That pinkish-white membrane creeping across your cat’s eye is the third eyelid, also called the nictitating membrane. It normally sits tucked out of sight in the inner corner of the eye, near the nose. When it becomes visible, something is causing the eye to retract, the membrane to rise, or the nerves controlling it to malfunction. A brief flash during sleep or drowsiness is normal, but if the membrane stays raised while your cat is alert, it points to an underlying issue that ranges from mild and self-resolving to serious enough to need prompt veterinary attention.
What the Third Eyelid Actually Does
Cats have an extra eyelid that people don’t. It’s a whitish-pink sheet of tissue that slides upward from the inner corner to shield the eyeball from physical damage, like scratches from brush or debris. Glands embedded in this membrane also produce part of the watery layer of tears, helping keep the eye surface moist. Under normal conditions, you’ll barely see it. It retracts on its own and stays hidden beneath the lower lid. When it extends and stays up while your cat is awake and active, that’s the sign something is off.
Haw’s Syndrome: The Most Common Cause
If both of your cat’s third eyelids are elevated at the same time, the most likely explanation is Haw’s syndrome. This condition is characterized by bilateral protrusion of the third eyelid along with slightly droopy upper eyelids, and it’s frequently tied to gastrointestinal problems like diarrhea or intermittently soft stools. In some cats the digestive symptoms are subtle or have already passed by the time the eyelids become noticeable.
The exact mechanism isn’t fully understood, but growing evidence points to a disruption in the communication network between the gut and the nervous system. Changes in gut bacteria appear to alter nerve signaling to the eyes, specifically the sympathetic nerves that normally keep the third eyelid pulled back. A 2024 case series published in Veterinary Ophthalmology documented cats as young as eight months developing Haw’s syndrome after weeks of intermittent loose stools, even when diarrhea wasn’t present at the time of examination.
Haw’s syndrome is not a disease of the eye itself. The eyes remain comfortable, the pupils look normal, and vision isn’t affected. In many cats, it resolves on its own once the underlying gut issue clears up, though addressing the digestive problem directly can speed recovery.
Horner’s Syndrome: A Nerve Problem
When only one eye is affected, Horner’s syndrome becomes a stronger possibility. This condition results from damage or dysfunction somewhere along the chain of sympathetic nerves running from the brain, down through the chest, and back up to the eye. In cats, these nerves actively hold the third eyelid in its retracted position, so when the signal is lost, the membrane rises.
Horner’s syndrome produces a distinct cluster of signs, usually all on the same side of the face:
- A smaller pupil in the affected eye, because the muscle that dilates the pupil loses its nerve supply
- A droopy upper eyelid, making the eye opening look narrower
- The eye sitting deeper in the socket, because muscles that normally hold the eyeball forward relax
- A raised third eyelid, which is the second most commonly reported sign
The causes vary widely. An ear infection, a chest injury, a neck wound, or even a blood clot can interrupt the nerve pathway at different points. In many feline cases, no cause is ever identified, and the condition resolves over weeks to months. Still, because some of the underlying triggers are serious, it warrants a vet visit to rule out treatable problems.
Dehydration and Weight Loss
A cat that is dehydrated or has lost significant body weight can develop sunken eyes. The eyeball is cushioned by a pad of fat behind it, and when that fat shrinks from weight loss, aging, or chronic illness, the globe sinks backward into the socket. When that happens, the third eyelid is no longer held in place by the eyeball pressing against it, so it drifts upward and becomes visible.
This is a mechanical issue rather than a nerve problem. The eye itself is typically healthy and comfortable, but the cat’s overall condition needs attention. Dehydration from kidney disease, hyperthyroidism, or simply not drinking enough can all produce this effect. If your cat’s eyes look sunken and the third eyelids are showing, body condition is the place to start looking.
Eye Pain, Injury, or Infection
The third eyelid also rises as a protective reflex when the eye is irritated or inflamed. A corneal scratch, a foreign body stuck under the lid, conjunctivitis, or an eye ulcer can all trigger it. In these cases, you’ll usually see additional signs: redness, colored discharge (yellow or green suggests infection), squinting, pawing at the face, or your cat holding the eye shut entirely.
This type of protrusion tends to affect one eye, and the cat is visibly uncomfortable. Unlike Haw’s syndrome, where the eyes look and feel fine despite the raised membrane, an irritated eye demands quick evaluation because corneal injuries can worsen rapidly.
Flat-Faced Breeds and Normal Variation
Brachycephalic breeds like Persians and Exotic Shorthairs have shortened, rounded skulls that change the geometry of the eye socket. Their prominent, shallow eye sockets mean the third eyelid can be slightly more visible at baseline. These breeds also have a high prevalence of chronic tearing due to compressed tear drainage pathways, and the resulting moisture around the inner eye corner can make the membrane more conspicuous. If your flat-faced cat has always had a slightly visible third eyelid with no other symptoms, it may simply be an anatomical quirk of the breed.
One Eye vs. Both Eyes
The pattern of involvement is the single most useful clue before you get to a vet. Both third eyelids showing simultaneously, with no eye pain or discharge, strongly suggests Haw’s syndrome, especially if your cat has had any digestive upset recently. One eye affected along with a smaller pupil and droopy lid points toward Horner’s syndrome. One eye affected with squinting, redness, or discharge suggests local eye injury or infection.
A brief appearance of the third eyelid as your cat wakes up from a nap is completely normal. During deep sleep, the third eyelid often extends across the eye. If you catch your cat in that half-awake moment, the membrane may still be partially visible for a few seconds before it retracts. This needs no attention at all.
What to Watch For
Some presentations are more urgent than others. A third eyelid that has been up for more than 24 hours while your cat is awake warrants a vet visit, even if the cat seems otherwise fine. Red flags that suggest a more immediate problem include eyes that are red or cloudy, any colored discharge, pupils that look different sizes from each other, squinting or holding an eye closed, decreased appetite, or any change in behavior like hiding or lethargy. Vision changes can be hard to spot in cats, but bumping into furniture or misjudging jumps are signs the eyes may not be working properly.