Anisocoria in dogs: what you need to know

You’ve probably spent hours gazing lovingly into your dog’s eyes, so you’d likely notice if something had changed — like if one of their pupils was suddenly bigger than the other. While pupils typically enlarge and retract to adjust to light, there’s a condition called anisocoria that occurs when your dog’s pupils become two different sizes. Aniscoria could be a symptom of a larger health problem, so we talked to two veterinarians to get the details: Dr. Aliya McCullough, Fetch’s Chief Veterinary Officer and Director of Veterinary Affairs, and Dr. Kelly Diehl, DVM, MS, Dipl. ACVIM (SAIM), a former vet and the senior scientific programs and communications adviser at Morris Animal Foundation. Here are the details on what pet parents should know about anisocoria in dogs.

Common causes and symptoms of anisocoria in dogs

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.

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,” says Dr. Diehl. “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
• Corneal ulcer
• Posterior Synechia (scar tissue that forms between the iris and lens, which can occur after uveitis)
• Congenital abnormality (abnormal development) of the iris

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. 

When it comes to symptoms of anisocoria in dogs, Dr. McCullough tells us that it depends on the cause. She says, “Anisocoria is a symptom itself, so other symptoms would be associated with the underlying condition causing it. For example, if the anisocoria is caused by glaucoma, other clinical signs could include eye redness, swelling, discharge, pain or loss or vision.”

Is sudden anisocoria in dogs considered an emergency?

If you notice a sudden change in the size of your dog’s pupils, it’s best to call the vet immediately. When anisocoria occurs suddenly, it’s crucial to get a diagnosis right away to decrease the chance of permanent eye issues for your pup. ‍

Is Anisocoria in dogs temporary? 

Ansicoria in dogs is not always permanent. “In some cases anisocoria is temporary and will resolve without treatment but a veterinarian should be consulted to make that determination,” Dr. McCullough says. 

However, if the anisocoria is caused by old age, it’s unlikely that the condition will disappear. “Anisocoria related to iris atrophy (shrinking or weakening of the colored part of the eye) is common in older pets and is likely to be permanent. Luckily, this condition does not impair their vision but they may be more sensitive to bright light.” 

How anisocoria is treated in dogs

Anisocoria alone isn’t usually uncomfortable for dogs, as long as the underlying issue isn’t causing discomfort. But anisocoria won't resolve until the underlying problem 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.