In evaluating the retinal vessels, one must remember that these are arterioles and not arteries. Since the walls of retinal arterioles are transparent, what is observed with the ophthalmoscope is a column of blood within them. The central light streak of normal arterioles is thought to represent the reflection of light as it strikes the interface of the column of blood and the concave vascular wall. In arteriolosclerosis (usually coexistent with hypertension), the lumina of the vessels are segmentally narrowed because of fibrous tissue replacement of the media and thickening of the basement membrane. Straightening of the arterioles and venous compression by arterioles (“A-V nicking”) are other signs of hypertension and arteriolosclerosis. In this circumstance the vein is compressed by the thickened arteriole within the adventitial envelope shared by both vessels at the site of crossing. Progressive arteriolar disease, to the point of occlusion of the lumen, results in a narrow, white (“silver-wire”) vessel with no visible blood column. This change is associated most often with severe hypertension but may follow other types of occlusion of the central retinal artery or its branches (see descriptions and retinal illustrations further on). Sheathing of the venules, probably representing focal leakage of cells from the vessels, can accompany inflammatory neuropathies, including idiopathic optic neuritis. Arterial and venule sheathing are also seen in leukemia, sarcoid, Behçet disease, and other forms of vasculitis.
In malignant, or accelerated, hypertension, there are, in addition to swelling of the optic nerve head and the retinal arteriolar changes noted previously, a number of extravascular lesions: the so-called soft exudates or cotton-wool patches, sharply marginated and glistening “hard” exudates, and retinal hemorrhages (Fig. 12-6). In many patients who show these retinal changes, analogous lesions are found in the brain (necrotizing arteriolitis and microinfarcts) and underlie hypertensive encephalopathy.
Microaneurysms of retinal vessels appear as small, discrete red dots and are located in largest number in the paracentral region. They are most often a sign of diabetes mellitus, sometimes appearing before the usual clinical manifestations of that disease. The use of the red-free (green) light on the ophthalmoscope helps to pick out microaneurysms from the background. Microscopically, the aneurysms take the form of small (20to 90-mm) saccular outpouchings from the walls of capillaries, venules, or arterioles. The vessels of origin of the aneurysms are invariably abnormal, being either acellular branches of occluded vessels or themselves occluded by fat or fibrin.