The periphery of the retina may harbor a hemangioblastoma, which may appear during adolescence, before the more characteristic cerebellar lesion. A large retinal artery may be seen leading to it and there may be a large draining vein. Occasionally, retinal examination discloses the presence of a vascular malformation that may coexist with a malformation of the optic nerve and basilar portions of the brain.
Ischemic Lesions of the Retina
Transient monocular blindness Transient ischemic attacks of visual loss involving all or part of the field of vision of one eye are referred to as amaurosis fugax or transient monocular blindness (TMB). They are common manifestations of atherosclerotic carotid stenosis but have other causes. An altitudinal horizontal border, or “shade,” is often, but not invariably, an aspect of the visual loss. The shade may rise or fall at the onset or cessation of the spell and occasionally remains throughout the episode. Fortuitous inspection of the retina during an attack may show segments of arteries that are filled with white material that migrate distally over many minutes. There can be stagnation of arterial and venous blood flow, which returns within seconds or minutes as vision is restored (Fisher). One interpretation of these observations is that an embolus to the central retinal artery and has broken up and moved distally. Fisher went on to discredit the theory of the time that transient monocular blindness was due to vasospasm of the retinal arteries.
Many attacks may precede infarction of a cerebral hemisphere, or they may abate without adverse consequence. In one series of 80 patients followed by Marshall and Meadows for 4 years, in an era prior to modern treatment of atherosclerosis, 16 percent developed permanent unilateral blindness, a completed hemispheric stroke, or both. Chapter 33 discusses this subject further.
Occlusion of the internal carotid artery usually causes no disturbance of vision, provided that there are adequate anastomotic branches from the external carotid artery or other sources to the ophthalmic artery. Occasionally, occlusion of the proximal internal carotid artery is marked by an episode of transient monocular blindness on the same side, just as a hemispheric transient ischemic attack may indicate recent acute carotid occlusion. Chronic carotid occlusion with inadequate collateralization is associated with an ischemic oculopathy, which may predominantly affect the anterior or posterior segment or both (see Young and Appen). In this case, insufficient circulation to the anterior segment of the globe is manifest by scleral vascular congestion, cloudiness of the cornea, anterior chamber flare, and low intraocular pressure, or sometimes high intraocular pressure if neovascularization of the iris (rubeosis iridis) occurs and compromises the outflow of aqueous humor. Ischemia of the posterior segment of the eye is manifest by circulatory changes in the optic nerve or by venous stasis. Light-induced amaurosis, in which periods of blindness are precipitated by exposure to bright light, is a characteristic symptom. Other signs of carotid disease may be present, for example, a local bruit over the carotid bifurcation.
Central or branch retinal artery occlusion Most often, ischemia of the retina can be traced to occlusion of the central retinal artery or its branches by thrombi or emboli—central or branch retinal artery occlusion (CRAO or BRAO). Occlusion is attended by sudden painless blindness. The retina becomes opaque and has a gray-yellow appearance; the arterioles are narrowed, with segmentation of columns of blood. A “cherry-red spot” appears in the fovea, where the retina is thinnest and the intact underlying choroidal circulation is visible (Fig. 12-8).