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Cats have a third eyelid to protect the surface of the eye, spread tears, and help remove dust and debris. Also known as the nictitating membrane or haw, this thin layer of tissue sits at the inner corner of each eye, closest to the nose.
In a healthy, alert cat, the third eyelid is usually difficult to see. It may briefly appear when a cat is sleeping, waking up, or deeply relaxed. However, a third eyelid that remains visible, especially when accompanied by redness, discharge, squinting, cloudiness, or behavioral changes may indicate an eye problem or another health condition.
What Is a Cat’s Third Eyelid?
A cat’s third eyelid is a thin, pale membrane located between the lower eyelid and the eyeball at the inner corner of the eye. Unlike the upper and lower eyelids, it moves diagonally across the surface of the eye.
The third eyelid includes:
- A thin protective membrane
- Supportive cartilage
- Conjunctival tissue
- A tear-producing gland
- Immune tissue that helps protect the eye
Humans retain only a small, nonfunctional fold of tissue in the inner corner of the eye. In cats, the nictitating membrane remains functional and plays an important role in eye protection and lubrication.
Why Do Cats Have a Third Eyelid?
1. It Protects the Eye
The third eyelid provides an additional physical barrier between the eye and potential hazards. It can help protect the cornea from:
- Dust
- Dirt
- Grass and plant material
- Small foreign objects
- Minor scratches
- Environmental irritants
This protective function was especially valuable to the wild ancestors of domestic cats, which moved through vegetation, hunted prey, and encountered other animals.
2. It Spreads Tears Across the Eye
As the third eyelid moves across the surface of the eye, it helps distribute the tear film. Tears keep the cornea moist, supply nutrients, reduce friction, and help maintain a clear optical surface.
The gland associated with the third eyelid contributes to normal tear production. This is one reason veterinarians generally try to preserve the gland whenever possible when treating third-eyelid disorders.
3. It Helps Clear Debris
The movement of the membrane can help sweep small particles and mucus toward the inner corner of the eye, where they can be removed more easily.
4. It Supports the Eye’s Immune Defenses
The third eyelid contains lymphoid tissue that contributes to local immune protection. This tissue helps the eye respond to microorganisms and other foreign material.
5. It Supports Clear Vision Indirectly
The third eyelid does not improve visual sharpness or give cats special night vision. Instead, it supports vision indirectly by helping keep the cornea moist, clean, and protected.
What Does a Cat’s Third Eyelid Look Like?
The third eyelid usually appears as a pale pink, whitish, or slightly gray triangular membrane extending from the inner corner of the eye.
Its appearance can vary slightly among cats, but it should not normally:
- Cover a large portion of the eye while the cat is alert
- Appear bright red or severely swollen
- Have a distinct round mass
- Be accompanied by thick discharge
- Cause the cat to squint or paw at the eye
Do not confuse the third eyelid with the conjunctiva, which is the thin tissue lining the eyelids and covering part of the eyeball. Both tissues may become more visible when inflamed.
When Is a Visible Third Eyelid Normal?
A small portion of the third eyelid may occasionally be visible without indicating disease.
Sleeping or Waking Up
The membrane may appear briefly when a cat is asleep, deeply relaxed, or just beginning to wake. It should usually retract as the cat becomes alert.
Deep Relaxation
Some cats show a small amount of third eyelid while resting comfortably. Temporary visibility in an otherwise healthy cat may not be concerning.
Temporary Eye Position Changes
The third eyelid can move forward when the eyeball temporarily retracts slightly into the socket. This may happen briefly during sleep or certain normal movements.
Persistent visibility is different from a membrane that appears for only a few moments. Contact a veterinarian when the third eyelid remains raised or repeatedly appears while the cat is fully awake.
Why Is My Cat’s Third Eyelid Showing?
A raised third eyelid is a sign rather than a specific diagnosis. Many different eye and health conditions can cause it.
Eye Irritation or Pain
Pain can cause the eyeball to pull slightly backward, allowing the third eyelid to move forward. Possible causes include:
- A scratch on the cornea
- A foreign object
- Trauma
- Inflammation inside the eye
- Glaucoma or other painful eye disease
Conjunctivitis
Conjunctivitis is inflammation of the tissues surrounding the eye. Affected cats may have:
- Redness
- Swelling
- Watery or thick discharge
- Squinting
- A protruding third eyelid
Conjunctivitis may be associated with viral or bacterial infections, environmental irritation, immune-mediated conditions, or other eye disorders.
Corneal Ulcer or Injury
A corneal ulcer is damage to the clear surface of the eye. It is often painful and may cause squinting, tearing, redness, cloudiness, sensitivity to light, and elevation of the third eyelid.
Corneal injuries can worsen quickly and should be assessed promptly.
Dehydration or Significant Weight Loss
Dehydration, severe weight loss, or loss of tissue behind the eye can cause the eyeball to sit farther back in its socket. This may make the third eyelid more visible.
A visible third eyelid alone cannot confirm dehydration. Other signs may include dry or tacky gums, reduced appetite, weakness, or changes in drinking and urination.
Systemic Illness
Illness affecting the entire body may cause both third eyelids to rise. This can occur when a cat feels generally unwell, weak, dehydrated, or affected by certain gastrointestinal, infectious, or neurological conditions.
Horner’s Syndrome
Horner’s syndrome results from disruption of nerves controlling structures around the eye. Signs commonly affect one side and may include:
- A raised third eyelid
- A smaller pupil
- A drooping upper eyelid
- An eye that appears slightly sunken
Horner’s syndrome can have several causes, including ear disease, trauma, nerve injury, or an unidentified cause.
Haw’s Syndrome
Haw’s syndrome describes elevation of both third eyelids, often in a cat that has or recently had gastrointestinal illness such as diarrhea.
The eyes themselves may not be painful or inflamed, but a veterinarian should still examine the cat to rule out other conditions.
Growth, Cyst, or Mass
A growth involving the eyelid, conjunctiva, third eyelid, or surrounding structures can change the membrane’s position or appearance. Although less common than inflammation or injury, masses require veterinary evaluation.
What Does It Mean If One or Both Third Eyelids Are Showing?
Only One Third Eyelid Is Visible
When only one eye is affected, the cause is more likely to involve that eye or the nerves on that side.
Possible explanations include:
- Eye injury
- Foreign material
- Corneal ulcer
- Conjunctivitis affecting one eye
- Horner’s syndrome
- A mass behind or around the eye
Both Third Eyelids Are Visible
When both are elevated, possible causes include:
- General illness
- Dehydration
- Weight loss
- Gastrointestinal disease
- Haw’s syndrome
- Severe fatigue or weakness
- Certain neurological disorders
The pattern can help guide a veterinary examination, but it cannot diagnose the cause by itself.
Common Third Eyelid Problems in Cats
Inflammation
The membrane may become red, swollen, or more visible because of infection, irritation, allergy, trauma, or inflammation affecting nearby eye tissues.
Injury
Scratches, bites, plant material, and other trauma can damage the membrane. Injuries may cause bleeding, swelling, tearing, or pain.
Prolapsed Third-Eyelid Gland
A prolapsed gland appears as a rounded pink or red mass at the inner corner of the eye. This condition is commonly called “cherry eye.”
Cherry eye is far more common in dogs than cats, but it can occasionally occur in cats. A red mass should be examined rather than assumed to be a simple irritation.
Cartilage Abnormality
The cartilage supporting the membrane can occasionally bend or develop abnormally, changing the third eyelid’s position.
Tumor or Abnormal Growth
Tumors involving the third eyelid are uncommon, but they can occur. Warning signs include a persistent lump, irregular tissue, bleeding, color change, or a slow-healing sore.
Warning Signs That Need Veterinary Care
Contact a veterinarian promptly when the third eyelid remains visible while your cat is awake, particularly when it is accompanied by another symptom.
Concerning signs include:
- Squinting or keeping the eye closed
- Redness or swelling
- Yellow, green, bloody, or excessive discharge
- A cloudy, bluish, or dull-looking eye
- Pawing or rubbing at the face
- A visible wound or foreign object
- Unequal pupil sizes
- Sudden vision changes
- A rounded red or pink mass
- Loss of appetite
- Vomiting or diarrhea
- Weakness or unusual hiding
- Recent trauma
When Is It an Emergency?
Seek urgent veterinary care for:
- Severe eye pain
- Sudden cloudiness
- A bulging or collapsed-looking eye
- Heavy bleeding
- A chemical exposure
- A penetrating injury
- Sudden loss of vision
- An eye that cannot close normally
- Severe weakness or collapse
Eye problems can progress quickly. Delayed treatment may increase pain and, in some cases, threaten vision.
How Are Third-Eyelid Problems Diagnosed and Treated?
Treatment depends entirely on the underlying cause. A veterinarian may examine the eyelids, third eyelid, cornea, pupils, and structures inside the eye.
Diagnostic tests may include:
- A fluorescein stain to look for corneal damage
- Tear-production testing
- Measurement of pressure inside the eye
- Evaluation of pupil responses
- Examination beneath the third eyelid
- Blood, urine, or fecal tests when systemic illness is suspected
- Imaging when disease behind the eye is possible
- Sampling or biopsy of an abnormal mass
Possible Treatments
Depending on the diagnosis, treatment may involve:
- Lubricating eye medication
- Antibiotic medication when a bacterial infection is present
- Antiviral treatment in selected cases
- Anti-inflammatory or pain medication
- Removal of a foreign object
- Treatment for dehydration or systemic illness
- Surgery for certain injuries, masses, or gland problems
Never use leftover eye drops or human eye medication unless instructed by a veterinarian. Certain medications, particularly steroid-containing drops, can worsen a corneal ulcer.
How to Check Your Cat’s Eyes at Home
A brief visual check can help you notice changes early.
- Use good lighting. Look at both eyes in a naturally lit room.
- Compare the eyes. Check whether the pupils, eyelids, and third eyelids look similar.
- Look for discharge. A small amount of clear moisture may occur, but thick or colored discharge is abnormal.
- Check the cornea. The front of the eye should look clear rather than cloudy or blue.
- Observe behavior. Watch for squinting, rubbing, hiding, appetite changes, or reluctance to move.
Do not force the eyelids open, press on the eyeball, or attempt to pull the third eyelid into view.
How to Support Healthy Eyes
Provide Complete and Balanced Nutrition
Feed a nutritionally complete cat food appropriate for your cat’s age and health. Cats require specific nutrients, including taurine, to support normal vision and overall health.
Do not add eye supplements unless recommended by a veterinarian.
Schedule Regular Veterinary Checkups
Routine examinations can identify eye changes, weight loss, dehydration, dental disease, and other health problems before they become advanced.
Learn more about why regular veterinary visits are important.
Reduce Environmental Irritants
Smoke, dusty litter, cleaning sprays, perfumes, and other airborne irritants may bother sensitive eyes. Use pet-safe household products and provide adequate ventilation.
Reduce the Risk of Eye Injuries
Keep sharp objects, thorny plants, chemicals, and unsafe toys away from your cat. Cats that go outdoors may face additional risks from fights, vegetation, debris, and infectious disease.
Know What Is Normal for Your Cat
Become familiar with your cat’s usual eye color, pupil size, tear production, and eyelid position. Recognizing normal appearance makes it easier to identify a sudden change.
Common Myths About a Cat’s Third Eyelid
Myth: Any Visible Third Eyelid Is an Emergency
A small amount may briefly appear while a cat sleeps or wakes. Persistent visibility or additional symptoms are more concerning.
Myth: The Third Eyelid Gives Cats Night Vision
It does not create night vision. Cats see well in low light because of other adaptations in the eye. The third eyelid protects and lubricates the eye.
Myth: A Visible Third Eyelid Always Means Dehydration
Dehydration is one possible cause, but eye pain, infection, gastrointestinal illness, nerve problems, weight loss, and other conditions can also raise the membrane.
Myth: Eye Drops From Home Are Safe to Try
Using the wrong medication can delay diagnosis or worsen certain conditions. Eye medication should only be used under veterinary direction.
Frequently Asked Questions
Why do cats have a third eyelid?
The third eyelid protects the cornea, spreads tears, helps remove debris, and contributes to the eye’s immune defenses.
Is it normal to see a cat’s third eyelid?
It may be briefly visible when a cat is sleeping, waking, or deeply relaxed. It should usually retract when the cat is fully alert. Persistent visibility should be assessed by a veterinarian.
Why is my cat’s third eyelid showing in one eye?
One-sided elevation can be associated with eye pain, injury, a foreign object, a corneal ulcer, inflammation, Horner’s syndrome, or disease behind the eye.
Why are both of my cat’s third eyelids showing?
Both membranes may rise because of dehydration, weight loss, gastrointestinal illness, Haw’s syndrome, general weakness, or another systemic condition.
Will a raised third eyelid go away on its own?
Temporary visibility during sleep may resolve normally. A membrane that remains elevated should not be left untreated without veterinary advice because the underlying cause may require care.
Does a visible third eyelid mean my cat is dehydrated?
Not necessarily. Dehydration can make the eyes appear sunken and reveal more of the membrane, but many other eye and health problems can produce the same sign.
Can cats get cherry eye?
Yes, but it is uncommon compared with dogs. It appears as a rounded pink or red mass at the inner corner of the eye and requires veterinary examination.
Is a protruding third eyelid contagious?
The protrusion itself is not contagious. However, some infections that cause conjunctivitis and third-eyelid elevation can spread between cats.
Should I try to clean or move my cat’s third eyelid?
No. Do not pull, push, or manipulate the membrane. You may gently wipe discharge from the surrounding fur with a clean damp cloth, but the eye itself should be examined when symptoms persist.
When should I take my cat to the veterinarian?
Arrange an examination when the third eyelid remains visible, repeatedly protrudes, or appears with redness, discharge, cloudiness, squinting, pain, appetite loss, diarrhea, weakness, or behavioral changes.
Final Thoughts
A cat’s third eyelid is a functional part of the eye rather than an unusual or unnecessary feature. It protects the cornea, spreads the tear film, clears debris, and supports local immune defenses.
Brief visibility during sleep or relaxation may be normal. However, a third eyelid that remains raised while the cat is awake can indicate eye pain, inflammation, injury, dehydration, gastrointestinal illness, nerve dysfunction, or another health problem.
Do not rely on appearance alone to diagnose the cause or apply unprescribed eye drops. Prompt veterinary care is the safest approach when the membrane remains visible or appears alongside pain, discharge, cloudiness, or changes in your cat’s health.
