Acute glaucoma is the one feline eye emergency where the clock, not the drug, decides the outcome. Normal intraocular pressure (IOP) in a cat runs 10 to 20 mmHg. Above roughly 30 mmHg the optic nerve head begins to lose axons, and at 50 to 60 mmHg sight can be gone in 12 to 24 hours. A cat that tracked a toy at breakfast can be blind by midnight with an eye that still hurts, which is why the visit belongs in the next 2 hours rather than on Monday.
The 24-hour line is about blood flow
Pressure damages the nerve only by squeezing its blood supply. Perfusion pressure is mean arterial pressure minus IOP, so an eye at 45 mmHg has lost most of the gradient that keeps retinal ganglion cells alive. Autoregulation holds flow steady to about 35 mmHg; past that, every 10 mmHg of extra rise cuts perfusion further. In referral practice, 40 to 50 mmHg held for 8 to 12 hours leaves permanent field loss, and 60 mmHg can do it in 4 to 6 hours. Once 24 to 48 hours pass uncontrolled, normalizing the IOP rarely brings vision back. It still removes pain, which is the second reason to go tonight.
The line is short because the cat cannot report it. There is no limping and usually no vomiting. Owners notice a wide pupil, a cloudy eye, or a cat that misses the food bowl by 3 to 4 cm. By the time the cornea looks blue-grey, the nerve has often been compressed for 12 to 24 hours.
Pressure bands and what each one means
Tonometry takes under 60 seconds per eye and is the only way to place a cat in one of these bands. Ask for both eyes at every visit.
| IOP (mmHg) | Usual reading | Vision at risk within | Act within | Recheck |
|---|---|---|---|---|
| 10 to 20 | Normal for a cat | No immediate risk | Routine exam in 12 months | 12 months |
| 21 to 25 | Borderline, early change | 8 to 12 weeks | 24 to 72 hours | 3 to 7 days |
| 26 to 35 | Suspect glaucoma | 2 to 7 days | 6 to 12 hours | 24 hours |
| 36 to 50 | Acute glaucoma | 12 to 24 hours | 2 to 6 hours | 1 to 2 hours |
| 51 to 70 | Severe attack, marked pain | 4 to 12 hours | Under 2 hours | Every 60 minutes |
| 71 and above | Globe at risk of rupture | 1 to 4 hours | Now, no delay | Continuous |
A 5 mmHg gap between the eyes matters as much as the absolute number. Cats often arrive with one eye at 18 mmHg and the other at 38 mmHg; the high eye is the emergency, the low eye is the warning.
Four signs you can see without a tonometer
No single sign confirms glaucoma, but 2 or more together justify an emergency call:
- The cornea turns hazy or blue-grey, like fog on glass, and blinking does not clear it.
- The pupil is wide and fixed. Shine a phone light in for 3 seconds; the affected pupil barely narrows while the other shrinks at once.
- The white of the eye looks bruised, with 3 to 5 engorged vessels, and the globe feels firmer than the other one through the lid. Do not press.
- The cat paws at its face, presses its head into a wall, squints, hides, or stops jumping onto a 60 cm surface it used 2 days ago.
Rule out uveitis first: there the pupil is small and the cornea is dull too, but pressure is low, often under 10 mmHg. A wide, unreactive pupil with a hazy cornea points the other way, and the two conditions need opposite treatment, so the eye must be measured rather than guessed at.
The second eye keeps its own 48 hours
About 1 in 2 cats that lose an eye to glaucoma develop it in the fellow eye within 1 to 2 years, and some within 3 months. That is a reason for a plan, not for despair. Every recheck should include tonometry in both eyes and a written IOP for the healthy one. When the good eye sits at 22 to 25 mmHg with a narrow drainage angle, many ophthalmologists start preventive treatment before it ever spikes. Recheck at 3 months, then every 6 to 12 months. If it does spike, the grace period is the same 24 to 48 hours, not more.
Hour by hour in the clinic
0 to 2 hours: tonometry in both eyes, fluorescein stain, slit-lamp exam, blood pressure and a pupil light reflex. 2 to 4 hours: pressure-lowering treatment starts, an agent by vein plus drops and sometimes a tablet, aiming under 20 mmHg within 1 to 2 hours. 4 to 12 hours: repeat IOP every 1 to 2 hours until it holds under 25 mmHg. 12 to 24 hours: judge vision with a dazzle reflex, menace response and pupil reflex, adding ultrasound if the cornea is too cloudy. 24 to 72 hours: choose the long game, whether laser, a drainage implant, or removal of an eye that is blind and still painful at 40 mmHg.
A blind eye is not a finished eye. With pressure controlled and no pain, some cats live with one for years. If the globe stays hard, the pain is constant and the cat eats less and sleeps more, and removal is kinder than keeping a 50 mmHg globe for appearance.
Costs and what surgery buys
An emergency exam with tonometry runs about 150 to 350 USD, and 24 to 48 hours of hospital care adds 600 to 1,500 USD. Referral surgery is the expensive part: laser treatment 1,800 to 3,500 USD, a drainage implant 2,500 to 4,500 USD, removal of the globe 800 to 1,800 USD. Ask for the number before consenting; typical figures sit in the cat cost guide. Surgery buys time rather than certainty: many saved eyes hold useful vision for 12 to 24 months and still need drops 2 to 3 times a day.
What to do in the next 60 minutes
- Cover the eye loosely with a clean damp cloth if the cat is pawing at it, and fit a cone if you have one.
- Call the clinic and say acute glaucoma suspect, one eye, pressure unknown. That phrase moves you ahead of a vaccine slot.
- Do not use leftover drops from another cat or a human bottle; the wrong drop worsens a corneal ulcer.
- Carry the cat in a closed carrier, keep the car dark, and hold the drive under 30 minutes if you can.
- Ask for both eyes to be measured, and for the two numbers written down before you leave.
Even a blind, hard eye is urgent, because pain control and the fate of the second eye both hinge on this visit. Two conditions that mimic parts of the picture are vestibular disease, with a head tilt and wide pupils but normal pressure, and urinary blockage, which also produces a cat that hides and stops eating within 24 hours.
Frequently asked
Can a cat recover vision after an acute glaucoma attack?
Sometimes, if pressure is brought under 20 mmHg within a few hours and the eye still responds to light. After 24 to 48 hours at 40 mmHg or above, the optic nerve damage is usually permanent, and treatment then aims at comfort and at protecting the second eye rather than at restoring sight.
How do I tell glaucoma from uveitis at home?
Look at the pupil. Glaucoma leaves it wide and slow to react, with a hazy cornea and a firm globe. Uveitis usually leaves it small and constricted, with low pressure rather than high. Both are painful and both need same-day assessment, but the treatments pull in opposite directions, so a tonometer reading decides.
Is one high reading enough to diagnose glaucoma?
One reading above 30 mmHg in a painful eye is treated as glaucoma until proven otherwise, but a single number can be falsely high if the cat struggles. Clinicians repeat the measurement, compare both eyes, and check the pupil light reflex and cornea before committing to a diagnosis.