One of Your Cat’s Pupils Is Bigger. When Is It Urgent?

If one of your cat’s pupils suddenly looks larger than the other, contact a veterinary clinic immediately. If the usual clinic is closed, call an emergency hospital. You do not need to establish which pupil is abnormal before seeking help.

The difference itself is the useful observation

VCA’s guide to anisocoria uses that word for unequal pupil size. Either pupil can be the one that is not behaving as expected. Possible causes include eye disorders and problems affecting the nerves that control the pupil. Appearance alone cannot distinguish them.

Tell the clinic when you first noticed the difference, when the eyes last looked the same if you know, and whether one eye is red, cloudy, held shut or has discharge. Mention a fall or other injury, disorientation, changes in balance and any current or recently used eye medicine. If you do not know when it started, do not turn the first sighting into a definite onset time.

Do not try to identify the cause at home

Keep the cat in a quiet, safe area while arranging transport. Do not shine a bright light repeatedly into the eyes, bring a finger near the cornea or move furniture to test vision. Do not reuse old drops unless the veterinarian instructs you to. The same owner-observed sign can require very different care.

A photo in ordinary room light may help document what you saw if it is quick and does not delay the call. Say which side is which from the cat’s point of view. Bring prior medical records if the change has been assessed before. A previously documented stable difference and a sudden new one are different histories; tell the clinic if today’s appearance is different from the known baseline.

Let the examination answer what the photo cannot

The veterinary team can examine the front and back of the eyes as appropriate and decide whether to check eye pressure or nerve function. Ask what the clinician wants you to monitor afterward and how soon to return if the difference changes. If the cat is also squinting or rubbing the eye, our guide to a painful-looking cat eye explains why using leftover medicine before assessment can be a problem. The call for a sudden pupil change comes first.

Written by PARTNER Editorial Team for general education. Contact your veterinary team for advice about your pet’s individual health and feeding needs.