Cat Third Eyelid Showing Why
By Dr. Zubair Khalid, DVM, MS, PhD ·
As a cat owner, you may occasionally notice a pale, whitish film creeping across your cat’s eye from the inner corner. That is the third eyelid (nictitating membrane), a protective structure normally hidden when your cat is awake and alert. When it remains visible or partially covers the eye, it often signals an underlying issue. Understanding why this happens helps you decide whether to monitor at home or seek veterinary attention.
Key Takeaways
- Protrusion of the feline third eyelid (nictitating membrane / haw) occurs when orbital fat volume diminishes, when the globe retracts due to ocular pain, or following disruption of sympathetic ocular innervation.
- Bilateral (both eyes) elevation in an otherwise alert cat is frequently caused by Haw's Syndrome (associated with gastrointestinal inflammation or viral enteritis) or systemic dehydration and fever.
- Unilateral (single eye) elevation points to localized ocular pathology: corneal ulceration, uveitis, foreign body, or Horner's Syndrome (ptosis, miosis, enophthalmos, and third eyelid elevation).
- Dehydration causes the retrobulbar fat cushion to shrink, allowing the eye to sink into the orbit (enophthalmos) and passively pushing the third eyelid across the cornea.
- Urgent veterinary evaluation is required if third eyelid protrusion is accompanied by blepharospasm (squinting), corneal cloudiness, yellow/green discharge, unequal pupils (anisocoria), or marked lethargy.
What Is a Cat's Third Eyelid and Why Is It Showing?
A cat's third eyelid is a pale, whitish membrane tucked into the inner corner of each eye. It usually stays hidden, so when you can see it, that is a signal worth paying attention to. The membrane slides across the eye to protect and lubricate it, and it becomes visible when the eye is pulled inward, when the eye is painful, or when the nerves that hold it back stop working properly. In many cases the cause is minor and treatable, but a visible third eyelid can also point to something that needs a vet.
A brief glimpse during blinking, sleep, or when your cat is relaxed is normal. What matters is whether the membrane stays out, whether one or both eyes are affected, and whether other signs appear.
One eye or two? This is the most useful clue. When only one eye shows the membrane, a local eye problem is more likely, such as a scratch on the cornea, a foreign body like a grass seed, inflammation inside the eye (uveitis), or Horner's syndrome. When both eyes show it in a cat that is otherwise bright and eating, the cause is more often whole body: stomach or gut upset, intestinal worms (the pattern called Haw's syndrome), dehydration, fever, or weight loss. Both eyes showing in a very sick or lethargic cat needs prompt care.
What you may see alongside it. Squinting, redness, cloudy cornea, yellow or green discharge, pawing at the face, unequal pupil sizes, a drooping upper lid, head tilt, vomiting, diarrhea, or poor appetite all change the picture. Pain and discharge point toward the eye itself. Vomiting and diarrhea point toward the gut. Unequal pupils or a head tilt raise the question of a nerve problem.
Is it an emergency? Seek care quickly if your cat is squinting hard, the eye looks cloudy or has a visible defect, there is thick discharge, the pupils differ in size, the cat is lethargic or not eating, or the membrane stays up for more than a few hours. A short episode in an otherwise well cat that resolves on its own can be watched, but a vet visit is still reasonable to rule out parasites or illness.
What you should not do. Do not use human or over the counter eye drops, and do not try to push the membrane back. Both can cause harm. Keep fresh water available and note how long the sign lasts and whether it affects one or both eyes, since that information helps your vet.
Quick Q&A: Common Questions
Question: Why is my cat’s third eyelid showing?
Answer: A visible third eyelid can be due to a benign condition called Haw’s syndrome (often linked to intestinal worms or gut upset), eye irritation or injury, dehydration, pain, or neurological problems. In many cases it is temporary and resolves once the underlying cause is treated. However, if accompanied by other symptoms like squinting, discharge, or lethargy, prompt veterinary evaluation is recommended.
Understanding the Third Eyelid
The third eyelid is a thin, T‑shaped piece of tissue located in the inner corner of each eye. It contains a gland that produces a portion of the tear film and helps distribute moisture across the eye. In healthy cats, the third eyelid remains retracted out of sight, held in place by the eye’s position and the surrounding muscles. When it becomes prominent, it usually means something is pulling the eye inward (as with dehydration or weight loss), or that the eye itself is painful, causing the eyelid to protrude protectively.
Common Causes of a Visible Third Eyelid
Haw’s Syndrome
Haw’s syndrome is a common, benign condition in which both third eyelids protrude simultaneously, often without any other eye symptoms. It is frequently associated with gastrointestinal upset, such as diarrhea or vomiting, and sometimes with intestinal parasites (especially tapeworms). The exact mechanism is unclear, but the condition typically resolves on its own within a few weeks once the gut issue is treated.
Eye Irritation or Injury
Any local irritation to the eye can trigger the third eyelid to move forward. Examples include corneal scratches, foreign bodies like grass seeds, conjunctivitis (pink eye), or feline herpesvirus. If only one eye is affected, an ocular problem is more likely than a systemic condition. Squinting, redness, discharge, or pawing at the eye often accompany these cases.
Systemic Illness or Dehydration
Systemic problems can cause the third eyelid to show in both eyes. Dehydration reduces the volume of fluid behind the eye, allowing the globe to sink slightly backward and the eyelid to move forward. Fever, severe pain from conditions like pancreatitis, or significant weight loss can also make the third eyelid more visible. Cats that are lethargic or have gastrointestinal symptoms may show this sign.
Neurological Issues
Less commonly, a prominent third eyelid may indicate a neurological problem, especially if the pupils are different sizes (anisocoria), the cat is dizzy tilting its head, or the eyelid protrusion is one‑sided. Conditions such as Horner’s syndrome (damage to the sympathetic nerves supplying the eye) can cause a drooping eyelid, constricted pupil, and visible third eyelid on the same side. Head trauma, ear infections, or tumors can also be responsible.
When to Seek Veterinary Care
A single, brief episode with a known trigger (like a recent stomach upset) may not be alarming, but you should contact your veterinarian if:
- The third eyelid stays up for more than a few hours.
- Only one eye is affected.
- Your cat is squinting, crying, pawing at the eye, or has discharge.
- There are other systemic signs: vomiting, diarrhea, fever, loss of appetite, lethargy.
- The pupils appear unequal or the cat seems off balance.
Even if both eyes are equally affected and your cat otherwise seems well, it is wise to schedule an appointment to rule out underlying parasites or illness.
What to Expect at the Vet Visit
Physical Examination
Your veterinarian will start with a general physical exam, checking your cat’s temperature, hydration status, body condition, and abdominal tenderness. They will also perform a thorough eye examination using an ophthalmoscope to look at the cornea, pupil, and lens. The vet may gently evert the eyelids to check for foreign bodies.
Diagnostic Tests
Depending on findings, tests may include:
- Schirmer tear test to measure tear production.
- Fluorescein stain to detect corneal ulcers or scratches.
- Bloodwork to assess hydration, organ function, and signs of infection.
- Fecal exam to check for intestinal parasites if Haw’s syndrome is suspected.
- Neurological exam if nerve damage is possible.
Home Care and Monitoring
If your vet diagnoses a benign condition like Haw’s syndrome and your cat is otherwise healthy, simple home care may be enough:
- Ensure fresh water is always available to prevent dehydration.
- Feed a balanced diet; address any gastrointestinal upset with veterinary guidance.
- Keep the eyes clean by gently wiping away any discharge with a damp, soft cloth.
- Monitor for changes: note if the eyelid retracts on its own, if the cat stops eating, or if symptoms worsen.
Never use over‑the‑counter eye drops without a vet’s approval, as some can irritate or damage the eye.
Prevention
While not all causes are preventable, you can reduce risks:
- Keep up with routine deworming and fecal checks as recommended by your vet.
- Provide a stress‑free environment to minimize flare‑ups of feline herpesvirus.
- Protect your cat from eye injuries by keeping sharp objects away and trimming claws.
- Maintain good hydration and nutrition, especially for older or sick cats.
- Have regular wellness exams to catch systemic issues early.
Seeing your cat’s third eyelid can be startling, but in many cases it is a temporary, treatable sign. By staying observant and seeking timely veterinary care, you can help your feline companion return to normal quickly and comfortably.
How the Third Eyelid Works and What Moves It
Picture the eye sitting in a cup of soft fat inside the bony socket. The third eyelid is anchored at the inner corner and, in a healthy cat, is held retracted by the position of the eyeball and by a small slip of cartilage and muscle. It sweeps across the eye during blinking and sleep. Three broad forces can push it into view.
The first is loss of the fat cushion. When the fat behind the eye shrinks, the eyeball settles deeper into the socket (enophthalmos), and the membrane passively rides forward. Dehydration and weight loss both shrink that cushion.
The second is pain or irritation of the eye itself. A painful eye is held partly closed and pulled back, and the membrane comes forward as a protective response. This is why a scratch or a foreign body often shows the membrane on just one side.
The third is nerve failure. The sympathetic nerves that run to the eye keep the upper lid lifted and the eyeball in normal position. When those nerves are damaged, the upper lid droops (ptosis), the pupil narrows (miosis), the eyeball sinks (enophthalmos), and the third eyelid rises. Together these four signs make up Horner's syndrome. The membrane can also rise when the muscles that normally retract it are weak or when the eye is unusually small.
A useful way to picture it: the third eyelid is like a window shade that is normally rolled up. It drops when the window frame shifts (fat loss), when someone pulls the frame inward (pain), or when the cord that holds the shade up is cut (nerve damage).
Worked Examples: Reading the Signs
Example 1: The healthy cat with a brief flash. You notice the membrane during a long blink or while your cat dozes in the sun. It retracts as soon as the cat is alert. No squinting, no discharge, normal appetite. This is normal and needs no action beyond routine wellness checks.
Example 2: The cat with gut upset. A young cat has had soft stools for two days and both third eyelids are showing evenly. The cat is eating and playful. This pattern fits Haw's syndrome, which is linked to gastrointestinal upset and sometimes intestinal worms. A vet visit with a fecal check is sensible. Treatment of the gut issue or worms usually resolves the membrane within days to a few weeks. It is not an emergency if the cat is bright.
Example 3: The squinting cat with one eye affected. One eye is half closed, the membrane is up on that side only, and the cat paws at its face. This suggests a local eye problem such as a corneal ulcer or a foreign body. This needs a same day or next day vet visit. Waiting risks worsening damage to the cornea.
Example 4: The dehydrated older cat. An older cat has been drinking poorly and has lost weight. Both membranes are up and the skin on the neck tents when gently pinched. Dehydration is likely, and the vet will check kidney function and other causes. Fluid therapy and treating the underlying illness are the priorities.
Example 5: The cat with unequal pupils. One pupil is small and the upper lid on that side droops, with the membrane showing. This is the classic Horner's pattern. Causes include middle ear disease, neck or head injury, and other nerve problems. A neurological exam and imaging may be needed.
Example 6: The cat that seems fine but shows both membranes for days. Even a well appearing cat with persistently elevated membranes deserves a check, because intestinal parasites and low grade illness can hide behind a calm exterior. A fecal exam and basic bloodwork are reasonable first steps.
Common Misconceptions
"The membrane is always a sign of serious disease." Not true. Brief flashes during blinking and sleep are normal. Many cases are benign and self limiting, especially the Haw's pattern in a bright cat.
"If both eyes are affected, it must be an eye problem." The opposite is more often true. Both eyes showing in an otherwise well cat usually points to a whole body cause such as gut upset, worms, dehydration, or weight loss. One eye showing is more likely to be a local eye problem.
"I can treat this with leftover or human eye drops." Do not. Some drops can worsen an ulcer or cause irritation. Only use medication prescribed by your vet for your cat.
"I should push the membrane back into place." Never do this. It can injure the eye and hides the problem from your vet.
"A visible membrane in a kitten is always harmless." Kittens can show the membrane with dehydration, worms, or infection. A kitten that is thin, has diarrhea, or is lethargic needs prompt care because small bodies dehydrate quickly.
"If the eye looks normal, the membrane can be ignored." The membrane can rise before other signs appear. Duration and whether one or both eyes are involved are the key details to report.
"Squinting is just sleepiness." Squinting, especially on one side, is a sign of eye pain until proven otherwise. It warrants a vet visit.
Diagrams in Words: What to Look For at Home
Diagram 1: The normal resting eye. Picture the eye from the front. The pupil sits centered, the upper and lower lids frame the eye, and the inner corner shows only a tiny pale sliver, if anything. No redness, no discharge. This is the baseline to compare against.
Diagram 2: Both membranes up. Both eyes show a symmetric pale curtain at the inner corner. The eyes themselves look clear. The cat is alert. This picture suggests a whole body cause rather than a single eye injury.
Diagram 3: One membrane up with squinting. One eye shows the membrane and is held partly closed. The surface may look dull or cloudy. The other eye is normal. This picture suggests a local eye problem and needs a vet.
Diagram 4: The Horner's picture. On one side, the upper lid droops, the pupil is smaller than the other side, the eyeball looks slightly sunken, and the membrane is up. All four signs on the same side point to a nerve problem.
Diagram 5: The dehydrated cat. Both membranes up, dull coat, tacky gums, and skin that stays tented when gently pinched at the neck. This picture calls for prompt veterinary care.
Diagram 6: The relaxed sleepy cat. The membrane shows while the cat is drowsy and retracts fully when it wakes and moves. This is normal and needs no action.
Special Situations: Kittens, Older Cats, and Flare Ups
Kittens. A kitten's small body loses water quickly. A kitten with both membranes showing, diarrhea, or poor feeding should be seen promptly. Worms are common in young cats, and a fecal check is a standard first step. Keep the kitten warm and offer water while you arrange the visit.
Older cats. In senior cats, weight loss and dehydration are common and both shrink the fat cushion behind the eye. A vet visit should include bloodwork to check kidney and thyroid function and a general check for dental disease and other chronic problems. Dental pain and poor appetite can contribute to weight loss and dehydration.
Cats with feline herpesvirus. Stress can trigger flare ups that cause eye irritation and a visible membrane. Keeping the environment calm, avoiding sudden changes, and following your vet's plan for flare ups help reduce recurrence. Do not start or stop medications without veterinary guidance.
Cats recovering from illness. After vomiting or diarrhea, dehydration can linger for a day or two. Offer water and monitor. If the membrane stays up or the cat remains lethargic, call your vet.
Cats with one eye affected after trauma. A blow to the head or a scratch can cause both local eye injury and nerve damage. Any one sided membrane with a drooping lid or unequal pupils after trauma needs prompt evaluation.
Households with multiple cats. Worms and some infections spread between cats. If one cat shows the membrane with gut signs, a fecal check for the household is reasonable, and routine deworming as advised by your vet helps prevent repeat problems.