Cat's Third Eyelid (Nictitating Membrane) Showing: Common Causes and Treatment

Sometimes you'll notice a white or semi-transparent membrane appearing on the inner corner of your cat's eye, looking like they're "rolling their eyes back"—this is actually the third eyelid (nictitating membrane) showing. The nictitating membrane is a normal structure of the cat's eye, usually hidden in the corner, but when it becomes visible, it often means your cat's body is sending some kind of signal.

## What Is the Third Eyelid?

The third eyelid is a thin membrane tissue located on the inner side of the cat's eye (near the nose). Its main functions are:

* Protecting the eyeball from dust and foreign objects
* Helping distribute tears during blinking to keep the eye moist
* Contains tear glands that produce about one-third of tear secretion

In healthy cats when awake, the nictitating membrane usually stays retracted in the eye corner and isn't very noticeable. It's only briefly visible during sleep, drowsiness, or just after waking up.

## Common Causes of Third Eyelid Showing

### Local Eye Problems

* **Conjunctivitis**: Eye inflammation, redness, swelling, increased discharge
* **Corneal injury**: Foreign body irritation or scratches
* **Eye infection**: Bacterial or viral infection
* **Cherry eye**: Third eyelid gland prolapse (less common)

### Systemic Issues

1. **Gastrointestinal discomfort**: Diarrhea, parasitic infections, etc. can cause the nictitating membrane to show
2. **Dehydration**: Insufficient water intake or severe diarrhea
3. **Fever or infection**: When the body is fighting disease
4. **Neurological problems**: Horner's syndrome affects eye nerves
5. **Overall weakness**: Malnutrition or chronic illness

### Haws Syndrome

This is a special condition where both eyes' third eyelids become noticeably visible simultaneously, but the eyes themselves appear normal. It's commonly seen after gastrointestinal tract infections (especially parasites) and usually resolves on its own within a few weeks.

## Accompanying Symptoms to Watch For

When the third eyelid is showing, watch for these conditions:

* Eye redness, swelling, tearing, or discharge
* Frequent blinking or rubbing eyes
* Decreased appetite or diarrhea
* Lethargy, increased sleeping
* Only one eye affected (may be a local problem)
* Both eyes affected simultaneously (may be a systemic issue)

## When to Seek Veterinary Care

Take your cat to the vet promptly in these situations:

* Third eyelid showing for more than 24 hours without improvement
* Accompanied by eye redness, swelling, discharge, or pain response
* Cat has other discomfort symptoms (not eating, diarrhea, fever)
* Nictitating membrane showing in only one eye
* Visible lesions or lumps on the nictitating membrane

## Home Observation and Care

If the third eyelid is only slightly showing and your cat's energy and appetite are normal:

* Watch to see if it resolves on its own within 12-24 hours
* Ensure your cat has adequate water intake
* Check if there have been recent diet changes or stressful events
* Don't use human eye drops without veterinary guidance

The third eyelid showing is not a disease itself, but rather a signal of the body's condition. The key is finding the underlying cause—it might just be lack of sleep or mild gastrointestinal discomfort, but it could also be an eye problem or systemic disease requiring treatment. When you're uncertain, consulting a veterinarian is the safest choice.

---
**References:**
* https://www.carnivoreraw.com/pages/third-eyelid
* https://www.3littlemeow.com/blog/posts/第三眼瞼外露反映主子身體問題
* https://www.jvs.com.tw/blog/posts/毛孩神秘的第三眼瞼
* https://vetster.com/en/wellness/why-is-my-cat-s-third-eyelid-showing
* https://www.catster.com/cat-health-care/cat-third-eyelid-showing/
* https://cats.com/haws-syndrome-in-cats

---