Most animals have a third eyelid to protect their eyes, and cats are no exception. This eyelid is normally hidden behind those yellow, green or sometimes blue peepers. When it protrudes abnormally, that's a sign that something is not right with Kitty. In some cats, slight protrusion is normal.
The Third Eyelid
The scientific name for the third eyelid is "palpebra tertia" or "nictitating membrane." It's also known as the haw. According to Scientific American, it consists of fold of tissue covered by the conjunctiva, the eye's mucus membrane facing the eyelid's inner surface on one side and the cornea on the other. It acts as the eye's lymph nodes, keeping dirt and other unwanted particles out of it. The third eyelid can cover the cornea, but when the cat is awake you'll only spot a tiny portion of it in the eye's inner corner.
If your cat's third eyelid protrudes, that's generally a sign that something's amiss with Kitty's health, but it may or may not be eye-related. If it protrudes for more than a short time, take your cat to the vet so she can determine the cause. If there has obviously been some type of trauma to the eye, don't wait but take the cat to the vet at once. As a general rule, if the third eyelid protrudes in one eye, something is wrong with that eye. If it's in both eyes, it indicates something wrong systemically.
Haw syndrome usually affects younger cats, often appearing after Kitty has had diarrhea or some other intestinal complaint. Since worms can also cause haw syndrome, be sure your cat is properly wormed. Haw syndrome should clear up without any treatment, once Kitty is on the road to recovery. If the protrusion is large enough to affect her sight, your vet can prescribe eyedrops to help shrink it.
Occasionally—more often in outdoor than indoor cats—the third eyelid falls prey to cancer, specifically squamous cell carcinoma. This is a type of cancer caused by exposure to the sun, and is most common in cats with white fur. The affected portion of the third eyelid will probably have to be removed, and your cat might require radiation therapy. Cryotherapy, in which a tumor is frozen, is also an option for some cancers.
In this condition, the cartilage in the third eyelid folds, creating discomfort for the cat as well as looking terrible. The cause is unknown, though for some reason Burmese are especially prone to the problem. Surgery can solve this problem—you'll be glad to know that nothing in your cat's eye need be removed, as was once the recommended treatment. Instead, a surgeon will reposition the problematic gland under the eyelid.
Very dehydrated cats may have third eyelid protrusion. In this case, the cat obviously looks unwell. Just giving her water and canned food is probably not enough if the dehydration has reached this point. Take this kitty to the vet for diagnosis and treatment.
Always check with your veterinarian before changing your pet’s diet, medication, or physical activity routines. This information is not a substitute for a vet’s opinion.
Jane Meggitt has been a writer for more than 20 years. In addition to reporting for a major newspaper chain, she has been published in "Horse News," "Suburban Classic," "Hoof Beats," "Equine Journal" and other publications. She has a Bachelor of Arts in English from New York University and an Associate of Arts from the American Academy of Dramatics Arts, New York City.