You glance over at your cat, and something looks different. That little pink or white film creeping across the corner of their eye is the third eyelid, also called the nictitating membrane. Normally, it stays tucked away, only appearing when they blink or sleep. When it stays visible, it’s your cat’s way of signaling that something needs your attention. Let’s walk through the most common reasons this happens and what you can do about it.
What Exactly Is the Third Eyelid?
Your cat has three eyelids per eye. The third one sits in the inner corner and acts like a windshield wiper, spreading tears, clearing debris, and protecting the eye. It’s controlled by a special nerve and can pop up when your cat is relaxed, sick, or in pain. If it’s showing and not during a sleepy blink, it’s worth investigating.
Common Causes: From Minor to Serious
There are several reasons a cat’s third eyelid might become visible. Some are harmless, others need a vet visit.
1. Eye Irritation or Injury
A speck of dust, a scratch from a playmate, or even an ingrown eyelash can make the eye sore. The third eyelid comes up to protect it. Look for squinting, redness, discharge, or pawing at the face. If you see these signs, a vet should check for corneal ulcers or foreign bodies.
2. Dehydration or Illness
When a cat is feeling under the weather, the third eyelid can become prominent. This is especially true with fever, vomiting, or diarrhea. Dehydration causes the eyes to sink slightly, making the membrane more visible. If your cat also has a poor appetite or seems lethargic, it’s time for a vet exam. For cats with chronic issues like allergies that can lead to sneezing or eye discharge, check out our guide on why your allergic cat sneezes so much for more context.
3. Haw Syndrome (Bilateral Protrusion)
This sounds fancy, but it just means both third eyelids are showing at the same time with no other eye symptoms. It often happens after a bout of diarrhea, a parasite infection, or even stress. The good news? It usually resolves on its own once the underlying issue is treated. But always have a vet rule out more serious causes.
4. Neurological Problems
Sometimes the issue is with the nerve that controls the eyelid. Conditions like Horner’s syndrome (which also causes a droopy ear and small pupil) or damage to the facial nerve can cause the third eyelid to protrude. This requires a veterinary neurologist or a thorough exam.
5. Weight Loss or Muscle Atrophy
If your cat has lost a lot of weight or muscle mass around the head, the eye socket becomes shallower, and the third eyelid can appear. This can happen with older cats or those with chronic diseases like hyperthyroidism or kidney disease. A vet check is important to address the root cause.
If your cat’s third eyelid is showing but they are acting normal, eating, and playing, you can gently check for debris by looking at the eye in good light. Never try to pull the membrane back yourself. If it doesn’t resolve in 24 hours, or if other symptoms appear, call your vet.
When Should You Worry?
You don’t need to panic, but you do need to pay attention. Make a vet appointment if you see any of these along with the visible third eyelid:
- Squinting, redness, or discharge from the eye
- Lethargy, hiding, or loss of appetite
- Vomiting or diarrhea
- Drooping of the eyelid or ear on the same side
- Your cat seems to be in pain or is pawing at their face
“The third eyelid is like your cat’s check engine light. It doesn’t tell you exactly what’s wrong, but it tells you to look under the hood.”
Can Allergies Cause the Third Eyelid to Show?
Yes, allergies can contribute. If your cat has itchy eyes from pollen, dust, or food, they may rub their face, leading to irritation and a protruding third eyelid. Food allergies in particular can cause skin and eye issues. If you suspect food is the trigger, our guide on signs of food allergies in cats can help you spot the clues. Additionally, if your cat’s allergies are causing skin irritation that leads to eye rubbing, you might need to manage their bathing routine. Learn more in our article about how often to bathe a cat with allergies.
What Will the Vet Do?
Your vet will start with a physical exam and look at the eye with an ophthalmoscope. They may use a fluorescein stain to check for scratches on the cornea, check tear production, and look for signs of infection. Depending on what they find, they might recommend eye drops, antibiotics, or blood work to check for underlying diseases like kidney issues or thyroid problems. For cats with chronic allergies that affect their overall health and longevity, you can read more about the life expectancy of a cat with allergies to understand the bigger picture.
Most cases of a visible third eyelid are treatable and not emergencies. The key is to notice it early and get a professional opinion. Your cat’s eyes are a window to their health, and that little membrane is just trying to tell you something.
