You notice a cloudy white film crossing the inner corner of your cat's eye — like half-closed blinds from inside. That is the third eyelid (nictitating membrane, "haw"). Healthy cats hide it almost completely. When it rides up and stays visible, the body is often signaling pain, dehydration, viral illness, or eye disease — not a camera trick.
Key points at a glance
- Both eyes haw up — think systemic illness — fever, dehydration, gut pain.
- One eye only — local eye problem — corneal ulcer, conjunctivitis, injury.
- With squinting, discharge, or hiding — vet same day.
- After anesthesia or deep sleep — brief haw normal; should retract when fully awake.
- Never pull the membrane — fragile tissue; exam by professional.
Common causes
| Pattern | Likely causes |
|---|---|
| Both eyes, cat lethargic | Upper respiratory infection, fever, dehydration |
| Both eyes, vomiting | GI pain, pancreatitis, toxin — urgent |
| One eye, squinting | Ulcer, scratch, glaucoma risk |
| One eye, no squint | Early Haw's syndrome, mild conjunctivitis |
| Intermittent after nap | Normal variant if bright and eating |
Haw's syndrome — bilateral third eyelid in young cats without other signs — often self-resolves in weeks; still needs vet baseline.
Associated symptoms to report
- Green or yellow eye discharge
- Not eating 24+ hours
- Drooling or lip smacking (nausea)
- Diarrhea or constipation with bilateral haw
- Dilated pupil or bulging eye — emergency
What happens at the vet
- Fluorescein stain — detects corneal ulcers.
- Eye pressure — rules out glaucoma.
- Temperature, hydration, abdominal palpation
- Blood work if systemic signs — kidney, liver, infection panels.
Home eye drops without diagnosis can worsen ulcers if steroid drops hit wrong condition.
Dehydration and haw
Skin tent slow, dry gums, sunken eyes plus haw — fluids may be needed. Encourage water; do not force syringe large volumes if vomiting.
When it can wait vs not
Wait 24 hours only if: haw appeared once after heavy sleep, cat bright, eating, playing, haw gone by afternoon.
Same day if: haw persistent 12+ hours, any squint, lethargy, or both eyes in a normally active cat.
The bottom line
A cat's third eyelid showing means something is off — localized eye injury, whole-body illness, or occasionally benign Haw's syndrome in young cats. Photograph both eyes, note appetite and litter use, and book prompt vet care when haw stays up with squinting or lethargy. Visible haw is a symptom to investigate, not a meme filter to ignore.
This article is for general education only and does not replace advice from a licensed veterinarian.