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Different pupil sizes in older cat.

Species: Cat
Breed: domestic shorthair
Age: More than 15 ye
I recently noticed that my cat's irises don't match... one is opened wide, the other isn't. I took him to the vet who checked with a penlight and said that both eyes reacted to light, but other than that, he couldn't tell me anything. Should I be concerned?


Online vet, Dr. Marie

Dr. Marie replied:

It's interesting that you have noticed this change in your cat's eyes. The medical term for what you are seeing is "anisocoria".

When a cat develops anisocoria at a senior age I usually do get concerned that there is something going on. There are a few possibilities.

One possibility is if the cat has a detached retina. This would be occurring in the eye that is dilated. Usually, this can be seen by the vet by looking inside of the eye with an instrument called an ophthalmoscope, but sometimes it is not obvious. The most common reason for a detached retina is high blood pressure. So, if this were my case I would likely be measuring Lucas' blood pressure. High blood pressure can cause a number of issues with cats and as such it really should be addressed.

Another possibility is a tumor affecting the optic nerve. (The optic nerve is the nerve that goes from the eye to the brain.) This is really hard to diagnose unless you have access to having a CT scan done. A CT scan can be expensive (usually around $1000 or so). Unfortunately, a tumor is probably one of the more common reasons for anisocoria in an older cat. The good news is that it is not likely to be painful. Many tumors can be very slow growing. If this is a tumor in this area you may start to see some neurological symptoms such as seizures.

We don't commonly see strokes in cats, but a small stroke could also cause a difference in pupil size. If this is the case, then Lucas may always be this way and it is nothing to worry about. Again, there is not much that can be done.

If this were my case I would likely check his blood pressure simply because if this were a blood pressure issue then there are things that can and should be done. But, if the blood pressure is normal then I would likely do nothing for now and keep an eye on him.

Another option for you would be for you to ask your vet for a referral to a veterinary ophthalmologist. These vets specialize in eye issues for cats and dogs and really should be able to tell you what is going on.

I hope everything is ok!

Dr. Marie.


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Disclaimer: Although Dr. Marie is a qualified veterinarian, the information found on this site is not meant to replace the advice of your own veterinarian. AskAVetQuestion.com and Dr. Marie do not accept any responsibility for any loss, damage, injury, death, or disease which may arise from reliance on information contained on this site. Do not use information found on this site for diagnosing or treating your pet. Anything you read here is for information only.

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Dr. MarieDr. Marie is a veterinarian who practices in a busy animal hospital in Ottawa, Ontario. She created Ask A Vet Question as a resource for good, accurate veterinary advice online. Dr. Marie treats dogs, cats, hamsters, guinea pigs, and rats. She has been a vet since 1999.

Is an online vet visit just as good as a trip to your veterinarian? No! But, many times, asking an online veterinarian a question can help save you money. While Dr. Marie can't officially diagnose your pet or prescribe medications, she can often advise you on whether a vet visit is necessary. You can also ask Dr. Marie for a second opinion on your pet's condition.