Cats have unequal pupil sizes: When pupil sizes are unequal

Most cat owners love the striking eye color that their own feline friends have. Bright greens, icy blues and golden amber are just one thing that sets cats apart from our canine companions. So if your cat has a pupil of a very different size in one eye than the other, you’ll be able to tell right away. Most owners will know right away that something is wrong, but what could possibly happen? What would cause one pupil to be larger than the other?

What is Aniscoria?

Anisocoria is a medical term for two pupils of different sizes, although it is full mouth. Anisocoria is not a disease itself, but a symptom of an underlying disease. Your cat’s pupil size can have a number of causes.

  • Corneal ulcer/injury
  • A brain or nervous system disorder that affects specific nerves in the cat’s eye (i.e. Horner’s syndrome)
  • Glaucoma (increased and dilated eye pressure in the affected eye)
  • Spastic pupil syndrome (possibly a symptom of feline leukemia virus
  • Degenerative changes in iris tissue with age
  • head trauma
  • exposure to chemicals or toxins

Depending on the underlying cause, you may not see any other symptoms, or you may see different accompanying eye symptoms. Cats with anisotropy may also have reddening of the sclera, clouding or bluishness of the cornea (the outermost layer of the eye), eye discharge, drooping eyelids, squinting, or rubbing/scratching at the affected eye. Your cat may also be less active than usual.

Whatever the cause, if your cat has a sudden onset of heteromydriasis, it is an emergency that requires immediate veterinary care. Failure to get your cat care right away can permanently damage your cat’s vision in the affected eye.

Diagnosing Causes of Feline Heteromydriasis

Most eye exams will start with three basic tests: Schirmer tear test, fluorescein staining, and intraocular pressure test. The Schirmer tear test will check the cat’s eye’s ability to produce tears. Certain disease processes inhibit tear production. Your cat’s eyes will actually try to compensate, but they may produce a sticky discharge instead of tears. Having said all that, you might think your cat just has funky tears when in fact he doesn’t! Staining the eyes with fluorescein dye can illuminate any ulcers on the cat’s cornea. The stain will collect in the ulcer itself, so even if the excess stain is flushed out of the eye, the stain will still fluoresce under the black light. Checking the cat’s intraocular pressure can check for glaucoma and uveitis. If your cat has glaucoma in one or both eyes, their pressure will be higher than normal. If your cat has uveitis in one or both eyes, their pressure will be lower than normal. Rest assured, veterinarians don’t use the dreaded “puff” test commonly found in human ophthalmology to check eye pressure.

If a standard three-eye test doesn’t reveal anything unusual, your veterinarian may wish to perform a more specialized test. A lightly scraped sample of the cat’s conjunctiva can be sent to an outside laboratory for histopathology, where a veterinary specialist will view the sample under a microscope. From this they can determine if there are any abnormal cells that could indicate benign (or malignant) growth. Your veterinarian may need to perform blood tests to rule out any systemic disease. They may also want your cat to get a skull X-ray, an MRI, or even have it seen by a veterinary ophthalmologist.

Treating Heterosis in Cats

How best to treat a cat’s heterodopia depends entirely on what’s causing the cat’s heterodopia. The specific treatment will be based on the specific disease. If your cat’s pupil size gap is caused by chemical or toxin exposure, removing the substance may reverse the pupil size gap. Some causes, such as Horner’s syndrome, are self-limiting, and an uneven pupil size may resolve on its own. For other causes, such as degenerative diseases, your cat’s heterodopia may never get better. Some causes of unequal pupil size may also require long-term medication. If your cat’s vision is affected due to unequal pupils, their vision may never return to normal.

Whatever the cause of your cat’s uneven corners of the eye, seeking immediate veterinary attention is vital to your cat’s vision and eye health. Whether you’re visiting an emergency veterinarian or a regular veterinarian, they can help you figure out what’s causing your cat’s disturbing eye symptoms and how best to fix the problem.


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