Localize the Lesion

Correct Answer: Occipital lobe
• A macular-sparing visual field defect involves a loss of vision in a significant portion of the visual field while preserving central vision.
• This type of defect typically results from damage to the contralateral occipital lobe.
• Macular sparing occurs because central vision is represented in a larger, more resilient area of the visual cortex, with a higher density of neurons and less susceptibility to damage than peripheral visual fields.
• Additionally, the macula is represented by the occipital tip. The tip has a dual blood supply from terminal branches of both the posterior and middle cerebral arteries and is therefore less susceptible to the impact of ischemia.

Correct Answer: Left occipital lobe
• This individual likely suffers from temporal crescent or half-moon syndrome presenting as a monocular loss of
temporal vision in a crescent shape.
• In general, the nasal visual field occupies about 60% of the visual horizon, while the temporal visual field extends to
nearly 100% of the horizon. As a result, 30-40% of the peripheral portion of the temporal visual field is unpaired,
as the contralateral eyes’ nasal fields do not cover these crescent-shaped areas. 1
• Each crescent is represented exclusively by the contralateral anterior occipital cortex, so a lesion in that region would result in temporal crescent syndrome.1
• Temporal crescent syndrome represents an exception to the typical pattern of retro-chiasmal lesions presenting as homonymous defects in visual fields.

Correct Answer: Right superior optic radiations

The visual fields indicate a left “pie on the floor” visual defect or left inferior quadrantanopia. This is typically associated with damage to
the right superior optic radiations (i.e. the right parietal pathway) since they represent the contralateral inferior visual fields.

By contrast, a lesion in the inferior optic radiations (answer choice B), also known as Meyer’s loop or the temporal pathway, presents as a “pie in the sky” defect or superior quadrantanopia.

Bitemporal hemianopia would be the expected visual field defect for an optic chiasm lesion (answer choice A), and a right occipital pole lesion (answer choice D) would likely present with macular sparing.

Correct Answer: Middle cerebral artery; posterior cerebral artery

Superior portions of the optic radiations are mainly supplied by branches of the inferior division of the middle cerebral artery.

Inferior portions receive blood primarily from the posterior cerebral artery. The posterior cerebral artery also dominates the blood supply to the optic radiations more posteriorly.

Correct Answer: From the occipital poles anteriorly

Myelination of each visual pathway begins only after the entire pathway has developed.

The post-geniculate visual pathway undergoes myelination starting from birth, and this process is generally complete by the fourth
month of life.

Myelination begins at the occipital poles and progresses anteriorly.

Note, however, that the occipital poles may not be fully myelinated until the twelfth month of life.

Correct Answer: Abducens nerve (CN VI) 

The abducens nucleus is located in the dorsal pons. Lesions here cause inability to abduct the eye, resulting in horizontal diplopia, most pronounced during lateral gaze to the affected side. Less prominent at near.

Correct Answer: Optic Nerve (CN II)

The optic nerve runs just posterior and superior to the cribriform plate in the anterior cranial fossa. A lesion in this area (i.e., meningioma) can compress CN II, leading to monocular vision loss and a relative afferent pupillary defect. The optic disc pallor supports chronic optic nerve compromise.

Correct Answer: Oculomotor nerve (CN III)

This patient’s findings—ptosis, dilated pupil, and impaired adduction/supraduction/infraduction—are classic for CN III palsy. CN III runs through the lateral wall of the cavernous sinus, making it vulnerable to mass lesions there.

• Trigeminal nerve (CN V) – Incorrect. CN V involvement explains facial pain and reduced corneal reflex, but it does not cause ptosis, ophthalmoplegia, or pupil dilation.
• Optic nerve (CN II) – Incorrect. The optic nerve lies in the anterior cranial fossa and would not be affected by a cavernous sinus lesion. Its damage causes visual acuity loss, not ophthalmoplegia.
• Abducens nerve (CN VI) – Incorrect. CN VI controls only lateral rectus; its involvement would cause horizontal diplopia, not the full oculomotor syndrome seen here.

Correct Answer: Glial cells

Unlike the intracranial optic nerve, the optic tracts are firmly attached to the brain throughout their course by glial cells.

Correct Answer: Anterior Choroidal Artery

The major blood supply to the optic tract comes from the anterior choroidal artery, which is a branch of the internal carotid artery, although other arteries often contribute to the blood supply. A direct artery may arise from the internal carotid artery.

Correct Answer: Nasally aligned optic tract fibers > contralateral pretectal nucleus > Edinger-Westphal nucleus

Following decussation, nasal fibers of the optic tract carry fibers of the path for the pupillary reflex pathway following the contralateral pretectal nucleus and into the Edinger-Westphal nuclei of the midbrain.

Correct Answer: Right optic tract

Bow-tie atrophy of the left eye indicates damage to the nasal fibers of the left eye. The right optic tract carries the crossing nasal fibers of the contralateral eye and, therefore, presents with temporal field loss in that eye. The nasal fibers correspond to the papillomacular bundle.

Correct Answer: Incongruous homonymous hemianopia

Lesions behind the body of the optic chiasm lead to homonymous hemianopic defects. Lesions closer to the occipital cortex are more congruous. Lesions anterior to the lateral geniculate body have localizing value.

Correct Answer: Visual cortex

• These constricted visual fields contain powerful localizing features–the “step-offs” in the visual field defects along the vertical meridian that are identical in the corresponding hemifields. In other words, these are BILATERAL CONGRUOUS HOMONYMOUS HEMIANOPIAS. Although the responsible lesions could lie anywhere in the retrochiasmal portion of the visual pathway on both sides, the most common location would be in visual cortex, where a single event—occlusion of the basilar artery or its two branches, the posterior cerebral arteries, could infarct both sides.
• Do not be surprised if the profound visual field loss is the patient’s only new deficit—that happens if the proximal posterior arterial circulation opens up quickly. Echocardiography showed an aortic valve vegetation, which could have been the source of emboli. Were the optic chiasm the site of damage, visual fields would have been variants of bitemporal hemianopia, not homonymous hemianopia. Bilateral optic nerve damage would cause visual field defect “step-offs” along the horizontal meridian, not seen here. Bilateral simultaneous retinal events—detachments or infarctions—are exceedingly rare and would not cause such symmetrical visual field defects with borders aligned to the vertical meridian. By the way, the patient need not have infarcted both sides of visual cortex at the same time. He could have had an old unilateral homonymous hemianopia about which he was unaware and now experienced a second event affecting only one cerebral hemisphere.

Correct Answer: Retina

• Look carefully at the shape of these defects, noting that they cross the vertical and horizontal meridians without a change in their width. In other words, there are no “step-offs.” These are incomplete “ring defects,” which are characteristic of outer retinal (pigment epithelium/photoreceptor) dysfunction in genetic disorders called “retinal dystrophies.” Very rarely, these defects could be caused by paraneoplastic or idiopathic autoimmune retinopathies.
• Electroretinography will confirm loss of rod function. For most forms of these conditions, there is regrettably no effective treatment yet. The defects become gradually more extensive over ensuing years. Patients also have impaired dark adaptation, but it develops so slowly that they make accommodations and do not suspect a disease. Ophthalmoscopy typically discloses an atrophic retinal pigment epithelium, black spicules, and attenuated arterioles. But do not be surprised if ophthalmoscopy shows a normal-appearing retina! This diagnosis is often long in coming, especially if ophthalmoscopic abnormalities are subtle or absent. In that case, the deficit is often blamed on psychogenic causes. Optic neuropathies would cause defects that have “step-offs” along the nasal horizontal meridian, a feature of nerve fiber bundle defects. Optic chiasm lesions and visual cortex lesions would show “step-offs” along the vertical meridian.

Correct Answer: Optic radiations

• This an incomplete left homonymous hemianopia in which the visual field defects have essentially the same extent in both eyes, earning the term “congruous.” Congruity, a term applied only to INCOMPLETE HOMONYMOUS HEMIANOPIAS, reflects the fact that axons derived from corresponding visual field points in the retinas of both eyes have come to lie next to one another and will therefore be harmed together by single lesions. That “neighborliness” of corresponding axons takes place gradually as the optic radiations proceed backwards toward the visual cortex. Thus, the lesion in this patient is likely to lie in the POSTERIOR part of the optic radiations.
• Of course, it could also lie within the visual cortex (but that was not one of your choices here!).

Correct Answer: Right anterior temporal lobe

These visual field defects have the following distinctive features: they are wedge-shaped in both eyes with one border aligned to the vertical meridian, the other extending out radially. You can see why they are called “homonymous hemianopic pie-in-the-sky” defects. They represent damage to the post-geniculate axons that curve around the anterior portion of the temporal horn of the lateral ventricle (“Meyer’s loop”) before joining up with the remaining axons of the optic radiations. If these defects had had borders aligned to the horizontal meridian, you would have called them “homonymous quadrantanopias,” and properly blamed the damage on a visual cortex lesion. In this case, the patient had undergone laser ablation of the anterior temporal lobe for intractable seizures. By the way, ablation extending beyond 4cm posterior to the anterior temporal tip is bound to cause such “pie-in-the-sky” visual field defects. The good news is that patients will not be impaired by them (or even notice them) unless the ablation extends back more than 6cm from the anterior temporal tip, in which case the root of the optic radiations may be damaged and the radial border of the homonymous defects will stretch closer to the horizontal meridian. Procedures in this region can also damage the anterior choroidal artery and its supply of the optic tract.

Correct Answer: Visual cortex

• No one would fault you for guessing “retina,” thinking that there was an overlooked abnormality–a retinal inflammation, separation, or scar. But that was not the case here. The defect derives from the other end of the visual system. Limited to the peripheral temporal field of the left eye, it is easy to miss unless you remember that the temporal field is normally more extensive than the nasal field. Standard static perimetry, which samples at most the central 30 degrees of the visual field, will always miss this defect. It is called a “temporal crescent defect,” and it occurs when a lesion—nearly always an infarct–damages the far anterior visual cortex in the opposite cerebral hemisphere
• Factoid: this is the only retrochiasmal visual field defect that is not a homonymous hemianopia! By the way, if a lesion can selectively damage the far anterior visual cortex, it can also selectively spare it. If you play the next video, you will see a “temporal crescent-sparing homonymous hemianopia” caused by a lesion that damaged only the mid-portion and posterior portion of visual cortex.
• Patients who have this defect will be able to see in the periphery of the damaged hemifield and believe that they are safe to drive. Not really!

Correct Answer: Visual cortex

• This is a right superior homonymous quadrantanopia—a defect pattern that is exquisitely localizing—to the left inferior visual cortex or its adjacent incoming axons. Notice three features of these defects: 1) they are on the same side of visual space (“homonymous”); 2) the defect borders are aligned to the vertical meridian (“hemianopic”); and 3) the defect borders are aligned to the horizontal meridian. Defects with borders aligned to BOTH MERIDIANS can arise only from lesions limited to the upper or lower banks of the calcarine fissure, a brain groove that separates the superior from the inferior primary visual cortex. In this patient, the lesion lies in the lower bank
• Although it is true that a lesion of the inferior portion of the optic radiations would cause a homonymous hemianopia that preferentially affects the superior field, it would not produce visual field defects aligned to the horizontal meridian because the superior and inferior optic radiation axons do not become distinct until they reach the calcarine fissure in the occipital lobe. By the way, you should not apply the term “quadrantanopia” unless the defects are homonymous hemianopias with borders aligned to both the vertical and the horizontal meridian. True quadrantanopias are almost always caused by infarctions in the domain of the posterior cerebral artery, which serves the upper and lower calcarine banks with separate branches. In this patient, MRI revealed restricted diffusion in the lower bank, indicating fresh infarction; the lower branch of the posterior cerebral artery must have been occluded. Embolism is always a consideration, with the heart as a source.

Correct Answer: Optic chiasm

• This is the most easily recognized visual field abnormality—a bitemporal hemianopia. It is the signature of an extrinsic or intrinsic lesion compressing, inflaming, infarcting, or expanding the optic chiasm.
• Actually, the most common cause of chiasmal damage is a mass lesion: pituitary adenoma, meningioma, and aneurysm in adults; craniopharyngioma and pilocytic astrocytoma in children. Why do such lesions produce this defect pattern? Because the crossing axons in the optic chiasm are especially vulnerable to any kind of insult! Patients are slow to notice the defects because the nasal field of each eye almost completely covers the temporal field of the other eye. Although they may eventually sense that peripheral vision is compromised, more often patients present with decreased vision in one eye because the lesion also compromises the function of an optic nerve. Visual fields may show a combination of nerve fiber bundle defects and temporal hemianopic defects (“junction pattern”).
• This patient had a pituitary tumor.
• The serum prolactin level was normal, eliminating prolactinoma as the type of adenoma. Tip: prolactinomas are treated initially with cabergoline, a dopamine agonist that can dramatically shrink the tumor and restore normal pituitary function, but the medication must be taken indefinitely or the tumor will recur. In this patient, transsphenoidal surgery yielded complete recovery of vision. Early diagnosis of this condition unquestionably improves visual outcome!

Correct Answer: Optic nerve

• The visual field defects are “cecocentral scotomas” (also called “centrocecal scotomas”) because they affect the visual field that lies between fixation and the physiologic blind spot. Whenever you encounter this pattern of visual field loss in both eyes, think mitochondria of retinal ganglion cells. The unmyelinated axons of those ganglion cells lie in the temporal portion of the optic disc, which is why that portion of the optic disc eventually appears pale.
• The pathology thins out the retinal nerve fiber layer around the fovea and between the fovea and the optic disc, which will be evident on optical coherence tomography (OCT). Toxic, metabolic (including nutritional deficiency), and hereditary conditions are behind this damage. Consider these common causes: ethambutol toxicity, thiamine (B1) and hydroxycobalamin (B12) deficiencies, alcoholism, starvation diets, non-adherence to vitamin supplementation after bariatric surgery, hereditary dominant (OPA 1) and Leber optic neuropathies.

This patient admitted to severe alcoholism. Treatment consisted of heavy thiamine dosing and abstinence. Vision is more likely to recover if the patient is adherent to the treatment regimen, if the optic discs have not developed pallor, and if OCT does not show thinning. Hence the importance of early diagnosis, especially in patients taking ethambutol, where scrupulous monitoring of visual acuity and color vision is advisable, so that the medication can be discontinued at the first sign of optic nerve toxicity.

Correct Answer: Somewhere in the retrochiasmal visual pathway

• Although this seems a relatively evasive answer, it is correct. You are looking at a complete homonymous hemianopia identified on confrontation and formal visual field testing. When a homonymous hemianopia is COMPLETE, you can localize it to the retrochiasmal visual pathway, but not to a specific region within that pathway! As the MRI is certifiably normal, you will wonder if the defect is of psychogenic origin. That presumption is difficult to disprove clinically. Some clinicians have developed maneuvers, such as having patients rapidly switch fixation back and forth between single targets displayed in opposite hemifields, but I have had a hard time convincing myself that I can exclude psychogenic “homonymous hemianopia.” Be careful here, as there are several retrochiasmal lesions that are either invisible on MRI or so subtle that they may be overlooked.
• Among those lesions are: 1) Recent hypoxic-ischemic injury owing to sudden systemic hypotension (MRI visual cortex volume loss may develop months later); 2) Creutzfeldt-Jakob disease, a prion protein disorder that may show subtle relevant MRI abnormalities only months later (positron emission tomography would earlier show a metabolically low visual cortex region); 3) focal encephalitis, also called “cerebritis” (small areas of high T2/FLAIR or cortical enhancement may be overlooked or dismissed); 4) Alzheimer disease with predominant volume loss in posterior cerebral hemisphere (relatively more MRI volume loss on the side opposite to the hemianopia may be overlooked); 5) focal metabolic encephalopathy in nonketotic hyperglycemia or mitochondrial cytopathy (which can be evanescent); and 6) optic tract lesion (because it is a narrow structure wedged between midbrain and temporal lobe, lesions here are often overlooked). This patient had signal abnormalities in the right optic tract and in the cervical spinal cord consistent with multiple sclerosis.

Correct Answer: Visual Cortex

• These small defects in the left hemifields are confined to the central degrees of the visual field. They are called “homonymous hemianopic paracentral scotomas.” Because they lie so close to the fixation point, they cause troublesome visual symptoms. They slow reading speed, as the patient consistently misses the first part of each word. You will be surprised to discover that the lesion occupied the entire back half of the visual cortex on the right side! Why would such a large lesion cause such small defects? Because the human brain devotes the entire posterior half of visual cortex to the central 10 degrees of the visual field, a phenomenon called the “magnification factor.” It testifies to the importance of “foveal vision” in primates.
• Such paracentral defects are routinely overlooked on standard static perimetry protocols, in which the test points are spaced far enough apart (“6 degrees of separation”) to miss the defects. Protocols that sample the central 10 degrees with test points at closer intervals are more likely to find these defects. In this patient, the defects were caused by posterior cerebral artery infarction, possibly related to embolism during the heart surgery.

Correct Answer: Optic Tract

• These visual field defects are confined to the left hemifields in both eyes and they have borders aligned to the vertical meridian, defining them as an incomplete “homonymous hemianopia.” But notice also that the defects in the two eyes are not of the same extent. If you overlaid them, they would not superimpose. This difference in the size of the two defects is called “incongruity.” If that incongruity is a true representation—and not simply an inconsistency in patient performance on the test—then the lesion must lie in the optic tract, where axons from corresponding points in the retinas of the two eyes have not yet come to lie close to one another.
• After all, the optic tract is early in the retrochiasmal portion of the visual pathway, which carries out the transformation of visual representation in the brain from monocular to hemifield. As the axons proceed farther posteriorly, through the lateral geniculate bodies and optic radiations to visual cortex, axons from corresponding retinal points become neighbors and incomplete homonymous hemianopias become more congruous. By the way, why did the patient say that she had lost vision “in my left eye?” Because patients will almost always blame a homonymous hemianopia on the eye on the side of the hemianopia! In this patient, brain MRI disclosed enhancement in the right optic tract and other signal abnormalities consistent with multiple sclerosis.

Correct Answer: Visual Cortex

• Notice three features of these visual field defects: 1) confined to the left hemifields in both eyes; 2) have discrete borders along the vertical meridian; 3) spare the central 10 degrees of the visual fields. These features define a “macular-sparing homonymous hemianopia,” one of the most localizing of all visual field defects! This pattern always derives from a lesion in the visual cortex that spares the posterior portion of that cortex, the termination point of axons carrying visual information from the central 10 degrees of the visual field.
• The patient was slow to notice these defects because of macular sparing, which allowed him to read at nearly normal speed and to ignore the more peripheral scotomas. He thought he could drive safely. However, in most jurisdictions, the finding of macular-sparing homonymous hemianopia would preclude a patient from being issued a driving license! The cause of these defects was an ischemic stroke in the distribution of the right posterior cerebral artery.
• The defects are permanent. He had atrial fibrillation and required long term rate control and anticoagulation to prevent future stroke.

Correct Answer: Parietal lobes

• This patient is simply unable to point accurately to objects in an array. Also, she forgets which ones she has already identified, so she miscounts. Her problem derives from a combination of visuospatial and attentional dysfunction. The term “simultanagnosia” has been applied to this deficit. The lesions lie in the inferior parietal lobules on both sides, interrupting the integration of visual and somatosensory information and the application of attention appropriate to the task. Called Balint syndrome, or Balint-Holmes syndrome, it most often arises acutely from biparietal stroke, usually in the setting of systemic hypotension (“watershed” or “border zone” infarction).
• When these deficits appear chronically, the most common cause is the “visual variant” of Alzheimer disease, sometimes known as “posterior cortical atrophy.”

Correct Answer: Left occipital-temporal lobe

• She is demonstrating the manifestations of “alexia without agraphia,” also known as “pure alexia.” It is a recognition disturbance for written language (and often other familiar symbols). It is not an aphasia (language disturbance). Written information cannot reach the posterior portion of the angular gyrus in the left temporal lobe. This disconnection happens most often when a lesion—almost always an ischemic stroke in the domain of the posterior cerebral artery—damages the left medial occipital lobe, causing a right homonymous hemianopia, and extends anteriorly enough to damage the splenium of the corpus callosum to interrupt transmission of visual information from the intact right visual cortex to the left angular gyrus. Patients can spell, understand spoken language, and express themselves normally, because those functions are generated in the angular gyrus without needing transmission from the visual cortex. Which critical neuro-ophthalmic abnormality was bound to be present (although not always!) and not mentioned in the vignette? A complete right homonymous hemianopia. This patient did have a left posterior cerebral artery stroke with forward extension into the splenial region.
• Over time, her reading deficit gradually improved, but she still hesitated. The homonymous hemianopia was permanent.

Correct Answer: Right pons

• This patient has a palsy of right horizontal conjugate gaze (“gaze palsy”) that involves saccades, pursuit, and the vestibulo-ocular reflex. Although you could postulate lesions affecting the extraocular muscles or the neuromuscular junction activated in right gaze, such a symmetrical deficit of conjugate gaze to one side will virtually always originate within the brain. If volitional and reflex gaze are both completely impaired, expect to find the lesion in the pons. You should be surprised that there are no other neurologic deficits–seventh nerve palsy, internuclear ophthalmoplegia, nystagmus, ataxia, skew deviation, or limb weakness. However, a lesion confined to a small area in the tegmental pons—in the location of the sixth nerve nucleus– might not produce any obvious accompanying signs (although you should look for them as “fellow travelers”). This patient had a hemorrhage from a right pontine tegmental cavernous malformation (“cavernoma”).
• The only effective treatment is surgical excision, which is hazardous. No treatment was undertaken in this case. The patient’s deficits gradually resolved enough to give him partial restoration of right gaze. He was advised to avoid any activity that would generate increased intracranial pressure (like weight-lifting, evacuative straining, and standing on his head!)

Correct Answer: Right cerebral hemisphere

In order to answer this question correctly, you had to know three things: 1) horizontal saccades are generated in the cerebral hemispheres and that they move the eyes toward the opposite side; 2) This patient is displaying a dissociation between loss of volitional leftward horizontal saccades and preservation of the leftward vestibulo-ocular reflex (“supranuclear ophthalmoplegia”); and 3) the vestibulo-ocular reflex pathway is confined motorically to the brainstem. Acute right hemisphere lesions (usually infarcts or hemorrhages), especially if they involve the parietal lobe or its efferent pathway, often produce ipsilateral gaze deviation and a supranuclear gaze disturbance, as described here. These phenomena may be manifestations of “motor neglect.” In fact, sensory aspects of hemispatial neglect are usually present, including extinction to double simultaneous stimulation in the visual, auditory, and tactile domains. Extinction of the visual field in one hemifield could, by itself, reflect a subtle homonymous hemianopia. But the presence of multimodal–visual, auditory, and tactile–extinction favors neglect over a topographic disorder of vision such as a homonymous hemianopia. Patients with homonymous hemianopias rapidly develop the ability to explore into their deficient hemifield and will bisect a line segment in the midline, especially if cued. Patients with hemispatial neglect consistently ignore stimuli in one hemifield and bisect a line segment more toward the side of the lesion. When the lesion is severe, neglect will extend to lack of awareness of contralateral limbs (“anosognosia”). Fortunately, the manifestations of neglect gradually lessen with time, but may never completely disappear. This patient had a right parietal infarct.

Correct Answer: Posterior commissure

• This is an important neuro-ophthalmic landmark! Lying at the junction of the thalamus and midbrain, it mediates upward gaze by connecting the rostral interstitial nuclei of the medial longitudinal fasciculus (RIMLF) and interstitial nucleus of Cajal (INC) on both sides.
• A lesion here SELECTIVELY interferes with upward gaze, sparing downward gaze, which is mediated more ventrally. When this commissure is damaged by an intrinsic lesion of the caudal thalamus or pretectal or tectal midbrain regions, or by an extrinsic lesion like a pinealoma, expect elements of a “dorsal midbrain syndrome” (also called “pretectal syndrome,” “Sylvian aqueduct syndrome,” or “Parinaud syndrome” if you are French, “Koerber-Salus-Elschnig syndrome” if you are German). The clinical features of this syndrome include lid retraction (“Collier’s sign”), dilated pupils with light-near dissociation, and sometimes esotropia, exotropia, or skew deviation. Why does a lesion here cause such a diverse set of clinical phenomena? Disruption of upgaze produces a “spill-over” activation of all extraocular muscles, pulling the eyes backwards into the orbit. Because the medial recti are the most powerful muscles, the eyes tend to converge. Dilated pupils with light-near dissociation (“tectal pupils”) are attributed to interruption of the pupillary light reflex pathway in the dorsal midbrain. Esotropia and exotropia result from disruption of the vergence pathway in that region, and skew deviation results from interruption of the vestibulo-ocular pathway as it reaches the midbrain. This patient had a pinealoma.
• Early diagnosis favors a better outcome of treatment. By the way, even with early diagnosis of pineal tumor, some features of a dorsal midbrain syndrome—especially upgaze deficiency and convergence-retraction–often linger. Why is this fact important? Because if upgaze deficiency and convergence-retraction are detected long after treatment, they should not be taken as signs of tumor recurrence or of a complication of radiotherapy, even if not previously documented!

Correct Answer: Right parietal lobe

In order to get the correct answer here, you must be aware that pursuit is mediated by a pathway that starts in the parietal lobe and generates pursuit eye movements ipsilaterally. When you move the optokinetic drum, you generate an obligatory pursuit movement in the direction of the moving stripes (the patient must be paying attention!). Each pursuit movement is met by an oppositely-directed conjugate saccadic movement that restores the eye position to straight ahead. You can use this device to generate repetitive jerk nystagmus. If you consistently see a discrepancy in nystagmus amplitude between rightwardly and leftwardly directed stripes, you must conclude that there is a unidirectional pursuit deficit caused by a lesion in the ipsilateral parietal lobe. By the way, the patient will not notice this pursuit deficit.

Correct Answer: Diencephalon

This patient has suffered severe damage to all parts of his brain, but the comitant esotropia with full ocular ductions suggests diencephalic dysfunction. There has been a breakdown of fusion, such that the balance between convergence and divergence has been upset. A lesion in the cerebral hemispheres could also upset that balance, but you were not offered that answer option here. Bilateral sixth nerve palsy is a common mistaken explanation for esotropia, but that should reduce abduction and cause more esotropia on side gaze than in primary gaze position (“incomitant esotropia”). Bilateral third nerve palsy would cause an exotropia. Bilateral fourth nerve palsy would cause a right hypertropia in left gaze and a left hypertropia in right gaze, features not present here.

Correct Answer: Midbrain

• You should find this combination of neuro-ophthalmic abnormalities challenging. Why? First, the complaint of blurred vision suggests that something is wrong with the visual pathway. But patients will often interpret mild misalignment of the eyes as blurred vision. The clue that the blurred vision is actually diplopia comes from the fact that covering either eye improves vision! Second, when eye movements are full, many examiners will assume that there is no disorder of ocular alignment. Not true. This patient has a vertical misalignment (“hypertropia”) discovered with the cover test. The challenge here is that the misalignment is small, so difficult to see. You could use the Maddox rod, which might show a consistent vertical displacement of the two images. Third, the oscillations of the eyes, present only in extremes of gaze, will often go unnoticed. This pattern of oscillations–called “nystagmus”– is important evidence of brainstem (rather than semicircular canal or otolith) dysfunction. The lesion is affecting one side of the pathway carrying the vertical vestibulo-ocular reflex. That pathway travels from the medulla to the rostral midbrain and diencephalon. A unilateral or asymmetric lesion here creates an abnormal “ocular tilt reaction,” which includes torsional displacement of the eyes and vertical misalignment.
• The patient may notice torsional displacement of the environment, and will report the vertical misalignment of the eyes (called “skew deviation”) as diplopia or blurred vision
• In this case, the cause proved to be multiple sclerosis. The brainstem did not show any MRI signal abnormalities, but there were characteristic T2/FLAIR signal abnormalities in the cerebral white matter.

Correct Answer: Cerebral peduncle

This is a classic “Weber syndrome” of third nerve palsy and contralateral hemiparesis. It arises from a lesion in the left third nerve fascicles as they pass through the cerebral peduncle, which carries corticopontine fibers that cross in the rostral pons and corticospinal fibers that cross in the medullary pyramids to innervate musculature on the opposite side of the body. Although most third nerve palsies have no accompanying neurologic abnormalities, occurring in the subarachnoid segment of the third nerve, you should recognize this particular “third nerve palsy plus” syndrome because it implies a lesion of the ventral midbrain, usually an infarction in the domain of an occluded paramedian arterial perforator, but inflammations and cancers can also do this. Aneurysm is not a consideration, but rarely Weber syndrome may be an early sign of rostral basilar thrombosis, so prompt vascular imaging and surveillance is necessary. MRI may show restricted diffusion in the ventral midbrain, yet very often the stroke is too small to show up. Even so, assume there is a stroke and direct your attention to future stroke prevention caused by disorders affecting small arterial vessels.

Correct Answer: Superior oblique muscle/tendon

• The pattern of ocular misalignment fulfills the features of the Bielshowsky “three-step test,” a strong indication of a “fourth nerve palsy.” In addition, the fact that the hypertropia in left gaze is greater when the patient is looking up than when he is looking down is critical to localizing the lesion: it lies in the superior oblique muscle or its tendon.
• Why does this part of the body give out? Consider that the anatomy of the superior oblique muscle and its tendon is awkward: the muscle passes forward along the medial orbital wall, passes through a sleeve of dura called the trochlea, and must turn more than 90 degrees to insert onto the top of the eye.
• It should not be surprising that the efficiency of its pulling power might decline as the years go by! Sometimes the diplopia in this condition can be palliated with a spectacle prism. But because of the incomitance, the prism is usually an imperfect or temporary solution, and eye muscle surgery the answer. Fortunately, a minor procedure (weakening of the inferior oblique muscle) is likely to provide relief of diplopia. Acquired causes of “fourth nerve palsy,” including trauma, tumor, and inflammation, would cause a hypertropia that is greater in downgaze than in upgaze. If you selected “neuromuscular junction” as your choice, you could hardly be faulted here, because myasthenia gravis can produce any pattern of ocular misalignment. In fact, the intermittency of the diplopia is likely to send you toward that diagnosis, but when the misalignment obeys the features of the three-step test, myasthenia is a less likely diagnosis. By the way, myasthenia gravis is not the only cause of intermittent diplopia. Here are other reasons for this phenomenon: 1) diplopia is present only in certain fields of gaze, and 2) fusion is breaking down because of fatigue or CNS depressants, which convert a phoria to a tropia.

Correct Answer: Dorello’s canal

• The likely cause of the diplopia is a right sixth nerve palsy. His ear abnormalities are likely caused by an infected middle ear (“otitis media”) with spread of the infection to the mastoid region. How can you connect that infection with the sixth nerve palsy? By assuming that the infection has spread through the petrous bone to its apex, where the bone forms the lateral wall of Dorello’s canal, the narrow space through which the sixth nerve travels to get from the subarachnoid space into the cavernous sinus.
• The association between middle ear infection and sixth nerve palsy was described long ago by Gradenigo and is now known as “Gradenigo’s syndrome.” The sixth nerve in the tight space of Dorello’s canal is vulnerable to a “compartment syndrome” as infections and cancers come from a lateral source in the petrous bone, from a medial source in the clivus (a favorite site for cancer metastasis), and from an inferior source in the sphenoid sinus (infections and cancer). You cannot be faulted here if you chose “transverse-sigmoid (dural) sinus junction” as your answer. Mastoid inflammation from middle ear infections can cause venous sinus thrombosis at the transverse-sigmoid junction, raising intracranial pressure and causing a secondary sixth nerve palsy. Why does high intracranial pressure cause a sixth nerve palsy? Because the high pressure drives the brain downward, tugging on the sixth nerve where it is tethered in Dorello’s canal. But in such cases, papilledema is usually present (and it was not in this boy). By the way, low intracranial pressure also drives the brain downward, which is why sixth nerve palsy also occurs in intracranial hypotension. A final tip: when intracranial pressure goes up, you might instead encounter a comitant esotropia with full ocular ductions, representing a breakdown in “ocular fusion.”

Correct Answer: Cavernous sinus

An incomitant esotropia with image separation greatest in right gaze suggests a right abduction deficit, even if you cannot see it. Many processes can cause an abduction deficit, but when you add an ipsilateral Horner syndrome, you are closing in on the place where the oculosympathetic pathway and the sixth nerve pathway come together—the cavernous sinus. In the cavernous sinus, the sixth nerve is the only cranial nerve lying within the venous lake (the other cranial nerves travel in the outer dural wall). The sixth nerve passes inferolateral to the carotid artery. On the outer wall of the carotid artery, you will find the oculosympathetic nerve. The combination of sixth nerve palsy and Horner syndrome is common with lesions arising within the cavernous sinus, including aneurysms and metastases.

Correct Answer: Mydriasis and light-near dissociation

A lesion of the ciliary ganglion in the orbit interrupts the parasympathetic supply to the iris sphincter and ciliary muscle, creating a pupil that does not constrict to light or to a target viewed at close range. Within weeks, pupil constriction to a target viewed at close range (“the near response”) is reestablished, but that constriction occurs slowly, creating the term “tonic pupil.” When gaze is redirected from a near to a distant target, pupil dilation is also slow. You might also note that the affected pupil is slightly oval and that the tonic constriction is segmental. Why do those phenomena occur? Because a lesion of the ciliary ganglion affects its nerve outflow segmentally; that is, one part of the iris sphincter is damaged more than other parts. Query: if you are confronted with a unilaterally dilated pupil, how would you know that it might be caused by a relatively acute ciliary ganglionopathy (before development of light-near dissociation and tonic features) rather than by exposure to a topical parasympatholytic agent? There are two clues: 1) in the short term, ciliary denervation causes asymmetric enlargement of the pupil (it has an oval shape) because the denervation is segmental, whereas parasympatholytic agents cause symmetric enlargement of the pupil (it has a round shape); and 2) a ciliary denervated pupil will constrict following instillation of pilocarpine 1%, whereas a pupil that is dilated because of topical exposure to a parasympatholytic agent will constrict minimally after instillation of pilocarpine 1%. Finally, here is a comment about the other answer choices. The combination of ptosis and supraduction deficit could represent a lesion of the superior division of the somatic portion of the third cranial nerve, which would occur intracranially, not in the orbit. The combination of ptosis and miosis suggests Horner syndrome, which arises from a lesion of the oculosympathetic pathway, which does not include the ciliary ganglion. The combination of mydriasis, adduction deficit, and lack of constriction to light or a near target is suggestive of a lesion of the intracranial third nerve that affects its inferior division.

Correct Answer: Third-order neuronal projection

• The first-order neuronal projection extends from the hypothalamus to the upper thoracic spinal cord. Lesions in the hypothalamus are rare and usually cause other neurologic manifestations. Lesions in the brainstem that cause Horner syndrome usually lie in the medulla and never cause Horner syndrome alone. Spinal cord lesions would also be expected to cause other neurologic manifestations. The second-order neuronal projection extends from the spinal cord to the superior cervical ganglion in the neck. Lesions here could cause Horner syndrome in isolation, but almost never acutely. The third order neuronal projection extends from the superior cervical ganglion to the eye. Acute Horner syndrome with ipsilateral facial or neck pain is most commonly caused by cervical carotid dissection, which damages the oculosympathetic axons lying on the outer edge of the vessel wall. The deformation of the carotid artery wall also causes turbulent flow, which may generate emboli that can cause cerebral stroke. The risk of stroke rapidly declines within the first 10 days after the dissection. Patients are usually placed acutely on prophylactic aspirin. The ciliary ganglion and nerves do not lie within the oculosympathetic pathway.

Correct Answer: Medulla

• The combination of Horner syndrome, hypertropia (from skew deviation), and ataxia is strong evidence of a lesion in the dorsolateral medulla. In that region, infarction is the overwhelmingly likely cause, secondary to occlusion of the ipsilateral vertebral artery and its posterior inferior cerebellar artery branch. A stroke in this territory, known as the “Wallenberg syndrome,” may also damage the inferior cerebellum. If the infarction is large, the cerebellum may swell enough to compress the medulla, creating a neurological emergency. The damaged cerebellum may have to be surgically excised. This is the most common brainstem stroke at any age, but especially in young adults. The underlying lesion may be idiopathic or traumatic dissection of the vertebral artery. By the way, can you guess whether the right eye or the left eye was the higher eye? Answer: the right eye. Use this rule: in skew deviation, the higher eye will always be on the side opposite to a medullary or caudal pontine lesion, whereas the higher eye will be on the same side as a rostral pontine or midbrain lesion.

Correct Answer: Right afferent pupil defect

A lesion in the optic tract is apt to cause a contralateral afferent pupil defect because axons coming from that eye and crossing into the opposite optic tract outnumber non-crossing axons. Such an afferent pupil defect is more likely to occur if the lesion damages the optic tract severely enough to cause a complete or nearly complete homonymous hemianopia. Why is this phenomenon important clinically? Because the combination of a unilateral complete homonymous hemianopia and an ipsilateral afferent pupil defect localizes the lesion to the contralateral optic tract, provided there is no evidence of an optic neuropathy on the side of the afferent pupil defect. Without that afferent pupil defect, you might not be able to localize a complete homonymous hemianopia to a particular region in the retrochiasmal visual pathway. Light-near dissociation occurs with lesions of the dorsal midbrain, not from lesions in the optic tract.

Correct Answer: Midbrain

• When the right eye is higher in left gaze and the left eye is higher on right gaze, you are looking at BILATERAL fourth nerve palsies, especially in the setting of head trauma. In concussive head trauma, the brain moves more than its coverings, including the dura. As a result, the dorsal midbrain slams into the rigid, knife-like inner margin of the tentorium cerebelli at exactly the place where the crossing fourth nerves exit from the brainstem. The fourth nerves are often injured together, although usually asymmetrically.
• Bilateral fourth nerve palsies create this pattern of “alternating hypertropia,” a large (at least 10 degrees) excyclodeviation, and an esotropia in downgaze. Imaging generally does not show a pertinent abnormality, although sometimes there is hemorrhage in the caudal dorsal midbrain.
• Recovery is delayed for months and is of variable degree. Eye muscle surgery may eventually be necessary.

Correct Answer: Right vestibular nerve

• The new imbalance, unidirectional horizontal rotary nystagmus, and the positive head impulse test strongly suggest an acute right peripheral vestibulopathy. It has upset the balance between the right and left vestibular pathway inputs. The right-sided lesion produces a drift of the eyes toward the right, which is met by compensatory leftwardly directed saccades, making up a left jerk nystagmus. The nystagmus remains left-beating in all three positions of horizontal gaze, a feature relatively unique to acute peripheral vestibulopathy.
• The positive head impulse test, which can be difficult to interpret in the presence of nystagmus, supports a peripheral lesion.
• The vestibular imbalance explains why the patient falls to her right. A common cause for this condition is “vestibular neuritis/neuronitis,” attributed to a virus. It can be frightening to patients and examiners and very unpleasant. In that condition, there is good evidence of reactivation of herpes simplex type 1 virus in the vestibular ganglia. A large trial showed no benefit of anti-viral therapy, but symptoms resolved more quickly in patients treated with prednisone 1mg/kg for 10 days. Therefore, steroid is often prescribed once the diagnosis is ascertained. Meclizine is also often prescribed for nausea. In this patient, the manifestations rapidly lessened and disappeared completely within 10 days.

Correct Answer: Optic Nerve

You are looking at a temporal visual field defect in the left eye that has a discrete border aligned to the vertical meridian that passes through fixation. The visual field in the right eye is normal. The localizing feature here is that the visual field defect has a border aligned to the vertical meridian, which places the lesion in the left optic nerve as it approaches the optic chiasm.

At that junction, axons derived from nasal retina have split off from those derived from temporal retina. Lesions there—no matter their nature—will preferentially damage the nasal axons to produce this unilateral temporal hemianopic visual field defect. If the lesion were situated farther posterior toward the optic chiasm, a temporal visual field defect would also be present in the other eye–a bitemporal hemianopia. This patient had a meningioma growing off the left clinoid process

Surgical removal led to normalization of the visual fields and disappearance of the afferent pupil defect. Early diagnosis—before the tumor has exerted severe compression of the optic pathway— is critical to the visual outcome in this circumstance. Many patients show unilateral temporal hemianopic defects on formal perimetry. How would you know that the defect reflects disease in the ipsilateral optic nerve near its junction with the optic chiasm? By finding an ipsilateral afferent pupil defect, as in this case.

Correct Answer: Left eye, temporal visual field

Correct Answer: 10 mm superior

The optic chiasm, which contains the crossing fibers of the optic nerves, is located 10 mm above the sella turcica. The proximity to the pituitary gland makes it a danger zone for pituitary macroadenomas, which can compress the optic chiasm, causing bitemporal hemianopia.

Incorrect Answers:

10 mm anterior. The optic chiasm is anterior to the hypothalamus and the tip of the 3rd ventricle.

10 mm inferior. The optic chiasm is inferior to the hypothalamus.

10 mm posterior. The optic chiasm is posterior to the optic nerves.

Correct Answer: Internal carotid

Correct Answer: Magnetic resonance imaging with contrast and fat-suppressed sequences

MRI of the brain and orbits with contrast, including fat suppression, is the best initial diagnostic modality to evaluate for a compressive lesion of the visual pathway, which is suspected in this case.

Incorrect Answers:

A. Computed tomography with contrast. CT is good for evaluating bony abnormalities (e.g., fractures) and hemorrhage but is not as good as MRI for evaluating soft tissues like the anterior visual pathways and brain.

B. Computed tomography angiography. CT angiography is a quick and accurate method for detecting cerebral aneurysms but would not adequately evaluate for other compressive lesions.

C. Magnetic resonance imaging without contrast. Noncontrasted MRI may miss smaller tumors and inflammatory lesions that may cause optic neuropathies, and contrast should be used whenever possible when evaluating for compressive lesions.

Correct Answer: Junctional scotoma, posterior optic nerve

A lesion at the posterior optic nerve produces a junctional scotoma— a visual field defect characterized by ipsilateral central and contralateral superotemporal field loss. This pattern is postulated to occur because of crossing fibers from one optic nerve loop, for a short distance, into the opposite optic nerve before joining the tract. This loop has been termed “Willebrand’s knee,” though whether it truly exists anatomically is debated.

Incorrect Answers:

Bitemporal hemianopia, optic chiasm. A bitemporal hemianopia arises from a compression of the optic chiasm.

Homonymous quadrantanopia, Meyer loop. The Meyer loop represents the portion of optic radiations in the temporal lobe. A lesion here causes a superior quadrantanopia.

Homonymous hemianopia, optic radiation. Contralateral homonymous hemianopia can arise from a defect in the optic tract, optic radiations, or primary visual cortex.