The outer layer that encloses the eye is composed of the clear cornea and the white sclera.  Inside the eye, the innermost layer is the nerve layer called the retina.  The middle layer of the eye – called the uvea or uveal tract – is the nutritional layer, rich in blood vessels.  It is made up of the iris (coloured portion), the ciliary body (the part that produces the fluid inside the eye called aqueous humor), and the choroid (provides nutrition to the retina).  When inflammation develops within the uveal tract, this is termed uveitis.  As specific segments of the uveal tract are affected, uveitis is further classified depending on the affected structure:

  • Iritis is inflammation of the iris.
  • Cyclitis is inflammation of the ciliary body.
  • Anterior uveitis or iridocyclitis is inflammation of both the iris and the ciliary body.
  • Choroiditis or posterior uveitis is inflammation of the choroid.
  • Panuveitis is inflammation of the iris, ciliary body, and choroid.

Due to its rich blood supply, the uveal tract is a natural target for diseases originating in other parts of the body. Because the cornea is normally clear, signs of disease may be seen inside the eye, often before signs develop elsewhere in the body. Additionally, uveitis can be caused from problems within the eye itself, such as cataracts or changes in the lens, corneal ulcers, or trauma.

Uveitis is not a disease, it is a condition or syndrome which indicates inflammation inside the eye.  It may cause vague clinical signs that may include blinking, squinting, watery discharge from the eye, and/or sensitivity to light without any obvious changes to the eye itself.  Some obvious signs of uveitis include:

  • Red, swollen conjunctiva (pink tissue around the eye)
  • Cornea appears dull or hazy blue
  • Iris becomes red or changes color
  • Pupil is constricted

Uveitis is diagnosed by an ophthalmic examination of the structures of the eye using instruments that magnify and illuminate. If it is suspected that the problem is being caused by another disease process elsewhere in the body (as opposed to problems within the eye itself as mentioned above), then blood profiles or other tests may be necessary to locate the cause. An ophthalmic examination consists of a visual inspection of the external and internal portions of the eye and the measurement of the pressure within the eye. Pressure inside the eye is maintained by fluid (aqueous humor) which is continually being produced by the ciliary body. When the ciliary body is inflamed, fluid production decreases, causing the pressure in the eye to drop below normal. In a normal eye, the aqueous humor flows forward, through the pupil, then drains into an opening (filtration angle) between the iris and the cornea where it leaves the eye. In some cases of uveitis, the draining of fluid can be blocked, increasing the pressure within the eye (a condition called glaucoma). Causes of blockage include:

  • Cellular debris produced in uveitis can block the drainage openings and inhibit the outflow of the fluid causing an elevation in the intraocular pressure.
  • The lens may adhere to the iris, blocking the flow of fluid through the pupil.

Once the uveitis resolves, glaucoma may still persist if the drainage structures were permanently damaged by the inflammation.  Alternatively, if the ciliary body was severely damaged by the inflammation, fluid production may cease entirely and the eye will begin to shrink.  For these reasons, regular recheck exams with the ophthalmologist are important.

Many different diseases can cause uveitis.  Diseases in the dog include lymphoma, bleeding disorders, ehrlichiosis, rocky mountain spotted fever, lyme’s disease, and brucellosis.  In the cat, causes include feline leukemia virus (FeLV), feline infectious peritonitis (FIP), feline immunodeficiency virus (FIV), toxoplasmosis, and/or other diseases.  In any animal, corneal ulcers, penetrating injuries, blunt trauma, or even a scratch may result in uveitis.  The lens can also cause uveitis if it has leaked some of its contents inside the eye, or if a cataract is rapidly forming or dissolving.  Uveitis can also occur after cataract surgery.  Further possible causes include local bacterial infection, immune mediated diseases, cancer, and parasitic diseases.  Treatment can be more specific if the actual cause of uveitis is known.  Unfortunately, in up to 70% to 80% of the cases, the cause of uveitis is never determined.

Medical treatment of uveitis must be aggressive to prevent glaucoma, scarring of the structures inside the eye, and blindness.  Different medications are used to control the original cause of the uveitis, if known, and to minimize the inflammation itself.  Eye drops and oral corticosteroids minimize the inflammatory process. Steroid-sparing drugs such as Imuran can help reduce the need for large amounts of oral corticosteroids and improve the outcome.  Corticosteroids may be administered by eye drops, injections under the conjunctiva, and orally depending on what structures in the eye are affected.  Drops in the eye must be postponed if damage to the corneal surface (such as an ulcer) is present because the corticosteroids prevent healing and can cause the ulcer to worsen.  If certain systemic diseases are suspected, oral corticosteroids may be postponed or avoided altogether.  Topically applied NSAIDs (non-steroidal anti-inflammatory drugs) will help reduce the inflammation within the eye.  Drops or ointments that dilate the pupil and relax the muscles within the eye help to reduce adhesions and pain.  This medication may not be used if glaucoma is present as it may further decrease the fluid drainage from inside the eye.  Antibiotics are only used when the cause has been localized to a systemic infection such as ehrlichia, lyme’s disease, and Rocky Mountain spotted fever.

The treatment of uveitis requires therapy to halt the inflammation of the uveal tract.  Follow up examinations ensure optimal therapy is being given and guard against possible complications.  Uveitis, if caught early and treated diligently and aggressively, will often resolve without serious consequences.  Unfortunately, in certain individual patients, the cause of uveitis is never determined and treatment may be life-long.  In other patients, uveitis is so severe that removal of the eye is necessary.  Lastly, in occasional patients, uveitis is self- perpetuating (causes more uveitis).  These patients are more difficult to control.