Conjunctivitis, the most common eye ailment in cats, is an inflammation or irritation of the mucus membranes that cover part of the eyeballs and line the eyelids.  It's generally caused by allergies to grass or pollen, irritants that get in the eye, or infection from a virus or bacteria.  "If caused by allergies, a clear discharge may develop; if caused by infection, the discharge may be green or yellow," says Ronald Riis, DVM, associate professor of ophthalmology at the Cornell University College of Veterinary Medicine.  "In either case, a cat with conjunctivitis will have a red and swollen eye and may squint or keep his eyes partially closed.  A cat's third eyelid also is often visible with conjunctivitis."

Both painful and a potential threat to vision, conjunctivitis is usually treated with antibiotics.  Since conjunctivitis can be a sign of a more serious illness, veterinarians likely will test for feline immunodeficiency virus, herpes virus infection, and feline leukemia virus if the cat doesn't respond to treatment within two weeks. 

Bloodshot and swollen eyes also can be signs of glaucoma, a leading cause of blindness in cats.  "As the disease progresses, pressure within the eye increases because the fluid outflow gets blocked," says Iris.  "The eye may tear, give off a discharge, look blue, or have enlarged pupils that are unresponsive to light."  The growing pressure not only causes pain as the eyeball swells but also compresses the retina and optic nerve; this condition will lead to blindness if not promptly treated.

Sometimes an eye injury or tumor will cause glaucoma, but it also can develop with age, particularly in certain breeds such as Persian, Siamese, and some domestic shorthairs.  To make a definitive diagnosis, the veterinarian will use a special instrument to measure eye pressure.  Glaucoma is usually a bilateral disease.  Although it may destroy one eye, the second eye can often be saved once the veterinarian has diagnosed that glaucoma is the cause. 

If diagnosed early (rare with the first-eye involvement), treatment may merely involve medicated eye drops to reduce the pressure in the eye.  If the case is more severe and vision is at stake, the cat may require hospitalization for aggressive treatment.  Glaucoma control may include cycloblation (either laser or cryosurgery), a procedure to intentionally damage the fluid-producing organ so that fluid production is reduced.  Surgery may also be needed to remove a tumor if present or to put in a shunt to drain the eye of excess fluid.

If vision is already lost, your veterinarian will recommend one of several procedures to ensure that your cat becomes pain free.  These procedures include surgically removing the inside of a blind eye and inserting a silicone implant inside the shell of the eye to maintain the eye's shape and movement.  If the pet can not undergo surgery because of age or illness, injecting high doses of antibiotics into the eye will kill the tissue that produces the liquid causing the pressure and pain.  Alternatively, the eye is removed and the lids stitched shut.

Uveitis, an inflammation of certain internal structures of the eye, is another condition that can lead to blindness.  Causes of uveitis are many, and treatment will vary depending on whether the cause is identified.  Usually the cause can't be determined, and anti-inflammatory drugs are used.  If uveitis goes untreated, it can lead to glaucoma and blindness.  Signs may be vague, such as blinking, squinting, discharge, eye cloudiness or redness, and iris color changes.  While the disease often can be cured, especially if detected early, it's sometimes chronic or may become so severe the eye must be removed.