What is a cat’s third eyelid?

Did you know that cats have more than two eyelids per eye? In addition to the upper and lower eyelids, there is a third eyelid that you don’t usually see. In some cases, a protruding third eyelid is actually a sign of disease or injury.

What is a cat’s third eyelid?

Cats and many other mammals have a third eyelid, called the nictitating membrane. The membrane is located at the corner of each eye, towards the center of the face. The third eyelid is usually retracted and invisible. Certain conditions may cause the third eyelid to protrude and partially cover the eyeball.

Causes of the appearance of the third eyelid in cats

It is uncommon to see a cat’s third eyelid. In many cases, the appearance of the third eyelid indicates a problem.

There are several reasons for a cat’s third eyelid to protrude. Some are considered normal, while others are problematic. In general, you should contact your veterinarian if you notice that your cat’s third eyelid is protruding for more than a few hours.

sleepy or relaxed state

It is normal for part of the third eyelid to show when the cat is very relaxed or tired. You may notice that the third eyelid is raised when your cat is sleeping or just waking up. Once the cat is awake and alert, the third eyelid should be retracted.If the third eyelid stays longer than

sedation or anesthesia

If your cat is under anesthesia or sedation during the medical procedure, the third eyelid will appear and partially cover the eye due to the extreme relaxation of the body. The effects may last for a few hours upon waking, but should wear off gradually. This is considered normal and not a problem unless the third eyelid is still up on the day of surgery.


The conjunctiva is a thin, transparent mucous membrane that lines the inside of the eyelid and covers the front of the eye. Sometimes called “pink eye,” conjunctivitis simply means inflammation of the conjunctiva. It can be caused by infection, allergies, injury or eye irritants. It is not uncommon for conjunctivitis to cause inflammation and protrusion of the third eyelid. Cats with conjunctivitis are usually treated with medicated eye drops or ointment containing antibiotics and/or steroids.

corneal ulcer

The cornea is transparent and covers the front of the eye, including the iris and pupil. Corneal ulcers are wounds in the cornea that can cause pain and inflammation in the eye and may cause the third eyelid to appear. Corneal ulcers are usually caused by eye injuries, such as abrasions, scratches, or punctures. Irritating or abrasive substances can also cause corneal ulcers. Corneal ulcers can quickly become very serious and require veterinary treatment. Cats with corneal ulcers usually require one or more eye medications and may also require oral medications.


The uvea is the middle part of the eyeball and contains many blood vessels. Inflammation of the uvea is called uveitis. It often makes the eyes look red and sometimes painful. The third eyelid may or may not be affected.


Glaucoma is a painful eye condition that causes increased pressure inside the eye. This is because the aqueous humor (fluid in the front of the eye) cannot drain properly. Excessive stress can cause damage to the optic nerve and can lead to blindness. Inflammation of the third eyelid can sometimes accompany glaucoma.

cherry eye

The third eyelid contains a gland that swells and looks like a rounded protrusion in the inner corner of the eye. This is called nictitating membrane prolapse or cherry eye. It is relatively uncommon in most cats, but is common in Burmese cats. Surgery is the treatment for cherry eye.

eye growth

Growths, tumors, lumps, and cysts in and around the eye can cause inflammation and protrusion of the third eyelid. If you notice unusual growth or swelling in or around your cat’s eye, contact your veterinarian to avoid further trouble.

Horner syndrome

Horner syndrome is a neurological disorder caused by neurological dysfunction. It affects the eyes and facial muscles, making the eyes appear asymmetrical. The third eyelid is usually very prominent in one eye. The eyelids are droopy and the eyes themselves look “inward”. Horner syndrome can be caused by trauma or a tumor, but sometimes the cause cannot be determined. This situation may resolve itself.

What to do if you see a cat’s third eyelid

If you notice that your cat’s eyelids are bulging and it doesn’t seem to be caused by sleep, relaxation, sedation, or anesthesia, it could mean something is wrong. Contact your veterinarian as soon as possible for advice. If your cat is showing other signs of illness, it is best to have your cat see a veterinarian for a checkup. Do not try to put anything in your eyes without veterinary advice as this can make things worse. Eye problems can easily get worse, so don’t delay seeing the vet.