Alexandra Van Der Woerdt,     New York, New York

Anatomy and Physiology

The uvea consists of the iris, ciliary body, and choroid. The uvea has many functions, including production of aqueous humor, regulation of the amount of light that enters the eye through constriction and dilation of the pupil, and supply of nutrients and oxygen to the nonvascularized or poorly vascularized portions of the eye. The uvea also is responsible for maintenance of the blood-aqueous barrier that protects the eye from toxins, infectious organisms, and exuberant inflammation. Inflammation starts either within the eye itself or in reaction to a disease process elsewhere in the body. Microorganisms or damaged tissue release inflammatory mediators such as histamine, serotonin, prostaglandins, and leukotrienes, which result in vasodilation and increased vascular permeability. This leads to breakdown of the blood-aqueous barrier. These inflammatory mediators also cause leukocyte activation and migration. Antigens are transported through the bloodstream to the spleen or other lymphoid tissues, which results in activation of T and B lymphocytes. These activated T and B lymphocytes are transported back to the eye through the bloodstream to the uveal tract. Uveitis resolves when inhibitory cytokines eliminate the inflammatory response and the initial offending antigen is removed. Chronic inflammation may occur when the initial offending antigen cannot be removed or the inflammatory response is not adequately suppressed.

Diagnosis

Clinical signs of anterior uveitis include blepharospasm, conjunctival hyperemia, corneal edema, aqueous flare, hypopyon, swelling of the iris, rubeosis iridis (neovascularization of the iris in chronic uveitis), decreased intraocular pressure (IOP), decrease or loss of vision, and miosis. Table 249-1 lists the differential diagnoses for the various clinical signs associated with anterior uveitis. Blepharospasm is a sign of ocular pain associated with spasm of the ciliary body musculature and intraocular inflammation. Conjunctival hyperemia is a nonspecific sign associated with many ocular diseases and cannot be used to rule in or rule out uveitis. Corneal edema is caused by temporary malfunction of the endothelial cells resulting in decreased removal of fluid from the cornea. Breakdown of the blood-aqueous barrier results in the leakage of protein and inflammatory cells into the aqueous humor. The anterior chamber is best evaluated for the presence of aqueous flare by illumination using a bright pinpoint or slit-beam light source held in close proximity to the cornea in a dark room. A normal eye shows a white light beam on the cornea, no light reflection from the aqueous humor, and a white light beam on the iris-lens. An eye with uveitis has increased protein and cells in the aqueous humor, which can be seen as light reflecting between the beam on the cornea and the beam on the iris-lens. Severe breakdown of the blood-aqueous barrier may result in the presence of white blood cells in the anterior chamber, a condition called hypopyon. These often settle at the bottom of the anterior chamber. It is important to remember that the presence of hypopyon does not necessarily indicate intraocular infection but is pathognomonic for intraocular inflammation. Inflammation of the iris and ciliary body causes a decrease in IOP because of a decrease in the production of aqueous humor and an increase in the removal of aqueous humor through the inflamed iris. Miosis results from elevated levels of prostaglandins in the aqueous humor that stimulate contraction of the iris sphincter muscle. Blepharospasm, corneal edema, aqueous flare, and miosis all may contribute to a decrease in vision.

Chorioretinitis often causes a decrease in, or loss of, vision. Secretion of fluid between the choroid and the retina may result in focal or complete (serous) retinal detachment. Focal areas of chorioretinitis are visible within the retina as hyporeflective lesions that often are round and may have a different color from the surrounding tapetal and nontapetal areas.