The vascular supply of the eye is through the ophthalmic branch of the internal carotid artery that supplies the retina, posterior (uveal) coats of the eye, and optic nerve head. This artery gives origin to the posterior ciliary arteries; the latter form a rich circumferential plexus of vessels (arterial circle of Zinn-Haller) located deep to the lamina cribrosa. The lamina cribrosa is a sieve-like scleral (dural) structure through which the axons of the central and nasal part of the disc run. This arterial circle supplies the optic disc and adjacent part of the distal optic nerve, the choroid, and the ciliary body; it anastomoses with the pial arterial plexus that surrounds the optic nerve. The other major branch of the ophthalmic artery is the central retinal artery. It issues from the optic disc, where it divides into four branches that supply the inner retinal layers; it is these vessels and their branches that are visible by ophthalmoscopy. A short distance from the disc, these vessels lose their internal elastic lamina and the media (muscularis) becomes thin; they are properly classed as arterioles. The inner layers of the retina, including the ganglion and bipolar cells, receive their blood supply from these arterioles and their capillaries, whereas the deeper photoreceptor elements and the fovea are nourished by the underlying choroidal vascular bed, by diffusion through the retinal pigmented cells and the semipermeable Bruch membrane upon which they rest. In up to a third of the population, a small cilioretinal artery may arise from either the choroidal circulation or from the circle of Zinn-Haller and supply the macula. In the case of a central retinal artery occlusion, the presence of this cilioretinal artery leads to the preservation of central acuity.
Abnormalities of the Retina
As indicated previously, the thin (100to 350-mm) retinal sheet and the optic nerve head are exteriorized parts of the central nervous system and the only part of the nervous system that can be inspected directly. In cases of visual loss it is important during funduscopic examination to carefully inspect the macular zone (which is located 3 to 4 mm lateral to the optic disc and provides central visual acuity). There are variations in the appearance of the normal macula and optic disc, and these may prove difficult to distinguish from disease. A normal macula may be called abnormal because of a slight aberration of the retinal pigment epithelium or a few drusen. With experience, the examiner can visualize the unmyelinated nerve-fiber layer of the retina by using bright-green (red-free) illumination. This is most often helpful in detecting demyelinating, toxic, or hereditary lesions of the optic nerve, which produce a loss of discrete bundles of axons as they converge to the disc.