The uvea (or the uveal tract) is the colored inside lining of the eye consisting of the iris, the ciliary body, and the choroid. The iris is the colored ring around the black pupil. The ciliary body is the set of muscles that contract and relax to allow the lens to focus on objects; it is also the major source of aqueous humor, the clear fluid within the eye. The choroid is the inner lining of the eyeball and extends from the ciliary muscles to the optic nerve at the back of the eye. The choroid also contains layers of blood vessels that nourish the inside parts of the eye, especially the retina. The anterior uvea refers to the portion in the front of the eye: the iris and the ciliary body. Diseases that affect the anterior uvea often affect the choroid as well.
Persistent Membranes across the Pupil
Persistent membranes across the pupil are the remains of the normal prenatal blood vessels that fill the region of the pupil. The persistence of colored strands across the pupil from one area of the iris to another, or to the lens or cornea, is common in dogs and occurs occasionally in other species. In Basenjis, the condition is inherited.
Atrophy of the Iris
A weakening and shrinking (atrophy) of the iris is common in older dogs and may involve the edge of the pupil or the connective tissue. Shrinkage of the edge of the pupil creates a scalloped border and a weakening of the sphincter muscle, which results in a dilated pupil or slowed reflexes of the pupil in response to light. Shrinkage of the connective tissue results in dramatic holes in the iris, and often, displacement of the pupil. Neither of these types of atrophy appears to affect vision. Animals lacking a functional sphincter muscle (which controls the opening and closing of the iris) may show increased sensitivity to bright light.
Cysts of the Iris
Cysts of the iris are usually free-floating, colored spheres in the liquid part within the eye. Although harmless in most breeds of dogs, cysts of the anterior uvea (the iris and the tissue and muscle surrounding the lens) in Golden Retrievers and Great Danes are associated with longterm inflammation of the uvea, abnormally high pressure within the eyeball (glaucoma), and the formation of cataracts. Therapy is rarely necessary in most breeds, but removal or rupture of a cyst may occasionally be required. Your veterinarian can assess your pet’s condition and recommend the best treatment.
Iris Coloboma
An iris coloboma is a hole in the iris that causes an irregularly shaped pupil. They occur occasionally in Australian Shepherds and are rare in other breeds. They are most common in eyes that have more than one color in the iris.
Inflammation of the Anterior Uvea
Inflammation of the iris and ciliary body (anterior uveitis or iridocyclitis), when severe, results in a contraction of the pupil of the eye, increased protein and cells in the anterior chamber, low pressure within the eye, an abnormally high level of blood (and redness) in the conjunctiva, swelling of the iris, low tolerance to light, and spasmodic winking. As a result, complications may occur, such as glaucoma, the formation of a cataract covering the lens of the eye, and cloudiness of the cornea. Inflammation of the membrane (the choroid) between the retina and the white of the eye (the sclera) frequently occurs at the same time.
Trauma and, rarely, tumors or worms within the eye are causes of inflammation of the uvea in only one eye. Common causes of inflammation of the uvea in both eyes include infectious diseases, such as infectious canine hepatitis (a viral infection), toxoplasmosis (a parasitic disease), fungal infections, and bacterial infections (for example, canine brucellosis, leptospirosis, and canine ehrlichiosis). Immune system problems may also cause this condition in dogs. Your veterinarian will want a thorough medical history of your pet to help in diagnosing this condition. Other diagnostic steps may include examination of the cornea for injuries, a physical examination, blood tests, and tests on fluid from your pet’s eye.
Treatment of the inflammation consists of topical medications to maintain iris movement capabilities, a darkened environment, and other prescription medications, such as steroids or antibacterial drugs. Your veterinarian will select the most appropriate treatment program for your pet’s specific condition.
Hyphema (Bleeding Within the Eyeball)
Bleeding inside the eyeball is called hyphema. It may appear as liquid or clotted blood. Causes include uveitis, traumatic injury, a tumor within the eye, detachment or tearing of the retina, high blood pressure, clotting disorders, birth defects, and glaucoma. Sudden, severe bleeding usually has a good outcome if the cause is identified and treated promptly. Recurrent or longterm hyphema has an unknown or poor outlook because glaucoma or blindness is likely. Although no drugs are available to treat hyphema directly, certain medications may help relieve the signs.
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