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The most common cause of retinal ischemia is carotid occlusive disease followed by other sources of embolism (e.g., the heart, aortic arch) and, rarely, hyperviscosity states (e.g., polycythemia vera). The mechanism of retinal ischemia in patients with carotid disease is either embolism to the central retinal artery or its branches or hypoperfusion of the retina caused by high-grade carotid stenosis or carotid occlusion. Transient retinal ischemia is manifested by transient monocular blindness (amaurosis fugax), which is classically but uncommonly described by patients as a "shade being pulled down" over the eye. It is more common for patients to describe fuzzy, blurred, or cloudy vision in the eye. When evaluating patients with a complaint of transient monocular visual loss, it is important to ask them whether they covered one eye and then the other eye to verify that the visual loss was monocular. Patients with homonymous hemianopia (e.g., from occipital ischemia), who do not cover one eye at a time to test their vision, often complain of monocular visual loss in the eye corresponding to the side of the hemianopia.
Clues to the mechanism of transient retinal ischemia may be obtained from the history. Sector or altitudinal field cuts with a horizontal meridian suggest embolism to the retina, whereas slow monocular dimming of vision, like a camera shutter, suggests hypoperfusion. A variant of this symptom is monocular visual dimming upon exposure to bright light, related to retinal hypoperfusion from ipsilateral carotid artery occlusion or critical stenosis.
The ophthalmoscopic examination may reveal other clues to the mechanism of ischemia. Hollenhorst plaques, which are yellowish, refractile retinal emboli composed of cholesterol flakes, suggest an arterial source of embolism. Platelet fibrin emboli, which are large and chalk-white and lodge in the central retinal artery over the disk head, may arise from a heart valve or complex carotid plaque. Septic emboli (Roth's spots), which appear as whitish cores surrounded by hemorrhage, suggest endocarditis.
Prolonged occlusion of the central retinal artery causes permanent damage to the retina. Central retinal artery occlusion usually causes total blindness that is usually painless. The pupil may fail to react because of diffuse retinal ischemia. The retina becomes pale and edematous, and the retinal vessels narrow. The optic disk becomes pale, but the fovea retains its color and stands out as a cherry red spot. Occasionally the actual embolus may be seen. Occlusion of a branch of the central retinal artery causes a segmental retinal infarct and visual loss in the form of a large scotoma. Although some studies suggest a low correlation between central retinal artery occlusion and ipsilateral carotid occlusive disease, more recent studies show a higher correlation. We suggest that all patients with central retinal artery occlusion undergo duplex carotid ultrasound to rule out carotid occlusive disease. If the carotid ultrasound is unrevealing, echocardiography should be performed to rule out a cardiac source of embolism.
A less recognized cause of retinal ischemia is venous stasis retinopathy (VSR). VSR is caused by chronic retinal hypoperfusion usually from internal carotid artery occlusion. VSR appears similar to diabetic retinopathy with dot and blot hemorrhages and cotton wool patches; they are distinguished by the fact that VSR usually is unilateral. Eventually VSR may lead to neovascular glaucoma and blindness unless the hypoperfusion is reversed. In this setting, if a stenotic external carotid artery is supplying blood flow to the orbit, external carotid endarterectomy should be considered. On the other hand, if blood flow to the eye is largely from limited collateral flow from the contralateral hemisphere through the anterior communicating artery, ipsilateral extracranial-intracranial bypass surgery may be considered.