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Neuro-Ophthalmology
A mechanistic understanding of vision impairment along with disturbances in pupillary and oculomotor control is often critical for diagnosing neurologic disorders. Although all physicians should be able to recognize neuro-ophthalmologic abnormalities, precise diagnosis and treatment commonly requires specialty consultation.
Vision
One of the most difficult diagnostic problems is vision loss that cannot be explained by obvious abnormalities of the eye. To evaluate such a patient properly, the examining physician must be familiar with the anatomy and physiology of the afferent visual system. The afferent visual pathways cross the major ascending sensory and descending motor systems of the cerebral hemispheres and in their anterior portion are intimately related to the vascular and bony structures at the base of the brain. Not surprisingly, localization of lesions within the afferent visual pathways has great value in neurologic diagnosis.
Anatomy of the Visual Pathways
Light entering the eye falls on the retinal rods and cones, which transduce the stimulus into neural impulses to be transmitted to the brain. The distribution of visual function across the retina takes a pattern of concentric zones increasing in sensitivity toward the center, the fovea. The fovea consists of a “rod-free” central grouping of approximately 100,000 slender cones. The ganglion cells subserving these cones send their axons directly to the temporal aspect of the optic disc, where they form the papillomacular bundle. Axons originating from ganglion cells in the temporal retina curve above and below the papillomacular bundle and form dense arcuate bands.
The arteries supplying the optic nerve and retina derive from branches of the ophthalmic artery. The central retinal artery approaches the eye along each optic nerve and pierces the inferior aspect of the dural sheath about 1 cm behind the globe to enter the center of the nerve. The artery emerges in the fundus at the center of the nerve head, from which it nourishes the inner two thirds of the retina by superior and inferior branches. Anastomotic branches derived from the choroidal and posterior ciliary arteries, the ciliary system, supply the choroid, optic nerve head, and outer retinal layers, including the photoreceptors. In about 10% of the population, the macula is supplied by a retinociliary artery, a branch of the ciliary system. Venous drainage from the retina and nerve head flows primarily through the central retinal vein, whose course of exit from the eye parallels that of entry of the artery.
What each eye “sees” is termed its visual field ( Fig. 392-1 ). The nasal side of the left retina and the temporal side of the right see the left side of the world, and the upper half of each retina sees the lower half of the world. Behind the eyes, the optic nerves pass through the optic canal to form the optic chiasm. In the chiasm, nerves from the nasal half of each retina decussate and join the fibers from the temporal half of the contralateral retina. From the chiasm, the optic tracts pass around the cerebral peduncles to reach the lateral geniculate ganglia. The orientation of the visual field is rotated 90 degrees in the lateral geniculate such that images from the inferior visual field project to the medial half, whereas images from the superior visual field project to the lateral half. The geniculocalcarine radiation initially fans out into superolateral and inferolateral projections, the latter passing around the lateral ventricle and for a short distance into the temporal lobe (Meyer loop) before turning posteriorly to reach the striate cortex of the occipital lobe. In the occipital lobe, the striate cortex (area 17) lies along the superior and inferior bands of the calcarine fissure, with macular fibers projecting most posteriorly to the occipital pole and more peripheral retinal projections lying more anteriorly.
Localization of Lesions within Visual Pathways
Monocular vision loss is due to a lesion in one eye or optic nerve. Binocular visual loss, on the other hand, can result from disease located anywhere in the visual pathways from the corneas to the occipital poles. Lesions involving the optic chiasm produce nonhomonymous visual abnormalities (e.g., the bitemporal hemianopia illustrated by lesion 3 in Fig. 392-1 ). Optic tract abnormalities are comparatively rare but produce characteristic visual changes. The fibers serving identical points in the homonymous half-fields do not fully commingle in the optic tract, so lesions damaging this structure produce incongruous homonymous hemianopia. Lesions of the geniculate nuclei, optic radiations, or visual cortex produce congruent hemianopic field defects that may go unrecognized unless the hemianopia intrudes on macular vision. Postgeniculate visual loss can be differentiated from pregeniculate visual loss by (1) a normal funduscopic appearance, (2) intact pupillary light reactions, and (3) appropriate lesions on brain imaging.
Examination of the Afferent Visual System
Visual function is most commonly assessed by “best-corrected visual acuity” ( Chapter 391 ). If visual acuity is not normal, it must be determined whether acuity can be improved with lenses or at least with the use of a pinhole. The normal reference is recognition of letters at an idealized 20 feet, and acuity charts are designed with even larger letters that are normally recognized at proportionally greater distances. Thus, if one reads letters at 20 feet no better than those normally perceived at 40 feet, vision is recorded as 20/40. Small visual charts that are easily carried in the physician’s case permit quick and fairly accurate bedside appraisal of acuity.
Visual fields can be tested at the bedside by confrontation, and rough estimates of their integrity can be made even in patients with reduced alertness. The fields should be tested individually for each eye because the pattern of visual field defects can provide important localizing information. A quick screen of the visual fields can be made by having the patient fixate on the examiner’s nose and identify the number of fingers flashed in each of the four visual field quadrants.
Common Causes of Visual Loss
Eye
The cause of monocular vision loss secondary to ocular and retinal lesions can often be detected by ophthalmoscopic examination or by measurement of intraocular pressure ( Chapter 391 ). Glaucoma caused by impaired absorption of aqueous humor results in a high intraocular pressure that usually produces gradual loss of peripheral vision, “halos” seen around lights, and occasionally, pain and redness in the affected eye. Retinal tears and detachments give rise to unilateral distortions of the visual image seen as sudden angulations or curves of objects containing straight lines (metamorphopsia). Hemorrhages into the vitreous humor or infections or inflammatory lesions of the retina can produce scotomas that resemble those resulting from primary disease of the central visual pathway.
Binocular vision loss secondary to retinal disease in younger subjects is often due to heredodegenerative conditions . Vascular diseases, diabetes ( Chapter 210 ), and age-related macular degeneration are causes in older patients. In most cases of pigmentary retinal degeneration , visual loss begins peripherally and slowly proceeds centrally. By contrast, macular degeneration (see Figs. 391-20 and 391-21 ) impairs central vision early in its course. A common variant in the complement factor H ( CFH ) gene is associated with a markedly increased risk for the development of age-related macular degeneration.
Optic Nerve
Acute or subacute monocular vision loss ( Table 392-1 ) as a result of optic nerve disease is most commonly produced by demyelinating disorders, vascular obstruction, neoplasm, or hereditary optic neuropathy. Demyelinating disease of the nerve head ( optic neuritis or papillitis ) produces disc edema along with loss of central vision in the affected eye only; subjectively unrecognized scotomas may sometimes be found in the other eye. Demyelination of the optic nerve behind the point where the retinal vein emerges ( retrobulbar neuritis ) initially leaves a normal-looking disc but a central or paracentral scotoma. With chronic demyelinating disorders, the optic disc becomes pale and atrophic.
TABLE 392-1
COMMON CAUSES OF TRANSIENT MONOCULAR VISION LOSS
| CATEGORY (TYPICAL DURATION) | CAUSES | DIFFERENTIAL FEATURES |
|---|---|---|
| Thromboembolism (1-5 min) | Atherosclerosis | Other atherosclerotic vascular disease, associated contralateral hemiparesis, angiography (carotid atheroma) |
| Cardiac | Valvular disease, mural thrombi, atrial fibrillation, recent myocardial infarction | |
| Blood dyscrasia | Blood tests positive for sickle cell anemia, macroglobulinemia, multiple myeloma, polycythemia, other | |
| Vasospasm (5-30 min) | Migraine | Ipsilateral headache, other classic aura, family history |
| Vascular compression (few seconds) | Increased intracranial pressure | Precipitated by position change, Valsalva maneuver, or pressure waves |
| Tumor | Associated with slowly progressive monocular visual loss | |
| Vasculitis (1-5 min) | Temporal arteritis | Associated with headache, polymyalgia rheumatica, palpable temporal artery, elevated sedimentation rate |
Optic neuritis can be an isolated syndrome or a manifestation of systemic diseases. , The clinical course and therapeutic response of optic neuritis depend on the underlying inflammatory mechanism. In more than 50% of patients initially seen with optic neuritis, typical symptoms and signs of multiple sclerosis eventually develop ( Chapter 380 ). Optic neuritis caused by multiple sclerosis is not responsive to steroids, but the underlying multiple sclerosis should be treated promptly. Some data suggest that phenytoin may be neuroprotective in patients with acute optic neuritis when used in doses of 4 to 6 mg/kg/day. Optic neuritis related to systemic lupus erythematosus ( Chapter 245 ), vasculitis ( Chapter 249 ), or sarcoidosis ( Chapter 83 ) may be steroid responsive.
Optic neuritis with an associated transverse myelitis is the clinical hallmark of neuromyelitis optica , which is a severe demyelinating disease often mistaken for multiple sclerosis but now recognized to be caused by anti–aquaporin 4 autoantibodies. Standard treatment options include rituximab (1-g infusions at an interval of 2 weeks) or azathioprine (3 mg/kg/day orally). More recently, satralizumab (a humanized monoclonal antibody targeting the interleukin-6 receptor at 120 mg subcutaneously at 0, 2, and 4 weeks, then every 4 weeks thereafter), eculizumab (a terminal complement inhibitor at 900 mg intravenously weekly for 4 doses followed by 1200 mg every 2 weeks), and tocilizumab (an interleukin-6 receptor inhibitor at 8 mg intravenously every 4 weeks) have been shown to reduce the risk of relapse. – Doses should be adjusted based on response and immune suppression.
Intraocular arterial occlusion may produce either central visual loss or an altitudinal field defect ( ischemic optic neuropathy ). Nonarteritic anterior ischemic optic neuropathy results from disease of the small vessels supplying the anterior portion of the optic nerve. The most common systemic disorders associated with it are hypertension (present in 50% of patients) and diabetes mellitus (present in 25%). Arteritic ischemic optic neuropathy is most commonly caused by giant cell arteritis ( Chapters 63 and 250 ) and should be considered in all patients who are older than 50 years of age. Oral glucocorticoids are the cornerstone of treatment for giant cell arteritis but are not helpful in patients with nonarteritic anterior ischemic optic neuropathy.
Tumors ( Chapter 175 ) invading the optic nerve or space-occupying lesions compressing it anywhere between the orbit and chiasm cause gradually decreasing central vision or a sector defect of the peripheral visual field. With such chronic lesions, the affected optic nerve becomes visibly atrophic.
Acute binocular vision loss resulting from bilateral optic nerve disease is most often caused by demyelinating disease or by toxic (methanol, tobacco, isoniazid) or nutritional factors (B vitamin deficiency, particularly of thiamine; Chapter 384 ). In younger persons and those lacking a clear history of toxic exposure, demyelinating lesions overwhelmingly predominate. Symptoms are of abrupt or subacute onset with visual blurring, which may progress rapidly to blindness within hours or days. There may be pain about the eyes, particularly with movement. Leber optic neuropathy , caused by a mutation in mitochondrial DNA, typically begins painlessly and centrally in one eye, with the second eye affected weeks to months later. Gene therapy can modestly improve vision in both the treated and untreated eye, albeit only for 3 years or so.
Papilledema is disc edema secondary to increased intracranial pressure ( Table 392-2 ). Vision is normal except under one of two circumstances: (1) acute transient episodes of amaurosis lasting a few seconds and attributable to acute increases in intracranial pressure (plateau waves); and (2) progressive loss of peripheral vision with long-standing, severe papilledema caused by compression of the optic nerve head. Idiopathic intracranial hypertension ( Chapter 175 ) is commonly seen in overweight women of childbearing age. Optical coherence tomography can help establish the diagnosis and differentiate papilledema from other optic disc abnormalities. Subacute or chronic binocular vision loss secondary to optic nerve disease can result from toxic and nutritional causes or from inherited optic atrophy . Visual loss is painless and primarily affects central vision; ophthalmoscopy shows optic atrophy.
TABLE 392-2
DIFFERENTIATION OF OPTIC NEURITIS FROM PAPILLEDEMA
| OPTIC NEURITIS | PAPILLEDEMA | |
|---|---|---|
| Central-cecocentral vision loss | Present | Absent |
| Distribution | Usually unilateral | Usually bilateral |
| Ocular pain on movement | Present | Absent |
| Direct light reflex | ±Reduced | Intact |
| CT and MRI of head | White matter plaques | Tumor, venous occlusion, etc. |
| Visual evoked responses | Abnormal | Normal |
| Lumbar puncture pressure | Normal | Elevated |
CT = computed tomography; MRI = magnetic resonance imaging.
Chiasm and Optic Tract
Patients with lesions of the optic chiasm or optic tract are often unaware of visual impairment until the deficit encroaches on central vision in one or both eyes. Intrinsic or extrinsic neoplasms and parachiasmal arterial aneurysms are the most common lesions in this location. Gliomas that arise within the chiasm or optic tract are rare in adulthood. Extrinsic lesions compressing the chiasm or tract include pituitary adenomas ( Chapter 205 ), dysgerminomas, craniopharyngiomas, meningiomas ( Chapter 175 ), and large aneurysms of the carotid or basilar artery ( Chapter 377 ). The diagnosis rests on finding the characteristic visual field abnormalities (bitemporal hemianopia for chiasm and incongruous homonymous hemianopia for optic tract lesions) and identifying the lesion with computed tomography or magnetic resonance imaging. Pituitary apoplexy secondary to acute hemorrhage into the gland ( Chapter 205 ) can result in sudden vision loss; prompt neurosurgical intervention under steroid coverage is required for most patients.
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