Uveitis is an inﬂammatory process involving the middle of the three layers in the eye. To understand uveitis, it is important to know the basic anatomy of the eye. The outer layer enclosing the eye is composed of the clear cornea and the white sclera. The innermost layer is the nerve layer or the retina. The middle layer, which is rich in blood vessels, is the uveal tract. It is composed of the iris in the front part of the eye, the ciliary body, which produces the ﬂuid (aqueous humor) inside the eye, and the choroid, which nourishes the retina in the back of the eye. Because of its rich blood supply, the uveal tract is a natural target for diseases originating in other parts of the body. When inﬂammation attacks specific segments of the uveal tract, the disease is further classified as iritis (inﬂammation of the iris), cyclitis (inﬂammation of the ciliary body) or choroiditis (inﬂammation of the choroid), depending on the affected structure. If all the structures are inﬂamed then it is called panuveitis (inﬂammation of all uveal structures of the eye).
Uveitis may produce vague signs that can include excessive blinking, squinting, watery discharge and photophobia (sensitivity to light) without any obvious changes to the eye itself. In more advanced cases, changes to the eye are visible without special instruments. The eye may appear dull, cloudy or red due to changes in the cornea or due to inﬂammatory cells accumulating inside the eye. Uveitis is usually diagnosed following an examination of the ocular structures by your veterinarian or veterinary ophthalmologist utilizing instruments, which magnify and illuminate the uveal tract. Once uveitis is diagnosed, a general physical examination should be performed in case the uveitis is actually an early sign of internal or systemic disease. The evaluation may include blood profiles or specific tests if a certain disease is suspected. Ocular examination consists of a visual inspection of the interior of the eye with a slit lamp and the measurement of ocular pressure. If the internal structures of the eye cannot be clearly visualized, ocular ultrasound may be performed to more clearly visualize the position of the retina and lens and to detect any abnormal masses or growths within the eye.
Ocular pressure is maintained by the aqueous humor (ﬂuid) produced by the ciliary body within the eye. Initially, if the ciliary body is inﬂamed, the ﬂuid production slows down and the ocular pressure drops. The aqueous humor produced in the eye normally drains through the angle between the cornea and the iris. The inﬂammatory debris produced in uveitis can block the drainage angle and result in increased intraocular pressure (glaucoma) over time. Once uveitis resolves, glaucoma can remain if drainage structures were damaged by the inﬂammation. A recheck of the eyes following uveitis is important for this reason.
Additionally, disease processes such as uveitis can lead to corneal ulcers (superficial to deep), ocular infections, corneal scarring, corneal vascularization, corneal mineralization, cataract, lens luxation, retinal detachment and keratoconjunctivitis sicca. Uveitis also can lead to secondary complications similar to those to which treatment for uveitis can give rise as discussed under “Prognosis”.
Uveitis is associated with many different diseases. Examples in the dog include Ehrlichiosis and Coccidioidomycosis, two systemic infectious diseases common to the southwestern United States. In the cat, uveitis can be a consequence of Feline Leukemia Virus, Feline Infectious Peritonitis or many other diseases. In any animal, penetrating injuries such as cactus spines or a cat scratch may produce uveitis. Inﬂammation of the uveal tract can occur when the lens capsule is breached (such as following surgery, trauma, or injury of the lens) or in the presence of cataracts where lens proteins leak out of the lens capsule into the eye. Other possible causes of uveitis are local bacterial infection, immune-mediated, and parasitic diseases. Treatment can be more specific if the actual cause is known. It is important to test for some infectious diseases to make sure there is not an underlying cause for the inﬂammation, but unfortunately, in up to 75% of the cases, the cause is never determined.
Uveitis must be treated aggressively in order to prevent glaucoma, scarring of the uveal structures, and possibly blindness. Different medications may be used to treat the underlying, original cause of the uveitis and to attempt to control the inﬂammation itself. Aspirin (not aspirin substitutes) and corticosteroids minimize the inﬂammatory process. Corticosteroids may be administered by injection under the lid of the eye, by drops in the eye, or as an oral medication, depending on the suspected cause of uveitis. Topical use must be postponed if damage to the corneal surface is present because the corticosteroids prevent healing of the ulcer. If certain systemic diseases are suspected, oral corticosteroids may be postponed until test results become available. Atropine dilates the pupil and helps prevent scarring of the iris. This medication may be contraindicated; however if glaucoma is present as it may further decrease the drainage of aqueous humor from the eye. Oral and topical antibiotics are employed when a bacterial infection is present in the eye.
The treatment of uveitis requires therapy to halt the inﬂammation of the uveal tract along with a search for the original cause of the disease. Many tests may be needed to determine possible causes and the results are important for proper treatment.
Treatment for uveitis can involve life-long topical and/or oral medications. Life-long topical medications seldom give rise to complications, which occur in less than 5% of these cases. Nevertheless, potential complications include, but are not limited to, inﬂammation of the pink tissue (conjunctivitis); corneal ulcerations (superficial to deep); corneal scarring, vascularization, and mineralization; ocular rupture secondary to ulceration; worsened inﬂammation inside the eye, secondary to infection or ulceration; glaucoma, secondary to the uveitis; retinal detachment or degeneration, secondary to uveitis or infection; ocular or orbital pain, secondary to uveitis, glaucoma or infection; eyelid rubbing; bleeding inside the eye (hyphema), secondary to uveitis or infection; tearing (epiphora); and/ or lens luxation, secondary to uveitis, glaucoma or infection. Some of these complications can lead to blindness. Some oral medications used to treat these disease processes may cause changes in behavior, gastrointestinal upset (diarrhea, vomiting, decrease/increase in appetite/thirst), panting, decreased white blood cell counts (such as leukopenia), and various changes in chemistry values (liver, kidney, potassium, to name a few).
Your awareness of your pet’s symptoms and compliance with recommendations for recheck examinations and periodic blood work help control these potential complications.