Neuro 13: Brainstem II: Eye Movements and Pupillary Control
Supranuclear control of eye movements:
extend from the brainstem and cerebellum to the forebrain and exert their influence on the final common output nuclei of CN III, IV, and VI. generates horizontal, vertical, and vergence eye movements
What do each of the subnuclei of the oculomotor nucleus control?
most rostral (anteiror and dorsal to the others) : The Edinger-Westphal nuclei, containing preganglionic parasympathetic fibers, form a V shape as they curve over the dorsal aspect of the oculomotor nuclei and fuse anteriorly in the midline ventral nuclei control the medial rectus dorsal nuclei control the inferior rectus intermediate nuclei control the inferior oblique medial nuclei control the superior rectus central caudal nucleus controls the levator palpebrae superior ventral nuclei- medial rectus (so not what you would think)
Lesions of the cerberal hemispheres normally impair eye movements in what direction, creating what kind of gaze preference? If the corticospinal pathways are involved, what kind of weakness can be expected?
normally impair eye movements in the contralateral direction, often resulting in gaze preference toward the side of the lesion ("Right way eyes") This gaze preference is typically accompanied by weakness contralateral to the cortical lesion so that the eyes look away from the side of the weakness
What is aniscoria?
pupillary asymmetry
Aniscoria w/ oculomotor lesions; What light conditions make it more obvious?
"blown pupil" lesions of the efferent parasympathetic pathway from the Edinger-Westphal nucleus to the pupillary constrictor muscle can cause impaired pupillary constriction, resulting in a unilateral dilated pupil. The anisocoria is more obvious in ambient light than in a darkened room There is a decreased or absent direct response when light is shone in the affected eye, as well as a decreased or absent consensual response when light is shone in the unaffected eye. there may be ptosis and eye movement abnormalities as well
Benign aniscoria:
A slight pupillary asymmetry of less than 0.6 millimeters is seen in 20% of the general population. This asymmetry can vary from one examination to the next, sometimes within a few hours. There are no other associated abnormal findings, such as dilation lag, changes in the asymmetry with lighting conditions, or eye movement abnormalities
In young children, because the visual pathways are still developing, congenital eye muscle weakness can produce
strabismus (dysconjugate gaze) that over time causes suppression of one of the images, resulting in amblyopia (decreased vision in one eye) early intervention is essential
Cover-uncover test: How do you name mild latent weaknesses?
Another helpful test for subtle eye muscle weakness is the cover-uncover test. Visual input normally helps maintain the eyes yoked in the same direction. Therefore, when an eye is covered while looking in the direction of a weak muscle, it may drift slightly back toward the neutral position. This mild latent weakness present only with an eye covered is called a phoria (as in exophoria, esophoria, etc.), in contrast to a tropia.
2 important clinical points to be made about the subnuclei of the oculomotor nerve;
(1) Unilateral weakness of the levator palpebrae superior or unilateral pupillary dilation cannot arise from unilateral lesions of the oculomotor nucleus. (2) Lesions of the oculomotor nucleus affect the contralateral superior rectus To summarize: a nuclear lesion of the oculomotor nucleus does not cause unilateral ptosis, unilateral dilated unresponsive pupil, or unilateral superior rectus palsy.
Pupillary dilation pathway:
(mydriasis) A descending sympathetic pathway from several lateral hypothalamic nuclei travels in the lateral brainstem and cervical spinal cord to reach thoracic spinal cord levels T1 and T2. This pathway is thought to be in approximately the same location in the brainstem as the spinothalamic tract because lesions of the spinothalamic tract tend to be associated with Horner's syndrome This descending sympathetic pathway activates preganglionic sympathetic neurons in the IML column of the upper thoracic cord Axons of the preganglionic sympathetic neurons exit the spinal cord via ventral roots T1 and T2 and skirt the apex of the lung before joining the paravertebral sympathetic chain via white rami communicantes. The axons ascend to synapse in the superior cervical ganglion. From there, postganglionic sympathetic fibers ascend through the carotid plexus along the walls of the internal carotid artery to the cavernous sinus, ultimately reaching the pupillary dilator muscle first aid 557
Cavernous sinus syndrome vs orbital apex syndrome: In cavernous carotid aneurysms, which CN is affected first?
A complete lesion of the cavernous sinus disrupts CN III, IV, and VI, causing total ophthalmoplegia, usually accompanied by a fixed, dilated pupil. Involvement of CN V1 and variable involvement of V2 causes sensory loss in these divisions of the trigeminal nerve. Horner's syndrome can also occur because of disruption of ocular sympathetics causes include: metastatic tumors, direct extension of nasopharyngeal tumors, meningiomas, pituitary tumors or pituitary apoplexy, anerysms. of the intracavernous carotid, cavernous carotid arteriovenous fistula, bacterial infection causing cavernous sinus thrombosis, aseptic thrombosis, idiopathic granulomatous disease (Tolosa-Hunt syndrome), and fungal infections such as aspergillosis or mucormycosis In cavernous carotid aneurysms or fistulas, the abducens nerve is often involved first because it lies closest to the carotid artery Orbital apex lesions produce the same deficits as cavernous sinus syndrome, but they are more likely to involve CN II also, causing visual loss, and often are associated with proptosis, or bulging of the eye, due to mass effect in the orbit. -> CN V2 is spared in orbital apex syndrome since it exits the cranium via the foramen rotundum Orbital apex syndrome can be caused by metastatic tumors, orbital cellulitis (bacterial infection), idiopathic granulomatous disease (orbital myositis or pseudotumor), and fungal infections such as aspergillosis. Since the cavernous sinus and orbital apex are contiguous, both structures can be affected by a single lesion. Impaired venous drainage in both disorders can cause vascular engorgement of the orbital structures. both = medical emergencies ; When MRI scans with contrast enhancement and lumbar puncture do not reveal a specific diagnosis and symptoms are progressive, emergency orbitotomy and biopsy are warranted In summary: cavernous sinus = III, IV, VI, V1, and variable V2 orbital apex = II, III, IV, VI, V1 but not V2
Red glass test:
A transparent piece of red glass is held over one eye, usually the right, and a small white light is held directly in front of the patient. The image seen by the right eye is therefore red, and the image seen by the left eye is white. The patient is then asked to follow the light as it is moved to nine different positions of gaze, and to report the locations of the white and red images. Normally the white and red images are fused in all positions of gaze image shows red glass test w/ oculomotor palsy
What nuclei constitute the somatic motor column?
CN III, CN IV, CN VI, and CNXII nuclei -> these nuclei all lie near the midline, adjacent to the ventricular system; for most, their fibers exit the brainstem near the midline, but w/ CN IV, the fibers exit dorsally
Case 9: A 23-year-old aerospace engineer developed mild headaches and difficulty looking up over the course of 3 weeks, so he went to see his family physician. On examination, his pupils were about 6 mm in diameter bilaterally and had minimal reaction to light, but they did constrict during accommodation. He was unable to look upward at all past the horizontal plane, but he had otherwise full eye movements in other directions. When attempting to look upward, or after closing and opening his eyes, he had retraction of the upper eyelids and convergence-retraction nystagmus of both eyes. Examination was otherwise normal. On the basis of the symptoms and signs shown in bold above, what syndrome does this patient have affecting his eye movements, and what is the usual localization of this syndrome? What is the most likely diagnosis?
Classic Parinaud's syndrome. usually caused by compression of or lesions in the dorsal midbrain and pretectal area Impaired upgaze probably caused by dysfunction of the upgaze portion of the vertical gaze center located in the dorsal part of the rostral midbrain reticular formation. Light-near dissociation probably occurs because fibers from the optic tract reaching the Edinger-Westphal nuclei travel via dorsal pathways including the posterior commissure and have been disrupted, while fibers descending from the visual cortex are relatively spared. Convergence and eyelid abnormalities w/ Parinaud's syndrome also localize to the rostral midbrain-pretectal region. Given the presence of a gradually progressive Parinaud's syndrome, the most likely diagnosis = pineal region tumor that has enlarged to compress the dorsal midbrain. In young children before the cranial sutures close, hydrocephalus can present w/ Parinaud's syndrome as the main abnormality. In an adult, if the hydrocephalus were severe enough to cause these findings, then intracranial pressure would likely be severely elevated as well, causing impaired consciousness. Therefore, the most likely diagnosis is a primary neoplasm of the pineal region.
Case 2: A 54-year-old man with a history of diabetes awoke one morning with horizontal diplopia that increased on gaze to the left and decreased on gaze to the right. He initially had some pain in the left periorbital area, which resolved after a few days. Exam was normal except for incomplete abduction of the left eye. He was able to move the left eye slightly past the midline toward the left; however, he was unable to fully "bury the sclera," as he could with the right eye when looking to the right. He had horizontal diplopia with no vertical component, which was worse on left gaze. On the basis of the symptoms and signs shown in bold above, where is the lesion? What is the most likely diagnosis?
Dysfunction of L lateral rectus muscle causing dysconjugate horizontal gaze and diplopia. Possible causes = dysfunction of L abducens nerve CN VI or lateral rectus muscle, or a mechanical problem in the orbit Given the pt's hx of DM, most likely dx = isolated abducens nerve palsy caused by microvascular disease.
Case 3: A 74-year-old man awoke one morning with vertical diplopia. The diplopia was relieved by covering of either eye and did not vary in severity at different times of the day. He had no history of head trauma. Past medical history was notable only for hypertension. Exam was normal except for a right hypertropia and incomplete downgaze with the right eye when looking medially. He was tested with a red glass over the right eye and had vertical diplopia, with the image from the right eye located below the image from the left eye. The diplopia worsened with downward and leftward gaze and improved with leftward head tilt On the basis of the symptoms and signs shown in bold above, where is the lesion? What is the most likely diagnosis?
Findings are compatible w/ R trochlear palsy. A disorder of the superior oblique muscle itself is possible the most likely cause of the isolated trochlear palsy in this pt is an idiopathic neuropathy of presumed microvascular origin.
Control of eye movements by the forebrain: The descending cortical pathways can travel directly to the brainstem or via relays in what midbrain region? - Frontal eye fields generate what kinds of movements, in what direction, via connections to what structure? - parieto-occipito-temporal cortex generate what kind of movement, in what direction, via connections to what structures? Cortical descending control of eye movements is heavily influenced by visual inputs arriving at -
Great image Descending cortical pathways travel either directly to the brainstem centers for horizontal, vertical, or convergence eye movements or via relays in the midbrain superior colliculi. The best-known cortical area that controls eye movements consists of the frontal eye fields. The frontal eye fields generate saccades in the contralateral direction via connections to the contralateral PPRF (paramedian pontine reticular formation, the second lateral gaze center) The parieto-occipito-temporal cortex regions are primarily responsible for smooth pursuit movements in the ipsilateral direction via connections with the vestibular nuclei, cerebellum, and PPRF; may make some contribution to contralateral eye movements as well Cortical descending control of eye movements is heavily influenced by visual inputs arriving at the primary visual cortex and visual association cortex
Pupillary constriction pathway:
Light entering one eye activates retinal ganglion cells, which project to both optic tracts because of fibers crossing over in the optic chiasm. (CNII) Fibers in the extrageniculate pathway continue in the brachium of the superior colliculus past the lateral geniculate nucleus to reach the pretectal area just rostral to the midbrain. After synapsing, axons then continue bilaterally to the Edinger-Westphal nuclei, which contain preganglionic parasympathetic neurons. Some of the crossing fibers travel in the posterior commissure The Edinger-Westphal nuclei lie just dorsal and anterior to the oculomotor (CN III) nuclei near the midline. Preganglionic parasympathetic fibers travel bilaterally from the Edinger-Westphal nuclei via the oculomotor nerves to reach the ciliary ganglia in the orbit From there, postganglionic parasympathetics continue to the pupillary constrictor muscles to cause the pupils to become smaller. Note that a light shone in one eye causes a direct response in the same eye and a consensual response in the other eye because information crosses bilaterally at multiple levels. (hence, pupillary light reflex direct & consensual)
Case 6: A 17-year-old male got into an argument with his sister while intoxicated, and she shot him in the neck with a steel pellet gun. He was brought to the emergency room and treated for a left pneumothorax (collapsed lung). The emergency room physician noticed unequal pupils and called a neurology consult. On exam, the patient had an entry wound at the base of his neck just above the left clavicle. There was no neck swelling. The right pupil was 4 mm, constricting to 3 mm in response to light, and the left pupil was 2 mm, constricting to 1.5 mm. The left eyelid had 3 mm of ptosis compared to the right. The left forehead felt smoother than the right, suggesting decreased sweat production. When the right neck was pinched, the right pupil dilated (ciliospinal reflex). Pinching the left neck caused no change in the left pupil. The remainder of the exam was normal. On the basis of the symptoms and signs shown in bold above, where is the lesion? What is the most likely diagnosis?
Horner's syndrome can be caused by lesion anywhere in the sympathetic pathway to the eye; given hx of penetrating neck wound, lesion is probably in the sympathetic chain or sympathetics of the carotid plexus the entry wound was low in the neck, making direct injury to the sympathetics in the carotid plexus unlikely. In addition, impaired sweating is more common w/ preganglionic lesions. The carotid artery may have been injured, causing a carotid dissection extending superiorly, resulting in Horner's syndrome. A lesion of the upper thoracic nerve root or spinal cord is unlikely given the absence of any other neurologic findings. Possible causes of Horner's syndrome in this pattern include direct traumatic injury to the sympathetic chain in the neck or carotid dissection. The most clinical localization = L sympathetic chain in the vicinity of the lower neck or lung apex, or L carotid plexus
Distinguishing lesions of the abducens nerve from lesions of the abducens nucleus/PPRF:
Lesions of the abducens nerve cause impaired abduction of the ipsilateral eye Lesions of the abducens nerve should be distinguished from lesions of the abducens nucleus, which produce an ipsilateral lateral gaze palsy involving both eyes because of the connections through the MLF -> Similarly, lesions of the PPRF cause an ipsilateral lateral gaze palsy (the input into horizontal gaze center is lost)
Pharmacological miosis and mydriasis: What drug classes cause small pupil size? dilated pupils? What pharmacological testing is useful in establishing diagnosis in cases of aniscoria? What specific pharmacological test helps r/o exposure to anticholinergic agents?
Numerous pharmacological agents can affect pupillary size and can cause confusion in diagnosis, particularly in the comatose patient. Opiates cause bilateral pinpoint pupils, and barbiturate overdose can also cause bilateral small pupils, mimicking pontine lesions Anticholinergic agents affecting muscarinic receptors, such as scopolamine or atropine, can cause dilated pupils. Pupillary dilation may be unilateral if topical exposure occurs in one eye only, mimicking uncal herniation. Pharmacological testing using eyedrops containing cocaine, hydroxyamphetamine, or pilocarpine (cholinergic agonist) can be useful for establishing the diagnosis in cases of anisocoria with equivocal or subtle findings -> When exposure to anticholinergic agents is suspected, 1% pilocarpine eyedrops can be useful because they cause pupillary constriction in parasympathetic lesions but cannot overcome pharmacological muscarinic blockade
Case 4: A 27-year-old man with no previous medical problems came to the emergency room because of 1 week of worsening left-sided headaches, left eye pain, and horizontal diplopia. He awoke 1 week prior to presentation with a severe left frontal headache. Two days later the headache had moved to his left eye, and he began noticing horizontal diplopia on rightward gaze. He went to his primary medical doctor and had an MRI scan, which was reportedly normal. Because of continued symptoms, he finally decided to come to the emergency room for further evaluation, where he was seen by a neurology consult resident. Examination was normal except for mild erythema of the left orbital conjunctiva and slightly decreased adduction of the left eye on right lateral gaze. He had horizontal diplopia and left eye pain on rightward gaze, and the rightmost image vanished when the left eye was covered. There was also minimal horizontal diplopia on leftward gaze, with the leftmost image vanishing when the left eye was covered. On the basis of the patient's symptoms and his findings on neurologic exam, what is the most likely location for a lesion in this patient causing horizontal diplopia? Given the pain and erythema of the left orbital conjunctiva, what are some possibilities for the diagnosis?
On right gaze: L eye pain, limited adduction, horizontal diplopia w R image vanishing when L eye was covered On L gaze: mild horizontal diplopia, w/ L image vanishing when L eye was covered Pain and erythema on L orbital conjunctiva The examination reveals bilateral limitations of horizontal eye movement of the L eye w/ more difficulty looking to the right. Relatively isolated dysfunction of both the medial and lateral rectus muscles would be difficult to explain on the basis of nerve or CNS lesions. One possibility would be a lesion that restricts movement of the L lateral rectus muscle, limiting its ability to stretch on R lateral gaze and decreasing its ability to contract on L lateral gaze. The differential diagnosis includes orbital trauma (none based on hx), thyroid disease (pain and subacute onset don't fit), myasthenia gravis (does not cause pain); or more likely, especially given pain and erythema, an infectious, inflammatory, or neoplastic disorder such as orbital cellulitis, orbital lymphoma, orbital myositis (orbital psuedotumor), sarcoidosis, Tolosa-Hunt syndrome, fungal infection, or cavernous sinus thrombosis in our pt: MRI revealed marked enhancement and thickening of L lateral rectus muscle, compatible w/ orbital myositis (pseudotumor)- relatively uncommon inflammatory condition of the extraocular muscles tx w/ oral steroids
Case 7: A 71-year-old man with a history of diabetes collapsed on the street and was unable to stand up, so he was brought to the emergency room by ambulance. On examination, he was awake but lethargic, had a rightward gaze preference, and was unable to move either eye past the midline toward the left. In addition, he had weakness of the right lower face and 2/5 strength in the right arm and leg, with an upgoing plantar response on the right. Examination was otherwise unremarkable. Lesions in which locations can cause the constellation of symptoms and signs shown in bold above? Given the patient's age and the time course of his presentation, what is the most likely diagnosis?
Pt has a combo of L horizontal gaze palsy (R gaze preference) and R hemiparesis constituting so called wrong-way eyes. can be caused by ongoing seizure activity in L cerebral hemisphere, by a lesion in the region of the L internal capsule and thalamus, or by a lesion in the L pons affecting the corticospinal tract and abducens nucleus. The patient's lethargy suggests possible mild involvement of the brainstem activating systems. Lesions in the vicinity of thalami causing wrong-way eyes usually do so in the setting of profound coma. Given the pt's age and hx of diabetes, the most likely diagnosis is an infarct in the L pons involving the L corticospinal and corticobulbar fibers as well as the L abducens nucleus (or PPRF). A hemorrahge in this location is also a possibility. In this pt, CT showed increased density at top of basilar artery- angiogram determined increased density was calcification rather than thrombosis. Area of L pons consistent w/ infarct from penetrating vessel arising from the basilar artery. Pt was found to have Afib and was treated w/ chronic oral anticoagulation.
Horner's Syndrome: What are the 3 classic syndrome signs? What light conditions is the aniscoria more obvious in? What would you see when removing the light after testing the pupillary reflex? How can the ciliospinal reflex serve useful in diagnosis? How can you differentiate preganglionic vs postganglionic lesions? (one clinical practice test, the other as a distinguishing feature of one vs the other) What are "pontine pupils"?
The classic syndrome consists of ptosis, miosis [pupillary constriction], and anhidrosis [absent sweating/flushing; skin feels smoother due to decreased moisture] Unlike oculomotor nerve lesions, anisocoria is more obvious in the dark than in ambient light the pupil still has a direct and consensual constricting response to light. However, there is a dilation lag relative to the normal pupil when the light is removed, and pupillary size is reduced. ciliospinal reflex: a painful pinch to the neck activates sympathetic outflow, causing pupillary dilation on the normal side but not on the side with Horner's syndrome. Horner's syndrome can be caused by lesions anywhere along the sympathetic pathway 1.Lateral hypothalamus or brainstem (e.g., infarct or hemorrhage) 2.Spinal cord (e.g., trauma) 3.First and second thoracic roots (e.g., apical lung tumor [Pancoast syndrome], disc herniation, or trauma) 4.Sympathetic chain (e.g., tumor or trauma) 5.Carotid plexus (e.g., carotid dissection) 6.Cavernous sinus (e.g., thrombosis, infection, aneurysm, or neoplasm) 7.Orbit (e.g., infection or neoplasm) Lesions proximal to the superior cervical ganglion, called preganglionic lesions, can be distinguished from postganglionic lesions by using hydroxyamphetamine/cocaine eye drops, which stimulate norepinephrine release and dilate the pupil for preganglionic, but not postganglionic, lesions. In addition, postganglionic lesions are not usually associated with anhidrosis because the sympathetics for sudomotor innervation diverge from the oculosympathetic pathway before the superior cervical ganglion. Large bilateral lesions of the pons are sometimes associated with pontine pupils, in which both pupils are small but reactive to light. This small pupillary size is probably caused by bilateral disruption of the descending sympathetic pathways.
How do the 3 nerves that control the extraocular muscles exit the skull?
The oculomotor (CN III), trochlear (CN IV), and abducens (CN VI) nerves pass through the cavernous sinus and then enter the orbit through the superior orbital fissure
Case 5: A 24-year-old woman with a history of pituitary adenoma suddenly developed severe headache, left forehead and cheek numbness, and inability to move the left eye. The patient presented 2 years previously with Cushing's syndrome and underwent two operations to resect a pituitary adenoma. She did well until 2 weeks before admission, when she started having left frontal headaches, especially around the left eye and nose. An MRI scan showed recurrent pituitary adenoma, and radiation therapy was planned. Two days prior to admission, however, she had onset of horizontal diplopia and was found by her endocrinologist to have a left CN VI palsy. She was admitted to an outside hospital and treated with steroids to try to reduce swelling. However, the next day she had a sudden worsening of her headache, with pain and numbness involving the left cheek and forehead; a dilated, fixed left pupil; and almost no movement of the left eye. She was therefore urgently transferred to a tertiary care center for further evaluation and treatment. Cranial Nerves: Right pupil 3 mm, constricting to 2 mm. Left pupil 6 mm, with no direct or consensual response to light. visual fields full. Fundi normal. Normal movements of the right eye, but the left eye had no extraocular movements and a marked left ptosis. She had diplopia in all directions of gaze. Sensation was slightly decreased to pinprick in the left forehead, eyelid, left bridge of the nose, and upper cheek. Face was symmetrical, aside from the left ptosis noted above. Shoulder shrug was normal, and tongue was midline. On the basis of the symptoms and signs shown in bold above, where is the lesion? Given the patient's history of a pituitary adenoma and the sudden onset of her deficits, what is the most likely diagnosis?
The pt has dysfunction of L CN III, IV, VI, and V1 constituting a L cavernous sinus syndrome. Lack of involvement of the optic nerve suggests disorder was not in the orbital apex. The most likely clinical localization = L cavernous sinus. Pt has hx of pituitary adenoma. Progress of her sx, however, is too rapid to be explained easily by extension of the tumor into the cavernous sinus. Sudden worsening of sx could be explained by hemorrhage into tumor or pituitary apoplexy. (pituitary apoplexy can occur in pts w/o a prev known hx of adenoma) In this pt: L hemorrhage extending from pituitary fossa into L cavernous sinus. ; pt taken to operating room for transphenoidal resection of hemorrhage and pituitary adenoma
Trochlear Palsy: What kind of diplopia is trochlear palsy? What finding would suggest severe weakness? What head position can help compensate for trochlear palsy? What view makes the diplopia the most severe? What test (other than the red glass test) can be useful for diagnosing trochlear palsy? (think no frills done by any dr anywhere type test) How do you interpret the results of this test? the trochlear nerve is the most commonly - cranial nerve.
The trochlear nerve produces depression and intorsion of the eye (SO4). Therefore, in trochlear nerve palsy there is vertical diplopia. If the weakness is severe, the affected eye may show hypertropia Patients with trochlear nerve palsy often report that they can improve the diplopia by looking up (chin tuck) and by tilting the head away from the affected eye (tilt in the direction of the good eye) because these maneuvers compensate for the hypertropia and extorsion, respectively the vertical diplopia is most severe when the affected eye is looking downward and toward the nose (looking nasally). diagnosing a fourth-nerve palsy usually involves demonstrating typical findings through the following four steps (Bielschowsky three-step test, plus the "missing step"): 1.The affected eye has hypertropia. 2.Vertical diplopia worsens when the affected eye looks nasally. 3.Vertical diplopia improves with head tilt away from the affected eye. 4.Vertical diplopia worsens with downgaze (less consistent than other steps). Another test that is sometimes useful is to have the patient look at a horizontal line (or pen). In a trochlear-nerve palsy, the patient will see two lines, with the lower line tilted. These two lines form an arrowhead with the "point" directed toward the affected side. The trochlear nerve is the most commonly injured cranial nerve in head trauma, probably because of its long course and thin caliber, making it susceptible to shear injury. Vascular or neoplastic disorders within the midbrain or near the tectum (e.g., pineal gland or anterior cerebellum) can also affect the trochlear nuclei or nerve fascicles. congenital fourth-nerve palsy, a relatively common cause of superior oblique weakness, is often latent for years except for minor head tilt and can later decompensate, leading to diplopia in adulthood. Other causes of vertical diplopia include disorders of extraocular muscles, myasthenia gravis, lesions of the superior division of the oculomotor nerve affecting the superior rectus, and skew deviation. Skew deviation is defined as a vertical disparity in the position of the eyes of supranuclear origin. Unlike trochlear palsy, in skew deviation the vertical disparity is typically (but not always) relatively constant in all positions of gaze. Skew deviation can be caused by lesions of the cerebellum, the brainstem, or even the inner ear. Other causes of head tilt include cerebellar lesions, meningitis, incipient tonsillar herniation, and torticollis.
Accomodation response- What 3 things happen for the accomodation response to take place? activated by what kind of visual signals? What do these signals activate?
This response occurs when a visual object moves from far to near, and it has the following three components: •Pupillary constriction •Accommodation of the lens ciliary muscle •Convergence of the eyes activated by visual signals relayed to the visual cortex From there, the pretectal nuclei are again activated, causing bilateral pupillary constriction mediated by the parasympathetic pathways(ciliary ganglia -> constrictor muscles) -> Contraction of the ciliary muscle of the lens is parasympathetically mediated by the same pathway.
Brainstem centers controlling vertical eye movements are located in what 2 areas: What portion of this region (ventral vs dorsal) is responsible for upward vs downward gaze? What nucleus is especially important for upgaze? What condition is associated w/ impaired vertical eye movements and midbrain atrophy?
Vertical eye movements are mediated by the superior and inferior rectus and superior and inferior oblique muscles Brainstem centers controlling vertical eye movements are located in the rostral midbrain reticular formation and pretectal area. The ventral portion of this region is thought to mediate downgaze, while the more dorsal region (in the vicinity of the posterior commissure) mediates upgaze (think about a 4-legged animal, their ventral surface is down and dorsal surface is up) One important nucleus that is thought to mediate downgaze is the rostral interstitial nucleus of the MLF -> Other lesser nuclei in this area that may also play a role include the nucleus of Darkschewitsch and the interstitial nucleus of Cajal Lesions such as infarcts or tumors of the dorsal parts of the vertical eye movement center cause impaired upgaze, while lesions in the ventral part cause impaired downgaze Progressive supranuclear palsy is associated with impaired vertical eye movements and midbrain atrophy
Oculomotor palsy: What position does the eye assume? What 2 other findings are associated with the oculomotor nerve's other functions besides extraocular motor control? What views would exacerbate the diplopia? What kind of diplopia is oculomotor palsy? Aneurysms in what location are most commonly associated w/ oculomotor palsy? What kind of herniation can cause oculomotor palsy? What condition in children is associated w/ oculomotor palsy?
W/ complete disruption of oculomotor nerve function, the only remaining movements of the eye are some abduction and some depression and intorsion the eye may come to lie in a "down and out" position at rest paralysis of the levator palpebrae superior causes the eye to be closed (complete ptosis) The pupil is dilated and unresponsive to light because of involvement of the parasympathetics the patient may report that the diplopia is worse when looking at near objects and better when looking at distant objects, since convergence is impaired. Red glass testing in third-nerve palsy generally reveals diagonal (oblique) diplopia that is most severe when looking up and medially with the affected eye common causes = diabetic neuropathy, microvascular neuropathy assoc w HTN and HLD, head trauma, compression of nerve by intracranial aneurysms, most often arising from junction of PComm w/ internal carotid artery other abnormalities in the subarachnoid space, cavernous sinus, or orbit, such as infection, tumor, or venous thrombosis. Herniation of the medial temporal lobe over the edge of the tentorium cerebelli can compress the oculomotor nerve (transtentorial uncal herniation also often causes coma and hemiplegia) Ophthalmoplegic migraine is a condition usually seen in children that causes reversible oculomotor nerve palsy. Lesions in the midbrain such as lacunar infarcts can also cause oculomotor palsy. disorders of the neuromuscular junction such as myasthenia gravis, can sometimes mimic the eye movement abnormalities and ptosis seen in oculomotor palsy. These patients should be considered to have a PComm aneurysm until proven otherwise. In all cases of CN III palsy an urgent CT angiogram (CTA) or magnetic resonance angiogram (MRA) should be done without delay. Pain is common but may be absent. Lesions of the oculomotor nerve can sometimes affect the superior division or inferior division in isolation
How does the lens change shape for viewing distant vs near objects?
When viewing a distant object, the ciliary muscle and pupillary constrictor muscle are relaxed When viewing a near object, the ciliary muscle and pupillary constrictor muscle contracts [parasympathetics] -> the lens is normally under tension from the suspensory ligament- when the ciliary muscle contracts it causes the suspensory ligament to relax, producing a rounder, more convex lens shape
wrong way eyes; 3 possible causes? (Why do each cause ipsilateral gaze weakness?)
certain clinical situations can cause the eyes to look toward the side of the weakness (contralateral to the lesion) Causes of wrong-way eyes include (1) seizure activity in the cortex, which can drive the eyes in the contralateral direction because of activation of the frontal eye fields, while also causing abnormal or decreased movements of the contralateral side of the body because of involvement of motor association cortex and other structures. for unclear reasons, (2) thalamic hemorrhage can disrupt the corticospinal pathways of the internal capsule, causing contralateral weakness, yet may also cause the eyes to deviate toward the side of the weakness. Lesions in the thalamic region causing wrong-way eyes are usually accompanied by deep coma lesions of the pontine basis and tegmentum can cause wrong-way eyes because disruption of the corticospinal fibers causes contralateral hemiplegia, while involvement of the abducens nucleus or PPRF causes ipsilateral gaze weakness
Adie's myotonic pupil:
characterized by degeneration of the ciliary ganglion or postganglionic parasympathetic neurons results in a mid-dilated pupil that reacts poorly to light. Some pupillary constriction can be elicited with the accommodation response, but the pupil then remains constricted and dilates very slowly, a condition described as a tonic or myotonic pupil. The cause is not known.
internuclear ophthalmoplegia (INO) (On PE, what type of eye adduction is spared? why?) What are common causes of INO? What kind of infarct would cause INO?
classic neurologic syndrome produced by an MLF lesion Lesions of the MLF interrupt the input to the medial rectus. Therefore, the eye ipsilateral to the lesion does not adduct fully on attempted horizontal gaze (a R INO means the R eye is unable to adduct - lack of signal to CN III/medial rectus of affected eye); In addition, for uncertain reasons there is also nystagmus of the opposite eye, possibly because of mechanisms trying to bring the eyes back into alignment. By definition, the side of the INO is the side of the lesion in the MLF. Since the ascending MLF crosses almost immediately after leaving the abducens nucleus the side of the INO is also the side on which eye adduction is weak. In an INO, eye adduction on the affected side is impaired with horizontal gaze but is often spared during convergence because the inputs to the oculomotor nucleus mediating convergence arise from the pretectal region and hence do not travel in the caudal MLF. common causes = MS plaques, pontine infarcts, or neoplasms involving the MLF A subtle INO can sometimes be detected only by testing of horizontal saccades in both directions and observation of a slight lag in the adduction of the eye on the affected side.
Cavernous sinus and orbital apex: Lies between what layers of the head? surrounds what? which 2 structures runs more medially in the sinus? What 4 structures run on the lateral wall? What other fibers run w. the carotid plexus? Where does the optic nerve sit in reference to the cavernous sinus? Where do all of these structures converge?
collection of venous sinusoids located on either side of the pituitary that receives venous blood from the eye and superficial cortex and ultimately drains via several pathways into the internal jugular vein Like other venous sinuses, the cavernous sinus lies between the periosteal and dural layers of the dura mater. surrounds the carotid siphon medial: abducens nerve (VI) & internal carotid artery lateral: III, IV, V1- run in sequence ; The maxillary nerve (CN V2) skirts the lower portion of the cavernous sinus and often runs through it for a short distance before exiting via the foramen rotundum sympathetic fibers traveling in the carotid plexus en route to the pupillary dilator muscle transverse the cavernous sinus as well The optic nerve lies just above the cavernous sinus and enters the orbital apex via the optic canal The orbital apex is the region where nearly all nerves, arteries, and veins of the orbit converge before communicating with the intracranial cavity via the optic canal and superior orbital fissure
Parinaud's syndrome: 4 main symptoms? What are 2 common causes? What "sign" is seen w/ hydrocephalus causing Parinaud's syndrome? What other condition coudl you see this eye deviation sign?
constellation of eye abnormalities usually seen with lesions compressing the dorsal midbrain and pretectal area. The four components of Parinaud's syndrome are: 1. Impairment of vertical gaze, especially upgaze. This may be due to compression of the dorsal part of the vertical gaze center. 2.Large, irregular pupils that do not react to light but sometimes may react to near-far accommodation. This light-near dissociation may occur as a result of disruption of optic tract fibers traveling to the Edinger-Westphal nuclei (most anterior and dorsal of the CN III nuclei) via dorsal pathways including the posterior commissure, while fibers for convergence descending from the visual cortex take a different route and are relatively spared. 3.Eyelid abnormalities ranging from bilateral lid retraction (Collier's sign) or "tucking" to bilateral ptosis. (M group of neurons) 4.Impaired convergence and sometimes convergence-retraction nystagmus, in which the eyes rhythmically converge and retract in the orbits, especially on attempted upgaze. Common causes = pineal region tumors and hydrocephalus -> can also be MS and vascular disease of the midbrain pretectal area Hydrocephalus can cause dilation of the suprapineal recess of the third ventricle, which pushes downward onto the collicular plate (tectum) of the midbrain. Thus, hydrocephalus, especially in children, can produce the bilateral setting-sun sign, in which the eyes are deviated inward because of bilateral sixth-nerve palsies and downward because of a Parinaud's syndrome. -> Similar downward and inward deviation of the eyes, sometimes referred to as "peering at the tip of the nose," can be seen in thalamic hemorrhage, where the mechanism is not known.
Skew deviation:
vertical disparity in the position of the eyes of supranuclear origin Unlike trochlear palsy, in skew deviation the vertical disparity is typically (but not always) relatively constant in all positions of gaze. Skew deviation can be caused by lesions of the cerebellum, the brainstem, or even the inner ear.
Convergence and divergence- what part of the brain provides descending inputs?
convergence = produced by medial recti divergence= by lateral recti The exact anatomical locations for centers in the brainstem controlling vergence have not been defined, but there appear to be separate pools of neurons in the midbrain reticular formation mediating either convergence or divergence movements. Vergence movements are under the control of descending inputs from the visual pathways in the occipital and parietal cortex and constitute part of the accommodation response
Diplopia; what can cause it? What presentation rules in eye movement abnormality vs rules out? What multi-image disorders cannot be caused by an eye movement abnormality? which image is seen by the abnormal eye?
double vision can be caused by: (1) mechanical problems such as orbital fracture with muscle entrapment; (2) disorders of the extraocular muscles such as thyroid disease, or orbital myositis (orbital pseudotumor); (3) disorders of the neuromuscular junction such as myasthenia gravis; or (4) disorders of CN III, IV, VI and their central pathways. Diplopia can also occasionally be caused by disorders involving the supranuclear oculomotor pathways such as internuclear ophthalmoplegia (INO), skew deviation, and ingestion of toxins such as alcohol or anticonvulsant medications. if the diplopia went away when the patient closed or covered one eye, this suggests an eye movement abnormality Monocular diplopia or polyopia (three or more images) can be caused by ophthalmological disease, disorders of the visual cortex, or psychiatric conditions, but not by eye movement abnormalities. On examination, eye movements are usually reported in degrees or millimeters from the primary position When an extraocular muscle is not working properly, dysconjugate gaze results, causing diplopia the image further from the midline and toward the direction of attempted gaze is always the one seen by the abnormal eye. For example, when looking at an object to the right, if one eye does not move to the right, then it will form a second image that appears displaced to the right.
What muscles control eye opening? eye closing? When do you see ptosis? What about specifically bilateral ptosis? How can you distinguish ptosis from facial weakness?
eye opening = striated skeletal muscle of the levator palpebrae superior (CN III) together with Müller's smooth muscle in the upper lid (sympathetics). -> frontalis muscle of the forehead (CN VII) performs an accessory role eye closure = orbicularis oculi muscle (CN VII). Ptosis can be seen in Horner's syndrome, oculomotor nerve palsy, myasthenia gravis, orbital mass, redundant skin folds assoc w/ aging (pseudoptosis) The ptosis in myasthenia is classically "fatiguing" and increases following sustained upgaze Causes of bilateral ptosis or closed eyes without loss of consciousness include nondominant parietal lobe stroke, severe neuromuscular disorders, dorsal lesions of the oculomotor nuclei affecting the central caudal nucleus, and voluntary eye closure associated with photophobia in migraine, meningeal irritation, or psychological causes. -> a nuclear lesion can only be bilateral Weakness of the orbicularis oculi caused by facial nerve or upper motor neuron lesions can cause a widened palpebral fissure that may be mistaken for ptosis in the opposite eye. -> in ptosis, the upper lid comes down farther over the iris in the affected eye, while in facial weakness, the palpebral fissure is widened mainly because of sagging of the lower lid in the affected eye
Case 1: A 48-year-old woman came to the emergency room with worsening left eye pain and intermittent double vision. Approximately 4 or 5 years previously, the patient had begun to have left frontal and left retro-orbital headaches that occurred intermittently at first, and then on an almost daily basis. An MRI scan was reportedly normal, and she was diagnosed with cluster migraine. The headaches continued to occur but were relieved by ibuprofen. One and a half years prior to presentation she began to have intermittent drooping of the left eyelid and dilation of the left pupil. She also noticed that her left eye occasionally drifted to the left, causing diplopia. This patient was very observant, and she noticed that her diplopia was worse when looking to the right. When covering each eye alternately, she reported that the two images did not overlap. In fact, the image from her left eye appeared to the right and slightly above the image from her right eye. These symptoms gradually progressed from intermittent to continuous, and her headaches were no longer relieved by up to 12 ibuprofen tablets per day, so she came to the emergency room. Neurologic exam: Mental status: Alert and oriented × 3. Speech fluent, with intact naming and repetition. 3/3 words recalled after 5 minutes. Cranial nerves: Visual fields full. Fundi normal. Right pupil 4 mm, constricting to 3 mm with direct and consensual light stimulation, and with accommodation. Left pupil 6 mm, with no direct or consensual response to light and no response to accommodation. Left eye had limited but not absent upgaze, downgaze, and adduction. Normal left eye abduction. Normal right eye movements. Left ptosis, with left palpebral fissure 6 mm and right 9 mm. Corneal reflexes intact. Facial sensation intact. Face symmetrical, other than left ptosis already described. Normal palate and tongue movements. Motor: No drift. Normal tone. 5/5 power throughout. Reflexes NL Coordination: Normal on finger-to-nose and heel-to-shin testing. Gait: Not tested. Sensory: Intact light touch, pinprick, vibration, and joint position sense. Normal graphesthesia; no extinction. On the basis of the symptoms and signs shown in bold above, where is the lesion? What is the most likely diagnosis?
findings compatible w/ oculomotor nerve CNIII) palsy. Although L eye adduction, upgaze, and downgaze were decreased, they were not absent, suggesting partial third-nerve palsy. In addition, there was no description of an abnormal eye position at rest. Third nerve palsy should be treated as an aneurysm until proven otherwise. Most common aneurysm causing CN III palsy occurs where the posterior communicating artery branches off the internal carotid. test: CT and cerebral angiogram Tx: L frontotemporal craniotomy ; dome of aneurysm seen to project posteriorly and inferiorly under the edge of the tentorium cerebelli ; clip across neck of aneurysm - the dome of the aneurysm immediately becomes less tense and could be safely opened w/ microscissors - some blood evacuated leading to decompression of adjacent structures
exotropia vs esotropia vs hypertropia: useful test for detecting?
forms of strabismus (eye misalignment) exotropia = abnormal lateral gaze of one eye esotropia = abnormal medial deviation hypertropia = Vertical deviation, usually described only with respect to the eye that is higher useful test = useful test for subtle dysconjugate gaze is to shine a flashlight from directly in front of the patient on both eyes simultaneously and then to examine the position of the reflection of the light on each cornea. Normally the reflection is symmetrical on the two corneas. When an eye is misaligned, the reflected light appears displaced in the opposite direction.
Brainstem circuits for horizontal eye movements: What is the MLF? Through this set up, what nucleus serves as the horizontal gaze center? What does this mean in terms of its role/communication? What is the second horizontal gaze center and where does it lie? What input does it provide?
generated by the lateral rectus and medial rectus muscles, which are controlled by the abducens and oculomotor nuclei, respectively the medial longitudinal fasciculus (MLF) interconnects the oculomotor, trochlear, abducens, and vestibular nuclei ; Through connections in the MLF, eye movements are normally yoked together, resulting in conjugate gaze in all directions. -> highly myelinated the abducens nucleus is a horizontal gaze center, controlling horizontal movement of both eyes in the direction ipsilateral to the side of the nucleus. Thus, some neurons in the abducens nucleus project to the ipsilateral lateral rectus muscle, while others project via the MLF to the contralateral oculomotor nucleus, which, in turn, activates the contralateral medial rectus muscle. In the pontine tegmentum near the abducens nucleus, an additional important horizontal gaze center called the paramedian pontine reticular formation (PPRF) provides inputs from the cortex and other pathways to the abducens nucleus, resulting in lateral horizontal gaze (lesions look the same as an abducens nucleus lesion)
one-and-a-half syndrome:
if a lesion involves both the MLF and the adjacent abducens nucleus or PPRF, there is a combination of an ipsilateral INO and an ipsilateral lateral gaze palsy. Thus, the ipsilateral eye cannot move at all horizontally, and the contralateral eye loses half of its movements, preserving only its ability to abduct, resulting in the quaint name one-and-a-half syndrome for this disorder.
locked-in syndrome and eye movements:
large pontine lesions can disrupt the bilateral corticospinal tracts and abducens nuclei, eliminating body movements and horizontal eye movements. However, sometimes the vertical eye movement centers in the midbrain are spared, allowing the patient to communicate entirely through vertical eye movements.
6 extraocular muscles and their origin and insertion: What movements do the oblique muscles perform?
lateral rectus, medial rectus, superior rectus, and inferior rectus muscles move the eye laterally, medially, superiorly, and inferiorly, respectively -> These muscles originate in a common tendinous ring at the orbital apex and insert onto the sclera. The superior oblique muscle originates on the sphenoid bone in the posterior medial orbit and passes anteriorly through the trochlea, a pulley-like fibrous loop on the medial superior orbital rim. It then inserts on the superior surface of the eye to produce intorsion, meaning movement of the upper pole of the eye inward the inferior oblique has no trochlea, but it originates along the anterior medial orbital wall and inserts on the inferior surface of the eye to produce extorsion, meaning movement of the upper pole of the eye outward.
How can you tell a pontine infarct affecting the exiting abducens fascicles from peripheral abducens nerve lesion?
lesions of the abducens nucleus in the pons produce not a simple abducens palsy but, instead, a horizontal gaze palsy in the direction of the lesion. In a gaze palsy, movements of both eyes in one direction are decreased; In addition, lesions of the abducens nucleus often affect the nearby fibers of CN VII in the facial colliculus, resulting in ipsilateral facial weakness
What other muscles are not for eye movement but other functions of the eye?
levator palpebrae superior: elevates eyelid pupillary constrictor and dilator muscles ciliary muscle: adjusts thickness of lens in response to viewing distance
Abducens nuclei: where is it located? At what region do the fibers exit the brainstem? what unique path does the abducens nerve take to exit the dura and then reach the cavernous sinus? what kind of injury is the abducens nerve particularly vulnerable to?
lie on the floor of the fourth ventricle under the facial colliculi in the mid-to-lower pons Abducens fibers travel ventrally to exit at the pontomedullary junction The abducens nerve must then follow a long course in the subarachnoid space, ascending between the pons and clivus. The abducens nerve then exits the dura to enter Dorello's canal, running between the dura and skull, under the petroclinoid ligament. It next makes a sharp bend as it passes over the petrous tip of the temporal bone to reach the cavernous sinus. abducens nerve is highly susceptible to downward traction injury produced by elevated intracranial pressure
Trochlear nuclei: where is it located? The nucleus is bound ventrally by what? What is unique about how the trochlear nerve exits the brainstem? At what level do the fibers exit? At this level, they are susceptible to compression via what kind of tumor? What makes the trochlear nerve easily injured? How do the fibers travel before reaching the cavernous sinus?
located in lower midbrain at the level of the inferior colliculi and the decussation of the superior cerebellar peduncle lie just ventral to the periaqueductal gray matter and are bounded ventrally by the fibers of the medial longitudinal fasciculus. The trochlear nerves are the only cranial nerves to exit the brain dorsally; unlike any other cranial nerve, the trochlear nerves exit the brainstem in a completely crossed fashion -> They travel caudally for a short distance and then cross to the opposite side before exiting at the level of the anterior medullary velum, where they are susceptible to compression from cerebellar tumors. The trochlear nerves are very thin and are relatively easily damaged by shear injury from head trauma. They travel through the subarachnoid space along the underside of the tentorium cerebelli and then enter the cavernous sinus to reach the orbit via the superior orbital fissure
M group of neurons- what do they mediate?
located in the midbrain near the nuclei for vertical eye movements Vertical eye movements are normally closely coordinated with movement of the upper eyelids in the same direction, except during eye blinks. This coupling is thought to be mediated by the M-group of neurons
Oculomotor nuclei - where are they situated? The fibers of CN III exit the brainstem in what fossa, between what 2 arteries? The parasympathetic fibers are particularly vulnerable to aneurysm arising from what artery?
located in the upper midbrain at the level of the superior colliculi and red nuclei, just ventral to the periaquaeductal gray matter Fascicles of the oculomotor nerve exit the brainstem as CN III in the interpeduncular fossa between the posterior cerebral and superior cerebellar arteries The Edinger-Westphal nuclei, containing preganglionic parasympathetic fibers, form a V shape as they curve over the dorsal aspect of the oculomotor nuclei and fuse anteriorly in the midline -> The parasympathetic fibers controlling pupil constriction run in the superficial and medial portion of the oculomotor nerve as it travels in the subarachnoid space (susceptible to compression from aneurysms, particularly arising from the nearby posterior communicating artery, PComm)
What is the classic feature of oculomotor palsy caused by aneurysm?
oculomotor palsy that involves the pupil The reason is thought to be that the parasympathetic fibers are located near the surface of the nerve, and if the nerve compression is severe enough to cause a complete oculomotor palsy, then the pupillary fibers should be involved as well. A complete oculomotor palsy that spares the pupil is often caused by diabetes or other microvascular neuropathy.
Abducens palsy: What kind of diplopia? What finding suggests severe weakness? In what view would the diplopia be worse? What finding would suggest a milder palsy? How does a pt compensate? The abducens nerve is particularly susceptible to what kind of injury?
produce horizontal diplopia severe: esotropia of the affected eye patients report that the diplopia is better when they are viewing near objects and worse when they are viewing far objects. (in contrast to CNIII palsy) & worse when the pt tries to abduct the affected eye In milder abducens palsy there may simply be incomplete "burial of the sclera" on lateral gaze. Some patients may tend to turn the head toward the affected eye in an effort to compensate for the diplopia Because of its long course along the clivus and over the sharp ridge of the petrous temporal bone, the abducens nerve is particularly susceptible to injury from downward traction caused by elevated intracranial pressure -> important early sign of supratentorial or infratentorial tumors, pseudotumor cerebri (idiopathic intracranial hypertension), hydrocephalus, and other intracranial lesions. -> abducens palsy seen with elevated intracranial pressure can be unilateral or bilateral. As with oculomotor and trochlear palsies, many cases of acute abducens palsy lack a demonstrated cause Other causes of horizontal diplopia include myasthenia gravis and disorders of the extraocular muscles caused by thyroid disease, tumors, inflammation, or orbital trauma.
What are 2 examples of reflex eye movements? What 3 structures are involved in reflex eye movement? How can an examiner iliict optokinetic nystagmus (OKN)? OKN is also referred to as - "_______ nystagmus" OKN is made up of what 2 supranuclear eye movements? What controls the slow phase vs fast phase?
reflex eye movements = include optokinetic nystagmus and the vestibulo-ocular reflex; Nystagmus is a rhythmic form of reflex eye movements composed of slow eye movements in one direction interrupted repeatedly by fast, saccade-like eye movements in the opposite direction. The cerebellum, vestibular nuclei, and cervical spinal proprioceptors influence ongoing voluntary eye movements and contribute to reflex eye movement The examiner can elicit optokinetic nystagmus (OKN) in the horizontal direction by moving a thick ribbon with vertical stripes (called an OKN strip) horizontally in front of the eyes -> the eyes alternate between smooth pursuit movements in the direction of stripe movement and backup (corrective) saccades opposite the direction of stripe movement in an attempt to stabilize the image. OKN is sometimes called "train nystagmus" because it can be observed in the eyes of fellow passengers as they watch the passing visual scene through an open window. The slow phase, or smooth pursuit phase, of OKN is mediated by the ipsilateral posterior cortex (parieto-occipito-temporal cortex ), with connections to the vestibular nuclei and flocculonodular lobe of the cerebellum projecting to the PPRF and abducens nuclei. The fast phase, or saccadic phase, of OKN is mediated by the frontal eye fields projecting ultimately to the contralateral PPRF. Therefore, lesions of the frontal cortex or anywhere in the saccadic pathways disrupt the fast phases of OKN, while the slow phases are disrupted by lesions in the smooth pursuit pathways. OKN testing is thus useful to check for subtle dysfunction in the eye movement pathways. OKN can also be elicited in the vertical direction.
Midbrain correctopia:
relatively rare condition, lesions of the midbrain can sometimes cause an unusual pupillary abnormality in which the pupil assumes an irregular, off-center shape.
Types of eye movements that are under supranuclear control: saccades vs smooth pursuit vs vergence [compare speed and voluntary vs involuntary control]
saccades = rapid eye movements reaching velocities of up to 700 degrees per second function to bring targets of interest into the field of view. (fast component) Vision is transiently suppressed during saccadic eye movements. Saccades are the only type of eye movement that can easily be performed voluntarily, although they can be elicited by reflexes as well. Smooth pursuit: eye movements are not under voluntary control, and they reach velocities of only 100° per second; They allow stable viewing of moving objects. vergence= eye movements maintain fused fixation of both eyes as targets move toward or away from the viewer; velocity is about 20deg per second.
Case 8: A 25-year-old woman had a 2-year history of multiple sclerosis with relapsing and remitting episodes of weakness and sensory deficits in her extremities. Two weeks prior to evaluation, she had onset of horizontal diplopia. Compared to previous exams, the new findings consisted of an abnormality of rightward horizontal gaze, such that the left eye did not adduct past the midline and the right eye had sustained end gaze nystagmus on abduction. In contrast, when convergence was tested, the left eye was able to adduct past the midline. Leftward horizontal gaze was normal. Draw the positions of this patient's eyes on leftward horizontal gaze, on rightward horizontal gaze, and during convergence. What is the location of this patient's lesion, and which side is it on? What is the most likely cause? During the next few days the patient gradually developed problems with leftward horizontal gaze as well, so neither eye would move past the midline when looking to the left. On rightward gaze, she continued to have no adduction of the left eye. Thus, the only remaining horizontal eye movement was right eye abduction, which continued to have end gaze nystagmus, as before. Draw the positions of this patient's eyes on leftward horizontal gaze and on rightward horizontal gaze following her clinical worsening. What is the location of this patient's lesion now?
see image Lesion 4 for leftward and rightward horizontal gazes; convergence was normal. These findings constitute a L INO localized in the L MLF. Given the pt hx, the most likely diagnosis is a MS plaque in the L MLF. ---------------- see image Lesion 5 for leftward and rightward gaze. These findings constitute a L INO plus a L horizontal gaze palsy (one-and-a-half syndrome.) Most likely her demyelinative plaque in the L MLF enlarged to involve the L abducens nucleus or PPRF. Tx w/ steroids
How does the oculomotor nerve split?
shortly after entering the orbit, the oculomotor nerve splits into two major branches. The superior division supplies the superior rectus and also innervates the levator palpebrae superioris, a muscle important for eyelid elevation. The inferior division supplies the medial rectus, inferior rectus, and inferior oblique muscles. also carries preganglionic parasympathetic fibers to the pupillary constrictor muscles and to the ciliary muscles of the lens
vestibulo-ocular reflex (VOR): What does this reflex do? What nuclei are involved? Where does this input go to to control extraocular movement? How could an awake pt demonstrate this reflex? How can the circuits involved in this reflex be tested in a pt who is comatose? (2) Why aren't "doll eyes" ilicited in an awake pt? What does the VOR suppression test test for?
stabilizes the eyes on the visual image during head and body movements -> serves to move the eyes at an equal velocity but in the direction opposite to head movement ; this keeps the eyes still in place and maintains visual fixation while the head is in motion Inputs from medial vestibular nuclei, travel in the MLF to control the extraocular nuclei awake pt: fixate on your finger and then turn your head side to side, noting the relatively stable image of your finger In patients who are comatose and therefore lack visual fixation, the integrity of brainstem circuits mediating the VOR is often tested with the oculocephalic maneuver to elicit "doll's eyes" or with cold water calorics -> A positive oculocephalic reflex means the pt moves their eyes opposite the rotation of their head (Doll's eyes) ; negative/abnormal if the eyes stay midline In the normal, awake individual, cerebellar circuits involving the flocculus and nodulus enable visual fixation to overcome the VOR. This is why oculocephalic testing does not produce "doll's eyes" in the normal, awake individual. Similarly, visual fixation can suppress nystagmus evoked by caloric testing -> In normal individuals the vestibulo-ocular reflex is often suppressed by visual inputs. For example, when normal individuals read a newspaper while riding a train as it pulls into the station, their eyes do not jump off the page even though their inner ears detect a large deceleration. In the VOR suppression test, patients fixate on an object moving with the head as it rotates (e.g., place a drinking straw in the patient's mouth and ask them to fixate on the far end of the straw as they turn their head from side to side). The presence of nystagmus indicates cerebellar dysfunction.
Afferent Pupillary Defect (Marcus Gunn Pupil) what is it characterized by? results from lesions in what structures? what test is useful in diagnosis? What is a hippus?
the direct response to light in the affected eye is decreased or absent, while the consensual response of the affected eye to light in the opposite eye is normal The afferent pupillary defect is caused by decreased sensitivity of the affected eye to light resulting from lesions of the optic nerve, retina, or eye [unilateral/asymmetric lesions]. Recall that lesions at or behind the optic chiasm would affect inputs from both eyes A useful way to detect an afferent pupillary defect is with the swinging flashlight test - flashlight is moved from the normal to the affected eye, and the affected pupil dilates in response to light This abnormal dilation should be distinguished from hippus, which is a normal, brief oscillation of the pupil size that sometimes occurs in response to light. optic nerve or retinal disease does not cause anisocoria, since the consensual response still causes both pupils to constrict or dilate together due to crossing fibers at multiple levels, despite any afferent defects on one side [hence, no aniscoria when comparing ambient light to dark room, eyes would appear normal until light reflex initiated]
light-near dissociation: classic example?
the pupils constrict much less in response to light than to accommodation abnormal light reflex but normal accomodation mechanism not known classic example of light-near dissociation is the Argyll Robertson pupil associated with neurosyphilis, in which, in addition to light-near dissociation, the pupils are also small and irregular. Light-near dissociation can also be seen in diabetes and Adie's myotonic pupil and as part of Parinaud's syndrome, which is associated with compression of the dorsal midbrain.
Muller's muscles (2)
the sympathetic pathway that controls pupillary dilation is also important in controlling the smooth muscle of the superior tarsal muscle (Müller's), which elevates the upper lid, causing a wide-eyed stare in conditions of increased sympathetic outflow. (note that the levator palpebrae superior is composed of skeletal muscle and also functions in eyelid opening under the control of CN III.) Sympathetics from the pathway also innervate the smooth muscle orbitalis (Müller's), which prevents the eye from sinking back in the orbit, as well as the cutaneous arteries and sweat glands of the face and neck. These various sympathetic functions are impaired in Horner's syndrome, causing ptosis
