Different pupil sizes in older cat.

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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