Feline uveitis: A review of its causes, diagnosis, and treatment

Many disorders—both ocular and systemic—can result in inflammation of the uveal tract. By quickly narrowing the causes and initiating appropriate treatment, you might be able to save an affected cat's sight.
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Nov 01, 2009

Uveitis is a common and painful ocular disease in cats that can eventually lead to blindness. Uveitis often occurs secondary to an acquired ocular or systemic disorder; however, in many instances the underlying cause is not identified despite extensive diagnostic testing. As such, it presents a great challenge for practitioners with respect to diagnosis and treatment. In this article, we review the pathophysiology, clinical presentation, causes, and treatment of uveitis in cats.

ANATOMY AND PATHOPHYSIOLOGY

The uvea, or vascular tunic of the eye, is composed of the iris and ciliary body anteriorly and the choroid posteriorly. The iris divides the anterior ocular compartment into anterior and posterior chambers and controls the quantity of light entering the posterior segment through the pupil. The ciliary body provides nourishment for and removes wastes from the cornea and lens via the production of aqueous humor. The choroid is the main source of blood and nutrition for the outer layers of the immediately adjacent retina.1

Uveitis is defined as any condition that involves uveal tract inflammation. Damage to the uveal tissue or vasculature2,3 causes the release of tissue factors and chemoattractants, resulting in vasodilation and changes in vascular permeability.3 These changes lead to a breakdown of the blood-aqueous barrier or the blood-retinal barrier, allowing protein and cellular accumulation.2

Classifying uveitis can help you determine what diagnostic steps will be required when presented with a cat that has uveitis. Anatomically, uveitis is classified as anterior if the iris and ciliary body are inflamed (iridocyclitis). Inflammation of the choroid is termed posterior uveitis or choroiditis. A full ophthalmic examination is required to determine which portions of the uveal tract are involved. Additionally, uveitis can be classified etiologically as being related to an underlying ocular disorder or secondary to a systemic disease process. Ocular disorders are typically unilateral and readily identified on complete ophthalmic examination. Systemic causes of uveitis often result in bilateral ocular involvement.

OPHTHALMIC EXAMINATION AND FINDINGS

A thorough ophthalmic examination is required to diagnose uveitis. Perform fluorescein staining to rule out the presence of a corneal ulcer, and measure intraocular pressure by applanation tonometry after applying topical proparacaine.

An anterior segment evaluation is best performed by using a hand-held slit lamp. Alternatively, the anterior chamber can be evaluated with a direct ophthalmoscope by using either the small spot or slit setting. Place the direct ophthalmoscope close to the eye, and focus it on the cornea. View the eye perpendicular to the light source to evaluate the anterior chamber for flare, cellular accumulation, or changes of the iris face. To evaluate for opacities of the aqueous, lens, or vitreous, you may also use retroillumination. To do so, stand at an arm's length from the patient, and obtain a tapetal reflex with a hand-held transilluminator. Opacities will block or diminish the tapetal reflex.


Table 1: Possible Ocular Signs and Complications in Cats with Uveitis
Perform the fundic examination by using direct or indirect ophthalmoscopy. Indirect ophthalmoscopy requires a focal light source (Finnoff transilluminator) and a hand-held lens. The most commonly used hand-held lenses are 20 or 28 diopters. The larger-number diopter lenses make it easier to visualize the fundus when the pupil is small. The image obtained by indirect ophthalmoscopy is less magnified than with direct ophthalmoscopy, but the larger field of view obtained makes it a better method of screening eyes for posterior segment involvement.

Clinical signs associated with uveitis in cats can vary widely and may not be as prominent as signs seen in other species (Table 1). Pain is often associated with acute uveitis and is observed clinically as blepharospasm, photophobia, enophthalmos, third eyelid elevation, or epiphora.2,3

Anterior uveitis


Figure 1. A slit lamp photo of a patient with ­anterior uveitis and resultant aqueous flare (arrowheads). Breakdown of the blood-aqueous barrier results in increased protein concentration of the aqueous humor, and the resultant turbidity (scattering of light) can be seen on oblique illumination of the anterior chamber. (Photo courtesy of Dr. Ellison Bentley.)
Aqueous flare, which is diagnostic of anterior uveitis (Figure 1), occurs secondary to the breakdown of the blood-aqueous barrier and increase in aqueous humor protein concentration.2,3 Purulent material or blood may also accumulate in the anterior chamber, resulting in hypopyon or hyphema, respectively (Figure 2).2


Figure 2. A patient with lymphosarcoma presenting with hyphema, hypopyon, and dyscoria (an abnormally shaped pupil) in the left eye. (Photo courtesy of Dr. Kirk Ryan.)
Inflammation of the corneal endothelium impairs the chloride-dependent active pump mechanism responsible for transporting fluid out of the stroma and results in corneal edema.4 Inflammatory cells within the aqueous humor may aggregate and deposit upon the corneal endothelium, resulting in keratic precipitates that are typically present on the ventral half of the cornea (Figure 3).2,3 Additionally, corneal vascularization can occur secondary to chronic inflammation.2


Figure 3. A slit lamp photo of a cat with FIP and anterior uveitis. Cellular debris is present along the endothelial surface of the inferior cornea resulting in "mutton fat" keratic precipitates (arrowheads). A superficial corneal ulcer is also seen centrally. (Photo courtesy of Dr. Ellison Bentley.)
Intraocular pressure is often decreased in patients with uveitis because of prostaglandin-mediated inflammation5 and edema of the ciliary body resulting in impaired aqueous humor formation or increased aqueous humor outflow through the uveoscleral route.6


Figure 4. A cat with FeLV infection demonstrating iridal swelling and dyscoria secondary to extensive posterior synechiae. Hypopyon is also present in the anterior chamber (arrowhead). (Photo courtesy of the University of Wisconsin-Madison.)
Changes in the iris can also occur with anterior uveitis. In response to the effects of prostaglandins and other inflammatory mediators on the iris sphincter muscle, miosis may be observed.2,3 The miosis is typically associated with ciliary muscle spasm, which contributes to ocular pain.2 Iridal swelling or iritis results from vasodilation, increased iris vessel permeability, and cellular accumulation, which often causes a change in iris color.2,3 Cellular accumulations may also give the iris a grossly swollen appearance.3


Figure 5. A cat with FIP and associated anterior uveitis. An iridal color change is the result of rubeosis iridis, or neovascularization of the iridal surface. Keratic precipitates are also present along the posterior aspect of the cornea inferiorly. (Photo courtesy of Dr. Eric Storey.)
In patients with chronic uveitis and resulting miosis and iridal swelling, the pupil margin may adhere to the anterior lens capsule, resulting in posterior synechiae (Figure 4).3 In these cases, the pupil margin appears irregular and the pupil fails to respond appropriately to light and dilating agents.3 If the entire pupil margin is involved, anterior movement of the aqueous humor is inhibited, resulting in aqueous humor accumulation behind the iris. This accumulation appears clinically as iris bombé, or an anterior ballooning of the iris, which predisposes the eye to secondary glaucoma development.3 Additionally, in patients with chronic inflammation, the iris may appear red, also known as rubeosis iridis, secondary to neovascularization of the anterior iridal surface (Figure 5).2