The third eyelid, also called the nictitating membrane, sits at the inner corner of the eye and normally helps protect and lubricate the surface of the eye. A small amount may show briefly while a cat is deeply asleep or just waking, but a membrane that stays visible while the cat is fully alert deserves attention.
The Key Question: Does It Disappear When the Cat Is Fully Awake?
If a cat’s third eyelid remains visible when the cat is awake, or if the eye also looks painful, red, cloudy, or abnormal, veterinary evaluation is appropriate. A brief appearance during sleep can be harmless, but persistent elevation can be associated with eye pain, inflammation, dehydration, systemic illness, or neurologic change.
When It Can Be Normal
A healthy cat may show a small amount of the third eyelid for a few moments after sleep, sedation, or deep relaxation. The key point is that the membrane should slip back into place as the cat becomes fully awake and active.
Common Causes of a Raised Third Eyelid
- Eye irritation or pain such as a corneal ulcer, conjunctivitis, or trauma.
- Systemic illness that causes weakness, dehydration, or weight loss.
- Gastrointestinal or general illness in cats that feel unwell overall.
- Neurologic or sympathetic nerve problems, especially when one eye looks different from the other.
One Eye vs. Both Eyes
If only one third eyelid is elevated, a local eye problem or nerve problem becomes more likely. If both are visible, the cause may be more generalized, especially if the cat seems weak, not eating, or dehydrated. This distinction is helpful but does not replace an examination.
What to Check at Home
Observe in normal light without prying the eye open. Look for squinting, a cloudy cornea, discharge, redness, unequal pupils, or signs that the cat is rubbing the face. Also note appetite, water intake, litter-box use, energy, vomiting, and diarrhea.
Warning Signs That Change the Urgency
| Finding | Why it matters |
|---|---|
| Squinting or obvious pain | Eye disease can worsen quickly |
| Cloudiness, discharge, or severe redness | Prompt eye examination is needed |
| Weakness, appetite loss, vomiting, diarrhea | May signal broader illness |
| Sudden vision change or disorientation | Urgent assessment is appropriate |
| Persistent third eyelid while fully awake | Not a normal sleep-related finding |
What Not to Do
Do not push the third eyelid back, do not use leftover eye medication, and do not apply human redness-relief drops. These steps can irritate the eye or delay correct treatment.
What the Veterinarian May Evaluate
The veterinarian may examine the cornea, tear production, eye pressure, pupils, and general health status. If the cat seems ill overall, hydration status, temperature, abdominal findings, and basic laboratory tests may also be considered.
Treat Persistence as the Clue
A third eyelid that shows briefly during sleep can be normal. A third eyelid that remains elevated in an awake cat, especially with pain or illness signs, deserves veterinary evaluation because the issue may involve the eye itself or the cat’s general health.
A photo helps most when it shows the cat fully awake
If the membrane appears only during sleep, take another look after the cat has been walking around normally for several minutes. A clear photo in ordinary room light can help show whether one eye or both eyes are affected and whether redness, discharge, or unequal pupils are present. Avoid flash and do not hold the eyelids open for a picture.
One elevated third eyelid plus squinting or a cloudy surface points attention toward the eye itself. Both third eyelids showing in a cat that is quiet, dehydrated, losing weight, vomiting, or having diarrhea can fit a broader illness pattern. These patterns help the veterinarian prioritize the examination, but they do not identify the cause on their own.
Do not treat the membrane as something that needs to be “put back”
The third eyelid is responding to an underlying condition; it is not usually the primary problem. Pushing it, applying leftover drops, or repeatedly touching the eye can add irritation. If the cat is rubbing the face, prevent further self-trauma as best you can and arrange veterinary care.
Related Virixoo Guides
Read why a cat’s eye may suddenly look cloudy. Read why a cat may keep one eye closed. Read why unequal pupils in cats need attention.
