Why Is My Cat's Third Eyelid Showing? Haw's Syndrome and Other Causes

Most cat owners have caught it at least once: a pale, faintly pink or grey film sliding in from the inner corner of one or both eyes, especially when the cat is drowsy or just waking up. That's the third eyelid, or nictitating membrane, and in a healthy cat it's almost never visible — it sweeps across to spread tears and clear debris, then retracts so smoothly you'd never know it was there. When it stays visible, partially or fully, that's not a quirk of a particular cat; it's a sign that something is disrupting the normal mechanism that keeps it tucked away, and it's worth figuring out what.

A vet walks through the main reasons a cat's third eyelid becomes visible, from Haw's syndrome to more serious causes.

What the Third Eyelid Actually Does

The nictitating membrane sits in the inner corner of each eye, underneath the upper and lower lids, and is largely hidden from view in a normal, healthy cat. It has three jobs: spreading tear film evenly across the surface of the eye with every blink, physically shielding the cornea from debris and minor scratches, and producing a portion of the tear film itself via a gland at its base. Because it's controlled partly by the same nerves and muscles that hold the eyeball forward in its socket, anything that affects that underlying system — dehydration, weight loss, nerve disruption, or local eye irritation — can cause the membrane to become visible even when the eye itself looks otherwise normal.

Haw's Syndrome: The Most Common Explanation

Haw's syndrome is the name for third eyelids protruding in both eyes at once, usually with no pain and no change in vision. It's not a disease in itself but a sign that the tissue and fat behind the eyeball has lost a small amount of volume — often because the cat has lost weight, is mildly dehydrated, or is recovering from a bout of diarrhea or a gastrointestinal upset. The eyeball sits very slightly further back in the socket as a result, and the third eyelids, no longer held taut, drift forward to fill the gap.

• Usually resolves on its own once the underlying cause — weight regained, hydration restored, GI upset settled — is addressed; no eye-specific treatment is typically needed.
• Both eyes affected roughly equally is the key clue that points toward Haw's syndrome rather than a problem with one specific eye.
• Cats aren't in obvious pain and continue eating, playing and behaving normally aside from the visible membranes — a cat that's squinting, pawing at an eye, or hiding is showing something else going on.

When Only One Eye Is Affected

A third eyelid showing in just one eye points away from Haw's syndrome and toward a local problem with that specific eye, which generally needs a same-week vet visit rather than a wait-and-see approach:

• Corneal ulcer or scratch. Pain triggers the third eyelid to sweep across and protect the injured surface; often comes with squinting, discharge, or a visibly cloudy patch on the cornea.
• Conjunctivitis or infection. Inflammation of the pink tissue around the eye — from a virus, bacteria, or irritant — commonly brings redness, discharge, and a puffy third eyelid along with it.
• Horner's syndrome. A disruption to the nerve pathway running from the brainstem to the eye, sometimes from an ear infection, a neck or chest injury, or occasionally no identifiable cause at all. Alongside the raised third eyelid, look for a constricted pupil, a slightly drooped upper eyelid, and an eye that looks sunken compared to the other side.
• A foreign body or trauma trapped behind the membrane itself — cats who've been outdoors, in a fight, or rubbing at one eye persistently should have the area checked directly.
• Cherry eye (prolapse of the third eyelid's own tear gland) is far less common in cats than in dogs but does happen, appearing as a firm pink lump at the inner corner rather than a flat membrane.

A closer look at what a visible third eyelid can mean and the situations that warrant a same-day vet call.

How Vets Work Out the Cause

Because "third eyelid showing" is a symptom rather than a diagnosis, a vet exam focuses on separating a whole-body cause from a local eye problem:

• Both eyes vs. one eye is usually the first and most useful sorting question, since it immediately narrows the likely cause toward Haw's syndrome or toward a local eye issue.
• A fluorescein stain test — a dye applied to the eye's surface — lights up under blue light if there's a corneal ulcer or scratch, which is often invisible to the naked eye otherwise.
• Pupil size and eyelid position on each side are compared, since asymmetry (one pupil smaller, one lid drooping) points toward Horner's syndrome and a neurological work-up.
• A general physical exam, including recent weight and hydration status, checks for the gut upset, parasites, or dietary change that commonly triggers Haw's syndrome in an otherwise well cat.

When to Get Your Cat Seen

• Same day: third eyelid in one eye only, visible pain (squinting, pawing at the eye, avoiding light), cloudiness on the cornea, or any injury to the eye area.
• Within a day or two: both third eyelids visible alongside vomiting, diarrhea, lethargy, or weight loss — the eyes are a symptom here, and the underlying cause needs addressing.
• Can be monitored briefly: both third eyelids slightly visible in an otherwise bright, eating, normally behaving cat, particularly right after waking from a nap — but if it persists more than a day or two rather than settling back to normal, it's still worth a call to the vet.

Treatment

Treatment follows the cause rather than the third eyelid itself, which almost always retracts on its own once the underlying issue resolves:

• Haw's syndrome tied to a recent GI upset typically clears within one to two weeks as the cat's weight and hydration return to normal, sometimes helped along by a short course of anti-nausea or gut-supportive medication from the vet.
• Corneal ulcers need topical antibiotic drops or ointment, applied on a strict schedule, and are checked with a repeat stain test to confirm healing before the medication is stopped.
• Conjunctivitis is treated according to its cause — antiviral or antibiotic eye medication for infection, or removing the irritant if it's environmental.
• Horner's syndrome itself isn't directly treatable; vets instead look for and address the underlying cause (an ear infection, for example), and many idiopathic cases resolve gradually over weeks to months on their own.

The Bottom Line

A visible third eyelid is the eye's way of signalling that something — locally in that eye, or more broadly in the cat's health — isn't quite right, even when nothing else looks obviously wrong. Both eyes affected in a cat who's otherwise eating and behaving normally is usually the least worrying version and often resolves by itself; one eye affected, or any sign of pain, discharge, or cloudiness, is the pattern that should prompt a same-week vet visit rather than a wait-and-see approach.

This article is general information, not veterinary advice. Talk to your vet about diagnosis and treatment tailored to your individual cat.