A cat holding one eye half closed is almost always in pain or discomfort. The squinting itself is a protective reflex called blepharospasm, where the eyelid muscles clamp down in response to irritation, injury, or inflammation somewhere in or around the eye. It can look subtle, like a lazy blink that never fully opens, or more obvious, with visible tearing and the cat pawing at its face. Either way, it signals something that needs attention.
The Most Common Causes
Several conditions can make a cat squint or hold an eye partially shut. Some are minor and resolve quickly; others can threaten vision if left untreated. The main culprits fall into a few categories.
Corneal injury or ulcer. A scratch on the surface of the eye is one of the most frequent reasons for sudden squinting. Cats get corneal scratches from rough play, a swipe from another cat, a twig, or even dust. Signs include squinting, watery or mucus-like discharge, cloudiness over the eye, and sensitivity to light. Your cat may rub the affected eye against furniture or with a paw. According to Cornell University’s College of Veterinary Medicine, vets confirm a corneal ulcer using a special dye (fluorescein) that sticks to damaged tissue and glows green under blue light, making even tiny scratches visible.
Conjunctivitis. Inflammation of the pink tissue lining the eyelid is extremely common in cats. The eye looks red, swollen, and watery. Bacterial or viral infections are typical triggers, and feline herpesvirus (FHV-1) is a particularly frequent offender. Cats that carry herpesvirus can have recurring flare-ups of eye inflammation throughout their lives, especially during periods of stress.
Foreign body. A piece of litter, a grass seed, or a tiny hair trapped under the eyelid will cause immediate squinting and tearing. Cats often paw at the eye repeatedly when something is stuck.
Allergies and irritants. Insect bites, food sensitivities, inhaled allergens, and environmental irritants like tobacco smoke can all inflame the eyelids. This type of inflammation (blepharitis) tends to cause puffiness and redness along the eyelid margins along with the squinting.
Deeper Eye Problems
If the squinting has come on gradually or your cat’s eye looks cloudy, discolored, or has changed size, the problem may be inside the eye rather than on its surface.
Uveitis. This is inflammation of the inner structures of the eye. The eye often looks cloudy or hazy because fluid, protein, or cells are leaking into the front chamber. Sometimes you can see blood or pus pooling behind the cornea. Uveitis can result from infections (including feline leukemia virus, FIV, or toxoplasmosis), trauma, or immune system disorders. It looks similar to glaucoma, but the key difference is that uveitis lowers the pressure inside the eye while glaucoma raises it. Your vet will measure eye pressure to tell them apart.
Glaucoma. Increased pressure inside the eye is painful and causes squinting, a visibly enlarged or bulging eye, and a dilated pupil that doesn’t respond normally to light. This is urgent because sustained high pressure damages the optic nerve and can cause permanent blindness.
When the Third Eyelid Is Showing
Cats have a translucent inner eyelid (the nictitating membrane or “third eyelid”) tucked in the inner corner of each eye. You don’t normally see much of it. When it slides partway across the eye, it can make the eye look half closed or give your cat a glazed, sleepy appearance.
A visible third eyelid in one eye often points to pain or irritation on that side, since the eye retracts slightly into the socket when it hurts, allowing the membrane to slide forward. If both third eyelids are protruding, a condition called Haws syndrome may be the cause. Haws syndrome involves a disruption of the nerve signals that control the smooth muscle fibers in the third eyelid. It sometimes accompanies diarrhea or gastrointestinal illness, but it can also appear in otherwise healthy cats and typically resolves on its own.
Horner’s Syndrome
If your cat’s half-closed eye is paired with a noticeably smaller pupil on that same side, plus a visible third eyelid, the combination points to Horner’s syndrome. This happens when the nerve pathway running from the brain to the eye is disrupted somewhere along its route, which passes through the neck and chest. The classic cluster of signs includes a drooping upper eyelid (ptosis), a constricted pupil, a sunken-looking eye, and third eyelid protrusion, all on the same side.
Horner’s syndrome itself isn’t an emergency, but it’s a signal that something is affecting that nerve pathway. Causes range from ear infections (the nerve runs close to the middle ear) to neck injuries, tumors, or sometimes nothing identifiable at all. Your vet will work backward from the eye to figure out where the disruption is happening.
Breed-Related Causes
Some cats are structurally predisposed to chronic squinting. Flat-faced (brachycephalic) breeds like Persians and Himalayans are prone to entropion, a condition where the eyelid edge rolls inward so that fur and eyelashes scrape against the cornea with every blink. Intact male Maine Coons can also develop entropion because of their pronounced facial jowls.
Entropion causes persistent irritation, tearing, and squinting that worsens over time. Left uncorrected, the constant friction leads to chronic inflammation, corneal ulcers, and potentially corneal sequestra (dead tissue on the corneal surface that appears as a dark brown or black spot). The good news is that surgical correction has a high success rate and permanently resolves the problem.
What to Watch For
Certain signs alongside the squinting suggest the situation is more serious:
- Yellow or green discharge indicates bacterial infection. Infected corneal ulcers produce mucopurulent (thick, yellowish) discharge and white or yellow discoloration within the cornea itself.
- A visibly cloudy or color-changed eye suggests uveitis, a deep ulcer, or glaucoma.
- A bulging or enlarged eye points to glaucoma or a mass behind the eye.
- Unequal pupil sizes can indicate Horner’s syndrome, uveitis, or nerve damage.
- Blood visible inside the eye signals significant trauma or severe internal inflammation.
- No improvement after 24 hours for mild, watery squinting. Minor irritation from dust or a brief encounter with an allergen often clears up within a day. If it doesn’t, the cause is likely something that won’t resolve without treatment.
What Happens at the Vet
A vet visit for a squinting eye is straightforward and usually quick. The exam starts with a close look at the eyelids, the surface of the eye, and the structures inside it using a bright light and magnification. The fluorescein dye test is standard: a small strip touched to the eye deposits an orange dye that turns bright green wherever the corneal surface is damaged. This painless test catches scratches and ulcers that are invisible to the naked eye.
If uveitis or glaucoma is suspected, your vet will measure the pressure inside the eye with a small instrument tapped gently against the corneal surface. Low pressure suggests uveitis; high pressure suggests glaucoma. For deeper issues, blood tests may be recommended to screen for underlying infections or immune conditions driving the inflammation.
Treatment depends entirely on the cause. A simple corneal scratch typically heals with antibiotic eye drops to prevent infection and sometimes a pain-relieving drop. Conjunctivitis from herpesvirus may require antiviral medication. Uveitis needs anti-inflammatory treatment to prevent lasting damage inside the eye. Entropion requires a minor surgical procedure to reposition the eyelid. In most cases, prompt treatment leads to a full recovery, but delays, especially with ulcers or glaucoma, can result in permanent vision loss.