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.
The following information isn’t intended to replace regular visits to your veterinarian. If you think your cat may have hyperthyroidism, please see your veterinarian immediately. And remember, please do not give any medication to your pet without talking to your veterinarian first.
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
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.
Eversion of the gland of the nictitans, known as cherry eye, occurs in cats,
especially the Burmese breed. For unknown reasons, the cartilage of the third
eyelid folds over, everting the gland. This condition is not only unsightly but
can be uncomfortable and may cause corneal ulceration in the cat.
Treatment: At one time, eversions of the nictitans gland were treated by
surgical removal, but that is not the ideal solution. Since part or all of the
gland was usually removed at the same time, there was decreased tear
production. This often led to secondary keratoconjunctivitis sicca. Now the
gland is generally repositioned surgically.
WebMD Veterinary Reference from "Cat Owner's Home Veterinary Handbook"