One eye half shut is a 2-hour problem, not a 2-day one. A cat cornea is 0.5 to 0.6 mm thick, and a defect that starts as a 2 mm scratch can melt and perforate inside 48 hours. This is also the one eye sign where home treatment does the most damage: a steroid drop on a scratch can turn a small defect into a ruptured globe overnight.
What to check in the first 2 hours
Use a phone flashlight and check 4 things: is the eye fully shut or only narrowed; is there discharge, and is it clear or yellow-green; is the third eyelid up 2 to 3 mm; are the 2 pupils the same size. Photograph both eyes in 1 frame under the same light, at about 30 cm, then repeat at the 2-hour mark. Fully shut, cloudy surface, or a 2 mm pupil difference means you stop watching and start driving.
One eye, not two, narrows the list fast
Both eyes together usually points to something systemic: herpesvirus, chlamydia, allergy, or fever. One eye usually points to a local hit: a scratch, a foreign body, a chemical splash, a lid turning in, or pressure inside that globe. The 3 groups below cover most one-sided squints, and each one needs a different first test, so do not treat them as one problem.
Group 1: corneal scratch and ulcer
The cornea has no blood vessels, so a 1 mm defect hurts more than a 1 cm cut on skin. Common causes: a claw during a fight, a branch, a grain of litter trapped under the lid, a groomer's nail, or a cat rubbing a dry eye. Signs are tearing, squinting, face rubbing, a grey or blue haze over the surface, and a pupil that stays small. An untreated scratch can pick up bacteria and begin to melt within 24 to 48 hours, and a melting ulcer may perforate less than 24 hours after that.
Group 2: lids, lashes, and trapped material
Entropion rolls the lid margin inward, so 1 to 2 mm of hair scrapes the surface with every blink. A grass awn or a litter fragment under the upper lid does the same. Chemical injury from a spray cleaner, a drain product, or plant sap burns the surface and the skin around it, so look for a wet or matted patch on the face and screen the product at /tools/toxic/ before you rinse with 200 to 500 ml of saline. Flat-faced cats with shallow sockets and a wide opening collect more of these than others.
Group 3: pressure and inflammation inside the eye
Uveitis, glaucoma, and lens luxation hurt deep inside and produce a squint with almost no discharge. Signs that separate them from a surface problem: a hazy cornea, a pupil that is fixed at mid-size or widely dilated, a red ring around the iris, a globe that looks bigger, and pain when the head is touched. Normal pressure in a cat runs about 15 to 25 mmHg, and a vet reads it in 30 seconds with a tonometer. Pressure above 30 mmHg held for 24 hours starts to cost vision. In cats, uveitis is often secondary to FeLV, FIV, toxoplasmosis, or lymphoma, so one squinting eye plus fever or weight loss needs blood work, not just drops.
The stain decides everything after it
Fluorescein is the test that sorts the 3 groups. One drop, a 1 to 2 minute wait, then a cobalt blue light: green means the surface is broken, and the defect gets measured in millimeters and drawn on a sketch. A central ulcer over the pupil, a deep or melting ulcer, and any ulcer in an eye that is painful to touch go to an ophthalmologist the same day. A negative stain does not clear the eye: herpes ulcers can hide under a dead flap of tissue, which is why a recheck stain at 24 to 48 hours is standard.
Never use human eye drops
Not the antibiotic left over from your own conjunctivitis, not last year's prednisolone bottle, not the red-eye drops that shrink vessels, not preserved artificial tears every 2 hours. Three reasons, in order of harm. Steroid on a scratch suppresses healing and feeds a melting ulcer. Preservatives in a 5 ml bottle sting a damaged surface and trigger more rubbing, which deepens the defect. Human antibiotic concentrations are built for a 70 kg body, and a 4 kg cat gets no useful dosing from them. A 2 mm ulcer treated with the wrong drop can perforate within 48 hours.
| Hours since noticed | What you see | Numeric trigger | Action |
|---|---|---|---|
| 0 to 2 | lid narrowed, surface clear, cat still eating | pupil difference 0 mm | photo both eyes, rinse 100 ml saline, no drops |
| 2 to 6 | eye fully shut, tearing, face rubbing | pupil difference 2 mm or more | vet same day, stain before any drop goes in |
| 6 to 12 | grey patch, yellow-green discharge | third eyelid up 3 mm | vet now, not in the morning |
| 12 to 24 | globe bulging, pupil fixed and wide | pressure 30 mmHg or higher | emergency, pressure check, vision at risk |
| 24 to 48 | defect not smaller on recheck | defect still 2 mm across | re-stain, refer to ophthalmology |
Four things to log at home
Which eye, plus the hour you first saw it shut. One photo of both eyes every 6 hours, same distance, same light. Discharge scored 0 to 3, with the color written down. And behavior at the bowl: head tilted while eating, food dropped from one side, or refusal to chew on that side. Those 4 lines tell a vet whether the problem sits on the surface, in the lid, or behind the pupil before the stain is even opened.
Red lines that mean same day
Eye shut for 2 hours or more with any cloudiness. Pupils that differ in size. A globe that bulges or feels hard. Green or yellow discharge. Pawing at the face or crying when the head is touched. Any eye problem in a kitten under 8 weeks, whose globe is soft and perforates fast. A squint plus fever, no appetite, or weight loss (see /symptom/cat-sunken-eyes/). A bulging eye with a fixed pupil is an emergency measured in hours.
Next steps
- Rinse 200 to 500 ml of plain saline if a chemical touched the eye, then call.
- Put a soft cone on now so a paw cannot add a second scratch.
- Skip every drop in the house, including the ones prescribed for a previous eye.
- Photograph both eyes at 0, 2, 6, and 12 hours with the same framing.
- Book the stain today if the pupil differs, the surface is cloudy, or the eye stays shut past 2 hours.
Bring the photos and the notebook. A stain, a pressure reading, and 12 hours of pictures answer in 10 minutes what a week of guessing at home cannot.
Terms mentioned in this article
Frequently asked
Can a cat's squinting eye wait until tomorrow?
Not if the eye is fully shut, the surface looks cloudy, the third eyelid is up, or the pupils differ by 2 mm. A cornea is 0.5 to 0.6 mm thick, and a defect can melt and perforate within 48 hours, so those signs mean a same-day stain rather than a night of watching.
Why can I not use my own antibiotic or steroid eye drops?
Steroid suppresses healing on a scratched cornea and can speed a melting ulcer toward perforation, while preservatives in a 5 ml bottle irritate the damaged surface and make the cat rub harder. Human concentrations are also built for a 70 kg body, so a 4 kg cat gains nothing and risks the globe.
What does the green dye test actually prove?
Fluorescein stains exposed corneal tissue, so a green spot means the surface layer is broken, and the defect can be measured in millimeters. A negative stain does not rule out disease, because herpes ulcers can sit under dead tissue, which is why a recheck at 24 to 48 hours is routine.