Cat Unequal Pupils: When Anisocoria Is an Eye Emergency
General guidance — not a diagnosis. When in doubt, call your vet.
Unequal pupils in a cat are called anisocoria. If the difference is new, persistent, painful, or paired with a cloudy or red eye, vision change, drooping eyelid, third-eyelid elevation, weakness, or trauma, seek veterinary care promptly. Sudden blindness, severe pain, collapse, or neurologic signs are emergencies.
When to See a Vet
Go to an emergency clinic now if:
- Anisocoria began after a head or eye injury.
- Your cat suddenly cannot see, bumps into objects, seems disoriented, or both pupils respond abnormally.
- The eye is very cloudy, enlarged, red, bulging, tightly closed, or obviously painful.
- Unequal pupils occur with collapse, seizure, circling, marked weakness, or difficulty walking.
- A chemical, medicated patch, eye drop, or toxic plant may have contacted the eye.
Contact your regular veterinarian the same day for a new persistent pupil difference even if your cat seems comfortable. Some eye diseases threaten vision before cats show dramatic pain.
For cat owners
An article can't ask about your cat.
Describe what you're seeing. You'll get a few follow-up questions about your cat, then where it lands — vet today, watch closely, or no visit needed — and what to do at home meanwhile.
First, check the lighting
Move your cat calmly into even, moderate light and look from directly in front without shining a bright beam into the eyes. Both pupils normally change with light, and an off-center angle can make them appear different. Compare a recent photo if available.
If the pupils become equal and stay equal, note what you saw and mention it to your veterinarian if it recurs. If one remains larger, take a photo in the same light and another in a dimmer room if this can be done without stress. Which pupil looks abnormal may become clearer when lighting changes, but home observation cannot localize the lesion reliably.
What can cause anisocoria?
Anisocoria is a sign produced when the iris or the nerve pathways controlling a pupil do not work normally. Painful inflammation inside the eye, called uveitis, often makes the affected pupil smaller. Feline uveitis can be subtle, painful, vision-threatening, and sometimes linked to systemic infection, immune-mediated disease, or cancer.[1] The cat uveitis guide explains accompanying haze, redness, and light sensitivity.
Glaucoma can enlarge a poorly responsive pupil and may cause cloudiness, redness, eye enlargement, behavior change, or pain. Most feline glaucoma is secondary to another eye disorder such as uveitis or intraocular neoplasia, rather than the primary inherited pattern familiar in some dogs.[2] A large pupil does not confirm glaucoma; pressure must be measured. Review the cat glaucoma signs and treat a painful or cloudy eye as urgent.
Horner syndrome makes the affected pupil smaller and can also cause a drooping upper eyelid, a sunken appearance, and elevation of the third eyelid. It results from disruption of the sympathetic nerve pathway, which travels from the brain through the spinal cord, chest, neck, and middle-ear region before reaching the eye.[3] This long path is why the problem may originate outside the eyeball.
Trauma, iris adhesions, retinal or optic-nerve disease, brain disease, congenital iris differences, age-related iris atrophy, and medication or chemical exposure can also cause unequal pupils. Some cats have a stable harmless difference, but that is a diagnosis made after important causes are excluded.
What should you observe at home?
Without touching the eye, record:
- Whether the smaller or larger pupil changes in bright and dim conditions.
- Redness, cloudiness, discharge, squinting, pawing, or a visible third eyelid.
- Whether your cat tracks a quiet moving object with each eye. Do not make sudden threatening gestures.
- Head tilt, facial asymmetry, ear discomfort, weakness, or altered walking.
- The exact onset and any fall, fight, grooming product, medication, or plant exposure.
A showing third eyelid can occur for several reasons; the cat third-eyelid guide helps document the pattern but cannot replace the eye examination.
What should you not do?
Do not use leftover eye drops. Steroid-containing drops can worsen some corneal infections or ulcers, and pressure-lowering or pupil-altering medicines are not interchangeable. Do not rinse the eye unless a veterinary professional or poison service advises it for a known exposure. Do not press on the globe, test vision by startling the cat, or wait several days to see whether a painful eye improves.
If a medication or chemical exposure is possible, bring the packaging or a clear label photo. Keep your cat indoors in a dim, uncluttered room and block stairs if vision seems impaired. Use a carrier for transport.
What will the veterinarian test?
An eye examination may include pupil and nerve responses, fluorescein stain for corneal defects, tear assessment, intraocular pressure, magnified inspection of the front of the eye, and retinal examination. If the eye is opaque, ultrasound may help assess structures behind it. Neurologic examination, ear evaluation, blood pressure, blood tests, infectious-disease testing, or imaging may be recommended depending on localization.
No single photograph can distinguish glaucoma, uveitis, Horner syndrome, retinal disease, or a neurologic problem. Prompt examination protects the eye and also helps find conditions elsewhere in the body that first appear as a pupil change.
This guide supports triage but cannot diagnose an individual pet; examination and testing may change the next step.
Frequently Asked Questions
Can cats naturally have different-sized pupils?
A stable difference can be benign or longstanding, but a new or unexplained difference should be examined before it is considered normal for that cat.
Which pupil is the abnormal one?
The larger pupil may be abnormal in bright light, while the smaller may be abnormal in darkness, but only an examination can reliably localize the problem.
Does anisocoria mean my cat had a stroke?
Not necessarily. Eye inflammation, glaucoma, nerve dysfunction, trauma, medication exposure, and other causes are possible. Neurologic signs make emergency assessment more important.
Is unequal pupil size painful?
Anisocoria itself is not the pain source, but conditions that cause it, including uveitis and glaucoma, can be painful and vision-threatening.
Can eye drops cause unequal pupils?
Yes. Some ophthalmic and other medications or chemicals can alter pupil size, including through accidental contact. Tell the clinic about every possible exposure.
Should I wait until morning?
Do not wait if there is pain, cloudiness, redness, trauma, vision loss, toxin exposure, or neurologic change. For a comfortable cat with a new persistent difference, call a veterinarian the same day for triage.
What photo is most helpful?
Take a straight-on photo of both eyes in even light without flash, then note the time, lighting, and associated signs. Do not delay care to get a perfect image.
References
- Townshend — Feline Uveitis: An Intraocular Lymphadenopathy. https://pubmed.ncbi.nlm.nih.gov/19237133/
- Grahn — Feline Glaucoma. https://pubmed.ncbi.nlm.nih.gov/36813391/
- Zwueste and Grahn — A Review of Horner's Syndrome in Small Animals. https://pubmed.ncbi.nlm.nih.gov/30651655/