Uveitis in Dog and Cats

Definition Uveitis can be defined as an inflammatory process of the uveal tract. Clinically, the iris and ciliary body usually react together, while the choroid behaves somewhat independently. Uveitis is thus generally classified into two categories: anterior uveitis or iridocyclitis and posterior uveitis or choroiditis (1). The uveal tract is a highly vascularized and heavily pigmented structure which constitutes the middle layer of the eyeball, between the outer fibrous tunic (sclera) and the inner nervous lining (retina). The size of the pupil is the result of a balance between the action of the dilator muscle (adrenergic innervation) and that of the sphincter muscle (cholinergic innervation). Clinically, it is particularly important to remember that the iris sphincter is much more potent than the dilator. The ciliary body is the only part of the uveal tract that cannot be visualized during a routine ocular examination: hidden from direct view by the iris, it is located too far anteriorly to be observed with the ophthalmoscope. The ciliary body has many roles, one of the most importent is the formation of vitreous humor. Any change in the composition or in the rate of flow of the aqueous humor can have serious deleterious effects. The choroid is the most posterior portion of the uveal tract. It has an important role for the nutrition of the posterior retinal layers (2).