When one of your cat’s pupils is noticeably larger than the other, the condition is called anisocoria, and it signals that something is affecting either the enlarged eye or the smaller one. Both pupils normally change size together in response to light, so a mismatch means one eye’s pupil isn’t responding correctly. This can range from a minor issue to a sight-threatening emergency, and if it appeared suddenly, it warrants a same-day veterinary visit.
Which Eye Is Actually the Problem?
This is the first thing to sort out, and it’s not always intuitive. The dilated eye might be the abnormal one, stuck open because of glaucoma or nerve damage. But the smaller eye could be the problem instead, constricted by inflammation or a different type of nerve disruption. Your vet determines this during the exam, but understanding the distinction helps make sense of the possible causes below.
Glaucoma: Pressure Building Inside the Eye
Glaucoma is one of the more urgent reasons a cat’s pupil stays dilated. Fluid inside the eye doesn’t drain properly, and pressure climbs. Normal eye pressure in cats runs about 12 to 19 mmHg. When pressure reaches 25 mmHg or higher, or when there’s a difference of 12 mmHg or more between the two eyes, glaucoma is strongly suspected.
A cat with glaucoma often has a visibly enlarged, bulging eye. The eye may look cloudy or bluish, and your cat might squint, paw at the affected side, or become less active. High pressure damages the optic nerve and retina, so without treatment, permanent blindness in that eye can follow quickly. This is the main reason sudden anisocoria is treated as an emergency.
Uveitis: Inflammation Inside the Eye
Uveitis is inflammation of the structures inside the eye, including the iris (the colored part that controls pupil size). Unlike glaucoma, uveitis typically makes the affected pupil smaller, not larger. That means the other eye, the one that looks dilated, may actually be the normal one.
What makes uveitis particularly important in cats is the list of infections that can trigger it. Feline leukemia virus, feline immunodeficiency virus, feline infectious peritonitis, toxoplasmosis, and bartonella are all common culprits. Fungal infections and even cancer inside the eye can also cause it. Your cat may squint, tear up excessively, and the eye may appear red or cloudy. Because uveitis lowers pressure inside the eye (the opposite of glaucoma), your vet can often distinguish the two conditions with a quick pressure reading.
Corneal Ulcers and Eye Injuries
A scratch to the surface of the eye, whether from a scuffle with another cat, a branch, or even a stray piece of litter, can cause a corneal ulcer. The damaged eye becomes painful, inflamed, and light-sensitive. You’ll typically notice your cat squinting hard on that side, with watery or mucus-like discharge. The cornea may look hazy.
Pain from a corneal ulcer causes the pupil on the injured side to constrict, so again, the dilated eye is usually the healthy one reacting normally while the injured eye clamps down. Your vet diagnoses this by applying a fluorescein stain, a harmless orange dye that glows green under blue light wherever the corneal surface is damaged.
Horner’s Syndrome: A Nerve Pathway Problem
Horner’s syndrome happens when the nerve pathway that controls pupil dilation on one side is disrupted. Because the dilating nerve stops working, the affected pupil stays small. The result is a cluster of signs all on one side of the face: a constricted pupil, a droopy upper eyelid, a sunken-looking eye, and the third eyelid (a translucent membrane cats have at the inner corner of each eye) creeping up and becoming visible.
The nerve pathway involved runs from the brain, down through the chest, and back up the neck to the eye. A problem anywhere along that route can trigger Horner’s syndrome. In cats, causes include ear infections, chest injuries, neck trauma, and occasionally tumors. In many cases, no underlying cause is ever found, and the condition resolves on its own over several weeks. It’s not painful, but the underlying cause sometimes needs treatment.
Other Possible Causes
Several less common conditions can produce uneven pupils in cats:
- Retinal detachment or degeneration. When the retina in one eye stops functioning, the pupil on that side may stay dilated because the eye no longer sends light signals to the brain properly. High blood pressure is a leading cause of retinal detachment in older cats.
- Iris atrophy. In senior cats, the iris muscle can thin and weaken with age, leaving the pupil stuck partially open. This is usually gradual and painless.
- Brain lesions or tumors. Anything affecting the brain’s control over one eye, whether a tumor, stroke, or infection, can cause one pupil to dilate or constrict abnormally. These cats often show other neurological signs like wobbliness, head tilting, or behavior changes.
What to Look for at Home
Before you get to the vet, note as many details as you can. These observations help narrow the diagnosis quickly:
- Which eye looks different, and how? Is one pupil large and round while the other is a narrow slit? Or is one eye squinting shut?
- Light response. In a dim room, shine a flashlight briefly toward each eye one at a time. A healthy pupil will shrink in response to light. If the dilated pupil doesn’t change at all when light hits it, that eye is more likely the problem.
- Redness or cloudiness. A red or cloudy eye points toward inflammation, injury, or pressure changes.
- Third eyelid visibility. If you can see a pinkish membrane creeping across the inner corner of one eye, that’s the third eyelid protruding, a hallmark of Horner’s syndrome.
- Behavior changes. Squinting, pawing at the face, hiding, bumping into furniture on one side, or reduced appetite all help your vet assess severity.
You can also try the cotton ball test: drop a cotton ball in front of your cat from slightly to each side and see if they track it with both eyes. A cat that consistently misses the ball on one side may have lost vision in that eye.
What Happens at the Vet
A veterinary eye exam follows a specific sequence. Your vet will check the pupillary light reflex in both eyes, then test the menace response (moving a hand toward each eye to see if your cat blinks). They’ll examine the surface and interior of the eye using a bright light and magnification.
Two tests are especially important. Tonometry measures the pressure inside each eye using a small handheld device gently tapped against the cornea. This reading tells your vet whether the eye pressure is too high (suggesting glaucoma) or too low (suggesting uveitis). Fluorescein staining checks for corneal ulcers by applying a dye that highlights any damage to the eye’s surface.
Depending on what these initial tests reveal, your vet may recommend blood work to check for infections like feline leukemia or toxoplasmosis, blood pressure measurement to rule out hypertension, or imaging if a neurological cause is suspected. Many causes of anisocoria are treatable, especially when caught early, so a prompt exam gives your cat the best chance of keeping their vision.