The retina and the optic nerve head are the only parts of the central nervous system that can be inspected during life with an ophthalmoscopic examination. Moreover, the ophthalmoscope allow us to inspect the superficial retinal vessels.
The vascular supply of the retina comes from the ophthalmic branch of the internal carotid artery, which in turn gives origin to the central retina artery. The latter, upon issuing from the optic disk, divides into four arterioles, which supply the four quadrants of the retina (see Figure 67.1-67.7).
The ganglion cells and bipolar cells receive their blood supply from these arterioles and their capillaries, whereas photoreceptor elements receive nourishment from the underlying choroidal vascular bed. These small vessels react in diseases like vessels of corresponding sizes in the brain. Since the walls of the retinal arterioles are transparent to the ophthalmoscope, what is seen is a column of blood cells. In hypertension and/or arteriosclerosis (usually coexistent with hypertension), the lumens of the vessels are narrowed because of fibrous tissue replacement of the media and thickening of the basement membrane. The light reflection from the vessel then has a different refractive index than the adjacent retinal tissue. Spreading of vessels, decrease of the size of the lumen, and arteriole-venous compressions are other signs of hypertension and arteriosclerosis (see Figure 67.1-67.8).
Capillary-venular aneurysms may develop, most often in diabetes mellitus (see Figure 67.9).
Since the central retina vein and artery share a common adventitial sheath, atheromatous plaques in the artery may result in thrombosis of the vein. Round or oval hemorrhages always lie in the outer plexiform layer and linear or flame-shaped ones in the superficial layer of the retina, as occurs in conditions with extremely high intracranial pressure.
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