The term uveitis refers to an infective or noninfective inflammatory disease that affects any of the uveal structures (iris, ciliary body, and choroid). The inflammation may be in the anterior part of the eye or in the posterior part, behind the iris and extending to the retina and choroid. Infective causes of posterior uveitis (choroidal) are toxoplasmal and cytomegalic inclusion disease, occurring mainly in patients with AIDS and other forms of reduced immune function. Noninfective autoimmune types are also common in the adult. Anterior uveitis is sometimes linked to ankylosing spondylitis and the human leukocyte antigen (HLA) B-27 marker, sarcoidosis, and recurrent meningitis (Vogt-Koyanagi-Harada disease); the posterior forms are associated with sarcoidosis, Behçet disease, and lymphoma.
Retinal diseases, particularly ARMD and diabetic retinopathy, are another important cause of blindness and are discussed further on, under “Other Diseases of the Retina.”
Knowledge of certain anatomic and physiologic facts is required for an interpretation of the neurologic lesions that affect vision. Light entering the eye traverses the inner layers of the retina to reach its outer (posterior) layer, which contains two classes of photoreceptor cells: the flask-shaped cones and the slender rods. The photoreceptors rest on a single layer of pigmented epithelial cells, which form the outermost surface of the retina. The rods and cones and pigmentary epithelium receive their blood supply primarily from the capillaries of the choroid. The rods function in the perception of visual stimuli in subdued light (twilight or scotopic vision), and the cones are responsible for color discrimination and the perception of stimuli in bright light (photopic vision). Most of the cones are concentrated in the macular region, particularly in its central part, the fovea, and are responsible for the highest level of visual acuity. Traquair described the rapid fall-off of acuity as the distance from the fovea increases as “an island of vision in a sea of blindness.” Specialized pigment molecules in the rods and cones absorb light energy and transform it into electrical signals, which are transmitted to the bipolar cells of the retina and then, in turn, to the superficially (anteriorly) placed neurons, or ganglion cells (Fig. 12-3). In the fovea, ganglion cells and other inner retinal structures are displaced (they are not found there) so that photoreceptors can be very highly concentrated to maximize central spatial acuity.
The axons of the retinal ganglion cells follow an arcuate course across the inner surface of the retina. Being unmyelinated, they are not visible, although fluorescein retinography shows a trace of their outlines; an experienced examiner, using a bright light and deep green filter, can visualize them through direct ophthalmoscopy. The axons of ganglion cells are collected in the optic discs and then travel posteriorly to form the optic nerves, optic chiasm, and optic tracts, finally synapsing in the lateral geniculate nuclei, the superior colliculi, the midbrain pretectum and the suprachiasmatic nucleus of the hypothalamus (see Figs. 12-2 and 12-3). The fibers derived from macular cells form a discrete group (the papillomacular bundle) that arrive at the temporal side of the disc and then assume a more central position within the nerve. These fibers are of smaller caliber than the peripheral optic nerve fibers and appear to be especially sensitive to toxic and metabolic injury. Damage to the papillomacular bundle produces the “cecocentral” scotoma (extending from fixation to the blind spot).