A cat that squints 1 eye, tears and avoids light has a corneal ulcer until proven otherwise, so 2 numbers rule the next 24 hours: a stain within 2 hours of the first squint, and a cone on within 1 hour, before any drop. The cornea is 0.5-0.6 mm thick in a cat, so a defect that looks like a 2 mm smudge can be 50% of the depth, and a 3 mm ulcer can perforate in 24-48 hours. Never start a steroid drop on a red, squinting eye: 1 dose of a 0.1% dexamethasone product can turn a 2 mm superficial ulcer into a melting one in 24 hours.
What a cat with an ulcer looks like
Four signs, in the order owners usually report them: squinting or a half-closed eye within 1-2 hours, tearing that wets the fur 2-3 cm below the eye, a white or gray haze over the pupil that was not there yesterday, and pawing at the face 5-10 times an hour. Cats hide eye pain well, so a cat that keeps 1 eye shut for 4 hours is already at the top of the pain scale it will show. A 3 mm pupil that will not dilate, or a cornea that looks blue rather than white, means the deeper layers are involved.
The 2-minute stain, and what the colors mean
Fluorescein is a dye that sticks only to exposed stroma, the layer under the outer epithelium. The vet applies 1 drop, waits 60-90 seconds, and looks under a cobalt blue light: green means an epithelial defect, and the size and shape are recorded in millimeters. A healthy cornea takes up no dye at all, so a 2 mm green dot is a positive result even if the eye looks normal in room light. The stain is safe, costs little, and takes less time than a nail trim.
Two things the stain cannot tell you: depth, and whether an infection is present. Depth needs a slit beam or a 3 mm probe, and infection needs a cytology swab, which takes 10-15 minutes at a lab that does it in-house. That is why a 2 mm ulcer that is not healing at 72 hours gets a second stain and a swab, not a stronger drop.
Four grades, four timelines
| Grade | Depth | Stain result | Recheck |
|---|---|---|---|
| 1 | 0.1-0.2 mm, epithelium only | Bright green, sharp edge | 3-5 days |
| 2 | 0.2-0.3 mm, into stroma | Green with a hazy rim | 48 hours |
| 3 | 0.3-0.5 mm, deep stroma | Green plus a dark center | 24 hours |
| 4 | 0.5 mm or more, descemetocele | Green with a clear bulla | Surgery in 24-48 hours |
Grades 1 and 2 usually close in 3-7 days with a cone, lubricant every 4-6 hours and a prescription antibiotic drop. Grade 3 needs a recheck at 24 hours because a melting ulcer can lose 0.1 mm of cornea per day. Grade 4 is a surgical case: a conjunctival graft or a 5-0 suture bridge, with a 7-14 day cone and a 10-20% risk of a scar that clouds part of the eye.
Why human eye drops are the main danger
Steroids shut down the local immune response and block the epithelium from sliding across the defect. A 5 mL bottle of 0.1% dexamethasone, sold for human allergies, is 10-100 times the strength a cat's cornea needs and is the wrong drug for any ulcer. In a cat, 1 dose can soften the stroma enough that the eye perforates within 24-48 hours, and a melting ulcer can go from 2 mm to 6 mm in 1 day. Antibiotic drops without a steroid are a different matter, but even those are prescription-only: the choice depends on whether the infection is bacterial, and a wrong drop for 48 hours lets a 1 mm defect become a 4 mm one.
The cone: 1 hour, not 1 day
An Elizabethan collar goes on before the first drop and stays on until the stain is negative, usually 5-10 days and often 14. The right size is 2-3 cm past the nose tip, snug enough that 2 fingers fit under the neck strap but not 3. A cat that can reach the eye with 1 paw will rub off the epithelium that took 48 hours to grow back, and a 2 mm defect re-opens in 1-2 hours of rubbing. Soft cones suit a calm cat; a determined cat needs a rigid one, and 1 in 5 needs a 10 cm extension.
Sleep and eating are fine with a cone on. Watch 3 things in the first 48 hours: the strap scraping the neck (check every 12 hours), bowls too deep to reach (switch to a 3 cm plate), and no grooming for 3 days, which is pain, not the cone.
The 24-hour rule
Any ulcer bigger, cloudier or more painful at 24 hours needs a visit that day, not at the weekend. The specific triggers are a defect that grows 1 mm or more in 24 hours, a cornea that turns from clear to gray, a pupil that shrinks from 5 mm to 2 mm, or a cat that stops eating for 24 hours. Deep ulcers hurt more, not less, so a cat that suddenly seems comfortable on day 3 can be a cat whose cornea has already perforated, and a perforation shows as a 1-2 mm dark spot with a flat anterior chamber.
Systemic pain relief is prescription-only and dose-dependent on weight and kidney values, so ask the clinic rather than splitting a human tablet. A cat with a 3 mm ulcer that is not controlled at 48 hours often needs a 5-7 day course of oral medication plus a recheck every 48 hours until the stain is negative.
What else can look like an ulcer
Three conditions mimic it and change the plan. A sequestrum, a 2-4 mm black spot on the cornea, is common in Persians and needs surgery rather than drops. Herpes keratitis causes branching 1-2 mm lesions that recur every 6-12 months and are treated with antiviral drops, not antibiotics. A 1-2 mm foreign body under the third eyelid, such as a grass awn, keeps re-scratching the cornea every time the cat blinks, 15-20 times a minute.
Two pages to keep open: the plant list, because a 5 mm fern or a fig leaf can cause a chemical burn that stains positive within 24 hours (houseplant emergencies), and the seizure page if the squinting came with 1-2 minutes of twitching or a fixed stare rather than pain (seizure clusters). Any product the cat walked through before the eye teared up is worth 1 minute on the toxin list (toxin lookup).
Five steps for the first 24 hours
- Put the cone on before anything else, sized 2-3 cm past the nose.
- Do not use any drop from the human medicine shelf, steroid or not.
- Get a fluorescein stain the same day; 2 minutes tells you more than 2 days of watching.
- Book the recheck at 24, 48 or 72 hours according to the grade, not by feel.
- Photograph the eye daily at 20 cm in the same light so a 1 mm change is visible.
Terms mentioned in this article
Frequently asked
How urgent is a squinting eye in a cat?
Same-day urgent. A fluorescein stain takes 2 minutes and shows a defect that room light hides, and a cat's cornea is only 0.5-0.6 mm thick, so a 3 mm deep ulcer can perforate in 24-48 hours. Put a cone on in the first hour, skip every drop from the human shelf, and book the stain today rather than watching for 48 hours.
Can I use my own steroid eye drops on my cat?
No. A 0.1% dexamethasone bottle sold for human allergies is 10-100 times the strength a cat's cornea needs, and steroids block the epithelium from healing. One dose can turn a 2 mm superficial ulcer into a melting one and perforate the eye within 24-48 hours. Antibiotic drops are prescription-only too, because the choice depends on whether the infection is bacterial.
How long does a cat have to wear the cone?
Until the stain is negative, usually 5-10 days and often 14 after a deep ulcer or surgery. The collar should extend 2-3 cm past the nose tip and fit so 2 fingers pass under the strap but not 3. A cat that reaches the eye with one paw can rub off epithelium that took 48 hours to regrow, and a 2 mm defect reopens in 1-2 hours.