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.
Some cats are born without the
ability to hear because of developmental defects in the hearing apparatus. Cats
may also be deaf in just one ear. Congenital deafness occurs most often in
white cats with blue eyes, and is the result of an incomplete autosomal
dominant gene. However, not all cats with blue eyes are deaf, and that includes
not all white cats with blue eyes. Longhaired cats with blue eyes have a higher
risk of deafness than shorthaired cats with blue eyes. White cats with the
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.