Why Does My Cat Have Different Sized Pupils?

By: Chewy EditorialUpdated:

Why Does My Cat Have Different Sized Pupils?


When I came home yesterday, my 13-year-old cat had one eye that was dilated and one that was normal. What could be wrong with my cat’s eyes? My cat isn’t acting weird. She eats, drinks and sleeps normally. She was licking a lot last night, but not this morning.


The term for different sized pupils is anisocoria, and this sounds like what you describe. Many things could cause anisocoria in your cat.

  • The most common cause of different-sized pupils in cats is anterior uveitis, a type of inflammation within the eye. The affected eye is the one with the smaller pupil.
  • Glaucoma is another common cause for differing pupil size in cats. Glaucoma is a condition in which increased pressure exists inside the eye. The affected eye has a larger pupil and will often bulge from the eye socket.
  • Horner’s syndrome is a condition where the innervation to the pupil is disrupted, causing the pupil in the affected eye to become smaller. The third eyelid tends to elevate a little in the affected eye.

Although your cat seems unfazed by the condition, anisocoria can be a sign of a significant underlying disease; a vet, or a board-certified veterinary ophthalmologist, should examine your cat promptly. Treatment depends on the underlying cause. Your vet may want to measure the pressure inside the eye using an instrument called a tonometer, to see if glaucoma is the cause. If uveitis is present, your vet might want to run some tests for infectious diseases; uveitis can sometimes have an infectious cause. Tumors can sometimes cause differing pupil size, and a 13-year-old cat is at increased risk of tumors compared to a younger cat. A CT-scan or a MRI may be warranted to rule out any lesions or growth in the brain that could be causing a cat’s differing pupil sizes.

Posted by: Chewy Editorial



By: Chewy EditorialUpdated: