Did you know that cats have more than two eyelids on each eye? In addition to the upper and lower eyelids, there is a third eyelid that is not normally visible. But, in those cases where that third eyelid protrudes, it is actually a sign of disease or injury.

What is the third eyelid in a cat?

Cats and many other mammals have a third eyelid called the “nictitating membrane”. This membrane is located at the corner of each eye towards the centre of the face. The third eyelid is usually retracted and not visible. Certain situations can cause the third eyelid to protrude and partially cover the eyeball.

Reasons for a cat’s third eyelid to become visible

It is rare to see your cat’s third eyelids. In many cases, the appearance of a third eyelid indicates that something is wrong. There are several reasons for a protruding third eyelid in cats. Some are considered normal, while others are problematic. In general, if you notice your cat’s third eyelid protruding for more than a few hours, you should contact your veterinarian.

Drowsiness or relaxed state

It is normal for part of the third eyelid to show when a cat is very relaxed or tired. You may notice that the third eyelid is raised while your cat is sleeping or just after waking up. The third eyelid should retract as soon as the cat is awake again. If that third eyelid remains raised for a long time after waking up, you should go to the vet with your kitten.

Sedation or anaesthesia

If your cat has been given anaesthesia or sedation for a medical procedure, the third eyelid will be visible and partially cover the eye due to the extreme physical relaxation. The effect may last for several hours after waking up, but should gradually disappear. This is considered normal and is not a problem unless the third eyelid remains raised after the day of the procedure.


The conjunctiva is a thin, clear mucous membrane that lines the inside of the eyelids 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 an eye irritant. 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 ointments containing antibiotics and/or steroids.

Corneal ulcers

The cornea is transparent and covers the front of the eye, including the iris and pupil. A corneal ulcer is a sore on the cornea that causes pain and swelling in the eye and may cause the third eyelid to appear. Corneal ulcers are usually caused by an injury to the eye, such as a scratch, scrape or puncture wound. Irritating or abrasive substances can also cause corneal ulcers. A corneal ulcer can quickly become very serious and requires veterinary treatment. Cats with corneal ulcers often need one or more types of eye medications and possibly oral medications as well.


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 eye very red and is sometimes painful. The third eyelid may or may not be affected.


Glaucoma is a painful eye condition that causes pressure to build up in the eye. This occurs because the fluid inside the front of the eye cannot drain properly. The excess pressure causes damage to the optic nerve and can result in blindness. Swelling of the third eyelid can sometimes accompany glaucoma.

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

If you notice that your cat’s eyelid is protruding and it doesn’t appear to be caused by sleep, relaxation, sedation or anaesthesia, then it probably means there is a problem. Contact your vet as soon as possible to seek advice. If your cat shows other signs of illness, it is best to take it to the vet for examination. Do not try to put anything in the eye without a vet’s recommendation, as this can make things worse. Eye problems can go from bad to worse, so don’t delay a visit to the vet!

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