All cats have the third eyelid. This structure may actually protect the eye health of your feline fur baby. When it shows itself abnormally, this is your warning to get your kitty to the vet’s office for an examination, because injury or illness could cause it to protrude.
Explanation of the Third Eyelid
The third eyelid, also called the “haw,” has long been thought of as either something that protects your kitty’s eyes or as something “extra,” like the human appendix. Scientific American says that vets in the early 1900s even removed this "irrelevant structure" to further examine a cat's eye, but this little flap of flesh actually protects your kitty’s eyes from dirt or other irritants and accidental injury. This third eyelid may also help to keep her eye surfaces moist as it redistributes her tears over the surfaces of her corneas.
When she’s either sleepy or relaxed, you may see both third eyelids move slowly across her eyes. This is when it functions as it’s supposed to. When she’s alert, those third eyelids are hidden inside her eye sockets with only a tiny portion showing in the inner corners of her eyes.
When both third eyelids are elevated in your cat’s eyes and they don’t come down, your vet may consider a diagnosis of haw syndrome. One thing -- this abnormal elevation can’t be caused only by dehydration, although it does happen with tummy illnesses. Instead, it’s more likely a problem with your kitty’s autonomic nervous system.
As your vet is thinking of different causes, he’ll run tests for different parasites and worms. He’ll also examine your kitty for a torovirus or chronic diarrhea. Lovely. If your vet does find that your cat has intestinal parasites or worms, he’ll deworm your kitty and this should help resolve the third eyelid problem.
Horner’s syndrome can be a little more serious, but it is treatable. This condition shows itself with sunken eyes, visibility of the third eyelid and small pupils. Some different causes include an injury or a cancer of a nerve in your kitty’s neck. If your kitty has a middle ear infection, this could also lead to Horner’s syndrome. This syndrome has no treatment, other than allowing it to resolve itself over time.
If your feline friend develops an eversion, or turning out of the gland of the nictitans, her third eyelid will fold over. This is a condition that requires veterinary attention, because if left untreated, your kitty could develop an ulcer of her cornea. It’s also uncomfortable for her and not very pretty to look at. Cherry eye can be treated with surgery to reposition this misbehaving gland. Vets used to completely remove this gland surgically, but that led to decreased tear production and something called secondary keratoconjunctivitis sicca. If you see a red third eyelid in your kitty, get her to the vet right away. If she’s a Burmese cat, she’s more likely to develop this condition.
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