What Is Anisocoria?
Anisocoria means the two pupils, the black centres of the eyes, are different sizes. It is a sign rather than a disease. The pupil gets smaller or larger in response to light, pain and signals from nerves that run from the brain, down the neck and back up to the eye, so a problem anywhere along that path, or inside the eye itself, can make one pupil look different from the other.
When it is an emergency
Sudden unequal pupils together with a head injury, wobbliness, a head tilt, seizures, a painful red or cloudy eye, or a pet who seems blind need a vet right away.
Common Causes
- Eye pain and inflammation: a scratch or ulcer on the cornea and uveitis usually make the affected pupil smaller; see also corneal ulcers
- Glaucoma: high pressure inside the eye usually leaves the pupil large and fixed, with a red or bluish eye; see glaucoma
- Nerve problems: Horner syndrome causes a small pupil with a droopy upper eyelid, raised third eyelid and sunken eye
- Brain disease or injury: head trauma, tumors, inflammation and strokes; see brain tumor symptoms in dogs and meningoencephalitis
- Other eye conditions: lens luxation, bleeding inside the eye, retinal disease and tumors
- Iris atrophy: age-related thinning of the iris in older pets, which can leave a pupil slightly larger and slower to react but is usually harmless
- Medication: eye drops such as atropine used during treatment deliberately dilate one pupil
- In cats, feline leukemia virus (FeLV) can cause a pupil abnormality sometimes called spastic pupil syndrome
What to Notice Before the Vet Visit
- Which eye looks different and whether the eye is red, cloudy, squinting or watering
- Whether the upper eyelid droops or the third eyelid covers part of the eye
- Any head tilt, circling, stumbling, behavior change or recent fall or knock
- When you first noticed it, and a photo in good light to show your vet
Diagnosis and Treatment
The vet will check both eyes in light and dark, measure eye pressure, stain the cornea for ulcers and do a neurological exam. Depending on findings, the next steps may be blood tests, blood pressure measurement, FeLV testing in cats, or a referral for an eye specialist or MRI. Our guide to veterinary specialists vs regular vets explains how referrals work. Treatment depends entirely on the cause, from eye drops for an ulcer or uveitis to urgent pressure-lowering drugs for glaucoma.
Frequently Asked Questions
Is anisocoria always serious?
No. Iris atrophy in an older pet or a pupil dilated by prescribed drops can be harmless. But because glaucoma and brain disease can look the same at first glance, any new difference in pupil size should be checked.
Can a pet be born with unequal pupils?
Yes, rarely. A congenital difference that never changes and causes no other signs is usually not a concern, but have it recorded at a wellness visit so new changes can be spotted.
Should I put eye drops in at home?
No. Some drops can worsen glaucoma or an ulcer. Use only what your vet prescribes after examining the eye.
If you notice your pet’s pupils are suddenly different sizes, do not wait to see if it passes. Find a veterinary clinic near you for an exam.