Although eyes are remarkably similar between domestic species, the feline eye presents some unique features both in normal anatomy and response to disease. In addition, there are a number of ocular conditions which occur only in the cat. In general, the cat eye exhibits a much less pronounced response to inflammation than seen in the dog. The cornea is slower to demonstrate edema or vascularization and corneal pigmentation is extremely rare. Thus, detection of the early stages of ocular disease in cats can be more challenging. Some of the more common conditions of the feline eye are discussed here.
Conjunctivitis in cats most often occurs secondary to systemic upper respiratory infections. Etiologic agents include: Herpesvirus, Calicivirus, Mycoplasma, Chlamydia. Often the ocular symptoms persist long after apparent resolution of the respiratory disease, possibly due to infection within the nasolacrimal duct. Diagnosis can be made by immunofluorescent antibody testing or virus isolation, however false negatives are common. A polymerase chain reaction (PCR) test is the most sensitive diagnostic method. A conjunctival or corneal scraping or (preferably) a biopsy is frozen in saline and sent overnight to a specially-equipped laboratory (currently Colorado State University). The test involves amplification of DNA within the sample to identify the presence of the virus. Antiviral medications available include idoxuridine (compounded), trifluridine (Viroptic®) and vidarabine (compounded) — virus sensitivity is variable and medication must be continued TID-QID for 3-6 weeks. Unresponsive cases should have an evaluation of the immune system including FeLV and FIV test.
|Common sequella to chronic conjunctivitis include: dry eye and/or conjunctival adhesions (symblepharon) which may result in occlusion of the nasolacrimal drainage puncta, protrusion of the third eyelid and narrowing of the palpebral fissure (possibly resulting in an impaired blink response).|
|Corneal Sequestrum is a form of corneal degeneration unique to the domestic cat. The clinical appearance is diagnostic — a localized black discoloration of the cornea, often associated with adjacent ulceration (see figure). The condition is thought to be inherited in the Persian and Himalayan breeds and may also occur secondary to chronic keratoconjunctivitis (particularly following Herpesvirus infection). The condition may be progressive; treatment requires superficial keratectomy for removal of the necrotic corneal stroma.|
|Eosinophilic Keratitis is an immune-mediated inflammatory condition of the feline cornea characterized by a fibrovascular response with or without ulceration. Diagnosis is made by corneal cytology which demonstrates an eosinophilic infiltrate. This is one of the rare conditions where topical corticosteroid therapy is indicated even in the face of corneal ulceration. In chronic cases, the lesion may become intensely proliferative and a superficial keratectomy may be required.|
|Uveitis in cats may occur spontaneously or secondary to systemic disease (see table). Spontaneous uveitis is often granulomatous, characterized by nodular lesions within the iris stroma and altered iris coloration (see figure). Flare and corneal edema are often absent but multifocal deposits (keratic precipitates) may occur on the corneal endothelial surface. The condition may be unilateral or bilateral and is usually responsive to steroids (systemic and subconjunctival). Long-term therapy is often required and secondary glaucoma is a major concern.|
|Iris melanoma is the most common primary intraocular tumor in cats. The condition often presents as a slowly progressive, diffuse increase in iris pigmentation (see figure). Ultimately the iridocorneal angle becomes involved and secondary glaucoma results. Early detection and enucleation is critical as these tumors metastasize early and aggressively (unlike the same disease in dogs where systemic dissemination is rare). Chronic uveitis in cats can also be associated with a benign form of iris hyperpigmentation which is non-proliferative in appearance. Distinction between these conditions is challenging but essential.|
|Glaucoma presents with much more subtle clinical signs in the cat compared to the dog. The cat eye is able to withstand much greater pressure elevation without evidence of corneal edema or scleral injection. Often the only sign may be a slight asymmetry in pupil size. Diagnosis of glaucoma in cats is challenging and requires tonometry which often reveals intraocular pressure of 35-55 mmHg (25 mmHg is the upper limit of normal). Although in the dog, this condition rapidly results in permanent retinal damage, cats may recover some vision even after several weeks of elevated pressure.|
|Retinal detachment is often an indication of underlying systemic disease. The condition is usually bilateral and presents with acute blindness and pupillary dilation. The most common cause is systemic hypertension (see separate forum section on this topic). All of the inflammatory conditions which cause uveitis in cats can also result in retinal detachment.|
|Systemic diseases causing|
uveitis (+/- retinal detachment) in cats
any metastatic neoplasia
Retinal degeneration occurs as an inherited condition less commonly in cats than in dogs. The condition has been identified as a specific genetic entity in the Abbyssian cat where rod/cone degeneration occurs which a highly variable age of onset (4 months to 6 years). A slowly progressive form of non-inherited retinal degeneration is associated with dietary taurine deficiency. The condition was originally described in 1975 and named Feline Central Retinal Degeneration (FCRD). Although most species are able to endogenously synthesize taurine from cysteine, this ability is limited in the cat. Commercial cat foods contain adequate levels of taurine and this form of nutritional retinal degeneration is seen most often in cats fed predominantly dog food.