Anisocoria in Cats: When Pupil Sizes Are Unequal

Close-up of a lazy cat
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Most cat owners love the striking eye colors their own feline friends possess. Brilliant greens, icy blues, and golden ambers are just one thing that makes cats stand apart from our canine companions. So if your cat has a one eye with a pupil that's a drastically different size than the other eye you will be able to tell right away. Most owners will know right away that something is not right, but what could possibly going on? What can cause one pupil to be larger than the other?

What Is Aniscoria?

Anisocoria, while a mouthful, is the medical term for when the pupils are two different sizes. Anisocoria, in and of itself, is not a disease but rather a symptom of an underlying condition. There are a variety of reasons your cat may have pupils that have different sizes. 

  • Corneal ulcer/injury
  • A brain or neurological disorder affecting the specific nerves running to your cat's eye (i.e. Horner's Syndrome)
  • Glaucoma (the affected eye will have an increased pressure within the eye and will be dilated)
  • Spastic pupil syndrome (can be a symptom of Feline Leukemia Virus
  • Degenerative changed to the iris tissue that can happen with aging
  • 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 ocular symptoms. cats with anisocoria may also have reddened scleras, a clouding or blue-tinted cornea (the outermost layer of the eye), eye discharge, a droopy eyelid, a squinting eye, or rubbing/pawing at the affected eye. Your cat also may be less active than normal.

Regardless of the cause, if your cat's anisocoria has sudden onset 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 the Reason for Your Cat's Anisocoria

Most eye workups will start with three basic tests: A Schirmer tear test, fluorescein stain, and an intraocular pressure test. A Schirmer tear test will check the ability of your cat's eyes to create tears. There are certain disease processes that will inhibit tear production. Your cat's eyes will actually try to compensate but instead of tears they may produce a thick, sticky discharge. All this to say, you may think your cat just has funky tears when in reality he has a lack of them! Staining the eye with fluorescein dye can illuminate any ulcers on your cat's cornea. The stain will collect in the ulcer itself so that, even when excess stain is flushed out of the eye, the stain will still fluoresce under a black light. Checking your cat's intraocular pressures can check for glaucoma as well as uveitis. If your cat has glaucoma in one or both eyes, they will have higher than normal pressures. If your cat has uveitis in one or both eyes, they will have lower than normal pressures. Rest assured, veterinary medicine doesn't check eye pressures with the dreaded 'air puff' test that is common in human ophthalmology. 

If the standard three eye tests don't reveal anything out of the norm, your vet may want to perform more specialized tests. Gentle scrape samples of your cat's conjunctiva can be sent off to an outside lab for histopathology, where a veterinary specialist will look at the samples under a microscope. From this they can determine if there is any abnormal cells present that could indicate a benign (or malignant) growth. Your vet may want to check a blood panel to rule out any systemic illnesses. They may also want your cat to get skull radiographs, an MRI, or even be seen by a veterinary ophthalmologist. 

Treating Your Cat's Anisocoria

How best to treat your cat's anisocoria depends entirely on what is causing your cat's anisocoria. A specific treatment will be based on the specific disease. If your cat's anisocoria is stemming from a chemical or toxin exposure, removing the the substance may reverse the anisocoria. Some causes, such as Horner's Syndrome, are self-limiting and the anisocoria may resolve on it's own. For still other causes, such as degenerative conditions, your cat's anisocoria may never get better. Some causes of anisocoria may also require long term medication. If your cat's vision is affected because of their anisocoria their vision may never return back to normal.

Regardless of the reason for your cat's anisocoria, seeking immediate veterinary attention is vital for your cat's vision and eye health. Whether you see an emergency veterinarian or your regular veterinarian, they can help you figure out what is causing your cat's troubling eye symptoms and how best to tackle the problem.