The Anterior Uvea

By | November 12, 2015

The irides of the puppy and kitten are often a different color than those of the adult. The blue-gray iris of puppies and kittens usually changes to the adult coloration within a few weeks. Iris color is ultimately related to the degree of stromal pigmentation and is influenced by coat color.

Congenital Abnormalities

Persistent Pupillary Membranes. Persistent pupillary membranes are strands of tissue that arise from the anterior iris surface and represent remnants of an embryonic vascular system. The persistent pupillary membranes may be confined to the iris surface or may extend from the iris to the cornea or lens (). Persistent pupillary membranes are inherited in the basenji.

Iris Cysts. Iris cysts are floating, fluid-filled vesicles that arise from the posterior iris epithelium and are usually found in the anterior chamber. Iris cysts may be unilateral or bilateral and singular or multiple in number.

Pupillary Abnormalities. A notch-like defect (coloboma) is occasionally seen in the ventronasal pupillary border of young dogs and cats, resulting in a keyhole-shaped pupil. An eccentric pupil (corectopia) may accompany multiple ocular defects, as occurs in the Australian shepherd. Eccentric pupils are usually oval and 1 to 2 mm off center ().

Heterochromia. Heterochromia is a variation in iris color. Heterochromia may describe zones of different colors in a single iris or may refer to the iris of one eye being different in color from the other. This difference in coloration is commonly seen in subalbinotic animals, including the merle-coated collie, Shetland sheepdog, Australian shepherd, harlequin Great Dane, Siberian husky, malamute, and Dalmatian dogs, and Siamese and white-coated cats. Variations in the degree of pigmentation in the retinal pigment epithelium and choroid may occur simultaneously. In white-coated cats, blue iris color is often associated with unilateral or bilateral deafness. Pale iris coloration may be a distinguishing feature of cats with Chediak-Higashi syndrome. Affected irides are pale yellow-green rather than the bold copper or yellow of the unaffected Persian cat.

Iridocorneal Abnormalities. Congenital mesodermal goniodysgenesis occurs in the iridocorneal angle of the basset hound. This mesodermal goniodysgenesis often forms extensive barriers across the iridocorneal angle, predisposing the animal to impaired aqueous outflow. Despite the presence of the congenital mesodermal goniodysgenesis, the onset of glaucoma does not usually occur in the basset hound until the dog is older than 6 months.

Acquired Abnormalities: Anterior Uveitis

Early recognition and treatment of anterior uveitis are important because the condition is painful and potentially blinding. Traumatic insult, toxicosis, and infectious diseases are common causes of anterior uveitis in young dogs and cats but not neoplastic, metabolic, or degenerative disorders. Anterior uveitis may result from disorders limited to the eye itself or may be secondary to systemic illness. Primary ocular abnormalities resulting in anterior uveitis are generally unilateral. Traumatic insult is probably the most common cause of a unilateral anterior uveitis in the young dog and cat.

The nature of intraocular foreign objects determines the characteristics of the intraocular inflammation. The most reactive materials include iron, steel, copper, and organic matter. Relatively inert materials include lead, glass, plastic, and rubber. Infection of extraocular tissues, such as a corneal ulcer, with breaching of the corneal/scleral barrier by pathogens or their toxins, may result in severe intraocular inflammation. Aberrant intraocular parasites, including Ancylostoma species, Toxocara species, and Cuterebra organisms, can cause anterior uveitis and subsequently endophthalmitis. Although Dirofilaria immitis occurs in the anterior chamber of dogs, affected animals are older than 6 months.

Anterior uveitis secondary to systemic illness is generally bilateral. Infectious diseases with secondary ocular effects are numerous. Anterior uveitis associated with canine adenovirus type 2 and canine distemper infections are uncommon, but canine herpesvirus commonly causes a panuveitis in young puppies. Feline infectious peritonitis is associated with prominent ocular signs. Feline leukemia virus and feline immunodeficiency virus infections rarely cause primary ocular disease. Intraocular hemorrhage and anterior uveitis are more likely secondary to severe anemia and metastatic lymphosarcoma, respectively. Ocular manifestations of bacterial infections may arise secondary to localized infections such as osteomyelitis or may represent systemic bacterial diseases such as seen in sepsis, leptospirosis, or brucellosis. Systemic fungal diseases are frequently associated with anterior uveitis but are uncommon in young dogs and cats. Feline toxoplasmosis should be considered in bilateral anterior uveitis. Rickettsial diseases to be considered include Rocky Mountain spotted fever and ehrlichiosis.

Anterior uveitis is often identified at least initially by the presence of lacrimation, blepha-rospasm, and enophthalmos — immediate indicators of ocular pain. Of the specific findings, conjunctival and episcleral hyperemia is the first to appear. Generalized corneal edema follows. Blood vessels may invade the deep corneal layers, forming a red paintbrush ring around the corneal circumference. The aqueous humor appears cloudy as protein enters the anterior chamber through the disrupted blood-aqueous barrier. Other changes in the anterior chamber include the influx of leukocytes (hypopyon), erythrocytes (hyphema), or fibrin strands. Collections of inflammatory cells on the inner corneal surface are referred to as keratic precipitates (). Changes in the pupil include miosis () and sluggish response to light. Adhesions of the iris to the lens (posterior synechiae) may cause permanent changes in the pupillary shape. Inflammation of the ciliary body causes decreased aqueous production and a decline in intraocular pressure.

Treatment of anterior uveitis is usually symptomatic care (). Corticosteroids are used to reduce inflammation as quickly as possible. Topical application of 0.1% dexamethasone or 1% prednisolone acetate is performed every 2 to 4 hours in the early management of anterior uveitis. Even hourly applications may be necessary for severely inflamed eyes. In animals with severe anterior uveitis, subconjunctival corticosteroids may be used. Methylpredniso-lone and triamcinolone acetonide are generally well tolerated by the eye. The usual volume is 0.2 ml of a 40-mg/ml concentration. For severe anterior uveitis without systemic infectious disease, oral corticosteroids may be considered, especially if the choroid is involved. The dosage of prednisolone, beginning at 1 to 2 mg/kg every 12 hours, should be gradually reduced as soon as ocular response allows. Abrupt cessation may be associated with a rebound of severe inflammation.

Pupillary dilation is accomplished by topical application of a parasympatholytic agent such as 1% atropine applied to effect. The inflamed iris will not respond as well to atropine as the unaffected iris, and the effects may diminish sooner than expected. Atropine reduces pain as uveal muscle spasm is relieved, prevents adhesions between iris and lens, and restores vascular permeability. Cats may salivate following application of atropine when the bitter drug exits the nasolacrimal system and is licked from the nose. If atropine is ineffective, 10% phenylephrine may be used in the regimen for its synergistic sympathetic effect. Therapy should be continued for 10 to 14 days beyond resolution of ocular signs.

 

Selections from the book: “Veterinary pediatrics: dogs and cats from birth to six months”. Johnny D. Hoskins. (2001)