Third Eyelid Showing in Cats: What Haw Syndrome and Eye Signs Mean

By Tailwisp Editorial TeamUpdated October 20, 20264 min read

Look into your cat's eyes, and you might see a pale or pink film sliding up from the inner corner. That is the third eyelid, also called the nictitating membrane or haw. It is normal for a moment when a cat is drowsy or just waking. If it stays in view, it is telling you something.

What is the third eyelid?

Cats have three eyelids: an upper, a lower and a third, which lies between the lower lid and the eye. The third eyelid sweeps across the eye, spreading tears, removing debris and protecting the cornea. It is usually hidden, and rises when the eye is irritated, the cat is relaxed or unwell, or the eye sinks in the socket.

Common causes

In one eye

• Eye injury or foreign body. See cat eye ulcers.
• Conjunctivitis. See conjunctivitis in cats.
• Glaucoma or uveitis. See cat glaucoma.
• Horner's syndrome, a nerve disorder that causes a drooping eyelid, a small pupil and a raised third eyelid on one side. It can be caused by ear disease, trauma, tumors or have no clear cause.
• Eye pain, or tumor behind the eye.
• Dental disease, such as a tooth root abscess. See cat dental disease.

In both eyes

• General illness, including infections, fever and pain.
• Dehydration and weight loss, because the eyes sink as fat behind them is lost. See cat weight loss causes.
• Haws syndrome, where both third eyelids rise, often with diarrhea or gut upset, mainly in young cats.
• Intestinal parasites. See worms in cats.
• Stress, sedation or anesthesia.
• Tetanus, rarely.
• Dysautonomia, a rare nerve disease.
• Normal for some cats, especially when they are very relaxed.

What to look for

• Is it one eye or both?
• Is there redness, discharge or squinting?
• Is the pupil different in size between the eyes?
• Is the cat eating, drinking and acting normally?
• Any diarrhea, vomiting or weight loss?
• Any recent injury, infection or anesthesia?

Take a photo, and bring it to the vet. Note how long it has been visible.

Diagnosis

The vet examines the eyes, checks the pupils, stains the cornea, measures eye pressure and tear production. Further tests, such as bloodwork, fecal tests, X-rays of the head or neck, or an ear exam, may follow, depending on the findings. See cat wellness exam: what to expect.

Treatment

Treatment depends on the cause: eye drops for eye disease, deworming, treatment for infection, fluids for dehydration, diet change or surgery for tumors. For Horner's syndrome, treatment targets the underlying disease, and in idiopathic cases, signs often resolve on their own over weeks to months.

At home

• Keep the cat comfortable and indoors.
• Do not use human eye drops.
• Wipe discharge gently with a clean, damp cotton pad.
• Offer water and normal food.
• Track changes with daily photos.

Prevention

Keep up with vaccinations, parasite prevention and checkups. Treat eye changes promptly.

Horner's syndrome explained

Horner's syndrome results from damage to nerves that run from the brain through the chest, neck and middle ear to the eye. Signs on one side are a smaller pupil, a droopy upper eyelid, a sunken eye and a raised third eyelid. It is not painful. Causes include ear disease, trauma, tumors and sometimes none can be found. Many idiopathic cases resolve within weeks to a few months. Your vet will look for a cause, since this can be linked with serious conditions.

Why weight loss shows in the eyes

When a cat is thin or dehydrated, the fat pad behind the eye shrinks, the eye sinks back slightly and the third eyelid becomes more visible. That is why both third eyelids may appear in a sick or underweight cat. Treat the cause, and the eyes return to normal. See cat weight loss causes.

Young cats with diarrhea

Some young cats, especially kittens and those recently adopted, have visible third eyelids along with loose stool. This is often called haws syndrome. It is usually linked with intestinal parasites or infection, and resolves with treatment. Bring a stool sample to the vet.

What not to do

Do not try to push the membrane back, and do not use human eye drops. Do not assume that it is harmless if the cat seems fine, since third eyelid changes can be an early sign of other disease. Take photos, and watch for other changes.

Tracking over time

Photograph both eyes each morning in the same light for a week. Note the cat's appetite, stool and energy beside each photo. If the membrane covers more of the eye each day, or one eye becomes red, call your vet sooner.

When to see a vet

See a vet if the third eyelid stays visible for more than a day or two, or the eye is red or painful. Use the US vet directory to find a clinic.

This article is general educational information, not veterinary advice. Every cat is different. If your cat is unwell, or you are unsure about a symptom, diet change or medication, contact your veterinarian.