That Mysterious Third Eyelid

Jun 5, 2011 by     23 Comments    Posted under: Tips & Advice

For many owners, the appearance of their cat’s third eyelid is cause for great concern and confusion.  Never fear- it can often indicate a problem, but with a little information, you can better determine why this might be happening and how quickly your cat needs professional medical attention.

First, a bit of background:  The third eyelid provides an extra layer of eye protection for cats and many other animals.  Other names for the third eyelid include the nictitating membrane, nictitans and haw.  Arising from the corner of the eye nearest the nose, the retractable third eyelid can be hidden from view or can extend across the surface of the eye.  It is white to light pink in color and lies on top of the eye, but underneath the eyelids.  It contains cartilage and a tear-producing gland at its base.  When irritated, it can appear reddened.

While birds and reptiles can actively move this protective eyelid into position, in cats the movement is passive. It is kept hidden by forward pressure of the eyeball in the socket. When danger to the eye is anticipated ( such as in a cat fight), cats use a special muscle behind the eye to pull it back into the socket slightly, allowing the third eyelid to quickly move up and across the surface of the eye. If the eye does become injured and painful, cats will use this special muscle to pull back the eye slightly and allow the third eyelid to cover the eye as protection.

Damage to the nerve control of the third eyelid will also result in a prominent (or more visible) third eyelid.  Damage affecting one eye can occur due to an injury or inflammation after surgery (especially ear or dental surgery).

If you notice that one of your cat’s third eyelids is covering one eye more than the other, it is likely that your cat has injured that eye.  Eye injuries are painful and can become serious quickly, so you should seek veterinary care right away.

What does it mean if both third eyelids are visible?  There are a variety of reasons for this to occur.  First of all, when cats are in a deep sleep or have been given a sedative, the third eyelids can become prominent.  If your cat has lost a lot of weight, the fat pad behind the eyes may also have decreased in size, changing the position of the eye in the socket and allowing the third eyelid to become visible.  Rarely, inflammation due to a neurologic, respiratory or intestinal infection can affect the nerve control of the third eyelid.   Your cat should be examined by a veterinarian in order to determine a likely cause and how best to treat the condition.

If you have never seen your cat’s third eyelid, and want to know what to look for, ask your veterinary at your cat’s next check-up.

Dr Diana Lafer

Dr. Diana Lafer founded Cats Limited in 1995. She earned her bachelor’s degree in biology from Wesleyan University and her veterinary degree from Cornell University. Dr. Lafer has a cat (Sparky), and a dog (Lucy). She enjoys spending time with her daughters, horseback riding, skiing, hiking, participating in triathlons, and volunteering for the Lakeville Pony Club.

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