The third eyelid is not normally seen, but it may become opaque and/or visible in response to illness or injury. A few cats will have their third eyelid up when they are totally relaxed and resting, so it is important to know what is normal for your cat. In these cats, the membrane will retract quickly when they alert or startle, and then stay retracted for awhile. The length of time the membrane is exposed may vary, as though the cat is blinking, or it may remain visible. When the nictitating membrane is visible over the inside corner of the eye, it is protruding.
When associated with a bulging eye, causes of protrusion of the nictitating membrane include infection in the tissue behind the eyeball (abscess), bleeding behind the eye (hematoma), and tumor.
Pancreatitis is an inflammation of the pancreas, which often primarily affects the exocrine or digestive enzymes. It can be acute or chronic, with the chronic form being more common in cats.
Pancreatitis can have many causes, including trauma, parasites, infection, and drug reactions. However, more than 90 percent of all pancreatitis cases in the cat have no identifiable cause. Siamese cats may have a genetic predisposition.
Unlike dogs, cats with pancreatitis do not usually show vomiting or abdominal...
When associated with a retracted or sunken eye, causes of protrusion of the nictitating membrane include any painful eye illness resulting in spasm of the muscles around the eye; spasm of these muscles when caused by tetanus; and dehydration or chronic weight loss that reduces the size of the fat pad behind the eye. When only one eye is involved, suspect an illness related to that eye; when both eyes are involved, suspect a systemic illness such as feline viral respiratory infection.
Horner’s syndrome can result in a sunken eye, prolapse of the third eyelid, and a small pupil. This can occur as a consequence of injury to (or cancerous involvement of) a nerve in the neck or a middle ear infection.
Treatment: There is no treatment, although the condition may resolve with time.
This is a rather common but temporary protrusion of uncertain cause of the third eyelid. It affects otherwise healthy cats under age 2 and is frequently preceded by a gastrointestinal illness.
Treatment: The protrusion clears up within a few months without treatment. During this time, if the film interferes with your cat’s vision, your veterinarian can prescribe an eyedrop solution containing 1 or 2 percent Pilocarpine, which reduces the size of the protrusion.