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Health & Wellness

What is anisocoria in dogs?

Unequal pupil sizes may be signaling a bigger problem.

Are you keeping an eye on the size of your dog's pupils? You know, the black part in the center of the iris (the colored part of the eye) where light enters the eye. It's an odd question but an important one all the same. That's because there's a condition called anisocoria, aka when your dog's pupils become two different sizes — and it's a symptom of larger health problems. If you notice one of your dog’s pupils is suddenly larger than the other, you should see your vet right away. 

“Many pet parents are the ones who discover anisocoria in their dogs because they note that their pet’s eyes look different,” Dr. Kelly Diehl, DVM, MS, Dipl. ACVIM (SAIM), a former vet and the senior scientific programs and communications adviser at Morris Animal Foundation, says.

When a dog has anisocoria, your vet will first try to answer two questions: Which eye is abnormal? And is the anisocoria a neurologic problem or an ophthalmic problem? Because the abnormal pupil can be either larger or smaller than the normal pupil, it’s important to determine which pupil is the dog’s normal size to help figure out what’s causing the anisocoria.

RELATED: Dog eye infections: symptoms and treatments

What causes anisocoria in dogs?

The muscles that dilate and constrict the pupil are controlled the same way as other muscles in the body — they receive impulses from the brain through nerves. While anisocoria in dogs isn't always an emergency, it’s often caused by head trauma or concussion, so you should contact your vet if one pupil looks suddenly bigger than the other.

“There are several causes for anisocoria, but most fall into one-of-two categories: something's wrong with the eye itself, or something's wrong in the nervous system,” Dr. Diehl says. “Some causes aren’t serious, such as some associated with normal aging, but other causes, especially ones centered in the brain, can be very serious.”

Several ophthalmic conditions (conditions relating to the eye) can cause anisocoria, including:

  • Uveitis
  • Ocular lymphoma (or any ocular cancer)
  • Degenerative iris atrophy
  • Glaucoma
  • Ocular pain

Anisocoria can occur from conditions that aren't strictly neurologic (in relation to the nervous system) or ophthalmic, either. Horner’s syndrome, when there's a disrupted nerve pathway on one side from the brain to the face and eye, can cause anisocoria in dogs. Other symptoms of Horner’s syndrome are an elevated third eyelid, recessed eyeball and droopy upper eyelid. In dogs, about half of the cases of Horner’s are called idiopathic, meaning that an underlying cause can’t be found. 

How to treat anisocoria in dogs

As long as the underlying cause is not uncomfortable, anisocoria alone isn’t usually uncomfortable. Like the pain associated with a broken bone, anisocoria won't resolve until the underlying problem causing it is resolved. 

“Really there’s no treatment specifically aimed at equalizing pupil size — it’s all aimed at the underlying condition.” Dr. Diehl says. “The good news is that anisocoria doesn’t need to be treated in many cases — if it’s associated with aging, there’s really no harm to the dog. If the anisocoria is associated with another problem, it may resolve once the underlying condition is addressed.”

The presence of anisocoria can mean that something very serious is going on with your dog’s health. So, if you notice your dog’s pupils are consistently two different sizes regardless of changing light, it’s important to schedule an appointment with your vet to have it checked out.

The Dig, Fetch Pet Insurance's expert-backed editorial, answers all of the questions you forget to ask your vet or are too embarrassed to ask at the dog park. We help make sure you and your best friend have more good days, but we’re there on bad days, too.

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