If you've noticed a pale, pinkish membrane sliding across your cat's eye from the inner corner, you're seeing the nictitating membrane—commonly called the third eyelid. This thin, protective tissue normally stays tucked away in the corner of your cat's eye, but when it becomes visible across part or all of the eyeball, it's called third eyelid protrusion or prolapse.

Haw syndrome is a specific condition where both third eyelids protrude simultaneously, creating a bilateral appearance that can be quite alarming for cat owners. The term "haw" comes from the visible membrane itself. Unlike many eye problems that affect just one eye, Haw syndrome typically affects both eyes equally, which is actually an important diagnostic clue. The third eyelid serves several protective functions: it distributes tear film across the eye surface, removes debris, and shields the cornea from injury. When functioning normally, you might catch a brief glimpse of it when your cat is drowsy or just waking up, but it should quickly retract.

Veterinarians distinguish between true Haw syndrome (usually benign and temporary) and third eyelid protrusion caused by more serious underlying conditions. True Haw syndrome most commonly affects young to middle-aged cats and often resolves spontaneously within weeks to months. The protrusion can range from covering just a small portion of the eye to extending across half the eyeball or more. While the appearance can be concerning, cats with uncomplicated Haw syndrome typically show no signs of pain or discomfort, their eyes remain clear without discharge, and they continue normal activities like eating and playing.

The condition was first formally described in veterinary literature in the 1980s, though cat owners and veterinarians had observed it for much longer. Research suggests that Haw syndrome may be more prevalent than previously thought, as mild cases often resolve before owners seek veterinary care. Understanding the difference between benign Haw syndrome and third eyelid protrusion indicating serious illness is crucial for appropriate response and treatment.

The reasons behind a visible third eyelid range from temporary and harmless to indicators of significant health problems. Gastrointestinal upset is the most common trigger for classic Haw syndrome. Veterinarians have observed strong correlations between mild digestive disturbances and bilateral third eyelid protrusion, though the exact mechanism remains somewhat mysterious. Theories include disruption of the sympathetic nerve supply to the eye or systemic effects of intestinal inflammation affecting the smooth muscle that controls the nictitating membrane.

Intestinal parasites, particularly tapeworms, have been implicated in many Haw syndrome cases. Some studies suggest that treating for parasites, even without confirmed diagnosis, often leads to resolution of the third eyelid protrusion. Viral infections, especially upper respiratory viruses common in cats, can also trigger the condition. Feline leukemia (FeLV) and other systemic viral diseases occasionally present with third eyelid changes among other symptoms.

Beyond Haw syndrome specifically, unilateral (one-sided) third eyelid protrusion often indicates different causes: Horner's syndrome, a neurological condition affecting the sympathetic nerves to the eye, causes a characteristic triad of symptoms including third eyelid protrusion, constricted pupil, and drooping eyelid on one side. Eye pain from corneal ulcers, foreign bodies, or conjunctivitis can cause the third eyelid to protrude protectively. Dehydration and significant weight loss can cause the fat pad behind the eye to shrink, allowing the eyeball to sink back (enophthalmos) and making the third eyelid more prominent.

More serious systemic conditions occasionally present with third eyelid changes. Tetanus, though rare in cats, can cause bilateral third eyelid protrusion along with muscle stiffness. Tumors behind the eye or affecting the nervous system may cause protrusion, usually unilateral. Feline infectious peritonitis (FIP), particularly the dry form, can cause eye changes including third eyelid visibility. If your cat shows other concerning symptoms beyond the eye changes—such as the refusal to eat, fever, difficulty breathing, or neurological signs—immediate veterinary evaluation is essential.

While classic Haw syndrome is often benign, certain signs indicate you should seek veterinary care promptly rather than waiting to see if the condition resolves. Immediate veterinary attention is warranted if: the third eyelid protrusion is unilateral (one eye only), which suggests a localized problem rather than systemic Haw syndrome; your cat shows signs of eye pain such as squinting, pawing at the face, or excessive tearing; there's discharge from the eyes, especially if yellow, green, or bloody; the eye appears red, cloudy, or the surface looks dull; your cat exhibits other symptoms like vomiting, diarrhea, lethargy, loss of appetite, or difficulty breathing; or the third eyelid protrusion appeared suddenly after trauma or injury.

For bilateral third eyelid protrusion without other symptoms, veterinarians typically recommend evaluation within a few days if the condition persists or worsens. During your veterinary visit, expect a comprehensive examination including: detailed eye examination with ophthalmoscope to check for corneal damage, foreign bodies, or internal eye problems; assessment of pupil response and eye movement; palpation of lymph nodes and abdominal examination; possible fecal examination to check for parasites; and blood work if systemic disease is suspected. Your veterinarian will ask about recent dietary changes, exposure to other cats, vaccination status, and any other behavioral changes you've noticed.

The diagnostic approach depends on accompanying symptoms and physical examination findings. For young, otherwise healthy cats with bilateral protrusion and possible mild digestive upset, veterinarians often recommend a conservative "watch and wait" approach combined with deworming. If Horner's syndrome is suspected, additional neurological examination and possibly imaging may be recommended to identify the cause. For cases with eye discharge or signs of pain, fluorescein staining may be performed to check for corneal ulcers.

Keep detailed notes about when you first noticed the third eyelid protrusion, whether it affects one or both eyes, any changes in your cat's behavior or appetite, recent dietary changes or new foods, and whether your cat has been outdoors or exposed to other animals. This information helps your veterinarian determine the most likely cause and appropriate treatment approach. Similar to how owners monitor other concerning symptoms like unusual hiding behavior, tracking the progression of third eyelid protrusion provides valuable diagnostic information.

Treatment for third eyelid protrusion depends entirely on the underlying cause. For classic Haw syndrome with suspected gastrointestinal involvement, the standard approach includes: broad-spectrum deworming medication (typically fenbendazole or similar), even without confirmed parasite diagnosis, as empirical treatment often resolves the condition; probiotic supplementation to support intestinal health; dietary management with easily digestible, high-quality food; and monitoring for spontaneous resolution over 4-8 weeks. Many veterinarians prescribe deworming as a first-line treatment because the connection between intestinal parasites and Haw syndrome is well-established, even when fecal tests come back negative.

If an underlying condition is identified, treatment targets that specific problem: Horner's syndrome treatment depends on the underlying cause (ear infection, trauma, tumor) and may resolve spontaneously in idiopathic cases over weeks to months; conjunctivitis or corneal ulcers require topical antibiotic or antiviral eye medications; upper respiratory infections may need supportive care, antiviral medications, or antibiotics for secondary bacterial infections; and systemic diseases require specific treatment protocols based on diagnosis. For eye pain or inflammation, your veterinarian may prescribe anti-inflammatory medications or pain relief.

The recovery timeline varies significantly: Uncomplicated Haw syndrome typically resolves within 4-8 weeks, though some cases persist for several months before spontaneously improving. The third eyelid gradually retracts over time rather than disappearing suddenly. Parasite-related cases often show improvement within 2-4 weeks of deworming treatment. Viral infection-related protrusion usually resolves as the infection clears, typically within 2-3 weeks. Horner's syndrome may take months to resolve, and some cases remain permanent if nerve damage is severe.

During the recovery period, monitor your cat for: gradual retraction of the third eyelid (even partial improvement is a positive sign); maintenance of normal appetite and activity levels; absence of new symptoms like discharge, redness, or squinting; and normal litter box habits. Follow-up veterinary visits may be recommended to ensure the condition is improving and no underlying problems have been missed. While waiting for resolution can be frustrating, most cases of true Haw syndrome do eventually resolve completely without intervention beyond deworming. However, if the condition persists beyond 8-12 weeks or worsens at any point, additional diagnostic testing may be warranted to rule out less common underlying causes.

While you can't prevent all causes of third eyelid protrusion, several strategies support overall eye and systemic health in cats. Parasite prevention is crucial: maintain regular deworming schedules, especially for cats with outdoor access; use veterinarian-recommended broad-spectrum parasiticides; prevent hunting and consumption of prey animals that may carry parasites; and have annual fecal examinations performed even in indoor cats. Many cases of Haw syndrome could potentially be prevented through consistent parasite control.

Digestive health support reduces the risk of gastrointestinal-related Haw syndrome: feed high-quality, easily digestible cat food appropriate for your cat's age and health status; avoid sudden dietary changes that can upset the digestive system; provide fresh water at all times to prevent dehydration; consider probiotic supplementation, especially during stressful periods; and monitor for signs of digestive upset like vomiting or diarrhea. Just as with other health conditions like inflammatory bowel disease, maintaining digestive health plays a broader role in overall wellness.

Regular veterinary care helps catch problems early: schedule annual wellness examinations (twice yearly for senior cats as outlined in senior cat care guidelines); keep vaccinations current to prevent viral infections; discuss any behavioral changes with your veterinarian promptly; and maintain dental health, as oral infections can sometimes affect the eyes. Early detection of systemic diseases improves treatment outcomes and may prevent complications including eye problems.

Environmental management reduces injury and infection risk: keep household chemicals and irritants away from cat areas; provide safe, supervised outdoor access or keep cats indoors to prevent trauma; maintain clean litter boxes and feeding areas to reduce bacterial exposure; and minimize stress through environmental enrichment and routine. Eye injuries from fights, accidents, or foreign bodies can cause third eyelid protrusion, so preventing these incidents protects eye health.

Learn to recognize normal versus abnormal eye appearance in your cat. Familiarize yourself with what your cat's eyes look like when healthy, including brief glimpses of the third eyelid during sleep or grooming. This baseline knowledge helps you identify changes quickly. While third eyelid protrusion often resolves without serious consequences, staying vigilant about your cat's overall health and seeking veterinary guidance when needed ensures the best outcomes. Regular observation and preventive care form the foundation of maintaining your cat's eye health and catching potential problems before they become serious.