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 one pupil that appears to be a different size than the other pupil you may notice the difference. Most owners will know right away that something is not right, but what could possibly be going on? What can cause abnormal pupil size?

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. 

  • A problem at the surface of the eye such as a corneal ulcer/eye injury/uveitis
  • A problem with the structures at the back of the eye (retina, optic nerve or optic tract), which may also affect vision in that eye
  • A brain or neurological disorder, such as head trauma or a tumor, affecting specific nerves running to your cat's eye
  • Inner or middle ear disease affecting the nerves that travel to the eye (which can also have additional signs known as 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 (Iris Atrophy)
  • Exposure to certain chemicals or toxins

Depending on the underlying cause you may not see any other symptoms or you may notice accompanying symptoms affecting the eyes or other parts of the body. Depending on the cause, cats with anisocoria may also have other eye abnormalities such as red eyes, a clouding or blue-tinted cornea (the outermost layer of the eye), eye discharge, a droopy eyelid, a squinting eye, or they may be rubbing/pawing at the affected eye. Your cat also may be less active than normal or have other behavioral or physical changes. The causes are so varied that it is possible to see many different scenarios.

Regardless of the cause, if your cat's anisocoria has a sudden onset, it is an emergency that requires immediate veterinary care. Eyes are so delicate that treatment should begin right away, otherwise you can risk permanent damage your cat's vision.

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. Staining the eye with fluorescein dye can illuminate any scratches, or 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 at the sight of the scratch. 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.

In addition to these 3 basic eye tests, your vet will want to assess your cat's vision and nerve functions. Your vet may want to perform more specialized tests as well including submitting a blood panel to rule out any systemic illnesses, as well as checking your cat's blood pressure. In some cases, imaging such as skull radiographs, or a CT scan or MRI can be a helpful tool as well. Other cases may be referred to a specialized 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 some types of 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 the underlying cause of its 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.

If you suspect your pet is sick, call your vet immediately. For health-related questions, always consult your veterinarian, as they have examined your pet, know the pet's health history, and can make the best recommendations for your pet.