Why Is My Cat's Third Eyelid Showing? Haw's Syndrome Explained

Cats have a third eyelid tucked into the inner corner of each eye that most owners never really notice, because it's normally supposed to stay out of sight. When both third eyelids suddenly ride up and stay visible across both eyes, it looks alarming, but in a young cat it's often a specific, well-documented condition that tends to resolve on its own.

What the third eyelid actually is

The nictitating membrane, more commonly called the third eyelid, is a thin membrane that normally sits retracted in the corner of a cat's eye. When it becomes persistently visible across both eyes, with the rest of the eye looking otherwise completely normal, vets call this Haw's syndrome.

Who typically gets it

Haw's syndrome tends to show up in young adult cats, generally under two years old. A 2024 clinicopathological study in Veterinary Record examining 10 cats with the condition found an average age of under 18 months, with every cat showing bilateral third eyelid protrusion and a drooping upper eyelid, but no other eye abnormalities: no constricted pupils, no redness, no pain.

The surprising gut connection

What makes Haw's syndrome distinctive is how often it shows up alongside diarrhea or general GI upset, despite the two seeming unrelated. In the same 2024 study, 4 of the 10 cats had diarrhea at the time of diagnosis. Cats.com notes that this gut-eye link is well documented but still not fully explained, though it's thought to involve the autonomic nervous system, the same pathway that governs both gut motility and the muscle tone that normally keeps the third eyelid retracted.

A genuinely new finding: diet as treatment

Newer research has actually turned this gut connection into a treatment approach. A 2025 case series in Veterinary Ophthalmology, from the same research group behind the 2024 study, treated five cats with Haw's syndrome using a gastrointestinal support diet formulated around gut microbiome health. All five cats showed complete resolution of their eye symptoms within 11 to 39 days, and none relapsed over close to a year of follow-up. The researchers propose that correcting an underlying imbalance in gut bacteria helps restore normal gut-brain-eye signaling, though this is a small case series, not yet a large controlled trial, so it's a promising early finding rather than a settled standard of care.

When it's something else: Horner's syndrome

Haw's syndrome is often confused with a different condition, Horner's syndrome, which can also cause third eyelid protrusion. Veterinary Partner (VIN) notes the key distinguishing features: Horner's syndrome typically affects only one eye and comes with a constricted pupil and a noticeably drooped upper eyelid on that same side, while true Haw's syndrome is bilateral with otherwise normal pupils. Third eyelid protrusion can also stem from pain, dehydration, or a mass behind the eye, which is exactly why a vet exam matters even when the pattern looks like a classic, self-resolving case.

What to actually do if you notice it

A young cat with both third eyelids showing, no other obvious eye problems, and possibly some recent loose stool fits the classic Haw's syndrome picture, and a vet visit is still the right first step to rule out the other causes above and confirm nothing more serious is going on. Most cases resolve within a few weeks with little to no treatment beyond addressing any concurrent GI upset, and the newer dietary research offers a promising option for cases that don't clear up on their own.

Frequently asked questions

Does Haw's syndrome hurt?
No, cats with true Haw's syndrome don't show signs of eye pain or discomfort, which is part of what distinguishes it from more serious eye conditions.

How long does it usually take to go away?
Classic Haw's syndrome often resolves within a few weeks on its own. In the small dietary treatment study, symptoms cleared within 11 to 39 days with intervention.

Should I be worried if only one eye is affected?
Yes, that's worth flagging specifically to a vet, since true Haw's syndrome is defined by both eyes being affected. A single affected eye points toward a different cause, like Horner's syndrome or a localized problem, that needs its own diagnosis.

Sources

• Fruchter, B. et al., "Clinicopathological findings in cats with haws syndrome," Veterinary Record, 195(10) (2024)
• Fruchter, B. et al., "Management of Haws Syndrome in Cats With Gastrointestinal Diet: A Case Series," Veterinary Ophthalmology, 29(2) (2025): e70097
• Cats.com, "Haw's Syndrome in Cats: Causes, Symptoms & Treatment"
• Veterinary Partner (VIN), "Haws Syndrome in Cats"

This article is for general information and isn't a substitute for advice from your own veterinarian, who can distinguish Haw's syndrome from other causes of third eyelid protrusion in your specific cat.