Understanding Your Cat's Third Eyelid and What It Means

In this guide

Cats have a feature most people never notice until it becomes visible: a third eyelid, also called the nictitating membrane, tucked into the inner corner of each eye. It normally stays out of sight, sweeping across the eye now and then to spread tears and clear away debris. When it stays raised and visible, it is worth paying attention.

When it is nothing to worry about

A slightly visible third eyelid when your cat is sleepy, half asleep, or just waking up is completely normal and usually settles once your cat is fully alert.

When it points to a problem

If the third eyelid stays raised while your cat is awake and active, especially in both eyes, it usually signals something else going on in the body rather than the eye itself. Common causes include:

• Gastrointestinal upset, sometimes called Haw's syndrome, often seen in cats under two years old and linked to a recent bout of diarrhoea or an intestinal parasite
• Dehydration, which can cause both third eyelids to sit slightly raised
• A respiratory infection or general fever, both common in cats with an active street or shelter background
• Local eye problems such as a corneal injury, conjunctivitis or a foreign body, usually affecting only one eye
• Nerve related conditions, including Horner's syndrome, which may come with other signs like a smaller pupil or a drooping eyelid on the same side

One eye or both eyes

As a general rule, one eye affected points towards a local eye issue that needs prompt attention, while both eyes together more often point towards a body wide cause such as an upset stomach, parasites or dehydration. Either way, it is a signal rather than a diagnosis on its own.

What to do

A third eyelid that stays up for more than a day or two, or that comes with other symptoms like diarrhoea, discharge, squinting or lethargy, is worth a vet visit. Vets can usually tell fairly quickly whether the cause lies in the eye or elsewhere in the body, and most underlying causes, including Haw's syndrome, resolve well once treated.

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