Neurology: A Clinician's Approach (Cambridge Medicine (Paperback)), 1st Ed.

5. Visual loss

Neuroanatomy

A brief discussion of the neuroanatomy of the visual system (Figure 5.1) is necessary to understand how to approach a patient with visual loss. Light enters the eye through the cornea, passes through the anterior chamber, the lens, and the vitreous to reach the retina. Images are projected upside down and backwards onto the retina: the inferior temporal retina, therefore, contains the image of the superior nasal part of space. The optic nerve enters the retina at the optic disc. Lateral to the optic disc is the macula, the center of which is the fovea, the area of greatest visual acuity. The optic nerve projects posteriorly: nasal optic nerve fibers (those that see the temporal field of vision) decussate in the optic chiasm while the temporal optic nerve fibers remain uncrossed. The optic chiasm gives rise to the optic tracts. The optic tract contains the representation of the contralateral half of visual space: the left optic tract therefore “sees” the temporal field of the right eye and the nasal field of the left eye. The optic tracts send fibers to the lateral geniculate body of the thalamus and to the pretectal nucleus of the midbrain. These pretectal fibers synapse with the Edinger–Westphal nucleus of the oculomotor nerve (which mediates pupilloconstriction; see Chapter 7) and decussate in the posterior commissure to reach the contralateral pretectal nucleus. The lateral geniculate body sends fibers to the occipital cortex via the optic radiations. The temporal optic radiations contain fibers from the superior visual fields and are known as Meyer’s loop, while the parietal optic radiations contain fibers from the inferior visual fields. The posterior occipital cortex contains the representation of macular vision. Progressively more anterior parts of the occipital cortex contain progressively more peripheral representations of visual space. The anterior part of the visual cortex contains a representation of the extreme temporal periphery in the contralateral eye (the temporal crescent) but lacks a homonymous nasal representation from the ipsilateral eye. The occipital cortex sends projections to the ipsilateral visual association cortices. These may be roughly divided into the “where” cortex of the parietal lobe, which processes spatial information, and the “what” cortex of the temporal lobe, which processes content.

History

Most patients with visual loss have ophthalmological problems such as cataracts, glaucoma, and macular degeneration. With some exceptions such as angle-closure glaucoma and retinal detachment, most of these conditions tend to develop slowly over months to years. The neurological conditions that affect vision, on the other hand, generally develop over a course of minutes, hours, or days, and often lead to evaluation in the emergency room. Similar to all other neurological processes, the key task in evaluating a patient with visual loss is to localize the problem. The most important elements of the history, beyond the tempo of symptom development, are whether pain accompanies visual loss and the exact pattern of visual field loss. This may be established to some degree by the history, but in most cases, the examination is more helpful. In some instances, it may be useful to “cheat” slightly by beginning with a brief examination in order to tailor the history appropriately.

Examination of the visual system

Visual acuity

The first step in examining the visual system is to measure visual acuity in each eye. In order to eliminate refractive errors, the patient should wear their eyeglasses or contact lenses when their acuity is tested. If they do not have their glasses, eliminate refractive errors by using a pinhole occluder or by poking tiny holes through an index card.

Figure 5.2

Figure 5.2 Testing for a right relative afferent pupillary defect. Note that in (E) and (F), the consensual light reaction is not shown. See text for more details.

Figure 5.1

Figure 5.1 Schematic of visual system neuroanatomy and common visual field defects. See text for more details.

Pupillary reactions

The anatomy of pupillary reactions is discussed in Chapter 7. In a patient with visual loss, examine the pupils in both light and dark. Make note of their size and regularity. Use a sufficiently bright flashlight when examining pupillary reactions: if the patient’s pupils constrict and then dilate quickly (hippus), the intensity of the light is probably too low. Observe for both the direct (pupillary reaction in the eye in which the light is shined) and consensual (pupillary reaction in the eye in which the light is not shined) reactions.

Relative afferent pupillary defect

Relative afferent pupillary defect (RAPD) almost always indicates an ipsilateral optic nerve lesion (Figure 5.2). To test for RAPD, examine the patient in a dimly lit room with a bright flashlight. Instruct the patient to focus on a distant target and shine the light into the eye that is being tested from below eye level. For a patient with a right RAPD:

1. At rest, both pupils should be equal (Figure 5.2A).

2. Shine the light in the right eye for approximately 3 seconds and observe the direct and consensual pupillary reactions (Figure 5.2B).

3. Remove the light and allow the pupils to dilate (Figure 5.2C).

4. Shine the light in the left eye for approximately 3 seconds and observe the direct and consensual pupillary reactions (Figure 5.2D).

5. Swing the light to the right eye and watch its reaction. If the pupil dilates even slightly on the right, the patient has a right RAPD (Figure 5.2E). The left pupil should also dilate (not shown).

6. Swing the light back to the left eye and look for constriction of the left pupil (Figure 5.2F). The right pupil should also constrict (not shown).

It is important to swing the flashlight back and forth several times in order to verify that RAPD is indeed present.

Color discrimination

The next step in assessing visual loss is to test color discrimination, as color vision is often lost early in patients with optic nerve disease. The most sensitive way to do this at the bedside is with Ishihara color plates. Because these are often not readily available, a simple way to examine for gross color vision defects is to test for red desaturation. Hold a bright red object in front of each eye in sequence and ask the patient to describe the color of the object and to note any color differences between the eyes. A patient with an optic neuropathy will perceive a bright red object as pink, black, brown, or washed out in appearance.

Visual field examination

Most gross visual field defects that bring a patient to acute neurological attention are detectable with bedside confrontation testing. Perimetry may be required, however, to detect more subtle deficits.

Central visual fields

Examine the visual fields in each eye independently. First, instruct the patient to look directly at your nose and test their central visual fields by asking them whether any parts of your face appear to be missing or blurred. Another good test of central vision is to ask the patient to trace their visual field deficit onto an Amsler grid or piece of graph paper.

Peripheral visual fields

Start by placing yourself approximately 2–3 feet away from the patient. Instruct them to close one eye and to look directly at your nose. For the first part of the examination, tell them that you will show them one or two fingers, and that it is their job to tell you how many they see. Quickly flash one or two fingers in each of the four quadrants of the visual field in succession. This is a gross test of each of the peripheral visual fields. Next, map out the visual fields using a red pinhead. Remind the patient yet again that they should be focusing on your nose and tell them to report the exact instant at which they perceive the redness of the pin. Sweep the pin from the periphery to the center of the visual field. If the pin is halfway between their viewing perspective and your own, you should both appreciate its redness at approximately the same time.

Common patterns of visual field deficits

Visual field examination is often time-consuming and subtle deficits may be missed if the fields are examined too hastily. Conversely, most visual field defects indicative of serious neurological disease are quite obvious. It may be helpful to screen for the following common patterns (Figure 5.1) of visual field loss:

• Monocular visual loss. This is the expected pattern of visual field loss in a patient with dysfunction of the eye or optic nerve.

• Central scotoma. In this pattern, the central portion of vision is lost while the periphery is preserved. This too implies dysfunction of the retina or optic nerve.

• Monocular altitudinal defect (not shown). Either the top or bottom half of vision is lost. This pattern suggests disease of the retinal vessels.

• Bitemporal hemianopsia. The temporal half of vision is lost in both eyes. This is the classical pattern produced by disorders of the chiasmal region such as pituitary adenomas and craniopharyngiomas.

• Junctional scotoma. A junctional scotoma occurs when a mass at the optic chiasm compresses the ipsilateral optic nerve and the decussating fibers from the contralateral nasal portion of the optic nerve. The result is ipsilateral monocular visual loss (or a central scotoma) and a contralateral temporal field cut.

• Homonymous hemianopsia. This is the typical pattern of postchiasmatic lesions: the same “half” of vision (e.g. right nasal field and left temporal field) is affected in both eyes. Postchiasmatic lesions that are relatively more anterior (e.g. in the optic tracts and anterior radiations) tend to be less symmetric than those that are more posterior (e.g. the posterior radiations and occipital lobes).

• Lateral geniculate lesions. These lesions may produce contralateral homonymous hemianopia. However, lesions of the anterior choroidal artery may produce a homonymous hemianopia with a spared horizontal central wedge, while lesions of the posterior choroidal artery may cause loss of the central wedge with sparing of the periphery. These deficits are present inconsistently and are often difficult to map out at the bedside.

• Upper quadrantanopsia. This is the loss of the superotemporal field in the contralateral eye and the superonasal field in the ipsilateral eye. It is the characteristic field cut produced by lesions of the temporal optic radiations.

• Lower quadrantanopsia. Loss of the inferotemporal field in the contralateral eye and the inferonasal field in the ipsilateral eye is the characteristic field cut of the optic radiations within the parietal lobe.

• Macular-sparing homonymous hemianopsia. Occipital lesions classically lead to homonymous hemianopsia with sparing of macular vision. The exact explanation as to why the macula is spared is uncertain. Possible but not completely satisfactory explanations include a dual blood supply (posterior cerebral and middle cerebral arteries) of the occipital cortex subserving foveal vision and bilateral representation of foveal vision in the occipital cortex.

• Temporal crescent defects. Anterior occipital lesions produce loss of vision in the extreme temporal periphery of the contralateral eye while sparing vision in the ipsilateral eye.

Funduscopic examination

The final step in evaluating the patient who complains of visual loss is funduscopic examination. Accurate funduscopic examination is essential to the diagnosis of monocular visual loss and may also be helpful in patients with binocular visual loss. It is important to visualize the fundi in the dark or to dilate the pupils pharmacologically. Important funduscopic abnormalities include:

• Central retinal artery occlusion. In the hyperacute setting, the optic disc appears normal or may show boxcar segmentation of blood within the retinal vessels. After approximately 1 hour, the retina takes on a white appearance. The vascular choroid (supplied by the posterior ciliary artery) shines through at the fovea, producing the classical cherry-red spot at the macula. Dull white (platelet–fibrin embolus) or bright yellow (cholesterol or Hollenhorst plaque) retinal emboli may be detected in the branches of the central retinal artery.

• Central retinal venous occlusion. The funduscopic appearance of central retinal venous occlusion is difficult to miss: the disc is blurred and the periphery of the fundus is smeared with hemorrhages.

• Optic disc pallor. This pattern reflects chronic disease of the optic nerve.

• Papilledema. Papilledema reflects increased intracranial pressure. In the earliest stages, papilledema is manifest by disc hyperemia and loss of venous pulsations. As the papilledema worsens, the disc becomes elevated and peripapillary vessel engorgement and hemorrhages develop. Chronic increased intracranial pressure leads to disc atrophy.

Monocular visual loss

The most common neurological etiologies of monocular visual loss are inflammatory and ischemic processes. It is important to consider these problems not only in patients with unilateral visual loss but also in patients with sequential bilateral visual loss.

Optic neuritis

Idiopathic optic neuritis

Optic neuritis is a common cause of acute to subacute visual loss in young people, especially young women. It is characterized by monocular vision loss associated with mild periocular pain upon eye movement. Visual loss occurs over a period of several days, and ranges from mildly reduced acuity to complete blindness with no light perception. Funduscopic examination is usually unremarkable in the acute setting. The vast majority of patients with optic neuritis have a complete or near-complete recovery of vision within a month. Further evaluation and treatment of optic neuritis is discussed in Chapter 22.

Atypical optic neuritis

Less common inflammatory and autoimmune causes of optic neuritis include sarcoidosis, Sjögren’s syndrome, systemic lupus erythematosus, and Behçet’s disease. These conditions may be distinguished from typical optic neuritis by the accompanying systemic symptoms. Devic’s disease (neuromyelitis optica) is characterized by bilateral optic neuritis and transverse myelitis, and is discussed further in Chapter 22.

Ischemic optic neuropathies

Temporal arteritis

Temporal arteritis is a systemic vasculitis that may result in blindness due to ischemia of the retina, choroid, or optic nerve. Patients with temporal arteritis are always older than 50, with a mean age of onset of about 70. Symptoms in addition to visual loss that suggest the diagnosis include headache, jaw claudication (pain with chewing), and scalp tenderness. Polymyalgia rheumatica, characterized by fever and aches in the shoulders and hips, accompanies temporal arteritis about half the time. Visual loss occurs in both eyes in up to 50% of patients: in approximately one-third of these, visual loss affects the fellow eye within 1 day, in another one-third within 1 week, and in the remaining one-third within 1 month. In patients with suspected temporal arteritis, check the erythrocyte sedimentation rate (ESR) and C-reactive protein (CRP) immediately.1 A normal ESR for a man is his age divided by two, and for a woman, her age plus ten divided by two.2 Unfortunately, recovery from visual loss related to temporal arteritis is rare, and the main purpose of treatment is to prevent visual loss in the fellow eye and other systemic symptoms. In order to give the patient the best chance of preserving their vision, start prednisone at 60–80 mg as soon as you consider the diagnosis. Because the morbidity of chronic steroid administration is high, it is advisable to arrange for temporal artery biopsy to confirm the diagnosis in almost all cases. Pathological changes of temporal arteritis remain visible on biopsy for as long as 2 weeks after initiating steroids.3 In patients with confirmed temporal arteritis, begin a slow steroid taper approximately 1 month after initiating therapy. Most patients require at least a low dose of steroids for approximately 1 year after starting treatment. Frequent examinations and ESR measurements are required for long-term monitoring purposes.

Nonarteritic ischemic optic neuropathy

Nonarteritic ischemic optic neuropathy (NAION) is a disorder of unclear etiology, which typically presents in older patients with painless monocular visual loss that develops over hours to days.4 Obviously, this presentation is similar to temporal arteritis, and several clinical clues must be used to distinguish between the two conditions. The degree of visual loss is typically milder in NAION than it is in patients with temporal arteritis. Patients lack headache or other symptoms of systemic disease. Binocular visual loss occurs in approximately 20% of patients, a much lower frequency than in temporal arteritis. ESR and CRP are normal. Unfortunately, there is no clearly proven therapy for NAION. Visual loss is usually permanent, but a minority of patients may improve spontaneously. Because the presumed mechanism of NAION involves ischemia of the posterior ciliary arteries, it is important to address risk factors for vascular disease to prevent systemic disease.

Less common optic neuropathies

Structural optic neuropathies

Compressive or infiltrative causes of optic neuropathies include trauma, tumor, abscess, or inflammatory lesions. It is important to recognize these causes of monocular visual loss quickly, as they require immediate consultation with an orbital surgeon.

Toxic and nutritional optic neuropathies

Methanol and ethylene glycol intoxication produce fulminant encephalopathies associated with bilateral severe optic neuropathies and a variety of systemic symptoms. Management of patients intoxicated with these substances usually requires consultation with multiple specialists including toxicologists and nephrologists. Ethambutol, disulfiram, and amiodarone are the most common medications that produce optic neuropathies. Vitamin B1 and B12deficiencies may also cause optic neuropathies.

Inherited optic neuropathies

Autosomal dominant and recessive optic neuropathies generally come to clinical attention in children. Leber’s hereditary optic neuropathy (LHON) is a mitochondrial disorder that affects mostly men, and may develop in adulthood.5It is passed down from mothers to their children in the mitochondrial DNA, and is characterized by painless, subacutely progressive visual loss. Visual loss becomes bilateral in almost all patients within several weeks to months. Some patients with LHON develop other CNS abnormalities, which may lead to a clinical picture resembling multiple sclerosis. Commercial testing is available for the most common mutations that cause LHON. The mainstays of therapy include cocktails of antioxidants (vitamin C, vitamin E, and coenzyme Q) and avoidance of tobacco and foods that contain high levels of cyanide such as cassavas. In some patients, vision may improve spontaneously.

Retinal ischemia and infarction

The ophthalmic artery is the first intracranial branch of the internal carotid artery. It gives rise to several branches including the central retinal and posterior ciliary arteries. The central retinal artery supplies blood to the retina via smaller branch arteries. The posterior ciliary arteries supply blood to the choroid, ciliary body, and iris. Reduced blood flow to the ophthalmic, central retinal, or branch retinal arteries may result in acute visual loss.

Central retinal artery occlusion is characterized by acute, painless, monocular visual loss. Transient ischemic attack involving the central retinal artery produces amaurosis fugax, in which the patient experiences visual blurring as if a shade is being pulled over the eye. This usually lasts between 5 and 20 minutes and resolves spontaneously. Branch retinal artery infarction causes visual loss in a sector of the visual field.

Ophthalmic artery infarction is clinically quite similar to central retinal artery occlusion, but may be associated with orbital pain and mydriasis due to infarction of the ciliary ganglion or iris sphincter. Because the choroid is also infarcted, the cherry-red spot (see above) is usually absent in ophthalmic artery infarction.

Unfortunately, there is no clearly effective treatment to reverse or reduce visual loss from central retinal or ophthalmic artery occlusion.6 Evaluation and treatment focuses on preventing cerebral ischemia secondary to ipsilateral carotid artery or cardiac disease (Chapter 21).

Migraine aura without headache

Approximately half of patients with migraine aura (Chapter 19) will have some element of visual loss.7 In a small minority of these patients, aura occurs without a subsequent headache. The aura typically develops over a few minutes and lasts for up to half an hour. Migraine aura without headache should be considered a diagnosis of exclusion unless the patient has a strong prior history of migraine with visual aura: all patients require a detailed evaluation for other, more serious causes of visual loss.

Angle-closure glaucoma

Although the diagnosis of angle-closure glaucoma should be fairly obvious to emergency room physicians and internists, it occasionally comes to the attention of a neurologist. A patient with angle- closure glaucoma usually has a red eye and appears to be in acute distress, clutching and covering the affected eye. Visual acuity is markedly decreased and the pupil is unreactive and in midposition. Measure the intraocular pressure and obtain an ophthalmological consultation as soon as angle-closure glaucoma is suspected.

Bitemporal hemianopsia and junctional scotoma

Bitemporal hemianopsia and junctional scotoma are caused by extension of pathology in the sella turcica into the adjacent optic chiasm. In many cases, visual loss is gradual and the patient may not report any problems beyond slight visual blurring. Headaches from mass lesions in this region are also common, and result from stretching of the diaphragma sellae. Because most of the pathology that involves the optic chiasm also involves the pituitary gland, accompanying endocrine disturbances are frequent. Common causes of sellar lesions in adults include pituitary adenoma, craniopharyngioma, and meningioma (Chapter 23). Uncommon sellar lesions include aneurysms of the circle of Willis, adenohypophysitis (classically secondary to sarcoidosis or tuberculosis), and pituitary abscess. Sellar region tumors sufficient to produce bitemporal hemianopsia almost always require the assistance of a neurosurgeon and an endocrinologist. Pituitary apoplexy is a rapidly developing, life-threatening syndrome discussed further in Chapter 19.

Homonymous upper quadrantanopsia

Lesions of the optic radiations within the temporal lobe (Meyer’s loop) produce visual field loss in the contralateral superotemporal quadrant and the ipsilateral superonasal quadrant. The classic setting in which this occurs is following anterior temporal lobectomy for refractory epilepsy. Although there are differing opinions concerning the degree of field cut produced by temporal lobectomy, a general rule of thumb is that resection produces no field defects if it is performed within 4 cm of the anterior temporal tip, a homonymous upper quadrantanopsia is the most likely defect with resections between 4 and 8 cm of the temporal tip, and a homonymous hemianopsia is most likely when the resection extends more than 8 cm posterior to the temporal tip.8 Other lesions in the temporal lobe that may produce homonymous upper quadrantanopsia include hemorrhages, arteriovenous malformations, and tumors. Any patient with this pattern of visual field deficits without a history of temporal lobectomy should undergo MRI of the brain with and without contrast to define the lesion.

Homonymous hemianopsia

Posterior cerebral artery infarction

Posterior cerebral artery (PCA) infarction is the most common cause of macular-sparing homonymous hemianopsia. Many patients with PCA infarctions do not recognize their deficits because central vision is spared or they may be able to compensate for their visual loss by simply moving their eyes. Some patients, however, note problems with reading or driving. In some cases, the deficits produced by PCA infarction that bring patients to clinical attention are confusional states or memory problems rather than visual field loss (Chapter 21).

Alexia without agraphia

Alexia without agraphia results from a lesion (usually a PCA infarction) of the left occipital lobe and the splenium of the corpus callosum. The patient can write but cannot read – even something that they themselves have just written! The explanation for this peculiar syndrome is as follows:

• The left occipital lobe lesion produces a right homonymous hemianopsia. Thus, there is no perception of written material in the right half of space.

• The callosal lesion disconnects the intact right visual cortex, which perceives written material in the left half of space from the language centers in the left hemisphere.

• The ability to write is retained because the language centers in the left hemisphere are still connected to the motor centers that govern the physical act of writing.

Cortical blindness

Bilateral occipital lobe infarction

Bilateral PCA infarction leads to complete blindness. Because funduscopic examination and pupillary reactions are normal, a patient with cortical blindness may be diagnosed as a malingerer. This is especially true when cortical blindness is accompanied by Anton’s syndrome, in which the patient fabricates a detailed, often preposterous visual environment.

Posterior reversible encephalopathy syndrome

Posterior reversible encephalopathy syndrome (PRES) is a severe encephalopathy produced by vasogenic edema, and is discussed further in Chapter 1.9 Patients with PRES rapidly develop an encephalopathy, blindness related to edema of the parietal and occipital lobes, and seizures.

Functional visual loss

Visual loss secondary to malingering or conversion disorders may be difficult to diagnose with routine bedside examination. Patients with psychogenic visual loss frequently wear sunglasses indoors, modeling their “blind person behavior” on those of well-known blind celebrities such as Ray Charles and Stevie Wonder. Pupillary reactions and funduscopic examination are normal in patients with functional visual loss. Psychiatric disease may mimic any organic pattern of visual loss, but common patterns include tunnel vision, complete blindness, and subtle bilateral visual loss. In tunnel vision, visual field constriction is identical regardless of the distance from which the patient is examined: a patient with tunnel vision describes the same field defect at 1 foot as at 20 feet. The patient with functional complete blindness will not fall or injure himself when attempting to traverse a path strewn with obstacles. Optokinetic drums or tapes may also be used to demonstrate preserved acuity in the patient who feigns blindness. The most difficult functional visual loss scenario to prove is the patient who complains of subtle bilateral visual loss. Diagnosing functional visual loss is often challenging, and formal ophthalmological evaluation is required in many cases.

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