A:
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.
