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Harrison’s Manual of Medicine



Clinical Assessment
Transient or Sudden Visual Loss

Clinical Assessment
Accurate measurement of visual acuity in each eye (with glasses) is of primary importance. Additional assessments include testing of pupils, eye movements, ocular alignment, and visual fields. Slit-lamp examination can exclude corneal infection, trauma, glaucoma, uveitis, and cataract. Ophthalmoscopic exam to inspect the optic disc and retina often requires pupillary dilation using 1% topicamide and 2.5% phenylephrine; the risk of provoking an attack of narrow- angle glaucoma is remote.
Visual field mapping by finger confrontation permits localization of lesions in the visual pathway (Fig. 10-1); formal testing using a perimeter may be necessary. The goal is to determine whether the lesion is anterior, at, or posterior to the optic chiasm. A scotoma confined to one eye is caused by an anterior lesion affecting the optic nerve or globe; the swinging flashlight test may reveal an afferent pupil defect. History and ocular exam are usually sufficient for diagnosis. If a bitemporal hemianopia is present, the lesion is located at the optic chiasm (e.g., pituitary adenoma, meningioma). Homonymous visual field loss signals a retrochiasmal lesion, affecting the optic tract, lateral geniculate body, optic radiations, or visual cortex (e.g., stroke, tumor, abscess). Neuroimaging is recommended for any pt with a bitemporal or homonymous hemianopia.

FIGURE 10-1. Deficits in visual fields caused by lesions affecting visual pathways.

Transient or Sudden Visual Loss
Amaurosis fugax or transient monocular blindness usually occurs from a retinal embolus or severe ipsilateral carotid stenosis. Prolonged occlusion of the central retinal artery results in the classic fundus appearance of a milky, infarcted retina with a cherry-red fovea. Any pt with compromise of the retinal circulation should be evaluated promptly for stroke factors (e.g., carotid atheromata, heart disease, atrial fibrillation). Vertebrobasilar insufficiency or emboli can be confused with amaurosis fugax, because many pts mistakenly ascribe symptoms to their left or right eye, when in fact they are occurring in the left or right hemifield of both eyes. Interruption of blood flow to the visual cortex causes sudden graying of vision, occasionally with flashing lights or other symptoms that mimic migraine. The history may be the only guide to the correct diagnosis. Pts should be questioned about the precise pattern and duration of visual loss and about other neurologic symptoms such as diplopia, vertigo, numbness, or weakness, which may help decide between compromise of the anterior or posterior cerebral circulation.
Malignant hypertension can cause visual loss from exudates, hemorrhages, cotton-wool spots (focal nerve fiber layer infarcts), and optic disc edema. In central or branch retinal vein occlusion, the fundus exam reveals engorged, dusky veins with extensive retinal bleeding. In age-related macular degeneration, characterized by extensive drusen and scarring of the pigment epithelium, leakage of blood or fluid from subretinal neovascular membranes can produce sudden central visual loss. Flashing lights and floaters may indicate a fresh vitreous detachment. Separation of the vitreous from the retina is a frequent involutional event in the elderly. It is not harmful unless it creates sufficient traction to produce a retinal detachment. Vitreous hemorrhage may occur in diabetic patients from retinal neovascularization.
Papilledema refers to bilateral optic disc edema from raised intracranial pressure. Transient visual obscurations are common, but visual acuity is not affected unless the papilledema is severe, long-standing, or accompanied by macular exudates or hemorrhage. Neuroimaging should be obtained to exclude an intracranial mass. If negative, an LP is required to confirm elevation of the intracranial pressure. Pseudotumor cerebri (idiopathic intracranial hypertension) is a diagnosis of exclusion. Most pts are young, female, and obese; some are found to have occult cerebral venous sinus thrombosis. Treatment is with weight loss, acetazolamide, and repeated LPs; some pts require lumboperitoneal shunting or optic nerve sheath fenestration. Optic neuritis is a common cause of monocular optic disc swelling and visual loss, although it rarely affects both eyes. If the site of inflammation is retrobulbar, the fundus will appear normal on initial exam. The typical pt is female, aged 15–45, with pain provoked by eye movements. Glucocorticoids, consisting of intravenous methylprednisolone followed by oral prednisone (Table 187-3), may hasten recovery in severely affected patients. If an MR scan shows multiple demyelinating plaques, treatment for multiple sclerosis should be considered. Anterior ischemic optic neuropathy (AION) is an infarction of the optic nerve head due to inadequate perfusion via the posterior ciliary arteries. Pts have sudden visual loss, often upon awakening, and painless swelling of the optic disc. It is important to differentiate between nonarteritic (idiopathic) AION and arteritic AION. The latter is caused by temporal arteritis and requires immediate glucocorticoid therapy. The ESR should be checked in any elderly pt with acute optic disc swelling or symptoms suggestive of polymyalgia rheumatica.
If the pt has diplopia while being examined, motility testing will usually reveal a deficiency in ocular excursions. However, if the degree of angular separation between the double images is small, the limitation of eye movements may be subtle and difficult to detect. In this situation, the cover test is useful. While the pt is fixating upon a distant target, one eye is covered while observing the other eye for a movement of redress as it takes up fixation. If none is seen, the procedure is repeated upon the fellow eye. With genuine diplopia, this test should reveal ocular malalignment, especially if the head is turned or tilted in the position that gives rise to the worst symptoms.
The most frequent causes of diplopia are summarized in Table 10-1. The physical findings in isolated ocular motor nerve palsies are:

Table 10-1 Common Causes of Diplopia

CN III: Ptosis and deviation of the eye down and outwards, causing vertical and horizontal diplopia. Pupil dilation suggests direct compression of the third nerve; if present, the possibility of an aneurysm of the posterior communicating artery must be considered urgently.

CN IV: Vertical diplopia with cyclotorsion; the affected eye is slightly elevated, and limitation of depression is seen when the eye is held in adduction. The pt may assume a head tilt to the opposite side (e.g., left head tilt in right fourth nerve paresis).

CN VI: Horizontal diplopia with crossed eyes; the affected eye cannot abduct.
Isolated ocular motor nerve palsies often occur in pts with hypertension or diabetes. They usually resolve spontaneously over several months. The apparent occurrence of multiple ocular motor nerve palsies, or diffuse ophthalmoplegia, raises the possibility of myasthenia gravis. In this disease, the pupils are always normal. Systemic weakness may be absent. Diplopia that cannot be explained by a single ocular motor nerve palsy may also be caused by carcinomatous or fungal meningitis, Graves’ disease, Guillain-Barré syndrome, Fisher’s syndrome, or Tolosa-Hunt syndrome.

For a more detailed discussion, see Horton JC: Disorders of the Eye, Chap. 28, p. 164, in HPIM-15.



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