Uneven pupil size:
Q: Dear Dr. Mike, My cat's pupils are not the same size; the left pupil appears to be larger than the right. I've heard this can have a number of explanations, one of them possibly being leukemia, although I have had him tested recently. Would you be able to tell me what some of the causes may be for different sized pupils on a cat? Thank You.
A: Uneven pupil size can occur for several reasons. An injury to the cornea of the eye with the smaller pupil is possible. Uveitis (inflammation of the eye) can cause the pupil in the affected eye to be smaller. Horner's syndrome is a neurologic disorder that makes the pupil of the affected eye smaller. Feline leukemia can cause pupillary spasms, also resulting in a difference in size of the pupils. Central nervous system injury can lead to difference is pupil size and in this case, the eye with the larger pupil is often the affected one.
A corneal ulcer often causes the cat to hold the eye partially shut. The normally clear cornea may be gray where the injury is, but it may remain clear. Some cats will avoid light when they have corneal ulcers. In many instances it is necessary to instill fluoroscein dye in the eye to make a corneal ulcer show up. The dye sticks to the damaged area. Anytime you suspect a corneal ulcer, the eye should be examined as quickly as possible.
Feline leukemia may affect the pupil size without any other visible disease or there may be signs of lethargy, depression, inappetance or signs of a generalized illness.
In Horner's syndrome, the third eyelid is usually at least partially visible in the affected eye and often covers 1/3rd to 1/2 of the eye. The eye may appear to be sunken into the socket. Horner's syndrome happens because there is injury to the sympathetic nerve to the eye. This can happen anywhere along the nerve's course and it courses from the brain down the neck, through the chest and back to the eye. So injury to the neck, chest or brain can lead to signs. A tumor in the chest can cause pressure on the nerve and produce signs. This syndrome also occurs in feline leukemia virus infected cats because chest tumors are more common in these cats. Horner's syndrome will sometimes appear for no apparent reason and then disappear in a few weeks. Gomez might be able to tell you what happened if he could talk -- a neck injury, bite wound or something like this are likely causes that could be easily missed.
There are probably a number of other reasons for uneven pupil size that I can't remember at the current time.
Your vet can help you distinguish between these problems. Sometimes, it is necessary to refer a patient to a veterinary ophthamologist or neurologist for a more definitive diagnosis.
Mike Richards, DVM
that is just too cute
