Refraction and Binocular Vision
Write short notes on the pinhole test
Uses a small opaque disk with a small 1-2mm hole in the centre It makes the image clearer by reducing the blur circle size also some reduction in spherical aberration Can be used at any stage it is useful as a check if whether a patients visual acuity can be improved
Define with the rule astigmatism
The highest refractive power is in the vertical meridian and can be corrected using a minus cylinder with its axis horizontal or with a plus cylinder with its axis vertical.
Define best vision sphere
This is when you use subjective tests to determine the spherical component of the refractive error. The aim is to find the most plus or least minus lens that is consistent with the patients best vision/visual acuity This lens will correct spherical ametropia completely or will place the circle of least confusion on the retina jn a patient with astigmatism.
Explain why the accurate measurement of interpupilary distance is an important aspect of a routine eye exam
1. Ensures patients glasses are centred correctly and that they are looking through the centre of the spectacles. 2. Allows trial frame to be correctly positioned as it avoids unwanted prismatic effect 3. Creates a record of facial growth over the years for children 4. May allow detection and monitoring of a pathological structural change
What is motor fusion State the normal values for a prism fusion range
Ability to maintain sensory fusion through a range of Vergence movement which may be horizontal, vertical or cyclovergence. Normal values Near. Blur. Break. Recovery. -ve (base in). 16 14. 12 +ve (base out). 35. 20 Distance -ve (base in). 12. 8 +ve (base out). 10. 18. 16
Define horopter and explain the physiological basis of the horopter in our Binocular Vision system
An imaginary structure/plane existing in Binocular Vision Represents the locus of all points in space whose images stimulate corresponding retinal points Any object placed on the horopter will have a constant image size on the retina no matter where it is located on the horopter. If image size is constant and the Binocular disparity remains zero then any object that is moved along the horopter should always appear to be the same distance from observer.
List two pathological or surgical reasons for hypermetropia
Aphakia— absence of the crystalline lens from the optic system of the eye, often leads to hypermetropia of > 10.00Ds Central senous retinopathy— elevation of sensory retina which leads to an effective shortening of the axial length Cornea plana Microphthalmos
What is axial myopia and refractive myopia
Axial myopia is when the axial length is too short for the power of the eye The refractive myopia is when the power of the eye is too weak with respect to the axial length
What is meant by a crossed cylinder lens
Constructed from a combination of a plus and a minus cylinder with perpendicular axes There are no standards for the colour of markings so always be careful to check with lens is which. The handle of the lens sometimes has the astigmatic interval marked on it.
What are the recommendations from BS of a standard chart
Design— letters should be of 5x5 sans serif design and should be chosen from a selection of letters with the same legibility Size— the minimum range of letter size should be from logMAR -0.1 (6//4.8) to logMAR 1.0 (6/60) Spacing— no letter should be repeated on any one line and letters should be centred and evenly spaced being equal to the width of the letters on that line Lines of letters should be separated by a gap not less than the height of the letters in the smaller line or 20mm.
Explain Helmholtz theory of decreased tension
During distance vision, the ciliary muscle is relaxed and the zonular fibres that cross the circumcentral space between the ciliary body and the lens equator are at a resting tension. With accommodative effort, contraction of the ciliary muscle relaxes the zonule allowing the lens to adopt its accommodated shape where the equetorial diameter is reduced and the anterior surfaces bulges forward. Helmoltz used an opthalmometer to observe and quantify changes in the crystalline lens during accommodation. He concluded that as the volume of the lens cannot change the diameter must be reduced.
Define principal visual direction
Each part of the retina has a given projection for a given distance of fixation. The fovea has p.v.d; independent of the direction of gaze
Explain what is meant by the term far point
Fat point is a point in space conjugate with the retina when the accommodation is fully relaxed.
Describe the neurological pathway which leads to the production of vergence eye movements
Infranuclear control 3 midbrain nucleai responsible for innervating the extraocular muscles: oculomotoror, trochlear and abducens nuclei. The occulomotor nucleaus innervates the medial rectus, inferior rectus, inferior oblique and the superior rectus. The superior oblique muscle is innervated by motor neurons in the trochlear nucleaus while the abducens nucleaus provides innervation to the lateral rectus. Supranuclear control: bilateral stimulation of the frontal and occipital eye fields have produced convergence eye movements leading to speculation that is common with saccadic eye movements. The frontal eye fields are involved in the production of voluntary vergence movement while responses produced by retinal disparity occur via the occipital cortex. 3 groups of cells: 1) encode vergence angle 2) encode vergence velocity 3) both vergence angle and velocity Found in the mesencephalic reticular formation, 2mm from the occulomotor nucleaus.
Classify esophoria and describe what would be seen on the cover test in each case.
Is a latent convergent squint It can be convergence excess (deviation is greater @near) It can be divergence weakness (deviation is greater @distance) It can be non specific where (deviation similar @near and @distance) In the cover test, when one eye is covered, the covered eye moves inwards towards the nose. On removal of the cover test it moves outwards to regain Binocular vision. If one eye has esophoria the other eye will too therefore will act the same when swapped. To dissociate the eyes fully and check the magnitude of the esophoria and the rate of recovery an alternate cover text can be performed.
Write notes on the distance duochrome test
It is used at various stages of the eye test and is based on the principle that red light (shorter wavelength) is refracted more than longer wavelength of green light Normally red and green filters are used and the dioptric difference between them is -0.5 ds In an emmetropic eye both the red and green filters will look equally clear In hypermetropic, over minused or under plussed eyes the targets on the green filter will appear clearer In myopic or under minuseed or over plussed eyes the red filter will appear clearer
Explain briefly the optics of a prism
Light is displaced towards the prism base Image is displaced toward the prism apex
Describe the normal procedure for fitting a trial frame
Measure the DISTANCE PD Adjust the trail frame to match PD loosen bridge height adjuster Adjust pantoscopic tilt angle to zero Place sides over patients ears Where possible shorten sides to place rear cells about 12mm from patients eyes Place bridge pad on patients nose Adjust bridge height so that lenses are centred vertically with patients eyes Lock bridge height adjuster
Write notes on the +1.00 blur test
Once the final sphere has been determined adding a plus 1 lense should reduce the VA on a logMAR chart by 3 or 4 lines In a snellen chart VA should be reduced to 6/9 or 6/12 if best vision sphere was 6/5 or 6/6 This test is useful for younger children to ensure you have not given them too much minus or too little power
Write short notes on the purpose and principles of the scheiner test
Opaque disc with two small holes around 1mm The Seperation of the holes is about 4mm smaller than the pupil diameter The disc is placed close to the eye, centred on the pupil and the patient views a distant spotlight For emmetropic eye 1 light is seen For hypermetropic eye 2 spots of light are seen but if the top hole is occluded the bottom light looks like it has disapeared For myopic eye 2 spots of light are seen but if the top hole is occluded it seems like the top light has disappeared
State the actions of the superior oblique muscle and the inferior rectus muscle and explain briefly where and why their main vertical action occurs.
Originated in upper,medial side of the orbit which abducts, depresses and internally rotates the eye.
Define fusion
The combining of images received by both eyes into one image. 2 types: sensory and motor. Sensory= ability to perceive 2 similar images, one on each retina, as one. Motor= ability to maintain sensory fusion through a range of vergence movements. Occurs when similar images are presented to non corresponding retinal areas and are situated close to the horopter. They will be fused with the appreciation of depth.
Explain emmetropisation
The components of the eye grow in a coordinated manner rather than independently The majority of infants are born with significantly refractive errors but these usually reduce and disappear as the eyes continue to grow and develop.
Explain what is meant by working distance in retinoscopy and explain how this can be compensated for
The end point of the Refraction should place the patients far point at or close to infinity However when the retinoscopy reflex is neutralised it is fine so at the practitioners arm length You correct this by either Subtracting the dioptric working distance from the rx found during retinoscopy to give final rx Or use the dioptric working distance lense and place it in the trial frame
How should the positive spheres be changed during Refraction
When altering plus lenses, place the new lense in the trial frame before removing the existing one
Define mixed astigmatism
When one focal line lies behind the retina and the other focal line lies in front of the retina.
Define normal retinal correspondence
When the Fovea have a common visual direction and retinal elements nasal to one fovea correspond with those temporal to the other fovea.
Define compound myopic astigmatism
When there is myopia in all meridians and both focal points lie in front of the retina.