Protrusion of Nictitating Membrane in Cats

Like many animals, your kitty has three eyelids to protect his eyes.
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Kitty apparently has mastered the slitted stink-eye look as he gazes upon the world, looking confident in his own superiority. But sometimes that squinty peeper signals an eye problem. If your kitty's third eyelid, called the nictitating membrane, is visible across part of his eye, he may need medical attention.

Third Eyelid Functions

The nictitating membrane is found in many species, including mammals, birds and reptiles, but is a vestige in humans, the little bump in the inner corner of the eye. In Kitty, it serves as a sort of windshield wiper and eye shield, moving across the eye to clear it of debris and protect it from damage. Some species can move this eyelid at will, but your kitty's lids move reflexively to protect his peepers. A muscle behind the eye pulls it back slightly to move the membrane into protective position in front. The eyelid also moves across the eye when your cat is very relaxed or sleeping, which you may notice if you look at your cat right before or after one of his many naps.


A protruding third eyelid often is the feline equivalent of a shiner. Cat disputes tend to develop into a flurry of growling, hissing and flying claws, and sometimes one of the combatants can get in a lucky swipe on his opponent's face. Any injury to your cat's eye tends to send the third eyelid into position, as it tries to keep the eye protected so it can heal properly. The third eyelid itself can be injured too, causing it to become visible from swelling.

Medical Cause

Many medical issues can affect your cat's eyes, causing discharge, crusting and appearance of the third eyelid. Conditions behind the eye itself, such as tumors, bleeding or infection, can weaken the muscle that controls its movement, causing the third eyelid to show. Horner's syndrome, Haw syndrome and cherry eye all affect the eye and cause protrusion of the third eyelid. With treatment of the underlying cause, the third eyelid usually withdraws to its hiding place in the corner of your kitty's eye.

When To Seek Treatment

The protrusion of your cat's third eyelid doesn't necessarily mean you must go scampering to the vet for treatment. Keep an eye on the lid, and see if it improves in a day or two. Between shed hair, dusty cat litter and sticking his face where it doesn't belong, your cat may have something in his eye, and the third lid is attempting to clear it. If the lid appears swollen or unusually red, or if other symptoms are present, such as discharge or a change in your cat's behavior, take him to your vet for an exam and treatment. In some cases, an ointment or antibiotic helps clear things up, while surgery may be necessary in other cases, depending on the injury.

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.

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