Dilated Pupils (Mydriasis) in Dogs

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Last updated on
3 min read

Key takeaways

The pupil is the dark circle located at the center of the iris (the colored portion) of dogs’ eyes. The size of dogs’ pupils varies depending on light levels. 

  • When one or both eyes dilate (when the pupil becomes larger), this is called mydriasis
  • While dilating in dim conditions or in certain mental states (such as fear) is normal; pupils that remain dilated continuously may be a sign of illness
  • Potential causes include poisoning, eye diseases, brain injury, and infectious disease
  • Causes of abnormal mydriasis are accompanied by additional symptoms, and often include incoordination and unresponsiveness to light
  • If required, treatment depends on the underlying cause and may include medications, hospitalization, or surgery in some cases
  • Diagnosis is through thorough ophthalmic exam, blood work, and diagnostic imaging
  • Prognosis varies widely depending on the underlying cause
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A closer look: Dilated Pupils (Mydriasis) in Dogs

The iris is the colored part of the eye and controls the opening (dilation) and closing (constriction) of the pupil. The pupil appears as a black circle in the center of the eye and is responsible for allowing light to reach the retina. Light signals are sent from the retina to the brain where they are translated, creating the sensation of vision.

Pupils dilate normally in response to changing light levels and as a result of certain physiological changes, such as during the natural fear response. Mydriasis can be unilateral (one eye only) or bilateral (both eyes). Abnormally dilated pupils may be unresponsive to light, or show partial response to light.

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Possible causes

In addition to head, brain, or eye injuries, there are many possible causes of abnormal pupil dilation.

Risk factors

Prolonged mydriasis in dogs is uncommon. If mydriasis is the only symptom present, it may not be an emergency, but veterinary attention is warranted. If accompanied by other symptoms such as excessive drooling, vomiting, or trouble walking, emergency veterinary care is required.

Testing and diagnosis

In cases of suspected mydriasis, the first step in diagnosis is a complete physical exam, including a thorough ophthalmic exam. Blood work, urinalysis, and diagnostic imaging may also be performed.

Other tests may be recommended based on the presence of concurrent symptoms and to rule specific conditions in or out.

Treatment depends on the underlying cause. Sometimes no treatment is necessary, while other times hospitalization, medication, or even surgery may be indicated.

Similar symptoms

Normal variation in pupil size may be mistaken for mydriasis. Mydriasis may be difficult to distinguish in dogs with dark-colored irises.

Associated symptoms

Concurrent symptoms vary widely and depend on the underlying cause.


María Besteiros - Writing for AnimalWised
Adrienne Farricelli CPDT-KA, DIP.CBST - Writing for PetHelpful
Dr. Bari Spielman - Writing for PetPlace
Ryan Llera, BSc, DVM; Tammy Hunter, DVM; Cheryl Yuill, DVM, MSc, CVH - Writing for VCA Animal Hospitals

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