 Fig. 6.7.6. Effect of stimulus geometry on visual acuity as a function of the logarithm of retinal illuminance: A, Landoltring and B, a grating. From Schlaer S1. 3. Contrast One of the important physical variables is the contrast of the stimulus from the surround. In general, greater the contrast, more sharply the pattern will be defined.12 In clinical practice, the contrast in retinal image may be reduced by the presence of stray light in the eye itself; as occurs due to scattering of light within the eye in patients with early cataract.13-15 4. Influence of wavelength Theoretically, the use of monochromatic as opposed to white light should provide more sharp image, because the monochromatic light should abolish chromatic aberrations. Flow ever, in general, the use of different coloured lights has little effect on visual acuity provided their luminance is adequately maintained.16 It indicates that physiological mechanisms, leading to suppression of the coloured fringes come into play. It has been argued that, because colour vision is mediated by at least three types of cones, the use of monochromatic light would impair visual acuity since, with green light, say, only the green-sensitive cones would be operative and so the retinal mosaic would be much grosser. But, there are circumstances in which the wavelength distribution can play a highly significant role in spatial discrimination. When differences are encoded by colour, as is the case in many maps, then these differences may be invisible, if coloured light is used. 5. Exposure duration The resolution capacity, in general, increases when the stimulus is of relatively longer duration. It is not related directly to the increased energy that results, if luminance is held constant. This effect might be related to the increased information-gathering capability of the cortical mechanisms for a longer time interval. The critical durations within which the visual system is able to summate energy for a threshold response varies from around 0.01 sec in the light-adapted eye to 0.1 sec in the dark-adapted eye, or at low luminances, when the criterion of threshold is light detection or luminance discrimination.17 Critical duration as a function of visual acuity threshold criterion for each of three levels of adaptation, i.e. scotopic, mesopic and photopic is shown in Fig. 6.7.7.18 Fig. 6.7.7. Critical duration as a function of visual acuity threshold criterion for each of three levels of adaptation: A, scotopic; B, mesopic; and C, photopic. From Brown JL.18 6. Interaction effects Interaction effects of the two targets reduce the visual acuity when the targets are too close together. It has been observed that when a competing target is presented within a few minutes of arc the resolution threshold may rise and even double. The diminution of performance is maximal at a distance of 2-5 minutes of arc and disappears for larger and smaller distances. Further, such an effect has been observed with ordinary visual acuity, vernier visual acuity as well as stereoscopic acuity OBSERVER-RELATED FACTORS 1. Retinal locus of stimulation Form sense is a function of cones. Therefore, form sense is most acute at the fovea, where there are maximum number of cones and decreases very rapidly towards the periphery (Fig. 6.7.8). In the foveal region, retina contains only cones (about 35,000) which are closely packed in an area of about one degree. Further, there is oneto- one relationship of these cones with ganglion cell fibres and higher up. In the peripheral retina, the sensitivity of the cones decreases and that of rods increases; out to an eccentricity of approximately 20 degrees.22 Further, in the peripheral retina, an increasingly large number of receptors converge upon a single ganglion cell, so that the retinal region for which stimulation can be discriminated independently becomes larger and consequently spatial resolution is reduced. Fig. 6.7.8. Regional variations of the visual acuity in the retina. N: Nasal retina; b: Blind spot; f: Fovea; T: Temporal retina. 2. Pupil size In an emmetropic eye, the point-spread function and thus the visual acuity (spatial resolution) remains almost constant when the pupil size is between 2.5 and 6 mm.23 Below 2.5 mm, due to diffraction, the point-spread function becomes progressively wider, and a reciprocal relationship is expected between minimum angle of resolution and pupil size. Beyond 6 mm pupil size, the aberrations begin to widen the point-spread function again. 3. Accommodation Youthful emmetropic eye with the accommodative reflex can perform successful visual function that requires scanning in depth. Thus accommodation is useful for spatial resolution at various distances of the target from the eye. After decreased accommodation with increasing age (after 40 years of age) though bifocal and trifocal glasses help to some extent, but are not a substitute for the range of accommodation that can be achieved by the youthful eye.24 4. Effect of eye movements The role of involuntary movements of the eye in determining visual acuity is not clear. It has been reported that if the image is somehow fixed on the retina, perception fades.25,26 Motion of the retinal image is actually essential for the maintenance of perception.27 5. Meridional variation in acuity With the help of grating or similar targets that allow the selective evaluation of the function of one meridian at a time, differences in visual acuity in the various retinal meridians have been widely reported. The resolution has been reported to be comparatively more sharp in horizontal and vertical meridia; although this is not universally so. Further, the differences hardly increase 15%. Uncorrected astigmatism has also been implicated resulting in meridional differences in the visual acuity.28 6. Optical elements of the eye The optical factors of the eye such as refractive errors, abnormalities of corneal curvature, abnormalities in the axial length of the eye and opacities in various elements of the eye may influence visual resolution capability. 7. Developmental aspects Psychophysical and electrophysiologic researches during recent years have established that visual acuity in infants develops much more rapidly than once thought. It has been reported that an infant's visual capacities are rather surprisingly well-established shortly after birth, and that adult levels are reached at approximately 2 to 3 years of age.29,30 MEASUREMENT OF VISUAL ACUITY As discussed earlier, the visual acuity is a highly complex function that consists of: Minimum visible, i.e. detection of presence or absence of stimulus, Minimum separable, i.e. judgement of location of a visual target relative to another element of the same target, and Minimum resolvable (ordinary visual acuity), i. e. the ability to distinguish between more than one identifying feature in a visible target. In clinical practice, the measurement of visual acuity is considered synonymous with the measurement of 'minimum resolvable'. (However, in theory, it is not so, as is clear from the above.) The threshold of the minimum resolvable is between 30 sec and 1 min of arc. Therefore, all the clinical tests employed to measure the visual acuity are designed taking into consideration the threshold of the one minimum resolvable. Based on this basic principle, many visual acuity charts have been developed. Examination with eye charts is quite satisfactory, but it is obviously incomplete. It emphasizes on f oveal vision, usually at one level of illumination, and adaptation at one distance. In order to be complete, it must be supplemented by other tests for near vision and at lower luminances and with some tests of peripheral function as well. It is important to realize that though assessing visual acuity in children may be difficult and often demands painstaking patience, its interpretation should be done in the light of the overall clinical picture. As no single test is dependable, one must try and use a battery of tests which may be repeated on subsequent follow-ups. The various visual acuity tests available can be grouped as follows: I. Detection acuity tests. These assess the ability to detect the smallest stimulus without recognizing correctly. Common detection acuity tests are: 1. 2. 3. 4. 5. 1. 2. 3. 4. 1. 2. 3. 4. 1. 2. 3. 4. 5. 1. 2. Dot visual acuity test Catford drum test Boek candy beads test STYCAR graded Ball's test Schwarting metronome test. II. Recognition acuity tests. These are designed to assess the ability to recognize the stimulus or to distinguish it from other competing stimuli. These include: A. Direction identification tests Snellen's E-chart test Landolt's C-chart test Sjogren's hand test Arrows test B. Letter identification tests Snellen's letter chart test Sheridan's letter test Flook's symbol test Lipman's HOTV test C. Picture identification charts (miniature toy test) Alien's picture cards test Beale Collins picture charts test Domino cards test Lighthouse test Miniature toy test of Sheridan D. Tests based on picture identification on behavioural pattern Cardiff acuity cards test Bailey Hall cereal test III. Resolution acuity tests 1. 2. i. ii. iii. 3. Optokinetic nystagmus (OKN) test Preferential looking test (PLT) Two alternative forced choice (2-AFC) test Operant variation looking (OPL) test Teller acuity cards (TAC) test Visually evoked response (VER) MEASUREMENT OF VISUAL ACUITY IN SCHOOL CHILDREN AND ADULTS Snellen's test types The distant central visual acuity is usually tested by Snellen's test types. The fact that two distant points can be visible as separate only when they subtend an angle of 1 min at the nodal point of the eye, forms the basis of Snellen's test types. It consists of a series of black capital letters on a white board, arranged in lines, each progressively diminishing in size. The lines comprising the letter have such a breadth that they will subtend an angle of 1 min at the nodal point. Each letter of the chart is so designed that it fits in a square, the sides of which are five times the breadth of the constituent lines. Thus at the given distance, each letter subtends an angle of 5 min at the nodal point of the eye (Fig. 6.7.9). The letter of the top line of Snellen's chart (Fig. 6.7.10) should be read clearly at a distance of 60 m. Similarly, the letters in the subsequent lines should be read from a distance of 36, 24, 18,12,9, 6, 5 and 4 m. Fig. 6.7.9. Principle of Snellen's test types. Fig. 6.7.10. Snellen's test types.. Landolt's test types It is similar to Snellen's test types, except that in it, instead of the letter the broken circles are used. Each broken ring subtends an angle of 5 min at the nodal point and is constructed similar to letter of Snellen's test types (Fig. 6.7.11). Fig. 6.7.11. Construction of Landolt's visual acuity target. With Snellen's letters, the end point consists of letter recognition; with Landolt's rings, it consists of the detection of the orientation of the break in the circle. Each method has advantages and disadvantages. Letter targets represent a practical visual test. However, the ability to recognize the target is influenced by literacy and past experience, even if the targets are somewhat blurred. Landolt's rings were designed to eliminate these factors and present a more objective test. However, since the gap can be placed in only four positions (up, down, left and right), guessing becomes an important factor. Also letter tests remain much less confusing for the patient and the examiner, since the identification of letters is both immediate and unequivocal. Procedure of Testing For testing distant visual acuity, the patient is seated at a distance of 6 m from the Snellen's chart, so that the rays of light are practically parallel and the patient exerts minimal accommodation. The chart should be properly illuminated (not less than 20 ft candles). The patient is asked to read the chart with each eye separately and the visual acuity is recorded as a fraction, the numerator being the distance of the patient from the letters, and the denominator being the smallest letters accurately read. When the patient is able to read up to 6 m line, the visual acuity is recorded as 6/6, which is normal. Similarly, depending upon the smallest line which the patient can read from the distance of 6 m, his vision is recorded as 6/9,6/12,6/18, 6/24, 6/36, and 6/60, respectively. If he cannot see the top line from 6 m, he is asked to slowly walk towards the chart till he can read the top line. Depending upon the distance at which he can read the top line, his vision is recorded as 5/60, 4/60, 3/60, 2/60 and 1/60, respectively. If the patient is unable to read the top line even from 1 m, he is asked to count fingers (CF) of the examiner. His vision is recorded as CF-3', CF-2', CF-1' or CF close to face, depending upon the distance at which the patient is able to count fingers. When the patient fails to count fingers, the examiner moves his hand close to the patient's face. If he can appreciate the hand movements (HM), visual acuity is recorded as HM +ve. When the patient cannot distinguish the hand movements, the examiner notes whether the patient can perceive light (PL) or not. If yes, vision is recorded as PL +ve and if not it is recorded as PL -ve. • • • • • • • Visual acuity equivalents in different notations Table 6.7.1 indicates different ways for specifying visual acuity levels: minimum angle of resolution; Snellen's acuity; efficiency rating; Snellen's fraction (i.e. the reciprocal of the minimum angle of resolution); and the logarithm of Snellen's fraction. LogMAR chart This is a modification of Snellen's chart, where each subsequent line differs by 0.1 log unit in the minium angle of resolution (MAR) required for that line. It has equal number of letters in each line. Used at a distance of 4 meters. Used for academic and research purposes. Decimal notation It converts Snellen fraction to a decimal. For example (Table 6.7.1) Snellen 20/20—decimal 1.0 Snellen 20/30—decimal 0.7 Snellen 20/40—decimal 0.5 Table 6.7.1. Visual acuity equivalents different notations in different MEASUREMENT OF VISUAL ACUITY IN PRESCHOOL CHILDREN Vision Tests in 3-5 years 1. Illiterate E-cutout test. This test is useful in children between 2Vfc and 3 years of age. The child is given a cutout of an E and asked to match this E with isolated Es of varying sizes. The first trial is not always successful. The mother may be instructed to teach E-game at home. When the child starts understanding the orientation of E, a visual acuity chart consisting of Es oriented in various directions may be used. 2. Tumbling E-test. It consists of different sizes of E in one of the four positions (right, left, upward and downward) on a dice (Fig. 6.7.12). Basically, it is similar to E-cutout test. Fig. 6.7.12. Tumbling E-pad test. Printed with large 20/200 E on one side and a series of five 20/20 tumbling Es on the other—calibrated to a 20 ft distance. 3. Isolated hand-figure test. Sjogren has replaced the E with the isolated figure of a hand, and in some children it works better than Es. 4. Sheridan-Gardiner HOTV test. It is another test similar to Ecutout test (Fig. 6.7.13). This is an initiative test, used to test vision in the age group of 2-5 years. The child is handed a card with HOTV and is asked to match the letters on the chart. Snellen's equivalent of 6/6-6/60 can be estimated using this method. Fig. 6.7.13. Sheridan Gardiner single letter optotypes. 5. Pictorial vision charts. When the child is able to verbalize, visual acuity chart showing pictures, rather than symbols may be used. Many such charts have been devised, and one should be chosen that presents pictures of objects with which the child is likely to be familiar. One of the examples is 'Kay picture test' (Fig. 6.7.14). The 'Allien preschool test' (Fig. 6.7.15), which presents picture in isolated form, is useful for this purpose. Fig. 6.7.14. Kay picture test. Fig. 6.7.15. Allen preschool test. 6. Broken ivheel test. A pair of cars in progressively smaller sizes, one of which has a wheel cut across, like Landolt C (broken wheel) is shown to the child and he is asked to identify the one with the broken wheel. 7. Boek candy bead test. The child is asked to match beads at 40 cm. Snellen's visual acuity equivalent of 20/200 is estimated by this method. 8. Light home picture cards. A chart containing an apple, house and umbrella (Fig. 6.7.16), arranged in Snellen's equivalents of 20/200 to 20/10 is used and the child is asked to identify the pictures along the lines. The test is carried out at 10 feet. Fig. 6.7.16. Light home picture cards. Vision tests in 2-3 Years 1. Dot vistial acuity test. Child is shown an illuminated box with black dots of different sizes printed on it. The smallest dot identified denotes the visual acuity of the child. 2. Coin test. In this test, the child is asked to identify the two faces of coins of different sizes held at different distances. 3. Miniattire toy test. In this test, the child is shown a miniature toy from a distance of 10 feet and is asked to name or pick the pair from the assortment. Vision tests in 1-2 Years 1. Marble game test. In children of 6 to 12 months of age, reaching or placing games can be used to estimate visual function. One such game is the 'marble game'. In it, the child is asked to place marbles in the holes of a card or in a box. This test is not intended to measure visual acuity of each eye, but rather to compare the functioning of the child's eye when one or the other is closed. The vision of an eye is then noted as being 'useful' or 'less useful'. 2. Sheridan's ball test. Mary Sheridan (1960) used a series of styrofoam balls of progressively smaller sizes. One records the smallest ball the infant can fixate and follow at a distance of 10 feet. Rolling the ball on a white or grey background and asking the child to pick it up, noting the smallest size to which he gives a good response, is a rough way of estimating visual acuity. 3. Other tests which can be used in this age group are Boek's candy test, Worth's ivory ball test. Measurement of visual acuity in infants Introduction Response to surrounding. By this age, a child should be aware and responsive to his surroundings and situations. A normal pupillary response, a positive blind response and an elicitable OKN indicate good visual acuity. Fixation behaviour can be determined accurately in this age group as the fovea develops completely by 3 months of age. Rough estimate of visual acuity from fixation pattern can be inferred as below: Gross eccentric fixation <CF 1 m Unsteady central fixation <6/60 Steady central fixation but not maintained 6/60-6/36 Central steady fixation, can maintain but prefers other eye 6/24-6/9 Central steady fixation, free alternation or corss-fixation 6/9-6/6 To call it as good fixation, it should be central, steady, and maintained preference of fixation in one eye denotes that poor vision in non-fixating eye. If the child habitually fixates with one eye, it indicates poor vision in the nonfixating eye and hence he will violently resist occlusion of the better eye. Refixation reflex. By 3-6 months, infants have adequate refixation reflex to permit cover test. An elicitable cover test indicates an underlying strabismus. Bruckner's method is helpful in children uncooperative to the cover test when an assessment is being carried out for small angle strabismus. By 6 months, the 'vergence response to interposition of loose base in/out prisms can be seen. Vestibulo-ocular reflex-induced nystagmus also helps in differentiating a seeing child from a blind one. The nystagmus usually persists for up to 5 seconds in a blind child. Optokinetic nystagmus (OKN) remains asymmetric till 4 months of age. Till this age group, it does not give the reliable equivalent of Snellen's visual acuity though it does help in gross visual assessment. Visually evoked response (VER) may be helpful in establishing the presence of cortical blindness and could give an estimation of the visual acuity of the child also. Forced choice preference gives optimum response at 3-12 months of age. From 9 months of age, infants find it hard to sustain interest in any target due to the phenomenon of habituation. Visual acuity tests for infants A few important tests are as below: 1. Optokinetic nystagmus test. In this test, nystagmus is elicited by passing a succession of black and white stripes through the patient's field of vision (Fig. 6.7.17). The visual angle subtended by the smallest strip width that still elicits an eye movement (minimum separable) is a measure of visual acuity. The only co-operation required in this test is that the infant be awake and hold both eyes open. It is reported that optokinetic nystagmus acuity is at least 6/120 in the newborn and improves fairly rapidly during the first few months of life, reaching to a level of 6/60 at 2 months, 6/30 at 6 months and 6/6 by 20-30 months.31 Fig. 6.7.17. Optokinetic nystagmus test for visual acuity (Courtesy: Dr. Elizabeth Joseph). 2. Preferential looking test. This test is based on the observation that, when presented with two adjacent stimulus fields, one of which is striped and the other of which is homogenous, infant will tend to look at the striped pattern for a greater portion of the time.32,33 Test procedures have been developed in which an examiner is hidden behind a screen on which one projects a homogenous surface on one side and black and white stripes on the other side.33-35 These two stimuli are alternated randomly. The observer is able to look through a hole in the screen at the eyes of infant, but is unaware of which target, stripes or homogenous field, is presented on which sides of the screen. The baby faces the screen (Fig. 6.7.18) and the observer records the direction of head movements in response to the appearance of the striped stimulus. The location of the striped pattern is varied at random from left to right, and fineness of the stripes is gradually reduced until there is no longer any correlation between the judged direction of infant's gaze and location of the striped pattern. Fig. 6.7.18. Preferential looking test. This method is especially suitable for infants up to four months of age. Older infants are too easily distracted. Visual acuity determined with this method has been reported to range from approximately 6/240 in the newborn to 6/60 at 3 months and 6/6 at 36 months of age.36 It must be well understood that grating acuity testing cannot automatically be equated with acuity testing based on recognition task, such as naming pictures or Snellen's letters. In normal children, grating acuity is better than recognition acuity.25,38 Further, it has been suggested that different neural processing mechanisms in the brain are involved with spatial discrimination and recognition tasks. Hence, it is not advisable to equate grating acuity with recognition acuity (Snellen's). 3. Visually evoked response (VER). Visually evoked response (VER) refers to electro-encephalographic (EEG) recording made from the occipital lobe in response to visual stimuli. VER is the only clinically objective technique available to assess the functional state of the visual system beyond the retinal ganglion cells. It is quite useful in assessing visual function in infants. Flash VER just tells about the integrity of the macular and visual pathway. Pattern reversal VER is recorded using some patterned stimulus, as in the checker board (Fig. 6.7.19). In it, the pattern of stimulus is changed (e.g. black squares go white and white become black but the overall illumination remains the same. The pattern reversal VER depends on form sense and thus gives a rough estimate of the visual acuity. VER studies have shown visual acuity in infants to be 6/120 at the age of 1 month which reached to 6/60 at 2 months and 6/6 to 6/12 at the age of 6 months to 1 year.29,39 Fig. 6.7.19. Technique of recording visually evoked response (VER) and record of normal VER pattern. The discrepancy between estimated visual acuity values with optokinetic nystagmus, preferential looking test and VER at 6 months of age must be kept in mind while performing these tests (Table 6.7.2). Table 6.7.2. Estimated visual acuity at different ages from Dabson and Teller31 and Hoyat et al40 4. Catford drum test. It is a detection acuity test useful in infants and preschool children. In this test, child is made to observe an oscillating drum with black dots of varying sizes (Fig. 6.7.20). The smallest dot which evokes pendular eye movements (not an optokinetic nystagmus) denotes the level of visual acuity. This test is unreliable since it overestimates the vision. Fig. 6.7.20. Catford drum for visual acuity. 5. Cardiff acuity cards test. This test or vanishing optotypes test can also be used in infants (Fig. 6.7.21). • • • Fig. 6.7.21. Cardiff acuity cards test. 6. Indirect assessment of vistml acuity. This test in infants can be made by observing the various milestones in the development of vision as follows: i. Blink reflex in response to sound is present since birth. ii. Menace reflex, i.e. reflex closure of the eyes on the approach of an object is usually present after the age of 5 months, if vision is normal. iii. Tests based on fixation reflex are useful in making a rough estimate of vision in infants. These include: Fixation behaviour test Binocular fixation pattern CSM method of rating monocular fixation. MEASUREMENT OF VISUAL ACUITY FOR NEAR Near vision is tested by asking the patient to read a near vision chart which consists of a series of different sizes of printer type arranged in increasing order and marked accordingly. Commonly used near vision charts are as follows: 1. Jaeger's chart. Jaeger in 1867 devised the near vision chart which consisted of the ordinary printers' fonts of varying sizes used at that time. Printers' fonts have changed considerably since then, however, it is now the general custom to use various sizes of modern fonts which approximate Jaeger's original choice. In this chart, prints are marked from 1 to 7 and accordingly patient's acuity is labelled as J1 to J7 depending upon the print he can read. 2. Roman test-types. The Jaeger's charts made from the modern fonts deviate considerably from the original standard, but they are probably sufficiently accurate for all practical purposes. However, to overcome this theoretical problem the 'Faculty of Ophthalmologists of Great Britain' in 1952 devised another near vision chart. It consists of 'Times Roman' type fonts with standard spacing. According to this chart, the near vision is recorded as N5, N6, N8, N10, N12, N18, N36 and N48. 3. Snellen's near vision test-types. Snellen introduced the so-called 'Snellen's equivalent for near vision' on the same principles as his distant types. The graded thickness of the letters of different lines is about 1/17th of the distant vision chart letters. In this event, the letters equivalent to 6/6 line subtend an angle of 5 minutes at an average reading distance (35 cm/14 in.). The unusual configuration of letters of this chart, however, cannot be constructed from the available printer's fonts. It can only be reproduced by a photographic reduction of the standard Snellen's distant vision test types to approximately l/17th of their normal size. Further, such a test, has never become popular. The graded sizes of pleasing types of passages from literature, the reading of which Procedure of testing For testing the near vision, patient is seated in a chair and asked to read the near vision chart kept at a distance of 25 to 35 cm, with a good illumination thrown over his left shoulder. Each eye should be tested separately. The near vision is recorded as the smallest type which can be read comfortably by the patient. A note of the approximate distance at which the near vision chart is held should also be made. Thus near vision is recorded as: NV = J1 at 30 cm (in Jaeger's notation) NV = N5 at 30 cm (in Faculty's notation) Near-vision equivalents in different notations These are shown in Table 6.7.3. Table 6.7.3. Equivalents visual acuity notations for near
Fundal coloboma classification ?
Ida manns classification
S/N ON FOVEA CENTRALIS ?
In this area , there are no rods, cones are tightly packed and other layers of retina are very thin. Its central part (foveola) largely consists of cones and there nuclei covered by a thin internal limiting membrane. All other retinal layers are absent in this region. In the foveal region surrounding the foveola, the cone axons are arranged obliquely (Henle's layer) to reach the margin of the fovea.
Describe International classification of retinoblastoma (ICRB) ?
International classification ofretinoblastoma (ICRB)= which is presently being followed worldwide to decide the treatment modality, is given below: Group A (very low risk): includes all small tumours <3 mm in greatest dimension, confined to retina, located >3 mm from fovea and > 1.5 mm from the optic disc. Group B (low risk): includes large tumours >3 mm in dimension, and any size tumours located <3 mm from fovea, and <1.5 mm from the optic disc margin. Group C (moderate risk): includes retinoblasroma with focal seeds characterized by subretinal and or vitreous seeds >3 mm from the retinoblastoma. Group D (high risk): includes retinoblastoma with diffuse seeds characterized by subretinal and or vitreous >3 mm seeds from the retinoblastoma. Group E (very high risk): includes extensive relinoblastoma characterized by any of the following: tumour touching the lens, neovascular glaucoma, tumour anterior to anterior vitreous face involving ciliary body and anterior segment, diffuse infiltratingtumour, opaque media with haemorrhage, tumour necrosis with aseptic orbital cellulitis, invasion of postlaminar optic nerve, choroid, sclera, orbit, and anterior clhamber, or phthisis bulbi.
Outer nuclear layer digram ?
It consists of nuclei of rods and cones.
what is Endophytic retinoblastoma ?
It grows inwards from the retina into the vitreous cavity. On ophthalmoscopic examination, the tumour looks like a well-circumscribed polypoidal mass of white or pearly pink colour. Fine blood vessels and sometimes a haemorrhage may be present on its surface. ln 1the presence of calcification, it gives the typical 'cottage cheese' appearance. There may be multiple growths projecting into the vitreous, which may seed into the gel.
External limiting membrane digram ?
It is a fenestrated membrane, through which pass processes of the rods and cones.
S/N ON 4. ENCEPHALOFACIAL ANGIOMATOSIS (STURGE-WEBER SYNDROME) ?
It is ch aracterised by angiomatosis in the form of port-wine stain (naevus llammeus), involvingone side of the face which may be associated with choroidal haemangioma, leptomeningeal angioma and congenital glaucoma on the affected side.
Describe Conservative tumour destructive therapy to salvage eyeball for RB ?
It is indicated when tumour is diagnosed in (stages A to D of lCRB). Present recommendations for the treatmem are multimodal therapy comprising primary systemic chemotherapy for chemoreduction followed by focal therapy for consolidation. Chemotherapy. Dose in mg/ kg body weight for chemored uction of retinoblastoma are as below: •Standard dose CVE regimen, recommended for group A, B, and, C palients, consists of 3-weekly, 6 cycles of carboplatin (18.6 mg) on day 1, vincristinc (0.05 mg) on day 1, and etoposide (5 mg) on day 1 and 2. • High dose CVE regimen recommended for group D patients, consists of 3-weekly, 6-12 cycles of carboplatin (28 mg) on day 1, vincristine (0.25 mg) on day 1 and etoposide (12 mg) on day 1 and 2. Focal therapy. Depending upon the location and size of the tumour, focal therapy can be chosen from the following modalities: Cryotherapy is indicated for a small tumour located anterior to equator. Laser photocoagulation is used for a small rumour located posterior to equator. Thcrmotherapy with diode laser is used for a small tumour located posterior to equator away from macula. Plaque radiotherapy is very effective against localised vitreous disease and for the elevated tumours when laser is ineffective. External beam radfotherapy (EBR), once the mainstay of treatment, is now reserved for diffuse disease in the only remaining eye. Note. If the above modalities are not available, the eyeball should be enucleated without hesitation.
T/T OF RP ?
It is most unsatisfactory; rather we can say that till date there is no effective treatment for the disease. 1. Measures to stop progression, which have been tried from time to time, without any breakthrough include: vasodilators, placental extracts, transplantation of rectus muscles in to suprachoroidal space, light exclusion therapy, ultrasonic therapy and acupuncture therapy. Recently vitamhi A (15000 IV, PO, qd of palmitateform) has been recommended to check its progression. 2 . Correct any refractive error, prescribe glasses. 3. Systemic acetazolamide (500 mg po) for associated cystoid macular oedema. 4. Stem cell therapy for retinitis pigmentosa, still under trial, is as below • Bone marrow derived mononuclear stem cells are injected intravitreally and their role as rescue therapy is being evaluated. • Embryonic stem cells, induced pluripotent stem cells and retinal progenitor cells are injected in the subretinal space and their role as replacement therapy is being evaluated. 5. Gene therapy trials, may end up in some fruitful outcome. 6. Retinal prosthesis (e.g. Bionic eye) may be of help for blind patients with retinitis pigmentosa. 7. Low vision aids (LVA) in the form of 'magnifying glasses' and 'night vision device' may be of some help. 8. Rehabilitation of the patient should be earned out as per his socioeconomic background. 9. Prophylaxis. Genetic counselling for no Consanguineous marriages may help to reduce the incidence of disease. Further, affected individuals should be advised not to produce children.
optic disc placed at what distance from fovea ? o.disc nasal o temporal to fovea ?
It is placed 3.4 mm nasal to the fovea.
what are classification names for Hypertensive Retinopathy ?
Keith and Wagner classification Modified Scheie classification Mitchell-Wong classification
which hypertensive retinopathy classification is predictive of risk for cerebrovascular disease, coronary artery disease and mortality
Mitchell-Wong classification
what is Mittendorf dot ?
Mittendorf dot represents remnant of the anterior end of hyaloid artery, attached to the posterior lens capsule. It is usually associated with a posterior polar cataract.
what is Mixed endophylic and exophytic ?
Mixed endophylic and exophytic growth pattern having features of both is also reported in some cases.
what are systemic Associations of retinitis pigmentosa ?
Most cases of retinitis pigmentosa (RP) are isolated, i.e. with no systemic features, such cases a relabelled as 'retinitis pigmentosa non-syndromic or simple'. About 25% have associated systemic diseases and such cases are labell1ed as 'syndromic retinitis pigmentosa'. A number of specific syndromes are described: l. Laurence-Moon-Biedl syndrome. 2. Cockayne's syndrome. 3. Refsum's syndrome. 4. Usher's syndrome. 5. Hallgren's syndrome. 6. Bassen-Kornzweig syndrome (Abetalipoproteinaemia), 7. Kearns-Sayre syndrome, 8. Friedreich's ataxia, 9. Bardet- Biedl syndrome, 10. NARP (neuropathy, ataxia, and retinitis pigmentosa), 11. neuronal ceroid lipofuscinosis, and 12. olivopontocerebellar degeneration.
what are the types of Non-tractional DME ?
Non-tractiottal DME. It may be of following types:a. q.Spongy thickening ofmacula (>290 μ), b. Cystoid macular oedema (CME}, and c. neurosensory detachment with or without (a) or (b) above. ote. Patients with above changes, seen on OCT, are fu rther divided into two types: • Centre involving diabetic macular edema with loss of foveal contour. • Non-centre involving diabetic macular edema, with thickening within 3000 um of the
ozurdex not used in
OZURDEX® should not be used if you have any infections in or around the eyes, including most viral diseases of the cornea and conjunctiva, including active herpes viral infection of the eye, vaccinia, varicella, mycobacterial infections, and fungal diseases. OZURDEX® should not be used if you have glaucoma that has progressed to a cup-to-disc ratio of greater than 0.8. OZURDEX® should not be used if you have a posterior lens capsule that is torn or ruptured. OZURDEX® should not be used if you are allergic to any of its ingredients.
PPX of RRD ?
Occurrence of primary retinal detachment can be prevented by timely application of laser photocoaguJation or ciyotherapy in the areas of retinal breaks and/or predisposing lesions like lattice degeneration. Prophylactic measures are particularly indicated in patients having associated high -risk factors like myopia, aphakia, retinal detachment in the fellow eye or histoiy of retinal detachment in the family.
what are fundal & ocular changes in leukaemic retinopathy include ?
Ocular involvement is more common with acute than chronic leukaemia. Characteristic features of leukaemic retinopathy include: 1• Fundus background is pale and orangish 2• Retinal veins are tortuous and dilated 3• Retinal arterioles become pale and narrow 4• Periuascular leukaemic infiltrates, seen as grayish white lines along the course of veins, are seen in latter stages 5• Roth's spot, i.e. retinal haemorrhages with typical white centre are very common 6• Subhyaloid haemorrhage, i.e. large preretinal haemorrhages may also be seen. Other ocular changes in leukaemia include: 1• Orbital infiltration, particularly in children presenting as proptosis 2• Ocular haemorrhages in the form of subconjunctival haemorrhage and hyphaema (bleeding in anterior chamber) 3• Iris changes in the form of iris thickening and iritis 4• Pseudohypopyon, i.e. collection of white cells in the anterior chamber.
D/D OF OCULAR ISCHAEMIC SYNDROME ?
Ocular ischaemic syndrome needs to be differentiated from 1.non-ischaemic CRVO, 2. diabetic retinopathy, 3. hypertensive retinopathy and 4. aortic arch disease caused by Takayasu arteritis, aonoarteritis, 5. atherosclerosis and 6. syphilis
what is Oguchi's disease ?
Oguchi disease, is an autosomal recessive form of congenital stationary night blindness associated with fundus discoloration and abnormally slow dark adaptation. On examination patients have normal visual fields but the fundus have a diffuse or patchy, silver-gray or golden-yellow metallic sheen and the retinal vessels stand out in relief against the background. A prolonged dark adaptation of three hours or more, leads to disappearance of this unusual discoloration and the appearance of a normal reddish appearance. This is known as the Mizuo-Nakamura phenomena and is thought to be caused by the overstimulation of rod cells.[4]
Intravitreal dexamethasone implant (Ozurdex) repeated after how many months ? effect of ozurdex last for how many moth ?
Ozurdex to be repeated every 2 months. Effect of Ozurdex lasts for about 3-6 months.
RETINA : GENERAL CONSIDERATIONS Initiation and transmission of visual sensation Stimuli for visual sensations Processes of initiation and transmission of vision Visual perceptions Light sense Form sense Sense of contrast Colour sense
PHYSIOLOGY OF VISION: GENERAL CONSIDERATIONS Physiology of vision is a complex phenomenon which is still poorly understood. The main mechanisms concerned with vision are: Initiation of vision (transduction), Transmission of visual sensation, and Visual perceptions. INITIATION AND TRANSMISSION OF VISUAL SENSATIONS Stimuli for visual sensations The rods and cones serve as sensory nerve endings for visual sensations. Stimuli for visual sensations may be divided, in a purely physical sense, into two types—inadequate and adequate. ▪ Inadequate stimuli produce glowing sensations called phosphenes. Mechanical stimulation by pressure on the sclera is an example of inadequate stimulus which produces pressure phosphene (which appears as a patch with contrasting border). Other examples of inadequate stimuli are rapid eye movements in dark (producing movement phosphene), passage of weak electric current through retina (producing electrical phosphene) and passage of X-rays or other ionizing radiations through the retina (producing radiation phosphenes). ▪ Adequate stimuli to vision are formed by visible portion of the electromagnetic radiation spectrum, i.e. 'the light'. It lies between ultraviolet and infrared portions from 400 nm at the violet end of the spectrum to 750 nm at the red end. The white light consists of seven colours denoted by 'VIBGYOR' (violet, indigo, blue, green, yellow, orange and red). Light ray is the term used to describe the radius of concentric waveforms. A group of parallel rays of light is called a beam of light. Processes of initiation and transmission of vision Light falling upon the retina is absorbed by the photosensitive pigments present in the rods and cones, and initiates photochemical changes which are described in detail in Chapter 6.3 Photochemistry of Vision. The photochemical changes trigger a sequence of events (electrical changes) that initiate the visual sensations. The retinal receptors are not just transducers of light into chemical and electrical signals; they are active processors of information. Thus the electrical potential changes produced and actively processed in the retina are transmitted through the ganglion cells and along the fibres of the optic nerve and other parts of the visual pathway to the visual cortex. The details of processes concerned are described in Chapter 6.4 Neurophysiology of Vision and Chapter 6.5 Electrophysiology of Retina and Visual Pathway. VISUAL PERCEPTIONS Visual perceptions are the functional elements of the vision—the sensations which result from stimulation of retina with light. These are of four kinds, namely the light sense, the form sense, the contrast sense and the colour sense. Light sense The light sense refers to the appreciation (awareness) of light, not only as such, but in all its gradations of intensity. The minimum brightness required to evoke a sensation of light is called the light minimum. It should be measured when the eye is dark-adapted for at least 20-30 minutes. Dark adaptation is the ability of the eye to adapt itself to decreasing illumination. The rods are much more sensitive to low illumination than the cones. This forms the basis of the Duplicity Theory of Vision, a theory which assumes that rods are used more in dim light (scotopic vision) and cones in bright light (photopic vision). These factors are discussed in detail in Chapter 6.6 Visual Adaptation. Form sense It is the ability to discriminate between the shapes of the objects. Cones play a major role in this faculty. Therefore, form sense is most acute at fovea, where there are maximum number of cones. Visual acuity recorded by Snellen's test chart is a measure of the form sense. It is described in detail in Chapter 6.7 Visual Acuity. Sense of contrast It is the ability of the eye to perceive slight changes in the luminance between regions which are not separated by definite border. For details see Chapter 6.8. Contrast Sensitivity. Colour sense It is the ability of the eye to discriminate between different colours excited by light of different wavelengths. It is a function of cones and thus better appreciated in photopic vision. Cones perform this function by different types of pigments which absorb red, green and blue wavelengths of light (primary colours). For details see Chapter 6.9 Colour Vision. • • • OTHER VISUAL PHENOMENA ASSOCIATED WITH PHYSIOLOGY OF VISION Some other phenomena associated with physiology of vision are included in the following chapters: Chapter 6.10 Critical Flicker Fusion Frequencies (page 298) Chapter 6.11 Entoptic and Allied Phenomena (page 303) Chapter 6.12 Field of Vision (page 316)
what is Painful red eye presentations of RB ?
Painful red eye presentations= When retinoblastoma is left untre ated during the quiescent stage, some patients may present with severe pain, redness, and watering. These symptoms occur either dueto acute secondary glaucoma or apparent intraocular inflammation or orbital cellulitis. • Acute secondary glaucoma may occur either due to tumour pushing the lens-iris diaphragm forward or tumour cells clogging the trabecular meshwork or neovascular glaucoma. In this stage, eyeball is enlarged (buphthalmos) with apparent proptosis, conjunctiva is congested, cornea become hazy, intraocular pressure is raised. •Apparent intraocular inflammation. Occasionally, picture simulating severe, acute uveitis usually associated with pseudohypopyon and/ or hyphaema may be the presenting mode (relinoblastoma masquerading as iridocyclitis). •Orbital inflammation, mimicking preseptal or Orbital cellulitis like presetation mayoccur with necrotic tumours. It does not imply extraocular extension and the exact mechanism is not known.
what is Peripheral retina ?
Peripheral retina refers to the area bounded posteriorly by the retinal equator and anteriorly by the ora serrata. Peripheral retina is best examined with indirect ophthalmoscopy and the Goldman three mirror contact lens examination (gives more magnified view).
WHAT IS PRIMARY RETINAL TELANGIECTASIA ?
Persistent dilation of retinal vessels in absence of other findings.
S/N on Pneumatic retinopexy ?
Pneumatic relinopexy is a simple out-patient procedure which can be used to fix a fresh superior RD with one or rwo small holes extending over less than two clock hour area in the upper two-thirds of peripheral retina. ln this technique after sealing the breaks wilh cryopex:y, an expanding gas bubble (SF6 or C3 F8 ) is injected in the vitreous. Then proper positioning of the patient is done so that the break is uppermost and the gas bubble remains in contact wilh the tear for 5-7 days.
ETIOLOGY OF RHEGMATOGENOUS RD ?
Predisposing factors for RRD include: 1. Age. The condition is most common in 40-60 years. However, age is no bar. 2. Sex. More common in males (M:F-3:2). 3. Myopia. About 40% cases of rhegmatogenous retinal d,etachmenr are myopic. 4. Aphakia: and pseudophakia. The condition is more common in aphakes and pseudophake than phakes. 5. Retinal degenerations predisposed to retinal detachment are as follows: • Lattice degeneration, • Snail track degeneration, • White-with-pressure anti white-without-or occult pressure, • Acquired or degenerative retinoschisis, and • Focal pigment clumps. 6. Trauma. It may also act as a predisposing factor. 7. Senile posterior vitreous detachment (PVD}. It is associated with retinal detachment in many cases.
WHAT ARE THE TYPES OF PRIMARY RETINAL TELANGIECTASIA ?
Primary retinal telangiectasia include: 1• idiopathic juxtafoveolar retinal telangiectasia, 2• Coats' disease, and 3• Leber's miliary aneurysm.
what are the types of primary retinal tumors ?
Primary retinal tumours= 1. Neuroretinal tumors • Retinobla:sroma • Astrocytoma 2. Retinal pigment epithelial(RPE) tumours • Congenital hypertrophy ofRPE • Congenital simple hamartoma of RPE • Combined hamartoma of retina and RPE • Adenoma and adenocarcinoma ofRPE • Hyperplasia and migration ofRPE simulating uveal melanoma 3. Retinal vascular tumours • Capillary haemangioma • Cavernous haemangioma • Racemose haemangioma • Vasoproliferative tumor 4. Primary vilreoretinal lymphoma.
RISK FACTORS OF ROP ? (PRIMARY & OTHERS)
Primary riskf actors= • Low gestation age( <32 weeks ) • Low birth weight ( <1500GM ) • Supplemental O2 Other risk factors= -light, vitamin E deficiency, -respiratory distress syndrome, -asphyxia, -shock, -acidosis.
(C/F) SYMPTOMS OF RRD ?
Prodromal symptoms include: • Dark spots (floaters) in front of the eye (due to rapid vitreous degeneration), and • Photopsia, i.e. sensation of flashes of light (due to irritation of retina by vitreous movements). Symptoms of detached retina are as follows: l. Localised relative loss in the field of vision ( of detached retina) is noticed by the patient in early stage which progresses to a total loss when peripheral detachment proceeds gradually towards the macular area. 2. Sudden appearance of a dark cloud or veil in front of the eye is complained by the patients when the detachment extends posterior to equator. 3. Sudden painless loss of vision occurs when the detachment is large and central.
RETINA : ANATOMY OF RETINA Gross anatomy Microscopic structure of the retina Blood supply of the retina Blood-retinal barrier
RETINA Retina, the innermost tunic of the eyeball, is a thin, delicate and transparent membrane. It is the most highly developed tissue of the eye. GROSS ANATOMY Extent. Retina extends from the optic disc to the ora serrata and has a surface area of about 266 mm2. Thickness. Thickness of retina at the posterior pole in the peripapillary region is approximately 0.56 mm, at the equator 0.18 to 0.2 mm, and at the ora serrata approximately 0.1 mm.1-4 Colour. Retina appears purplish-red due to visual purple of the rods. After death of a person, the retina appears white opaque. Regions. Grossly, on ophthalmoscopic examination, the fundus can be divided into three distinct regions (Fig. 6.1.1): Optic disc, Macula lutea, and Peripheral retina (general fundus). Fig. 6.1.1. Gross anatomy of the retina. OPTIC DISC Optic disc is a pale-pink, well-defined circular area of about 1.5 mm diameter. Colour of the disc is seldom uniformly pink and the tint shows considerable variations within normal limits. At the optic disc, all the retinal layers terminate except the nerve fibres, which pass through the lamina cribrosa (sieve-like sclera) to run into the optic nerve. In comparison to the rest of retina, the optic disc appears pale due to lamina cribrosa, medullated nerve fibres behind it and absence of vascular choroid. In the centre, where the nerve fibres are thinnest, the white lamina shines more brightly. The grey spots in the lamina, when they are seen, are due to the non-medullated nerve fibres reflecting less light than the white connective tissue fibres. Physiological cup of the optic disc is a depression seen in it. The central retinal vessels emerge through the centre of this cup. The cup varies in size, shape, position and depth in different eyes. Sometimes there is scarcely any physiological cup, in which case the disc is more uniformly pink and the central vessels may have already divided before they come to the surface. Increase in the size of the cup and/or difference in the size of cup of two eyes should be watched with suspicion and investigated to exclude glaucoma. MACULA LUTEA Macula lutea (yellow spot) is a comparatively dark area 5.5 mm in diameter, situated at the posterior pole of the eyeball, temporal to the optic disc between the temporal vascular arcadae. Histologically, macula is the region with more than 1 layer of ganglion cell nuclei. Tripathi and Tripathi2 have recommended the term area centralis for this area. They have reported that in fact area centralis is a horizontally ellipsed area demarcated approximately by the upper and lower arcuate temporal retinal vessels. It corresponds to approximately 15° of the visual field and that the photopic vision and colour vision are primarily the functions of this area. Oxygenated carotenoids, in particular lutein and zeaxanthine, accumulate within the central macula and cause yellow colour. These carotenoids have antioxidant capabilities and also function to filter the blue wavelengths of light, possibly preventing photic damage. Macular area/macula lutea/area centralis comparises of three main areas (Table 6.1.1): Fovea, Parafovea, and Perifovea. Table 6.1.1. Anatomic terminology and features of the macular area Fovea also labelled as fovea centralis is the central depressed part of the macula. It is about 1.50 mm in diameter and about 1.55 mm in thickness. It corresponds to 5° of visual field and is the most sensitive part of the retina.2 Margo fovea, margin of fovea can be seen biomicrioscopically as a ring-like reflection of the internal limiting membrane. Thus, the fovea represents an excavation in the retinal centre and consists of a margin, a declinity of 22°(clivus) and bottom(foveola). Foveola (0.35 mm in diameter) forms the central floor of the fovea. It is situated about 2 disc diameter (3 mm) away from the temporal edge of the optic disc and about 1 mm below the horizontal meridian. Thickness of foveola is 0.15 mm (150 μm) and is characterized by the underlying foveal nuclear cake resulting from the centripetal migration of the photoreceptors and centrifugal lateral displacement of the biopolar and ganglion cells, (see microscopic structure, Page 177 Fig. 6.1.9). Umbo is a tiny depression in the very centre of the foveola which corresponds to the ophthalmoscopically visible foveolar reflex, seen in most normal eyes. Loss of the foveolar reflex may be an early sign of damage. Greatest concentration of cones is found in the umbo, an area of 150-200 μm diameter, referred to as the central bouquet of cones. Clivus refers to the declinity extending from the margin of fovea to the margin of foveola. It comprises peripheral vascular and central foveal avascular zone(FAZ). Foveal avascular zone (FAZ) is located inside the fovea but outside the foveola, its exact diameter is variable and its location can be determined with accuracy only by fluorescein angiography. Parafovea refers to a belt that measures 0.5 mm in width and surrounds the foveal margin. This area, histologically, is characterised by normal architecture of retinal layers which include 4-6 layers of ganglion cells and 7-11 layers of biopolar cells. Perifovea refers to a belt that measures 1.5 mm in width and surrounds the parafoveal area. This region is histologically characterized by several layers of ganglion cells and six layers of bipolar cells. PERIPHERAL RETINA The peripheral retina, also known as extra areal periphery, can be divided into four regions:1,2 Near periphery. The near periphery refers to a circumscribed region of about 1.5 mm width around the area centralis (macula lutea). Mid-periphery. The mid-periphery occupies a 3 mm wide zone around the near periphery. Its outer limit correspondes to the equator. Far periphery extends from equator to the ora serrata. The width of this belt varies, depending upon the ocular size and refractive error. The average circumference of the eye is 72 mm at equator and 60 mm at the ora serrata. The average width of the far periphery belt is about 6 mm. Since it is customary to chart the peripheral retinal pathologies in clock hours, so 1-clock hour corresponds to 5-6 mm of far peripheral circumference. Thus, the belt of peripheral retina can be divided into 12 squares of 6 × 6 mm in size. Extereme periphery refers to the area of ora-serrata and pars plana. Ora serrata. It is the serrated peripheral margin where the retina ends and ciliary body starts. The dentate processes, consisting of tooth-like extensions of the retina on the pars plana separated by oral bays, are well marked on the nasal half of the retina. At the ora, the sensory retina is firmly attached both to the vitreous and retinal pigment epithelium. Ora serrata is 2.1 mm wide temporally and 0.7- 0.8 mm wide nasally. Its distance from the limbus is 6.0 mm nasally and 7.0 mm temporally. It is located 6-8 mm away from the equator and 25 mm from the optic nerve on the nasal side. The ora serrata is a zvater shed zone between anterior and posterior vascular systems. So, peripheral retinal degenerations are more common. MICROSCOPIC STRUCTURE OF THE RETINA Retina consists of ten layers from two distinct functional components, the pigment epithelium and the sensory retina. These ten layers from without inward are (Fig. 6.1.2): Retinal pigment epithelium Layer of rods and cones External limiting membrane Outer nuclear layer Outer molecular (plexiform) layer Inner nuclear layer Inner molecular (plexiform) layer Ganglion cell layer Nerve fibre layer Internal limiting membrane Fig. 6.1.2. Microscopic structure of the retina. • • RETINAL PIGMENT EPITHELIUM (RPE) It is the outermost layer of retina. It consists of a single layer of hexagonal-shaped cells containing pigment. The RPE cells show fine mottling due to unequal pigmentation of the cells and this is responsible for granular appearance of the fundus. RPE is firmly adherent to the underlying Bruch's membrane (basal lamina of the choroid) and loosely attached to the layer of rods and cones of the sensory retina. The potential space between RPE and the sensory retina is called subretinal space. A separation of the RPE from the sensory retina is called retinal detachment, and the fluid between the two layers is called subretinal fluid (SRF). Electron microscopy shows that adjacent RPE cells are connected with each other by tight junctions (zonulae occludentes and zonulae adherentes) and constitute the outer blood-retinal barrier. The RPE cells at the fovea are taller, thinner, and contain more and larger pigment granules than elsewhere in the fundus, thereby giving a dark colour to this area. RPE cells, in cross-section, can be differentiated into apical and basal configurations. Apical part of RPE cells is formed by the microvilli which project between the rods and cones processes. At apical end of each RPE cells, overlie about 45 photoreceptors. Melanin grannies are concentrated in the apical end of each RPE cell. The melanin is found in cytoplasmic granules called melanosomes. This pigment serves to absorb stray light and minimize scatter within the eye. The other major RPE pigment is lipofuscin, which accumulates gradually with age. It is thought that lipofuscin in the RPE is derived from the aged or damaged outer segment lipids that have been ingested and then digested by RPE. Basal membrane of each RPE cell has convulated infolds to increase the surface area for the absorption and secretion of material, and lies in contact with the Bruch's membrane of choroid. Functions of RPE • • • • • • • • Plays important role in photoreceptor renewal and recycling of vitamin A, i.e. visual pigment. Maintains integrity of subretinal space by forming outer bloodretinal barrier and actively pumping ions and water out of this (subretinal) space. RPE is involved with transport of nutrients and metabolites through the blood-retinal barrier and elaboration of the extracellular matrix.5 Phagocytic action by phagocytosis and digestion of photoreceptors. Provides mechanical support to the processes of photoreceptors and thus maintains rectinal adhesions. Manufactures pigment which presumably has an optical function in absorbing light. Regenerative and repairative function after injury and surgery. Electrical homeostasis as a result of asymmetrical transport properties of the apical and basal membranes. NEUROSENSORY RETINA The neurosensory retina consists of 3 types of cells and their synapses arranged in nine layers, which from without inward are described below. • • • • • • Layer of rods and cones (neuroepithelium) This layer contains only the outer segments of photoreceptor cells arranged in a palisade manner (Fig. 6.1.2). Density and distribution of photoreceptors Rods and cones (photoreceptors) are the end organs of vision which transform light energy into visual (nerve) impulse. Rods contain a photosensitive substance visual purple (rhodopsin) and subserve the peripheral vision and vision of low illumination (scotopic vision). Cones also contain a photosensitive substance and are primarily responsible for highly discriminatory central vision (photopic vision) and colour vision. Number of photoreceptors. There are about 120 million rods and 6.5 million cones. Distribution of cones. The highest density of cones is at fovea with an average of 199000 cones/mm2, but their number is highly variable and ranges from 100000 to 324000 cones/mm2 with the highest density in an area as large as 0.032 degree2,6 The number of cones falls off rapidly outside the fovea; being only 6000 cones/mm2 3 mm away from fovea and about 4000 cones/mm2 10 mm away. Cone density is 40-45% greater on the nasal than on the temporal aspect of human retina, and slightly lower in the superior than in the inferior retina at the mid-periphery.7 Distribution of rods. Rods are absent at the fovea in an area of 0.35 mm (rod-free zone) which corresponds to 1.25° of the visual field; but are present in a large number (160,000/mm2) in a ring-shaped zone 5-6 mm from the fovea. These are maximum below the optic disc (170,000/mm2) and their number reduces towards the periphery. The entire nasal retina has 20- 25% more rods than does temporal retina, and the superior retina has 2% more than the inferior retina.6 • • Structure of photoreceptor The long axis of the photoreceptor is oriented perpendicular to the retinal surface. Each photoreceptor consists of: Cell body and nucleus (which lie in the outer nuclear layer), cell process that extends into outer plexiform layer and Inner and outer segments (which form the layer of rods and cones). • • Rod cell Each rod (Fig. 6.1.3) is about 40-60 μm long. Fig. 6.1.3. Microscopic structure of a rod cell. Outer segment of the rod is cylindrical, highly retractile, transversly striated and contains visual purple. It is composed of numerous lipid protein lamellar discs stacked one on top of the other and surrounded by a cell membrane. The number of discs varies between 600 and 1000/rod and each disc is 22.5-24.5 nm in thickness.1 The discs contain 90% of the visual pigment is scattered on the surface of plasmalemma. The outer segment is attached to the inner segment by a cilium with a characteristic of 9 + 0 configuration (nine doublets around the periphery with no central microtubule). Inner segment of the rod is thicker than the outer segment. It consists of two regions: ellipsoid and myoid. Ellipsoid (the outer portion) is adjacent to the outer segment and contains abundant number of mitochondria. Myoid (the inner portion) contains the glycogen as well as the usual organelles. Outer rod fibre arises from the inner end of rod, which passes through the external limiting membrane and swells into a densely staining nucleus—the rod granule (lies in the outer nuclear layer); and then terminates as inner rod fibre (lies in the outer molecular layer) which at its end has an end bulb called the rod spherule that is in contact with the cone foot. Cone cell Each cone cell (Fig. 6.1.4) is 40-80 μm long. It is largest at the fovea (80 μm) and shortest at the periphery (40 μm). Fig. 6.1.4. Microscopic structure of a cone cell. Cone outer segment is conical in shape, much shorter than that of rod and contains the iodopsin. The lamellar discs, which are narrower than those of the rods, maintain continuity with the surface plasma membrane. There are about 1000-1200 discs/cone. Cone inner segment and cilium are similar to the rod structures; however, the cone ellipsoid is very plump and contains a large number of mitochondria. Unlike rod, the inner segment of the cone becomes directly continuous with its nucleus and lies in outer nuclear layer. A stout cone inner fibre runs from the nucleus which at the end is provided with lateral processes called cone foot or cone pedicle (lies in the outer plexiform layer). Interphotoreceptor matrix (IPM) and Interphotoreceptor retinoid binding protein (IRBP) ▪ Interphotoreceptor matrix (IPM) occupies the space between the photoreceptor outer segments and the retinal pigment epithelium. It is a complex structure consisting of proteins, glycoproteins, GAGs and proteoglycans such as chondroitin sulphate. The IPM has a diverse range of functions, including retinal attachment and adhesin molecular trafficking, facilitation of phagocytosis and probably photoreceptor outer segment alignment. Interphotoreceptor retinoid binding protein (IRBP) accounts for 70% of the soluble proteins in the IPM. In humans, it is produced by photoreceptors (mainly cones) and can bind all-trans-retinol, 14-cisretinol, α-tocopherol, retinoic acid and cholesterol. Although the primary function of IRBP is the efficient transport of retinoids between the photoreceptors and the retinal pigment epithelium, it may also serve to minimize fluctuations in retinoid availability, and to protect the plasma membranes from the damaging effects of high retinoid concentrations. IRBP is not the only binding protein found in the retina. Cellular retinoid binding proteins (CRBP) are a subgroup of the fatty acid binding proteins that orchestrate reisomerisation in the retinal pigment epithelium and may also have a role in early retinal development. Cellular fatty acid binding proteins (cell FABP) are also found in the retina. They protect retinal processes from toxic effects of fatty acids and take part in cell growth and differentiation. External limiting membrane In low magnification, it appears as a fenestrated membrane extending from the ora serrata to the edge of the optic disc; through which pass processes of the rods and cones. Electron microscopy studies show that the external limiting membrane is formed by the junctions (zonulae adherentes) between the cell membrane of photoreceptors and Muller's cells and thus it is not a basement membrane. • • • • Outer nuclear layer This layer is primarily formed by the nuclei of rods and cones; cone nuclei are somewhat larger (6-7 μm) than the rod nuclei (5.5 μm) and lie in a single layer next to the external limiting membrane. Rod nuclei form the bulk of this multilayered outer nuclear layer except in the cone dominated foveal region. Number of rows of nuclei and thickness of this layer varies from region to region as follows: Nasal to the disc—8 to 9 layers of nuclei and 45 μm thickness. Temporal to disc—4 rows of nuclei and 22 μm thickness. Foveal region—10 rows of nuclei and 50 μm thickness. Rest of the retina except ora serrata—one row of cone nuclei and 4 rows of rod nuclei with a thickness of 27 mm. Outer plexiform layer This layer contains the synapses between the rods spherules and cone pedicles with the dendrites of the bipolar cells and processes of the horizontal cells (Fig. 6.1.5). In other words, this layer marks the junction of the end organs of vision and first-order neurons in the retina.2 It is thickest at the macula (51 μm) and consists predominantly of oblique fibres that have deviated from the fovea and is also known as Henle's layer. Fig. 6.1.5. Cell connections in outer plexiform layer. • • • • • • • • • • • • • • Inner nuclear layer Under microscope, this layer resembles the outer nuclear layer except that it is very thin. This layer disappears at fovea and in rest of the retina consists of the following: Bipolar cells Horizontal cells Amacrine cells The soma of the Muller's cells Capillaries of the central retinal vessels. i. Bipolar cells (neurons) Bipolar cells are neurons of first order of vision. The body of the bipolar cells consists entirely of the nucleus which lies in the inner nuclear layer. Their dendrites arborize with the rod spherules and cone pedicels in the outer plexiform (molecular) layer and their axons arborize with the dendrites of ganglion cells in the inner molecular layer. On the basis of morphology and synaptic relationship, nine types of bipolar cells are seen under light microscopy:8 Rod bipolar cells Invaginating midget bipolar cells Flat midget bipolar cells Invaginating diffuse bipolar cells Flat diffuse bipolar cells On-centre blue cone bipolar cells Off-centre blue cone bipolar cells Giant bistratified bipolar cells Giant diffuse invaginating bipolar cells. Rod bipolar cells. These have large soma and profuse dendrites which arborize only with the rod spherules. Axons of these bipolar cells have synapses with the soma of up to four ganglion cells (Figs 6.1.5 and 6.1.6). These constitute about 20% of the total bipolar cells. Midget bipolar cells. Invaginating midget cells are relatively small and make connections only in the triads of cone pedicle. Their dendrites deeply invaginate the cone pedicle. The flat midget bipolar cells resemble the invaginating midget bipolar cells except that they do not invaginate but make a superficial contact with the cone pedicle. Their axons synapse with a single ganglion cell (Figs 6.1.5 and 6.1.6). Fig. 6.1.6. Cell connections in inner plexiform layer. Flat and invaginating diffuse bipolar cells. Their dendrites make contact with the cone pedicle only but not with their triads and their axons synapse with a number of ganglion cells of all types (Fig. 6.1.6). Blue-cone bipolar cells. There are two types of blue cone bipolar cells—the ON-centre or BBb variety and OFF-centre or BBa variety. Like the diffuse bipolar cells, blue cone bipolars innervate more than one pedicle.8 The axon terminal of ON-centre cells arborize in stratum 5 and that of OFF-centre cell arborize in stratum 1 of the inner plexiform layer. • • Giant bipolar cells. These are of two types—giant bistratified and giant diffuse invaginating bipolar cells which are distinguished by the extent of their dendritic spread. Giant diffuse bipolar cells are morphologically similar to flat diffuse cells except for the difference in their dendritic fields.8 ii. Horizontal neurons Horizontal neurons are flat cells having numerous horizontal associative and neuronal interconnections between photoreceptors and bipolar cells in the outer plexiform layer. These are of two types —A and B: Type A horizontal cells have seven groups of dendrites which have contact with triad of seven cone pedicles and their single axon has contact (perhaps) with distant cone triad. Type B horizontal cells' dendrites have contact with rod receptors only and (perhaps) their axons with the distant rod cells. iii. Amacrine cells These cells are situated within the innermost part of this layer. These have a piriform body and a single process which passes inwards in the inner plexiform layer and forms connections with the axons of the bipolar cells and the dendrites and soma of the ganglion cells. Thus they perform an integrative function similar to that of the horizontal cells (Fig. 6.1.6). iv. Muller's cells The nucleus and cell bodies of the Muller's cells are located within the inner nuclear layer. Fibres from their outer ends extend up to the external limiting membrane and those from their inner ends reach up to the internal limiting membrane. In contrast to most other elements of the retina which possess either a photoreceptive or neural function, the Muller's cells provide structural support and contribute to the metabolism of the sensory retina. Their role in various layers is as follows: • • • • • • • • In external limiting membrane, junctions between the terminal parts of the fibres form the outer ends of the Muller's cells and cell membrane of the photoreceptors form the external limiting membrane. In outer nuclear layer, the Muller's cells provide reticulum around the cells somata. In outer plexiform layer, the major processes of Muller's cell produce side branches which form the horizontal extending reticulum. In inner nuclear layer, lie their cell bodies and secondary branches form the reticulum around the various cell somata. In inner plexiform layer, they play the same role as in outer plexiform layer. In layer of ganglion cells, provide reticulum around cell somata. In nerve fibre layer, their processes interweave with axons of ganglion cells. In internal limiting membrane, the inner fibres of the Muller's cells take part in the formation of this membrane. v. Other glial cells In addition to Muller's cells, the retina contains other glial cells— astrocytes, microglia and (rarely) oligodendrocytes. The astrocytes are most abundant and are located around the blood vessels. • • • Inner plexiform layer This layer essentially consists of synapses between the axons of bipolar cells (first order neurons), dendrites of ganglion cells (second order neurons) and the processes of integrative Amacrine cells (Fig. 6.1.6). Fibres from the Muller's cells courses vertically through this layer and their side branches form the horizontal extending reticulum. This layer is absent at the foveola. • • • • Ganglion cell layer The cell bodies and the nuclei of the ganglion cells (second order neurons of visual pathway) lie in this layer. Throughout most of the retina, the ganglion cell layer is composed of a single row of cells, except in the macular region where it becomes multilayered (6-8 layers of the cells) and on temporal side of the disc where it has two layers. Ganglion cell layer is absent in the region of foveola. Ganglion cells have been variously classified. A few of the poptdar classifications are as follows: W, X and Y ganglion cells (see page 211). P (P1 and P2) and M ganglion cells. OFF-centre and ON-centre ganglion cells. Monosynaptic and polysynaptic ganglion cells. Monosynaptic or midget ganglion cells These cells predominate in the central retina. Dendrite of each such cell synapses with the axon of the single midget bipolar cell (Fig. 6.1.6). Polysynaptic ganglion cells These cells lie predominantly in peripheral retina. These cells have large dendritic fields and so synapse with multiple bipolar cells. There are complex connections between the dendrites of ganglion cells, axons of bipolar cells and amacrine neurons in the inner plexiform layer. The axons of the ganglion cells form the nerve fibre layer and then after passing through optic nerve, chiasma, and optic tracts, ultimately synapse with cells in the lateral geniculate body (third order neuron of visual pathway). • • • • Nerve fibre layer (stratum opticum) Nerve fibre layer essentially consists of the unmyelinated axons of the ganglion cells which converge at the optic nerve head, pass through lamina cribrosa and become ensheathed by myelin posterior to lamina. In addition to axons of the ganglion cells (centripetal nerve fibres), this layer also contains the following: Centrifugal nerve fibres, which are thicker than the centripetal nerve fibres. Their exact origin and termination is not known. Processes of Midler's cells, which interweave with the axons of the ganglion cells. Neuroglial cells that are present in the nerve fibre layer are categorized as macroglia and microglia. Macroglia are constituted by two types of astrocytes (fibrous and protoplasmic) derived from the neural crest. Microglia are small cells that are derived from the mesodermal invasion of the retina at the time of vascularization. Macroglia have a structural role in the retina while microglial cells take the role of wandering tissue histiocytes in response to tissue injury and phagocytose debris which is carried to the vasculature for removal from the retina. Retinal vessels lie in the nerve fibre layer but as a rule do not project on the surface of retina. A rich bed of superficial capillary network is present in this layer (Fig. 6.1.11). Features of nerve fibres The nerve fibres vary in their thickness from 0.5 to 2 μm and are nonmyelinated. The cytoplasm of the axons contains microtubules, fine fibrils, mitochondria and occasional vesicles. Arrangement of nerve fibres in the retina In contrast to the remaining fibres of the sensory retina, which course perpendicular to the surface of the retina, the fibres within the nerve • • • fibre layer course parallel to the surface in the following manner (Fig. 6.1.7): Fibres from the nasal half of the retina come directly to the optic disc as superior and inferior radiating fibres (srf and irf). Fibres from the macular region pass straight in the temporal part of the disc as papillomacular bundle (pmb). Fibres from the temporal retina arch above and below the macular and papillomacular bundle as superior and inferior arcuate fibres (sat and iaf) with a horizontal raphe in between. Fig. 6.1.7. Arrangement of nerve fibres in the retina. Arrangement of nerve fibres of the optic nerve head Optic nerve comprises about 1.2 million nerve fibres. Fibres from the peripheral part of the retina lie deep in the retina but occupy the most peripheral (superficial) part of the optic disc. While the fibres originating closer to the optic nerve head lie superficially in the retina and occupy a more central (deep) portion of the disc (Fig. 6.1.8). • • • • • • • Fig. 6.1.8. Arrangement of nerve fibres at the optic nerve head. Thickness of nerve fibre layer at the disc Thickness of the nerve fibre layer around the different quadrants of the optic disc margin progressively increases in the following order: Most lateral quadrant (thinnest) Upper temporal and lower temporal quadrant Most medial quadrant Upper nasal and lower nasal quadrant (thickest) Clinical significance of distribution and thickness of nerve fibres at the optic disc margin Papilloedema appears first of all in the thickest quadrant (upper nasal and lower nasal) and last of all in the thinnest quadrant (most lateral). Arcuate nerve fibres which occupy the superior temporal and inferior temporal quadrants of optic nerve head are most sensitive to glaucomatous damage, accounting for an early loss in corresponding regions of visual field. Macular fibres occupying the lateral quadrant are most resistant to glaucomatous damage and explain the retention of the central vision till end. • • • • Internal limiting membrane Internal limiting membrane (ILM) mainly consists of a PAS positive true basement membrane (unlike external limiting membrane) that forms the interface between retina and vitreous. The fibrils of the vitreous merge with the internal lamellae of this membrane. Externally, the basal foot processes of the Muller's cells abut with the membrane and probably play a role in its formation. Thus the internal limiting membrane consists of four elements: Collagen fibrils; Proteoglycans (mostly hyaluronic acid) of the vitreous; Basement membrane; and Plasma membrane of the Muller cells and possibly other glial STRUCTURE OF FOVEA CENTRALIS Infoveal region, there are no rods, cones are larger, in abundance and tightly packed, and other layers of retina are very thin (Fig. 6.1.9). Its central part (foveola) largely consists of cones and their nuclei covered by a thin internal limiting membrane. All other retinal layers are absent in the foveolar region. In the foveal region surrounding the foveola, the cone axons are arranged obliquely (Henle's layer) to reach the margin of the fovea. The ganglion cell layer in 2 to 6 layers thick. Fig. 6.1.9. Microscopic structure of the fovea centralis. • • • • • • BLOOD SUPPLY OF THE RETINA Outer four layers of the retina, viz. pigment epithelium, layer of rods and cones, external limiting membrane, and outer nuclear layer get their nutrition from the choriocapillaris. Inner six layers of retina, viz. outer plexiform layer, inner nuclear layer, inner plexiform layer, layer of ganglion cells, nerve fibre layer and internal limiting membrane get their supply from the central retinal artery. Outer plexiform layer gets its blood supply partly from the central retinal artery and partly from the choriocapillaris by diffusion. Fovea is an avascular area mainly supplied by the choriocapillaris. Macular region gets its blood supply by small twigs from the superior and inferior temporal branches of central retinal artery. Sometimes, cilioretinal artery (a branch from the ciliary system of vessels) is seen originating in a hook-shaped manner within the temporal margin of the disc. It runs towards the macula and supplies it; thus, when present, it helps to retain the central vision in the event of occlusion of the central retinal artery. Retinal vessels are end arteries, i.e. they do not anastomose with each other. However, anastomosis between the retinal vessels and ciliary system of vessels does exist (in the neighbourhood of the lamina cribrosa) with the vessels which enter the optic nerve head from the arterial circle of Zinn or Haller. This arterial circle is formed by an anastomosis between 2 and 4 or more short posterior ciliary arteries and lies in the sclera around the optic nerve. From it, numerous branches pass forward to the choroid, inward to the optic nerve and backward to the pial network. Branches which pass inward, invade lamina cribrosa and also send branches to the optic nerve head and the surrounding retina. • • • Central retinal artery Central retinal artery, the first branch of ophthalmic artery, arises near the optic foramen and courses ahead with 5-6 right angle bends as follows (Fig. 6.1.10): Fig. 6.1.10. Course of central retinal artery. OD: optic disc, PR: prelaminar region, LC: lamina cribrosa, D: dura, A: arachnoid, P: pia, ON: optic nerve, SAS: subarachnoid space, CRV: central retinal vein. Outside the optic nerve. It runs a wavy course forward, below the optic nerve, but adherent to the dural sheath to about 10- 15 mm behind the eyeball, where at a point along the inferomedial aspect of the nerve it bends upwards to pierce the dura and arachnoid, from both of which it receive covering. In the subarachnoid space. It bends forwards and after a short course it again bends upwards at nearly right angle and invaginates the pia to reach the centre of the nerve. The entering vessel is thus clothed by the pia along with the pial vessels. It is also surrounded by a sympathetic nerve plexus (nerve of Tiedemann). In the centre of optic nerve. The artery bends forwards and then in company with the vein, which lies on its temporal side, it passes anteriorly and pierces and lamina cribrosa to appear inside the eye. • • In the optic nerve head. It lies superficially in the nasal part of physiology cup, covered only by that layer of glial tissue (connective tissue meniscus of Kuhnt) which closes the physiological cup. Here, it divides into two branches—a superior and an inferior, each of which subdivides into a temporal and a nasal branch at or near the margin of the optic disc. In the retina. The four terminal branches of central retinal artery namely, the superior nasal, superior temporal, inferior nasal and inferior temporal, divide dichotomously as they proceed towards the ora serrata, where they end without anastomosis. • Arrangement of retinal capillaries The terminal fundus arterioles bend sharply and dip almost vertically into the retina, forming the capillary network arranged as follows: ▪ In most of the extramacular funds, there are tivo retinal capillary netivorks—a superficial and a deep. Superficial capillary network lies at the level of the nerve fibre layer and the deep network lies between the inner nuclear layer and the outer plexiform layer (Fig. 6.1.11). The deep capillary network is more dense and complex than the superficial. There are anastomotic capillaries which run from one to the other. Peripherally, as the ora serrata is approached, the capillary network is reduced to a scanty single layer. Fig. 6.1.11. Arrangement of retinal capillaries. ▪ In the parafoveal zone, the capillary network is especially well developed and is threelayered. However, there exists a capillary-free zone in the fovea, known as foveal avascular zone (FAZ) of about 500 μm in diameter. ▪ In the peripapillary region, the capillary network becomes fourlayered to support the extremely thick nerve fibre layer characteristic of this region. BLOOD-RETINAL BARRIER The endothelial cells of a normal retinal capillary are closely bound together about the lumen by intercellular junctions of the zonula occludens type. These junctions normally prohibit a free flow of fluids and solutes from the vascular lumen into the retinal inter stitium and thus form a blood-retinal barrier. Presence of this barrier is confirmed by absence of fluorescein leakage from these capillaries. The endothelial cells of retinal capillaries are encircled by a basement membrane around which is present a layer of pericytes (mural cells). Pericytes are also surrounded by a layer of basement membrane. Normally, the endothelial cells and pericytes are present in a one to one ratio in young individuals. However, in certain diseases, such as diabetes mellitus, there occurs a relative decrease in the number of pericytes. On the other hand with increasing age, there occurs a gradual decrease in the number of endothelial cells.
what is Laurence-Moon-Biedl syndrome ?
RP + obesity, + hypogenitalism, + polydactyly and + mental deficiency.
what is Cockayne's syndrome ?
RP + progressive infantile deafness, + dwarfism, + mental retardation, + nystagmus and, + ataxia.
whta is Hallgren's syndrome ?
RP + vestibulocerebellar ataxia, + congenital deafness and + mental deficiency.
C/F OF CAPILLARY HAEMANGIOMA ?
Retinal capillary haemangioma may be detected on routine fund us examination or because of visual symptoms due to macular exudates.
what is Mitchell-Wong classification?
Severity of hypertensive retinopathy h as been graded into mild, moderate and malignant, which is predictive of risk for cerebrovascular disease, coronary artery disease and mortality
what are Siegrist streaks ?
Siegrist streaks are formed due t o linear configuration of the pigment along the choroidal arterioles. These are formed due to fibrino id necrosis associated with malignant hypertension.
S/N on Stickler syndrome ?
Stickler syndrome, also known as hereditary arthro ophthalmopathy, is an autosomal dominant conncclivc tissue disorder characterized by following features: I Ocular features are as below 1. Vitreous is liquified and shows syneresis giving appearance of an optically-empty vitrous cavity. 2. Progressive myopia is very common. 3. Radial lattice like degeneration associated with pigmcntary changes and vascular sheathing. 4.Bilateral retinal detachment may occur in 30% cases ( commonest inherited cause of retinal detachment in children). 5. Ectopia lentis is occasionally associated. 6. Pre-senile cataract occurs in 50% cases. II Orofaclal abnormalities include flattened nasal bridge, maxillary hypoplasia, cleft palate and high arched palate. 111 Arthropathy is characterized by stiff, painful, prominent and hyperextensible large joints. Other features include deafness and mitral valve prolapse.
S/N ON enucleation ?
Surgical technique I . Separation of co11jtt11ctiva and Tenon's capsule (Fig. 12.38A): Conjunctiva is incised all around 1.h e limbuswith the help ofspringscissors. Undermining of the conjunctiva and Tenon's capsule is done combinedly, a ll around up to the equator, using blu nt-tipped c urve d scissors. This manoeuvre exposes the extraocular muscles. 2. Separation ojextraocular muscles (Fig. 12.388): The rectus muscles are pulled out one by one with the help of a muscle hook and a 3- 0 silk sutme is passed near the insertion of each muscle. The muscle is then cut with the help ofrenotomy scissors leaving behind Fig. 12.38 a small stump carrying the suture. The inferior and superior oblique muscles are hooked out and cut near the globe. 3. Cutting of optic nerve (Fig. 12.38C): The eyebaJl is prolapsed out by stretching and pushing down the eye speculum. The eyeball is pulled out with the help of sutures passed through the muscle stumps. The cnucleation scissors is then introduced along the medial wall up to the posterior aspect of the eyeball. Optic nerve is felt and then cut with the scissors while maintaining a constant pull on the eyeball. 4. Removal of eyeball. The eyeball is pulled out of the orbit by incising the remaining tissue adherent to it and haemostasis is achieved by packing the orbital cavity with a wet pack and pressing it back. 5. T11serting an orbital implant (Fig. 12.380 ): Preferably an orbital implant (made up of PMMA Medpor or hydroxyapatite) of appropriate size should be inserted into the orbit and sutured with the rectus muscles. 6. Closure of conj unctiva and Tenon's capsule is done separately. Tenon's capsule is sutured horizontally with 6-0 vicryl or chromic catgut. Conjunctiva is sutured vertically so that conjunctiva! fornices are retained deep with 6-0 silk sutures (Fig. 12.38£) which are removed after 8-10 days. After completion of surgery, antibiotic ointment is applied, lids are closed and dressing is done 'Ari th firm pressure using sterile eye pads and a bandage.
SYMPTOMS OF OCULAR ISCHAEMIC SYNDROME ?
Symptoms include: 1• Loss of uision, which usually progresses gradually over several weeks or months 2• Transient black outs (amaurosis fugax) may be noted by some patients. 3• Pain, ocular or periorbital, may be complained by some patients. 4• Delayed dark adaptation may be noted by few patients.
RETINA : VISUAL ADAPTATION : 6.6 VISUAL ADAPTATION INTRODUCTION Light minimum Minimum retinal illumination Quantum catch and stimulus threshold Minimum flux of energy Minimum amount of energy Increment threshold DARK ADAPTATION Dark adaptation curve Mechanism of dark adaptation Factors influencing dark adaptation LIGHT ADAPTATION Time course of light adaptation Mechanism of light adaptation Red versus cone light adaptation VALUE OF LIGHT AND DARK ADAPTATION IN VISION MEASUREMENT OF DARK ADAPTATION Psychophysical measurements Electrophysiologic measurements
The human visual system responds to a truly remarkable range of luminance which is summarized in Fig. 6.6.1. The level at which light can just barely be detected is only one billionth of the level at which exposure to light may cause injury. The human eye in its ordinary use throughout the day is capable of functioning normally over an exceedingly wide range of illumination by a highly complex phenomenon termed as the visual adaptation. The process of visual adaptation primarily involves dark adaptation (adjustment in dim illumination) and light adaptation (adjustment to bright illumination). The terms light and dark adaptation are relative and indicate the change in sensitivity which the retina is making, rather than any static condition. For example, if the illumination in a room is that of moderate daylight and a person goes out into the sunshine, his retina undergoes light adaptation. On the contrary, if the person from same room with moderate daylight illumination goes into a dark room, his retina undergoes dark adaptation. The change which takes place is always that which best enables the retina to function under the new conditions. Before discussing the processes of light and dark adaptation, it will be useful to get familiar with certain related terms given below. These will be helpful in understanding the subject of visual adaptation. Fig. 6.6.1. Range of luminance to which the human eye responds, with the receptive mechanism involved. Light minimum The minimum brightness required to evoke a sensation of light is called the light minimum. It should be measured when the eye is dark-adapted for at least 20-30 minutes. In a fully dark-adapted state, the minimum stimulus necessary to evoke the sensation of light is also called the absolute threshold. Minimum retinal illumination It is another way of defining the threshold stimulus. The minimum retinal illumination required to evoke the sensation of light, in addition to the luminance of stimulus also takes into consideration the pupil size regulating amount of light entering the eye (and thus the threshold expressed in trolands), spectral composition of the light and even the number of quanta falling on unit area of the retina in unit time.1 Quantum catch and stimulus threshold Like matter, light also has an atomic structure. The atoms of light are called 'quanta'. A single quantum is caught by a single molecule of rhodopsin in the rods, and each catch produces a change that can excite the whole rod.2 However, it has been seen that for the flash to be reliably detected, it needs about six quanta to be caught in a rod cluster. Thus the threshold for detection is about six quanta close together in space and time. Minimum flux of energy If an effective point source of light is used, the image is concentrated on to a point on the retina, so that the concept of retinal illumination loses its application; instead, then, we define the threshold as the minimum flux of light energy necessary for vision, i.e. the number of lumens per second or, better, the number of 'quanta' per second entering the eye. Experimentally, a value of 90 to 144 quanta per second has been obtained.3 Minimum amount of energy When a very brief flash of light is used as the stimulus (less than 0.1 sec) we may express the threshold as simply the total number of quanta that must enter the eye to produce a sensation. Thus there are three thresholds, the minimum retinal stimulation, the minimum light flux and the minimum amount of energy. Of these, it is the last from which we are able to compute just how many quanta of light a single receptor must absorb to be excited. Increment threshold The increment threshold (Fig. 6.6.2) is the additional light energy necessary to just perceive a difference in illumination. In dim light, the energy is low, but in bright light a large increment of energy is needed before we perceive any change. Adaptation provides a continual adjustment of retinal sensitivity so that the increment threshold remains a relatively constant percentage of the background level.4 Fig. 6.6.2. Adjustment of rodincrementthresholdtodifferent levels of background illumination in human eye (from Aguliar and Stiles). DARK ADAPTATION Dard adaptation is the ability of the eye to adapt itself to decreasing illumination. When one goes from bright sunshine into a dimly lit room, one cannot perceive the objects in the room until some time has elapsed. During this period, eye is adapting to low illumination. The time taken to see in dim illumination is called 'dark adaptation time'. The rods are much more sensitive to low illumination than cones. Therefore, rods are used more in dim light (scotopic vision) and cones in bright light (photopic vision). Dark adaptation is a much slower process than light adaptation, and the sensitivity of the eye to dim stimulus gradually increases over many minutes in the dark. Temporal summation: Bloch's law of vision The completely dark-adapted retina needs to absorb several quanta of light within a restricted time and area before a sensation can be elicited. The critical period of temporal summation is about 0.1 seconds. During this period, a given amount of luminous energy will have the same effect, regardless of its distribution in time. This is Bloch's law, which can be expressed as Bt = k, where B is the luminance, duration of the stimulus, and k a constant. Beyond the critical duration, temporal summation does not occur and the effect of a test light becomes dependant on luminance alone. The critical period varies with stimulus size, background luminance, the type of task (about 0.1 s for detection, 0.4 s for discrimination) and wavelength (about 0.25 s for blue targets, 0.1 s for red). The Broca-Sulzer and Troxler effects The Broca-Sulzer effect. When a light is turned on, there is a critical period during which its apparent brightness undergoes temporal summation; the apparent brightness falls to a plateau once the critical period has expired. This is why a short flash (shorter than the critical period, so that no plateau is reached) can appear brighter than a longer flash of the same luminance. This is the Broca-Sulzer effect. ▪ The Troxler effect. It occurs when a spot of light is projected on to the retina and held completely stationary. The light appears to fade away and disappears, and because this can occur without the bleaching of an appreciable fraction of retinal photopigment, a neural rather than photochemical mechanism is likely. DARK ADAPTATION CURVE Dark adaptation curve plotted with illumination of the test object in vertical axis and duration of dark adaptation along the horizontal axis shows that visual threshold falls progressively in the darkened room for about half an hour until a relative constant value is reached (Fig. 6.6.3). The dark adaptation curve plotted with retinal sensitivity along the vertical axis and duration of dark adaptation along the horizontal axis. Figure 6.6.4 shows that sensitivity of retina is very low on first entering the darkness, but within 1 minute the sensitivity has increased tenfold, that is, the retina can respond to light of one-tenth the previously required intensity. At the end of 20 minutes, the sensitivity has increased about 6000-fold, and at the end of 40 minutes, it has increased about 25,000-fold. Fig. 6.6.3. Dark adaptation curve plotted with illumination of test object in vertical axis and duration of dark adaptation along the horizontal axis. Fig. 6.6.4. Dark adaptation curve plotted with retinal sensitivity along the vertical axis and duration of dark adaptation along the horizontal axis. The typical dark adaptation carve as shown in Figs 6.6.3 and 6.6.4 is obtained when the region of retina tested contains both rods and cones (e.g. about 11 degrees from the fovea). Fig. 6.6.5 shows the course of dark adaptation in 110 normal persons.5 It can be seen that the decrease in threshold of the retina (Figs 6.6.3 and 6.6.5), i.e. increase in sensitivity of retina (Fig. 6.6.5), proceeds in two steps. The first is rapid, of short duration, and small in extent. The second is slow, more prolonged, and larger. This indicates that two processes are at work, each having different characteristics, and that the break in the curve is the point at which one process is about to finish and the second one is just commencing. The analyses have revealed that the first plateau of the curve represents cone threshold (reached in about 5 minutes) and the second plateau represents rod threshold (reached after about 30 minutes). The inflection of the dark adaptation curve where the rod limb begins is called the cone-rod break or alpha point, and it usually occurs after seven to ten minutes of adaptation. The final rod phase of adaptation does not begin until 93% of rhodopsin has already regenerated.6 Fig. 6.6.5. The course of dark adaptation in 110 normal persons. Fairly marked differences in the final stage of dark adaptation are seen. Upper and lower curves represent the extremes of dark adaptation in normal subjects. Shaded area between the upper and lower curves represents the mean of dark adaptation in normal subjects. From Mandeibaum.5 Though all the chemical events of vision occur about 4 times as rapidly in cones as in rods, the cones do not achieve anywhere near the same degree of sensitivity as the rods. Therefore, despite rapid adaptation by the cones, they cease adapting after only a few minutes, while the slowly adapting rods continue to adapt for many minutes and even hours, their sensitivity increasing tremendously. In addition, a large share of the greater sensitivity of the rods is also caused by convergence of as many as 100 or more rods on to a single ganglion cell in the retina; these rods summate to increase their sensitivity. MECHANISMS OF DARK ADAPTATION The process of adaptation is primarily based on the changes in visual pigment; however, several other mechanisms work together in both light and dark. Visual pigment mechanism Dark adaptation basically involves a reversal of the mechanism of light adaptation, but the process is much slower than the light adaptation. When the person remains in darkness for a long time, the retinal and opsins in the rods and cones are converted back into the light-sensitive pigments. Furthermore, vitamin A is reconverted back into retinal to give still additional light-sensitive pigments, the final limit being determined by the amount of opsins in the rods and cones. Thus the regeneration of visual pigments requires the expenditure of metabolic energy and an exchange of material with the pigment epithelium. The details of regeneration of visual pigments are described in Chapter 6.3 on Photochemistry of Vision (page 200). The sensitivity of rods is approximately proportional to the antilogarithm of the rhodospin concentration, and it is assumed that this relationship also holds true in cones. In other words, the relationship of threshold sensitivity to visual pigment concentration is logarithmic (Fig. 6.6.6) and is much more critical for rods than cones. Bleaching 50% of the appropriate pigment elevates cone threshold by a factor of 30, but it elevates rod threshold by 100 million.7 Fig. 6.6.6. Logarithmic relationship between visual sensitivity (measured as dark-adaptated threshold) and concentration of rhodopsin. After Rushton.7 Dark adaptation on a molecular level There is an approximate relationship between the amount ofrhodopsin bleached and the state of dark adaptation. Ths is delineated in the Dowling-Rushton equation: log(z)/A = aB, where A is the threshold in complete dark adaptation, B is the fraction of bleached rhodopsin, and a is a constant of proportionality. Dark adaptation and regeneration of rhodopsin are dependent on the local concentration of 11-cis retinal. The limiting factor for recovery after a bleach is the rate at which it is delivered to opsin in the bleached photoreceptors. A healthy RPE is essential to this process and the age-related decline in dark adaptation may be related to a failing RPE. • • Other mechanisms of light and dark adaptation In addition to the adaptation caused by changes in concentration of rhodopsin and cone pigments, certain other mechanisms given below also play some role. Change in pupillary size This can cause adaptation of approximately 30-fold because of changes in the amount of light allowed through the pupil. Neural adaptation The mechanism of neural adaptation involves neurons of the visual chain in the retina itself. That is, when the light intensity first increases the intensities of the signals transmitted by the bipolar cells, the horizontal cells, the amacrine cells, and the ganglion cells are all very intense. However, the intensities of most of these signals decrease rapidly. Although the degree of this adaptation is only a few fold rather than the many thousand-fold that occurs during adaptation of the photochemical system, this neural adaptation occurs in fraction of a second, in contrast to the many minutes required for full adaptation by the photochemicals. Neurological adaptation is based on following two mechanisms: Feedback inhibition is the neurologic mechanism of adaptation in which excitation of one cell stimulates other cells that make inhibitory connection to it. Thus after a few milliseconds of activity, the excitation is turned off by the inhibitory feedback. Another mechanism of adaptation lies within the neuron itself, in as much as the response to stimulation is often non-linear. For example, doubling the intensity of the stimulus may produce a response less than twice as large, and from a functional standpoint, this lessened response is an adaptation of the stimulus. ▪ The existence of neural mechanisms of adaptation is demonstrated by folloiving example: In Oguchi's disease (a rare dystrophy), dark adaptation of the rods is very prolonged (requiring hours) in spite of the fact that rhodopsin regenerates at a normal ratio. In electroretinogram, a-wave is normal (which rules out the photoreceptors as the site of adaptation defect), but the b-wave is suppressed until dark adaptation has taken place. Thus, Oguchi's disease must involve neural and not photochemical mechanisms of adaptation, and the defect must be proximal to the photoreceptor.8 FACTORS INFLUENCING DARK ADAPTATION A. Factors related to preadapting light 1. Luminance (intensity) of the preadapting light The lower the adapting luminance, the more rapidly the cone threshold drops during early part of dark adaptation and the sooner the rod branch appears. With further reduction in the adapting luminance, the initial plateau of the cone branch becomes shorter and then disappears entirely as the rod branch comes in at an ever earlier time9 (Fig. 6.6.7). Fig. 6.6.7. Effect of progressive decrease in luminance (intensity of the preadapting light) on dark adaptation curve. For explanation see text. After Hecht et al.9 2. Duration of the light used to preadapt the eye An increase in the degree of light adaptation, by prolonging the exposure, causes a decrease in the slope of subsequent curve for rod dark adaptation. Fig. 6.6.8 depicts a significant difference between light adaptation duration of 5 min and 10 or 20 min. Increasing duration of light adaptation beyond 10 min has no further effect on the later scotopic branches of the recovery process.10 • Fig. 6.6.8. The course of dark adaptation following each of six different durations of exposure to an adapting luminance of 333 millilamberts. From Wald and Clark.10 3. Energy of the light used to preadapt the eye The total energy of the light depends upon the level of luminance and duration of exposure. So the same total energy can be obtained by using either higher adapting luminance of shorter duration or lower luminance of longer duration. Following observations have been made in the course of dark adaptation curve in relation to energy of light used to preadapt the eye: A reciprocal relation between flux and dark adaptation duration has been reported for duration between 3 msec and 3 sec of exposure.11 In other words, an increase in the total energy of the light used to preadapt the eye causes a decrease in the slope of the subsequent curve for adaptation and a displacement of the curve to the right on the time axis (Fig. 6.6.9).12 • Fig. 6.6.9. The effect of increasing both the intensity and duration of the preadapting illumination, (a) filled circles 4 min × 20 millilamberts and open circles 0.2 min × 447 millilamberts (both having almost equal energy of light), (b) filled circles, 4 min × 110 millilamberts and open circles, 1 min × 447 millilamberts. From Haig.12 It has also been concluded that when the total energy of light adaptation is constant, the overall form of the subsequent darkadaptation curve is also roughly constant and is independent of the way adapting energy is distributed in time (Fig. 6.6.10), at least from adapting duration of a few milliseconds up to Fig. 6.6.10. Dark adaptation following light adaptation duration of 600 msec (open circles) and 30 sec (filled circles) having the same total quantum energy at the retina. From Pugh.14 4. Wavelength distribution (colour) of the adapting light The fact that the wavelength of the adapting light influences the course of dark adaptation curve has been well explained in practice as below. It has been common practice in occupations in which work has to be performed under dim illumination (e.g. pilots during night flying) to keep the eyes exposed to red light by use of red goggles in an effort to preserve the dark adaptation of the retina. The possible explanation of the effect of red goggles is that their use prevents the short waves of the spectrum (blue end of spectrum) from reaching the rods, and thus preventing their bleaching (since only the rods are affected by the blue end of the spectrum). Therefore, in order that the sensitivity of the rod mechanism be maintained, the blue end of the spectrum must be eliminated. Fig. 6.6.11 shows the course of dark adaptation curves following light adaptation with deep blue, yellow and deep red coloured lights.15 Fig. 6.6.11. Course of dark adaptation curve following light adaptation with deep blue (square), yellow (triangles) and deep red (circles) coloured lights. From Brown.15 B. Factors related to the test stimulus 1. Wavelength (colour) of the test stimulus The course of dark adaptation curve varies with the character of the light used to test the threshold, and this variation also depends upon the region of the retina tested, i.e. whether cones or rods are predominant. Figure 6.6.12 shows the course of dark adaptation curve measured with each of seven different spectral distributions of the test stimulus with 10 fields located 5 degree above the fovea. Since the luminance is based on the spectral activity of the cones, so the cone branch of dark adaptation curve with different wavelength is superimposed in Fig. 6.6.12. On the other hand, the portions of dark adaptation curves representing the rod branch are displaced along the log luminance axis in amounts corresponding to the difference in the effectiveness of lights of different wavelengths with constant luminance, as stimuli for rods.16 Fig. 6.6.12. Course of dark adaptation curve measured with each of seven different spectra! distributions of the test light with 1° test fieldlocated5°above the fovea. From Kohlrausch, A.16 2. Duration of exposure of retina to test It has been reported that the visual system is capable of summating energy of a stimulus over a limited time interval. Within that interval, luminance and flash duration are reciprocally related, i.e. a decrease in the luminance can be compensated by proportionate increase in duration up to a critical value and vice versa. In general, the duration of exposure to the testing light must be kept constant because, with short exposures, the retina obeys the law of reciprocity. A faint light may fail to excite the photoreceptors, if its duration is very short, but reach the threshold level of stimulation if its duration is increased. 3. Region of the retina where test stimulus is applied There exist marked regional differences in the response of retina to adaptation due to difference in the relative sensitiveness of rods and cones in different parts of the retina. The central region of the retina (fovea) undergoes dark adaptation, but to a much less degree than the peripheral regions. Figure 6.6.13 shows the thresholds for centrally fixated areas of different size during dark adaptation. It will be noted that the curve of a centrally fixated area of 2 degrees is quite different from that involving a 20 degrees field. Since only cones are present in the central 2 degrees area of retina, so the curve from this area indicates only cones threshold (reached in about 5 minutes). While the curve obtained from stimulation of central 20 degrees area of the retina consists of two components. The first plateau of the curve is steep and represents cone threshold (resembles the curve obtained from central 2 degrees area of the retina). The second plateau of the curve obtained from stimulation of central 20 degrees area of retina represents rod threshold (reached in about 30 minutes).17 Fig. 6.6.13. Course of dark adaptation curve for centrally fixated area of different sizes. From Hecht et al.17 C. Factors related to the individual Vitamin A deficiency Severe deficiency of vitamin A elevates the threshold for dark adaptation curve due to depletion of photosensitive pigment (Fig. 6.6.14). Though there occurs an elevation of threshold throughout the dark adaptation process, the elevation of the later rod branch of the curve is more prominent than that of the early cone branch of the curve.18 However, it has been reported that not all subjects deprived of vitamin A show changes in dark adaptation, even though the deprivation is carried to a point, at which characteristic changes in the skin are produced. Though the dark adaptation curve is abnormal in vitamin A deficiency, but because of the tediousness of ensuring standard conditions in making these tests and the wide range of normal values, it is doubtful that this method will be used to measure deficiency of vitamin A, especially since better chemical methods for determining it in the blood have now been devised. Fig. 6.6.14. Course of dark adaptation curve following four durations (14, 21, 25 and 35 days) of experimental vitamin A deficiency compared with a normal (0 days) dark adaptation curve. From Hecht and Mandelbaum.18 2. Effects of anoxia on dark adaptation Anoxia is associated with a significant elevation in dark adaptation threshold.19 Following administration of 100% oxygen, there occurs a rapid recovery of sensitivity of levels associated with a normal level of oxygen pressure. 3. Effect of tobacco inhalation Tobacco inhalation in the form of cigarette smoke is associated with elevation of dark adaptation threshold.20 Factors responsible include presence of carbon monoxide in the cigarette smoke and constriction of retinal vessels induced by nicotine which probably interfere with the oxygen metabolism of the retina. Quick recovery is reported following administration of oxygen. 4. Effect of anaesthesia on dark adaptation It has been reported that dark adaptation process is progressively retarded when it is measured repeatedly after a series of short bleaching flashes under halothane anaesthesia.21 The progressive retardation after each successive bleach occurs up to about seven bleaches, after which an equilibrium state is reached. This effect has been attributed to some form of interference by the anaesthesia with the replenishment of a retinal ingredient that is essential for pigment regeneration. 5. Effect of opacities in ocular media on the dark adaptation curve Threshold for dark adaptation process is higher in patients with opacities in the ocular media than for normal subjects. It occurs because of the fact that less light is able to reach the retina, if the ocular media are cloudy. 6. Dark adaptation in retinal degenerations Threshold for dark adaptation process is found to be raised when the measurements are made from the retinal area showing degeneration. 7. Dark adaptation in myopia Raised threshold of dark adaptation has been reported in patients having myopia between 5 to 10 dioptres. It does not seem to depend upon myopic degeneration in the fundus. As yet, no good explanation for this association is available. Most of such patients also complain of a slight degree of night blindness. 8. Dark adaptation in glaucoma There occurs no significant change in dark adaptation in early stages of glaucoma, till there occur no field defects. However, delayed dark adaptation is noted in patients with glaucoma having visual field LIGHT ADAPTATION When one passes suddenly from a dim to a brightly lighted environment, the light seems intensely and even uncomfortably bright until the eyes adapt to the increased illumination and the visual threshould rises, the process by means of which retina adapts itself to bright light is called light adaptation. Unlike dark adaptation, the process of light adaptation is very quick and occurs over a period of 5 minutes. Strictly speaking, light adaptation is merely the disappearance of dark adaptation. The visual system must detect contrast over a huge range of light intensities at least 12 log units. The area of the pupil can vary only 16-fold (1.3 log units) and therefore its role in visual adaptation is limited. Light adaptation is a form of "automatic gain control". Over a range of at least 3 log units of cone function, the intensity increment required for detection, DI, where DI/ I is a constant, is know as the Weber-Fechner relation, and the constant is called the Weberfraction. The relationship breaks down at higher light levels, when saturation occurs (Fig. 6.6.15). Light adaptation works very quickly, gets faster as background intensity increases and is dependent on calcium ion flux; it is abolished when calcium is buffered inside photoreceptors. The light-adapted eye is maximally sensitive at about 555 nm. TIME COURSE OF LIGHT ADAPTATION Crawford22 studied the time course of light adaptation by employing 'increment threshold measuring' technique (as mentioned earlier in the introduction, the increment threshold refers to the additional light energy necessary just to perceive a difference in illumination). The time course of light adaptation as shown in Fig. 6.6.15 reveals following features: Fig. 6.6.15. The light adaptation curve. (From Louise Bye). ▪ Anticipatory effect. Threshold for detection of test flash is seen to rise even before the adapting light is turned on (Fig. 6.6.16A). This anticipatory effect can be explained by the fact that the cortical effect of turning on a bright adapting light occurs more quickly than the cortical effect of presenting a weak test stimulus. ▪ Transient effect. A rapid elevation in the test flash threshold is observed immediately on turning on the adapting light (Fig. 6.6.16B). This transient effect is attributed to a high level of transient neural activity which so occupies the available pathway for transmission of information that higher flash luminances must be employed, if their presence is to be detected. As the system adapts to the light, the neural activity is reduced and the threshold level falls (Fig. 6.6.16C). ▪ Photochemical effect. As shown in Fig. 6.6.16D, after a period of 2 or 3 min, depending upon the luminance of the adapting light, threshold begins to rise again as the light adaptation process continues, finally reaching a maximum after five or ten minutes. The late elevation of threshold after it passes through a minimum value is attributed to the photochemical adaptation process and represents the time required for the rates of bleaching and regeneration of photopigments to reach a balanced state. Fig. 6.6.16. Time course of light adaptation measured by an increment threshold technique. From Crawford.22 MECHANISM OF LIGHT ADAPTATION Visual Pigment Mechanism When a person remains in bright light for a long time, a large proportion of the photochemicals in both the rods and cones is reduced to retinal and opsins. Furthermore, much of the retinal of both the rods and cones is converted into vitamin A. Because of these two effects, the concentration of the photosensitive chemicals is considerably reduced, and the sensitivity of the eye to light is even more reduced. This is called light adaptation. For details of the photochemistry of bleaching of the visual pigments see page 200. It is important to note that rod sensitivity (in contrast to that of cones) decreases dramatically as visual pigment is bleached, and a loss of only 7% of rhodopsin brings cone function into play (as mesopic or mixed rod and cone vision).6 After some time of exposure to bright light, i.e. when bleaching of photopigments is going on, there occurs isomerization of the immediate products of bleaching back into photosensitive form by the light itself. This process is called 'photoregeneration'. As shown in Fig. 6.84 at point 'D' on the time course of light adaptation, after the rates of bleaching and regeneration of photopigments reach a balanced state, the threshold again starts rising.22 Other mechanisms Other mechanisms which play an important role in dark adaptation, also play an even more important role in light adaptation. Since, when a light is turned on, the chemical changes are overshadowed by the more rapid neural adjustments, so light adaptation is primarily a neural phenomenon. The photoreceptors provide part of the adjustment of sensitivity and of increment threshold that occurs with light adaptation. Additional adaptive effects are provided by the horizontal cells (which feedback upon the photoreceptors with inhibitory signals) and probably by other retinal elements. Thus light adaptation and the neural adjustment of sensitivity, which take place rapidly and continuously as our gaze wanders between lighter and darker parts of the scenery, are just as critical to vision as the regeneration of visual pigment which is more talked about in clinical ophthalmology. In addition to the neural mechanism, the pupillary mechanism also plays a role by controlling the amount of light entering the eye. RED VERSUS CONE LIGHT ADAPTATION There are important differences between the adaptation properties of rods and cones. Rods show light adaptation only over a limited range of light intensity. Once the light becomes moderately bright, the rod receptor potential saturates (Fig. 6.6.2).4 Cones, on the other hand, continue to adapt and to respond to brighter and higher light up to the point of retinal damage. Furthermore, each of the three colour systems (erythrolabe, chlorolabe and cyanolabe) seems able to adapt independently of one another.23 VALUE OF LIGHT AND DARK ADAPTATION IN VISION Between the limits of maximal dark adaptation and maximal light adaptation, the eye can change its sensitivity to light by as much as 500,000 to 1,000,000 times, the sensitivity automatically adjusting to changes in illumination. Since the registration of images by the retina requires detection of both dark and light spots in the image, it is essential that the sensitivity of the retina always be adjusted so that the receptors respond to the lighter areas but not to the darker areas. An example of maladjustment of the retina occurs when a person leaves a movie theatre and enters the bright sunlight, for even the dark spots in the images then seem exceedingly bright, and, as a consequence, the entire visual image is bleached, having little contrast between its different parts. Obviously, this is poor vision, and it remains poor until the retina has adapted sufficiently for the darker areas of the image to longer stimulate the receptors excessively. Conversely, when a person enters darkness, the sensitivity of the retina is usually so slight that even the light spots in the image cannot excite the retina. After dark adaptation, however, the light spots begin to register. As an example of the extremes of light and dark adaptation, the intensity of sunlight is approximately 10 billion times that of starlight; yet the eye can function both in bright sunlight and in starlight. MEASUREMENT OF DARK ADAPTATION 1. PSYCHOPHYSICAL MEASUREMENTS The most commonly employed procedure for the measurement of dark adaptation is the determination of threshold for light detection as a function of time in the dark after the examination of an adapting field. Based on this principle, the Goldmann-Weekers adaptometer is the instrument used ordinarily for testing dark adaptation. To begin with the use of adaptometer, the subject is preadapted to a standard amount of light and then is presented with a series of light flashes (localized 11 degrees below fixation in the standard instrument). The intensity of the test flashes is controlled by neutral density filters and the threshold at which the test light is just perceived is plotted against time. Under these conditions, the dark adaptation curve shows two typical plateaus (see page 248). When the test spot is focussed on the foveola, where rods are absent, there is only a cone plateau. Though the dark adaptation test is subjective, but poor cooperation or malingering is easily recognized. It must be kept in mind that a record of dark adaptation is complementary to the electro-retinogram because adaptometry is a focal test (which must be remembered in interpreting results from patients with patchy disease), and thus in some instances may be a 2. ELECTROPHYSIOLOGIC MEASUREMENTS There are a variety of electrophysiologic measurement techniques which may be used in the study of dark adaptation. These are useful in that they permit an investigation of purely sensory aspects of the process in selected neural locations. While psychophysical measurements involve the entire system and include some motor response system as well as the sensory components. The following electrophysiological tests may be used in the study of dark adaptation: 1. Electroretinogram. Details of the ERG are described on page 226. With ERG record, dark adaptation can be measured in terms of the magnitude of the retinal action potential.24 2. Visually evoked cortical potential (VECP) records (employing a red stimulus light to emphasize cone function and a low luminance blue light to emphasize rod function) are quite comparable with psychophysical data on dark adaptation.25 3. Single unit responses. Although, this technique is not applicable to human subjects, the use of microelectrode permits recording from single cell in various regions of the visual nervous system from retina to cortex. The process of dark adaptation has been studied in single cells of the cat retina26 and the results provide a basis for identifying specific cellular mechanisms that underlie the psychophysical processes which have been so extensively studied in man. 3. REFLECTION DENSITOMETRY Reflection densitometry is a procedure by which photopigment concentration can be measured in human eyes with a non-invasive technique. This procedure was developed by Rushton27,28 and Weale.29 These measurements are particularly useful in furthering our understanding of visual adaptation. A comparison of the logarithm of threshold during dark adaptation with the fraction of photopigment present in the retina at various times following light adapting illustrates the correspondence between the two (Fig. 6.6.17).30 Fig. 6.6.17. Comparison of the logarithm of threshold and the fraction of rhodopsin regenerated during dark adaptation. The psychophysical threshold represented by open circles were obtained using the afterflash procedure.
ozurdex Common Side Effects in Diabetic Macular Edema ?
The most common side effects reported in patients with diabetic macular edema include: cataract, increased eye pressure, conjunctival blood spot, reduced vision, inflammation of the conjunctiva, specks that float in the field of vision, swelling of the conjunctiva, dry eye, vitreous detachment, vitreous opacities, retinal aneurysm, foreign body sensation, corneal erosion, inflammation of the cornea, anterior chamber inflammation, retinal tear, drooping eyelid, high blood pressure, and bronchitis.
S/N ON ANGIOMATOSIS RETINAE (VON HIPPEL LINDAU SYNDROME) ?
This is a rare condition affecting males more often than females, in the third and fourth decade of life. •Angiomatosis involues retina, brain, spinal cord, kidneys and adrenals. • Clinical course of angiomatosis rerinae comprises vascular dilatation, tortuosity and formation of aneurysms which vary from small and miliary to balloon-like angiomas, followed by appearance of haemorrhages and exudates, resembling eventually the exudative relinopathyof Coats. Massive exudation is freque ntly complicated by retinal detachment which may be prevented by an early destruction of angiomas with cryopexy or photocoagulation.
T/T OF EALES DISEASE ?
Treatment of Eales' disease comprises: l. Medical treatment= • Oral corticosteroids for extended periods is the mainstay of treatme nt during stage of active inflammation. • Antitubercular therapy course h as also been recommended in selective cases. 2. Laser photocoagulation= of the retina either PRP or feeder vessel photocoagulation is indicated in stage of neovascularizion. 3. Vitreoretinal surgery= is required for non-resolving vitreous haemorrhage and tractional retinal detachment.
RETINA : FIELD OF VISION : VISUAL FIELDS: GENERAL CONSIDERATIONS Definition, limits and analogy Central versus peripheral fields Methods of assessment of visual fields KINETIC PERIMETRY Types Indications Technique STATIC PERIMETRY Advantages of automated static perimetry over manual kinetic perimetry Automated perimetry: Terminologies and variables Testing strategies and patterns Evaluation of HFAsinglefield printout
VISUAL FIELDS: GENERAL CONSIDERATIONS DEFINITION, LIMITS, AND ANALOGY Definition ▪ Field of vision or visual field refers to the sum total of visual perception for an eye fixed on a stationary object of regard with the head and body held fixed in position. Thus, visual field is a threedimensional area of subject's surrounding that can be seen at any one time around an object of fixation. ▪ Field of gaze. The visual field is different from the "field of gaze", in which the eye is permitted to have freedom of rotational movement while the head and body are kept in a constant position. ▪ Field of vieiv. Here both the eye as well as the head can be moved. • • Normal limits Normal uniocular visual field. The normal uniocular field of vision with a 5 mm white colour object extends to approximately 60° nasally (toward the nose, or inward) from the vertical meridian in each eye, to 100° temporally (away from the nose, or outwards) from the vertical meridian, and approximately 60° above and 75° below the horizontal meridian (Fig. 6.12.1 A). Macida corresponds to the central 13° of the visual field; the fovea to the central 3°. Field for blue and yellow colour is raughly 10° less and that for red colour is about 20° less than that of white. Normal binocular visual field reaches 180° in the horizontal plane (160° for monocular vision) and 135° in the vertical plane. Fig. 6.12.1. (A) Extent of normal visual field; (B) Traquir's normal hill of vision. Classic analogy Tire visual field can be compared with a topographic map of an island. Traquair proposed the concept of "an island of vision surrounded by a sea of darkness". The height of the island correlates to the sensitivity of the retina. Vision perception is most sensitive at the fovea, and sensitivity decreases towards the periphery. In the "island of vision", a deep well represents the blind spot, roughly 15° temporal to the point of fixation. The blind spot is an absolute visual field defect caused by the optic nerve head, which has no overlying retina (Fig. 6.12.1B). • • • • CENTRAL VERSUS PERIPHERAL FIELDS The visual field can be divided into central and peripheral fields: Central field includes an area from the fixation point to a circle 30° away. The central zone contains physiological blind spot on the temporal side. Peripheral field of vision refers to the rest of area beyond 30° to outer extent of field of vision. Isopter. Visual field, i.e. the three-dimensional hill of vision can be divided into many isopters depending upon the perception sensitivity. Thus, each isopter can be defined as a threshold line forming points of equal sensitivity on a visual field chart. Scotoma refers to an area of loss of vision totally (absolute scotoma) or partially (relative scotoma) in the visual field. METHODS OF ASSESSMENT OF VISUAL FIELDS Campimetry This refers to examination of the visual field projected on to a flat surface, e.g. on a transparent screen or flat-panel monitor. It is best suited to examination of the central visual field, up to approximately 20° of eccentricity. Perimetry This refers to examination of the visual field performed with a hemispherical surface on to which the visual field is projected. The standard unit of measurement in the visual field is the differential light sensitivity (DLS). Basic principle underlying all forms of perimetry is same. The eye to be examined is positioned at the geometric centre of the hemisphere, such that all points on its inner surface are equidistant from the eye. The surface is uniformly illuminated and test objects are small spots of light that are projected on top of the adapting background. Indications for perimetry Indications for visual field testing include visual field deficits, vision loss, headache, and neurologic deficits. Perimetry is typically the only clinical procedure that evaluates the status of the afferent visual pathways for locations outside the macular region. It is used especially for detection of glaucoma. Types of perimetry Kinetic perimetry. In this form, test objects used are fixed in size and brightness. Urey are moved from non-seeing areas into seeing portions of the visual field, the subject is asked to respond when the object first becomes visible. Static perimetry. In this form, stationary test objects that vary in size and brightness, but never move are employed. Brightness of the test object at a point is gradually increased till the patient responds. Basic differences between the two forms of perimetry are enlisted in Table 6.12.1 and shown in Fig. 6.12.2. Table 6.12.1. Differences between kinetic and static perimetry Kinetic Static Kinetic Static Stimulus presentation From non-seeing to seeing Can be presented from seeing to nonseeing or vice versa Optimal stimulus speed 4 deg per sec Independent of reaction time Sensitivity for detection of shallow focal loss Likely to be missed, especially if in periphery More sensitive than kinetic Time required for test Dependent on patient reaction time Manual static more time consuming than automated static Fig. 6.12.2. Kinetic versus static perimetry KINETIC PERIMETRY Kinetic perimetry is still an indispensable diagnostic method in a number of clinical situations where the periphery needs to be tested or where automated static perimetry reaches its limits. Types Kinetic perimetry is of following types: 1. Manual kinetic perimetry Prototype is Goldmann perimeter. In 2007, the production of the Goldmann perimeter ceased. 2. Semiautomated kinetic perimetry Prototype is Octopus 101. Strong agreement has been shown with Goldmann perimetry for type and location of defect. 3. Automated kinetic perimetry Prototype is Octopus 900. The Octopus 900 provides 90° full field projection perimetry with a range of 47 decibels and capable of performing both kinetic and static perimetry programmes (Fig. 6.12.3). Fig. 6.12.3. Octopus 900 kinetic perimetry. • • • Indications Indications include: Assessment of young children Patients with poor vision or severely restricted visual fields Patients with brain injury involving the posterior hemispheres of the brain. • • • • Technique Technique of automated perimetry is summarized below: Using the built-in telescope, the examiner tracks the patient's eye for good fixation. A moving stimulus is presented from a non-seeing area to a seeing area. It is repeated at various points around the clock and a mark is made as soon as the point is seen. These points are then joined by a line, an "isoptre". The process is repeated with a point of lesser luminescence and another isoptre is created. Thus, a number of isoptres are plotted so the end result is a chart of the maximal peripheral vision for decreasing level of brightness. The results differ depending on photopic, mesopic, or scotopic background luminosity. For current Octopus kinetic perimetry, the visual field strategy currently utilised when screening with Octopus perimeter is a 5°/sec stimulus speed for peripheral and central visual field isopters using I4e and I2e targets along with a 3°/sec stimulus speed using I4e target for blind spot mapping and further evaluation of field loss area. This is coupled with suprathreshold static assessment within the central visual field using the I4e target. STATIC PERIMETRY Static perimetry requires presenting a stimulus of a constant size of varying intensity at a fixed location to determine the threshold sensitivity at that locus. Static perimetry is performed with an automated perimeter. This technique is performed at every designated location in a preselected area and pattern, and a map of threshold sensitivities for a given field of vision is generated. • • • • ADVANTAGES OF AUTOMATED STATIC PERIMETRY OVER MANUAL KINETIC PERIMETRY Test administration is more standardized and, therefore, more reproducible. Less input from a technician is required, minimizing testing variability. Reliability is improved with automated fixation monitoring and reduced examination time for static testing compared with manual static testing. Patient dependability maybe quantitated and statistically assessed. • • • • • AUTOMATED PERIMETRY: TERMINOLOGIES AND VARIABLES Luminous intensity Current standard unit of luminous intensity is candela/m2 (cd/m2). Unit used in HFA is apostilb (asb), which is an European unit; Apostilb (asb) is a unit of brightness per unit area and is defined as 35-1 candela/m2. Americans use millilambert instead of apostilb. 10,000 asb = 1 lambert or 3183 cd/m2. 1 asb is the least intense stimulus that can be seen foveally. • • • • • • • Concepts of luminous intensity: Log Basic mathematics of log tables: 101= 10 102 = 100 103 = 1000 104 = 10000 105 = 100000 106 = 1000000 10,000 × 100 = 1,000,000 or 104 × 102 = 106 (Note: 4 + 2 = 6) In considerations of luminosity, log scale (geometric progression) is used because sensation relates to factors rather than addition of intensity, e.g. if a patient has failed to see a 1000 asb stimulus, increasing intensity by 1 asb will not have any meaning as the difference would be very little to appreciate. However, if it is increased by 1/10 of a log factor, it will increase illumination by 25% and 3/10 of log factor will double the intensity. Decibel Decibel is not a unit of luminosity. It describes retinal sensitivity, i.e. less the light required to be perceived by retina at a particular point, more is the sensitivity of retina at that point. In a normal person, it is maximum at fovea. One dB is described as one-tenth of log unit. Maximum intensity of stimulus that HFA can produce is 10000 asb. Figure of 10000 can have a maximum log of 4 (10000 = 104). Since 1 dB is 1/10 of log, maximum possible sensitivity is 40 dB which is equivalent of 1 asb which is the minimum intensity of stimulus which can be perceived by a young trained observer and thus this 40 dB is Automated perimeter variables 1. Background illumination. HFA uses 31.5 apostilb (asb) background illumination. Apostilb (asb) is a unit of brightness per unit area (and is defined as 35-1 candela/m2). 2. Stimulus intensity. HFA uses projected stimuli which can be varied in intensity over a range of more than 5% log units (51 decibels) between 0.08 and 10,000 asb. In decibel notation (dB), the value refers to retinal sensitivity rather than to stimulus intensity. Therefore, 0 dB corresponds to 10,000 asb and 51 dB to 0.08 asb (Fig. 6.12.4). In contrast to kinetic perimetry, the higher numbers indicate a logarithmic reduction in test object brightness, and hence greater sensitivity of vision (Fig. 6.12.4). Fig. 6.12.4. Stimulus intensity scales compared. 3. Stimulus size. HFA usually offers five sizes of stimuli corresponding to the Goldmann perimeter stimuli I through V. Unless otherwise instructed, the standard target size for automated perimetry is equivalent to Goldmann size III (4 mm2) white target. Stimulus duration. Stimulus duration should be shorter than the latency time for voluntary eye movements (about 0.25 seconds). HFA uses a stimulus duration of 0.2 sec while Octopus has 0.1 sec. Concept of threshold, suprathreshold and infrathreshold ▪ Threshold. Minimal intensity of light at which a stimulus is perceived by the visual system at a specific location in the field of vision. In statistical terms, it is the point on a frequency-of-seeing curve at which a stimulus is perceived 50% of the time. ▪ Suprathreshold. Intensity of light at which stimuli are perceived greater than 95% of the time on the frequency-of-seeing curve, or any stimulus brighter than threshold. ▪ Infrathreshold. Intensity of light at which stimuli are perceived with a frequency of less than 5%, or any stimulus weaker than threshold. A threshold value is determined by presenting stimuli of gradually increasing and decreasing intensities, determining the dimmest stimulus presented that is seen and the brightest that is not seen, and either averaging these values or using the last seen or not-seen stimulus as the threshold value. Threshold sensitivity may vary with retinal adaptation (background illumination), stimulus colour and size, duration of stimulus presentation, and degree of stimulus movement. • • • TESTING STRATEGIES AND PATTERNS Testing strategies Two basic categories of testing strategies are offered by most instruments: suprathreshold screening and threshold testing. 1. Suprathreshold screening This enables rapid discovery of gross field defects. A fixed stimulus that is presumed to be threshold at approximately 30° is used for the entire test area, without adjusting for decreased sensitivity as per the normal contour of the hill of vision. The tested points are recorded as either seen or not seen. ▪ Indications for screening include: Baseline testing for a patient before ocular or neurosurgical operative procedures Routine examination for occupational licensing Localization of a postchiasmal neuro-ophtlralmologic lesion. II. Threshold testing Threshold strategies report actual retinal sensitivities at each point tested in numeric form (dB). As in the screening strategies, the sensitivities are displayed in a pattern corresponding to that employed in the test. Along with actual values, a display of differences from expected values or depth of defect may be provided. If the actual values are within 4 to 5 dB of expected values, they are considered within the limits of normal variability and may be recorded as "normal". Visual threshold is the physiologic ability to detect a stimulus under defined testing conditions. The normal threshold is defined as the mean threshold in normal people in a given age group at a given location in the visual field. It is against these values that the machine compares the patient's sensitivity. Thresholds are reported in decibels in a range of 0-50. Fifty decibel (dB) is the dimmest target the • perimeter can project. 0 dB is the brightest illumination the perimeter can project. The lower the decibel value the lower the sensitivity; the higher the decibel value, the higher is the sensitivity. Threshold testing provides more precise results than suprathreshold testing and is thus preferred by most clinicians, although it takes more time and the equipment often costs more. Common threshold testing strategies 1. Full threshold testing. This testing modality involves performing the bracketing ("staircase") process at every point tested (4-2 on the Humphrey and 4-2-1 on the Octopus perimeter). A stimulus brighter than the patient's expected threshold is presented at a point. If the stimulus is seen, stimuli of successively reduced intensity are presented until the stimulus is not seen. Then stimuli of increasingly higher intensity are presented until one is seen. Most instruments use 4 dB increments until threshold is crossed for the first time. Subsequent changes in intensity are usually of 2 dB steps. A point somewhere between suprathreshold and infrathreshold is determined to be threshold for that point. Most perimeters bracket until threshold is crossed twice, from the suprathreshold and infrathreshold directions. Several points in the field are in the process of being thresholded at the same time. 2. FASTPAC is a rapid thresholding strategy in the Humphrey Field Analyzer: This uses a 3 dB step size and estimates the threshold after only one crossing, but thresholds the four primary seed points in a similar manner to standard full-threshold testing (Fig. 6.12.5). • • Fig. 6.12.5. Bracketting in FASTPAC versus full threshold. Half of the points start at 1 dB brighter and the other half start at 2 dB dimmer than the predicted threshold values. After a single crossing, the threshold is estimated. Thresholds of locations that differ from expected values by 4 dB or more are then re-estimated. Studies have shown that FASTPAC, underestimates mean deviation and pattern standard deviation when compared with the standard algorithm. 3. Swedish interactive thresholding algorithm (SITA). SITA strategy uses continuous estimation of threshold values and measurement errors throughout the test, using advanced visual field models and mathematical analyses performed in real-time. Models are initially based on prior visual fields. Staircase procedures are used to alter stimulus intensities at predetermined test point locations. Test time is further reduced by eliminating catch trials to determine the frequencies of false-positive answers and by using a more effective timing algorithm. The SITA Standard and SITA Fast strategies are available as software implemented on the Humphrey Field Analyzer II. Note. Threshold testing is preferred over screening strategies for patients in whom a defect is likely to be present (e.g. those with optic neuritis accompanied by reduced acuity, glaucoma patients with pathologic-appearing optic discs). A. • • • • B. • • • • C. • • • • D. Testing Patterns Basic testing patterns are chosen depending on the area of field to be tested and the density and distribution of points within. The options for areas to be tested include the central 30°, the peripheral 30° to 60° area, full 60°, central 10°, and central 5°. Special test configurations are also available, including glaucoma patterns (central 30° plus peripheral nasal step area and temporal periphery), peripheral nasal step area, temporal crescent region, and neurologic patterns for examining the vertical meridian. Standard test programmes used with satic threshold strategy on the Humphrey Field Analyzer can be grouped as below: Central field tests Central 30-2 test Central 24-2 test Central 10-2 test, and Macular test. Peripheral field tests Peripheral 30/60-1 Peripheral 30/60-2, Nasal step, and Temporal crescent Speciality tests Neurological-20, Neurological-50, Central 10-12, and Macular test Custom tests EVALUATION OF HFA SINGLE PRINTOUT Tire standard HFA printout is obtained using a software called Statpac printout. For the purpose of evaluation, the Humphrey single field printout (Statpac printout) with central 30-2 test can be studied in eight parts I to VIII as described below (Fig. 6.12.6). I. Patient data and test parameters: general information At the top of printout page (part I) are printed: Patient's data [name, date of birth, eye (right/left), pupil size and visual acuity] Test parameters (test name, strategy, stimulus used, background). For interpreting any field, first look at the general information for the folloiving reasons: The data provided is about both the testing conditions and demographics. It helps to identify the patient. It is important to look at the test pattern and test strategy being used as tests using different test strategies may not be comparable. Correct name and date of birth of the patient not only identify the patient but are absolutely essential to enable comparison with normative data for the present field and also with future fields to detect change. The date and time of testing is given on top right. It is advisable to record the date in DD/MM/YYYY format as it is most familiar format in our country. Eye being tested should be confirmed as a wrongly entered eye would not allow fixation monitor tests and a wrong normogram for the eye would identify blind spot as scotoma. Pupil size is automatically detected in newer models. Fields may be carried out on a normal or a dilated pupil but not while it is dilating. Visual acuity of the patient and trial lens selection are given. Not using proper trial lens in cases obviously requiring the same may cause generalized depression of fields. Stimulus size, colour and background brightness should be noted as an improper stimulus size will misrepresent fields and certain STATPAC features may not be available for nonstandard parameters. • • A larger stimulus size may be rquired in patients with poor vision or coloured stimuli may be used for certain retinal or optic nerve conditions. Test duration, if prolonged, indirectly indicates that the patient was taking too long as points were thresholded repeatedly which could be due to unreliable responses, fatigue or increased variability in defective fields. A SIT A Standard test typically takes around 7 minutes. Foveal threshold. A look at the foveal threshold just under the test duration can yield useful information. A good foveal threshold but a poor visual acuity may indicate improper refraction while vice versa may indicate early foveal damage. • II. Reliability Indices Patient reliability may be appraised by several parameters. These include the fixation losses, false-positive and -negative responses, and degree of response variability. During the examination, rest periods as well as additional instructions may be needed to ensure optimal reliability. In addition, if machine-determined parameters of reliability suggest that the patient performed poorly but fixation was observed to be of good quality, the field may be of more value than would have otherwise been determined. 1. Fixation losses Methods to monitor fixation i. Blind-spot monitoring technique. It is done by repeatedly representing the stimulus to blind spot and noting the response. Fixation losses are recorded as a fraction, with the number of losses of fixation in the numerator and the number of blind-spot presentations in the denominator. It is also known as Heijl-Krakau method. ii. Sensor monitoring. If eye movement sensors are used, an absolute number of fixation losses is recorded. iii. Manual monitoring of fixation by the examiner. iv. Gaze tracker. It is printed at the bottom of the test result (Fig. 6.12.7) Fig. 6.12.7. Gaze tracking. Upward deviation going fully up indicates 10 degrees gaze error (fixation loss). • • Downward deviation indicates blinks (wherein the tracker could not measure fixation). An even smooth line, with minimal excursions implies good fixation. Reliability status ▪ Fixation losses when exceed 20% the test results are considered unreliable. If this arbitrary cutoff point be increased to 33%, which would decrease the number of fields deemed unreliable without significantly affecting the sensitivity or specificity of the test. ▪ Pseudofixation losses may be seen from high refractive errors due to lens-induced shift in blind spot location. 2. False-positive and false-negative responses ▪ False-positive response is recorded when a patient signals that a stimulus has been seen when one has not been presented, usually in response to an audible click rather than a visual stimulus. It gives an unusually white out graph (white scotoma). ▪ False-negative response is registered when a patient fails to respond to the presentation of a stimulus at a given location that is significantly brighter than the previously determined threshold at that point (supra threshold). It gives a clover leaf-like pattern of graph. Reliability status Examinations in which false responses of greater than 33% are registered have been considered unreliable. A higher rate of falsenegative responses has been shown to occur in patients with glaucomatous field loss compared with normal. Cut off values of reliability indices The visual field examination is considered unreliable, if three or more of the following reliability indices have below mentioned values: • • • • • Fixation losses ≥ 20%, False-positive error ≥ 33%, False-negative error ≥ 33%, Short-term fluctuation ≥ 4.0 dB, Total questions ≥ 400. III. Grayscale Grayscale simulation of the test data is depicted in part III of the printout (Fig. 6.12.6). Grayscale printout is generated by delegating different symbols to actual decibel values of threshold sensitivities, with a specific symbol representing a particular range of stimulus intensities. Darker areas indicate a lower differential light sensitivity and lighter areas indicate zones of higher sensitivity. The resultant printout assigns a retinal sensitivity to every area of field tested, but in reality only a limited number of points were actually thresholded. To allow for continuity, values of presumed retinal sensitivity are interposed to unassessed area of the field. It gives only a gross assessment of the field. The darker the printout, the worse is the field. The grayscale provides the field defects at a glance. However, in general we do not make a diagnosis based on the grayscale. The main emphasis on statistical help shows in part IV to VIII of the printout (threshold values). IV. Total deviation plots Total deviation plots provide the deviation of patient's threshold values from that of age corrected normal data. The two total deviation plots are numeric value plot and the probability plot (grayscale symbol plot). ▪ Numeric value plot. Simple threshold sensitivities are measured in decibels and given in the numeric plot. Values above 40 dB are generally not expected and may be indicative of a trigger happy field. The actual numeric values may not be concentrated upon for day-today interpretation of field but only with specific purposes. In other words, numeric value plot represents the differences in decibels. SA zero value means that the patient has the expected threshold for that age. Positive numbers reflect points that are more sensitive than average for that age; whereas negative numbers reflect points that are depressed compared with the average. ▪ Probability plot (grayscale symbol plot). In the lower part of part IV of the printout, the total deviation plot is represented graphically. The darker the graphic representation the more significant it is. Note, hr general, the total deviation plot is an indicator of the general depression and is not capable of revealing the hidden scotomas that may be present in the overall depressed field. V. Pattern deviation plots The two pattern deviation plots (numeric pattern deviation plot and probability pattern deviation plot) shown in part V of the printout are similar to total deviation plots except that here Statpac software has corrected the results for the changes caused by cataract, small pupil, etc. VI. Global indices Global indices are depicted in the part VI of the printout. Global indices refer to some calculations made by Statpac software to provide overall guidelines to help the practitioner to assess the field results as a whole rather than on point-to-point basis as shown in the total deviation and pattern deviation plots. Below mentioned four global indices are provided with the full threshold programme which summarize the status of visual fields at a glance. Principally, the global indices are used to monitor progression of glaucomatous damage rather than for initial diagnosis. 1. Total deviation or me an deviation. The mean sensitivity is an average value of threshold sensitivity for all points tested. The mean deviation is the difference between mean sensitivity obtained and that expected (Fig. 6.12.8). Mean deviation should be in the range of zero in normals. It is more affected by generalized decreases in sensitivities rather than by small, localized defects but is increased in the presence of any defect. Mean sensitivity may also be decreased by media opacities, significant pupil constriction, blur, and unreliability. Fig. 6.12.8. Total/mean deviation: Relative visual field sensitivity = Normal field sensitivity—Measured field sensitivity. 2. Pattern standard deviation. Pattern standard deviation reflects the regional non-uniformity of a visual field, or frequency of deviation of sensitivity values after adjusting for the mean defect of the entire field (Fig. 6.12.9). It is a measure of variability within the visual field, i.e. it measures the difference between a given point and adjacent points. A high value indicates an irregularity to the expected normal hill of vision, suggesting localized defects. Therefore, a normal value is in the range of zero. A low value is also found in a field with a diffuse but regular decrease in threshold sensitivities. It actually points out towards localized field loss and is most useful in identifying early defects. It loses its advantage in marked depression. Fig. 6.12.9. Pattern deviation: Individual visual field sensitivity. 3. Short-term fluctuation (SF). It is a measure of the variability between two different evaluations of the same 10 points in the field. It is not available with SIT A strategy. A high SF means either decreased reliability or an early finding indicative of glaucoma. 4. Corrected pattern standard deviation (CPSD). It is the PSD corrected for SF. It indicates the variability between adjacent points that may be due to disease rather than due to intra-test variability. VII. Glaucoma Hemifield test (GHT) Glaucoma Hemifield test compares 24-2 visual fields into 10 regions, with 5 inferior regions representing mirror images of 5 corresponding superior regions (Fig. 6.12.10). These clusters of points have been developed based on the anatomical distribution of the nerve fibres and are specific to the detection of glaucoma. Differences between corresponding superior and inferior zones are compared with the differences present in the population of normal controls. Possible test outcomes are: Fig. 6.12.10. Glaucoma Hemifield test. ▪ Outside normal limits. The GHT outside normal limits denotes that either the values between upper and lower clusters differ to an extent found in less than 1% of the population or any one pair of clusters is depressed to an extent that would be expected in less than 0.5% of the population, hr other words, the GHT is described as "outside normal limits" when differences between a matched pair of corresponding zones exceeds the difference found in 99% of the normal population, or when both members of a pair of zones are more abnormal than 99.5% of the individuals with the normative population. ▪ Borderline. The GHT is considered borderline when the difference between any one of the upper and lower mirror clusters is what might be expected in less than 3% of the population. In other words, the GHT is described as borderline when matched pairs of zones are abnormal at the 97th percentile within the normative database. ▪ General reduction of sensitivity. VFs are described to have generalized reduction of sensitivity when both conditions for "outside normal limits" are not met, and the best region of the VF is depressed to a level at the 99.5th percentile within individuals of the normative database, hr other words, GHT is considered to have generalized reduction sensitivity, if the best part of field is depressed to an extent expected in less than 0.5% of the population. ▪ Abnormally high sensitivity. The GHT is described as having abnormally high sensitivity when the overall sensitivity in the affected region of the VF is better than 99.5% of individuals within the normative population, hr other words, abnormally high sensitivity is labelled when the best part of the visual field is such as would be expected in less than 0.5% of the population. ▪ Within normal limits. VFs are described as being within normal limits when none of the above conditions are met. VIII. Actual threshold values Actual threshold values are shown in part VIII of the printout may be inspected for airy pattern or scotoma when clinical features are suspeciant and even if all the seven other parts of the printout are normal. ▪ Scotoma by definition is the depressed part of the field as compared to the surrounding and not as compared to normal. ▪ Sensitivity of the test is lost when the actual test threshold values are below 15 dB.
what is 'pseudoglioma.' ?
Various condilions other than retinoblastoma, which present as leucocoria are collectively called as 'pseudoglioma.'
S/N on lntravitreal anti-VEGF drugs given in DR ?
Vascular endothelial growth factor (VEGF) plays a pivotal role in the etiopathogenesis of diabetic maculopathy and retinopathy. Anti-VEGFs, e.g. Bevacizumab ( 1.25 mg) and Ranibizumab (0.5 mg) when given intravitreally in 0.1 mL vehicle lead to improvement in vision in >40% cases and stabilize vision in another >40% cases. These drugs should be preferred over laser therapy particularly in patients with: • Focal CME involving centre of fovea, • Diffuse DME • DME with neurosensory detachme Anti-VEGFs are also recommended before panretinal photocoagulation (PRP) in patients with POR and diffuse DME. Note. Effects of the anti-VEGFs last for 4-6 weeks and frequent injections are warranted. Ln addition, the cost factor and risk of endophthalmitis are the deterrent of anti-VEGF therapy.
what is Vascular loop ?
Vascular loop or a thread of obliterated vessel may sometimes be seen running forward into the vitreous. It may even be reaching up to the back of the lens.
Choroidal rupture ? tear occures in which structer ? m/v a/w ? complication ?
When the eye is compressed along its anterior-posterior axis, the eyewall becomes stretched in its horizontal axis due to hydraulic displacement of the vitreous. Tear= 1. Bruch's membrane, which has little elasticity, may tear, along with the overlying 2. retinal pigment epithelium (RPE) and 3. underlying choriocapillaris. Associated adjacent subretinal hemorrhage is common. Choroidal ruptures may be single or multiple, commonly in the periphery, and may be concentric to the optic disc. Ruptures that extend through the central macular area may cause permanent visual loss. There is no immediate treatment. Occasionally, choroidal neovascularization (CNV) develops as a late complication in response to the damage to Bruch's membrane. A patient with choroidal rupture near the macula should be alerted to the risk of CNV and advised to use an Amsler grid for self-testing. If subfoveal CNV is present, it is generally treated with an anti-VEGF agent, although photodynamic therapy can be used in selected patients. Thermal laser photocoagulation is rarely employed for nonsubfoveal lesions. Subfoveal surgery for CNV in patients with choroidal rupture complicated by CNV is less commonly done given the effectiveness of anti-VEGF agents.
C/I OZURDEX ?
_______CONTRAINDICATIONS_______________ • Ocular or periocular infections (4.1) • Glaucoma (4.2) • Torn or ruptured posterior lens capsule (4.3) • Hypersensitivity (4.4)
what are hard exudates ? layer ?
are lipid deposits in the outer pleidform layer of retina which occur following leaky capillaries in severe hypertensive retinopathy. They appear as yellowish waxy spots with sharp margins. They are generally seen in posterior pole and maybe arranged as macular-fan or macular-star. They are also temporary and may disappear in 3-6 weeks. opl layer.
ORIGIN OF RETINAL ASTROCYTOMA ?
astrocytes within the retinal nerve fibre layer.
another name for retinal astrocytoma ?
astrocytic hamartoma
in relation to o.disc,foveal placed below by how many dimeter ?
l mm below the horizontal meridian
what are types according to Clinico-angiographic classification of diabetic maculopathy ?
l. Focal ex11datiue maculopatlry 2. Diffuse exudatiue maculopathy. 3. Ischemic maculopatlty. 4. Mixed maculopathy.
what are main pathogenic factors for hypertensive retionopathy ?
l. Vasospasm. 2. Arteriosclerotic changes 3. Increased vascular pemieability 4. Raised intracra11ial pressure
PATHOGENESIS OF HYPERTENSIVE RETINOPATHY ?
l. Vasospasm= - Arteriolar narrowing due to vasospasm is the primary response to raised blood pressure and is related to the severity of hypertension (acute hypertensions).• Vasospasm of retinal arterioles occurs in pure form in young individuals, but is affected by the preexisting involutional sclerosis in older patients. - Vasospasm of choroidal uessels causes choroidal and RPE ischaemia, which manifests as hypertensive choroidopathy. - Vasospasm of peripapillary choroid leads to optic nerve head ischaemia, manifesting as hypertensive optic neuropathy. 2. Arteriosclerotic changes= which manifest as changes in the arteriolar reflex and A-V nipping result from thickening of the vessel wall and are a reflection of the duration of hypertension (chronic hypertension). In older patients, arteriosclerotic changes may preexist due to involutional sclerosis. 3. Increased vascular pemieability= results from hypoxia causing breakdown of inner blood re tinal barrier and occurs in severs hypertension and is responsible for haemorrhages, exudates, focal retinal oedema, macular oedema, focal intraretinal periarterial transudates (FlPTs), and disc oedema. 4. Raised intracranial pressure= occurs in malignant or accelerated hypertension and manifests as hypertensive optic neuropathy, characterized by papiJloedema and optic nerve ischaemia.
WHAT IS COLOBOMA OF THE OPTIC DISC ?
lt results from the failure in closure of the embryonic fissure. It occurs in two forms. Minor defect is more common and manifests as inferior crescent, usually in association with hypermetropic or astigmatic refractive error. Fully developed coloboma typically presents inferonasally as a very large whitish excavation, which apparently looks as the optic disc. The actual optic disc is seen as a linear horizontal pinkish band confined to a small superior wedge. Defective vision and superior visual field defect is usually associated.
DECREASE VISION IN CAPILLARY HAEMANGIOMA DUETO ?
macular exudates
importance of OCT-based classifica tion of diabetic macular edema ?
management diifers according to type . The OCT classification has a bearing on the management since non-tractional DME is treated conservalively whereas tractional DME is purely treated by pars-plana vitrectomy (PPV} with removal of posterior hyaloid.
what is Reese-Ellsworth classification ?
prognostic significance for the control of local disease has become irrelevant with the availability of newer chemotherapeutic agents and modes of therapy.
PORN IS CAUSED BY ?
progressive outer retinal necrosis caused by an aggressive variant of varicella zoster virus,
Endophytic reti11oblnstoma differentiated from ?
retinal tumours in tuberous sclerosis and neurofibromatosis, astrocytorna and a patch of exudative choroiditis.
what is primary response of arteriole to raised BP ? how hypertensive choroidopathy, hypertensive optic neuropathy occures ?
vasospasm >>> arterio narrowing >>> 1. choroidal and RPE ischaemia >>> hypertensive choroidopathy. 2. Vasospasm of peripapillary choroid >>> hypertensive optic neuropathy.
optic disc is vericaly oval or horizontaly oval ?
verticaly oval
S/N ON VIRAL RETINITIS ?
zoster retinitis, progressive outer retinal necrosis (PORN)caused by an aggressive variant of varicella zoster virus, and acute retinal necrosis (ARN) caused by herpes simplex virus 2 (in patients under the age of 15 years) and by varicella zoster virus and herpes simplex virus-1 (in older individuals) have become more conspicuous in patients with AIDS (HIV infection).
what are the Effects of microangiopathy producing DR ?
• Breakdown of blood-retinal barrier leads to retinal oedema, haemorrhages, and leakage oflipids (hard exudates). • Weakened capillary wall produces microaneurysms, and haemorrhages. • M icrovascularocclusions produce ischaemia and its effects, cotton-wool spots (due to infarct of nerve fibre layer) and arteriovenous shunts, i.e. intraretinal microvascular abnormalities (IRMAs).
anti-VEGF drugs preferred over laser therapy particularly in patients with ?
• Focal CME involving centre of fovea, • Diffuse DME • DME with neurosensory detachment • Anti-VEGFs are also recommended before panretinal photocoagulation (PRP) in patients with POR and diffuse DME.
S/N on Macular photocoagulation ?
• Focal photocoagulation (Fig. 12.15A). It is the treatment of choice for focal DME not involving the centre of fovea. • Grid photocoagulation (Fig. 12. l 5B). It is no more the treatment of choice for diffuse DME. lt may be considered only for recalcitrant cases not responding to anti-VEGFs and intravitreal steroids. Note. Macular photocoagulation is contraindicated in ischemic maculopathy and tractional DME.
Acute hypertensive retinopathy Fundus changes include ?
• Marked arteriolar narrowing due to spasm of the arteriolar wall, in response to sudden rise in blood pressu re. • Superficial retinal haemorrhages, flame shaped, appear in the posterior pole. • Focal intiraretinal periarteriolar transudates (FlPTs) are small, white, focal oval lesions occurring due to the deposition of macromolecules along the major arterioles. These result due to break down of bloodl-retinal barrier following dilatation of terminall arterioles as a result of sudden rise in blood pressure in malignanr hypertension. • Cotton wool spots are also more marke d in malign ant hypertensive retinopathy. • Microaneurysms, shunt vessels and collaterals may also develop as a result of capillary obliterations.
Vascular and haematological changes seen in diabetes mellitus are ?
• Thickening of capillary basement membrane • Capillary endothelial cell damage • RBCs: deformation and rouleaux formation • Increased stickiness of platelets • Increased plasma viscosity Loss of capillary pericytes