What can the cat’s third eyelid tell us?

The third eyelid of the cat, I believe, can be an initial diagnostic tool for the cat owner in respect of cat health – I am not recommending anything other than the ability to spot a health problem. This is because the third eyelid is not normally seen but it can become opaque and/or visible when the cat is ill or injured.

A cat’s third eyelid or nictitating membrane. Photo by Editor B

The third eyelid is called the nictitating membrane. It’s purpose is to protect the eye. It also helps in the production and distribution of tears.

Sometimes, however, the cat will have their third eyelid up and partially over their eye when totally relaxed and resting. This is normal and can be excluded when using the third eyelid for assessing whether your cat is ill. When the third eyelid is over the eye, under these circumstances, it will quickly retract when the cat is alert.

Haw Syndrome is quite a common condition which causes the third eyelid to protrude. It affects cats that are otherwise healthy and which are under 2 years of age. It often occurs after a gastrointestinal illness. Apparently, the protrusion clears up without treatment, within a few months. A veterinarian can prescribe eye drops, which will reduce the amount the third eyelid covers the eye.

A part of the structure of the third eyelid is a gland which is important in the production of tears. Cherry eye refers to the condition when this gland protrudes from the eye or it becomes prolapsed (falls out of place). Another description is, “eversion of the gland of the nictitans” (eversion: turns inside-out).

Apparently, this condition is more likely to affect Burmese cats amongst the cat breeds. The cartilage of the third eyelid folds over, turning the third eyelid inside out. It can cause the cornea (the surface of the eye) to become ulcerated. Veterinarian attention must be sought.

A rare autonomic nervous system disorder called “Key-Gaskell Syndrome” causes the third eyelid to prolapse. This is a serious condition.

The nictitating membrane can protrude across the eye due to infection in the tissue behind the eyeball, bleeding behind the eye or a tumor. These conditions also cause the eye to bulge.

The eye can retract (become sunken) and the muscles surrounding the eye can spasm. If this occurs in both eyes the advice is to suspect a “systemic” illness i.e. a viral respiratory infection. If one eye is affected suspect: chronic weight loss and dehydration.

Conclusion: We should expect to see an eye without seeing the nictitating membranee except sometimes when the cat is resting or sleeping. If it is visible routinely or for long periods, I would take him/her to the veterinarian for a checkup.

  • Note: I have deliberately kept this post short and straightforward because this is about veterinary care but it is sensible if a cat owner is at least aware of these things.
  • More: Feline Eye Disease
  • Source: Various but mainly this book.
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Michael Broad

Hi, I'm a 71-year-old retired solicitor (attorney in the US). Before qualifying I worked in many jobs including professional photography. I have a girlfriend, Michelle. I love nature, cats and all animals. I am concerned about their welfare.

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  • My 9 month old cat just started with Haw Syndrome. She is very active, playful and eats without problems. She doesn't seem to have any health set backs or issues. I found her at 6 weeks old
    She had 1st set of vaccinations, and got her fixed at 7 months old.
    Should I take her to a vet, even though she seems very normal, except for this 3rd eye thing?

    • Hello Ruth. Thank you for visiting and commenting. This condition is quite common. It is a temporary protrusion of the third eyelid and is a temporary condition. The cause is uncertain apparently. It affects otherwise healthy cats under the age of two. Sometimes it is preceded by a gastrointestinal illness. The protrusion clears up within a few months without treatment. During those two months if the film interferes with your cat's vision a veterinarian could prescribe an eye drop solution which you might ask your vet about. This will reduce the size of the protrusion. This information comes from a highly regarded veterinary book. I hope it helps and good luck.

  • I didnt even know cats had a third eye so ive learned something. I guess thats why they have there eye half open sometimes.

  • I rarely see this on my cats unless they are falling asleep or just waking up. It can look pretty strange. Quite clever really that they have to eyelids. Cats are just amazing aren’t they.

    Gigi is getting more and more playful these days. It’s getting better. I found her a toy mouse which she loves. She is very picky about playing and toys. She just doesn’t go for alot fo stuff, but the right kind of mouse toy and she goes nuts and even get out of breath playing with it. She is doing well and I am proud of her. :)

    • Thats so great shes getting enjoyment out of that Mouse. You must be so proud of her as i know you will be. Its great to see shes enjoying things alot more. :)

    • Marc, I'm so pleased for you about Gigi, you seemed really worried about her for a while so this is wonderful news :)

      • Yea its wonderful news that you can help her out of her depression. Its like that instinct of knowing something was wrong and trying to find whats happening n Fixing it

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