Unequal Pupils (Anisocoria) in Dogs

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Last updated on
3 min read

Key takeaways

Anisocoria is the term for unequal pupil size in dogs. One pupil is normal, while the other is either constricted or dilated. 

  • Anisocoria is common in dogs experiencing Horner’s syndrome, but it also results from a variety of conditions and injuries that either affect one eye directly or the parts of the nervous system that control pupil size
  • Diagnosis often includes a physical examination, ophthalmologic exam, urinalysis, diagnostic imaging, and blood work
  • Treatment depends on the underlying cause and prognosis ranges from complete resolution to permanent blindness
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A closer look: Unequal Pupils (Anisocoria) in Dogs

Anisocoria is common because it is caused by a myriad of medical conditions. Essentially any illness or injury affecting one eye or the nerves to that eye has the potential to lead to anisocoria. The underlying causes for anisocoria range from minor to serious. If anisocoria develops suddenly, immediate veterinary care is warranted to minimize the risk of blindness.

One common cause of anisocoria and Horner’s Syndrome is a significant neck injury, which is often associated with dogs that are tied out using a leash and collar.

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Possible causes

Some topical eye medications may also lead to anisocoria.

Risk factors

Anisocoria does not vary in severity. The underlying causes of anisocoria have associated symptoms with varying degrees of seriousness.

The collection of these five concurrent symptoms is called Horner’s syndrome.

  • One constricted pupil (miosis)
  • Elevated third eyelid on the affected eye
  • Retraction of the eyeball into the socket
  • Drooping of the eyelid
  • A subtle increased blood flow and warmth to the ear and nose on the affected side

This is not a disease itself, but a name for the collection of these symptoms. Horner’s syndrome is very common in dogs showing anisocoria.

Testing and diagnosis

Diagnostic testing for dogs with anisocoria typically includes:

  • Physical examination
  • Ophthalmologic examination
  • Blood tests
  • Diagnostic imaging
  • Urinalysis

Investigation of anisocoria often requires referral to a veterinary ophthalmologist and/or neurologist.

Treatment depends on the underlying cause.

The prognosis also depends on the underlying cause, and can vary from a good with treatment, to irreversible eye injuries causing blindness.

Similar symptoms

Having two pupils of unequal sizes is not easily misinterpreted. The larger challenge is determining which eye is the abnormal one. One pupil may be dilated compared to the normal one, or it may be constricted.

Associated symptoms


Wendy Brooks, DVM, DABVP - Writing for Veterinary Partner
Dr. Bari Spielman - Writing for PetPlace
Melissa Boldan, DVM - Writing for PetMD
Ryan Llera, BSc, DVM; Tammy Hunter, DVM; Cheryl Yuill, DVM, MSc, CVH - Writing for VCA Animal Hospitals

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