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Anisocoria in Dogs

What Is Anisocoria in Dogs?

Anisocoria, a condition characterized by unequal pupil sizes in dogs, indicates a deviation from normal eye function. The pupil, a small dark aperture at the eye’s center, regulates the amount of light entering the eye, facilitating vision.

In dim lighting, a healthy pupil dilates to permit more light, aiding in discerning shadows and navigating dark environments. Conversely, in bright conditions, the pupil contracts to reduce light exposure, ensuring clear vision for the dog. Normally, pupils change size symmetrically, adjusting simultaneously to light variations. Anisocoria occurs when one pupil significantly differs in size from the other.

The iris, the colored portion encircling the pupil, houses the muscles and nerves governing pupil size. Disorders affecting these nerves and muscles can disrupt the ability to regulate pupil size effectively. Additionally, certain brain conditions may interfere with the transmission of signals necessary for proper pupil contraction.

While many of these conditions are chronic, sudden onset of anisocoria warrants immediate veterinary attention, particularly if it follows trauma. Although anisocoria is uncommon and may be benign, rapid onset demands urgent medical evaluation due to potential underlying issues.

Why Do Dogs Have Different-Sized Pupils in Anisocoria?

When a disease or injury affects the nerves or muscles controlling pupil size, it can result in one pupil being abnormally dilated or constricted. While changes in lighting contribute to alterations in pupil size, the sympathetic and parasympathetic nervous systems also play crucial roles.

The sympathetic nervous system, known as the “fight or flight” response, activates in response to danger or intense physical activity. This activation causes pupils to dilate, allowing more light into the retinas and improving vision. Consequently, frightened dogs often exhibit dilated pupils. Conversely, the parasympathetic nervous system, known as the “rest and digest” response, becomes active during periods of relaxation or while a dog is eating or resting, causing pupils to constrict.

The process of altering pupil size involves the coordination of various components including nerves, muscles, the brain, and the retina. Disruption in any part of this pathway can lead to abnormalities, which may manifest on the surface of the eye, within the iris muscles, along the eye-connected nerves, within the retina, or even in the brain itself.


Anisocoria, characterized by unequal pupil sizes in dogs due to one pupil being either abnormally dilated or constricted, presents various clinical signs depending on its underlying cause. Alongside asymmetrical pupil sizes, one or more of the following symptoms may be observed:

  • Redness of the conjunctiva, the white part of the eyeball
  • Squinting
  • Eye drainage
  • Changes in eye coloration
  • Drooping of the eyelid or face
  • Head shaking
  • Elevation of the third eyelid
  • Lethargy or decreased activity level


Anisocoria in dogs can stem from various conditions, some of which are chronic and relatively harmless, while others are more severe:

  • Iris atrophy: This condition involves changes in the iris, commonly observed in senior dogs of small breeds like Chihuahuas, Miniature Poodles, and Miniature Schnauzers.
  • Corneal injury: The cornea, the eye’s clear surface, can sustain injuries such as corneal ulcers, resulting in pain and reduced pupil size in the affected eye.
  • Head trauma: Incidents like car accidents, being kicked by large animals, or falls can elevate intracranial pressure due to brain bruising or bleeding, leading to anisocoria by compressing eye-related nerves.
  • Glaucoma: Abnormal pressure increase inside the eye characterizes glaucoma, a genetic condition potentially causing optic nerve damage, anisocoria, and blindness.
  • Cancer: Various cancers, including meningiomas, a common brain tumor in dogs, can disrupt eye nerve function, resulting in unequal pupil sizes. Additionally, tumors within the eye can cause anisocoria.
  • Horner’s syndrome: This syndrome manifests as one-sided facial drooping, an elevated third eyelid, and a constricted pupil due to damage to the sympathetic nervous system from disease or injury.
  • Middle ear infection: Deep ear infections can inflame surrounding nerves, leading to a small pupil on the affected side.
  • Uveitis: Inflammation of the uvea, the eye’s middle layer, can cause pupil constriction, eye cloudiness, squinting, and redness. Uveitis can result from infections or immune-mediated diseases.
  • Other trauma: Less evident forms of trauma, such as choke collar usage or deep ear flushing, can inadvertently damage nerves involved in pupil constriction, causing changes in pupil size.


Anisocoria in dogs is typically diagnosed through an ophthalmic examination conducted by a veterinarian. Using an ophthalmoscope, they will examine the interior of the dog’s eyes. Bright lights may be used to assess the normal pupillary light response, determine the affected pupil, and observe pupil constriction and dilation. A comprehensive physical examination will precede any further diagnostic steps.

Additionally, veterinarians may conduct various tests on the dog’s eyes. These may include staining the cornea with fluorescein dye to identify ulcers, evaluating eye pressure, and assessing tear production. Depending on the suspected underlying cause, they may suggest additional tests such as blood work or imaging, particularly if there are concerns about systemic diseases.


The treatment of anisocoria in dogs varies depending on the underlying cause, ranging from no specific treatment to surgical intervention.

Many conditions contributing to anisocoria, such as corneal injuries or damage to sympathetic nerves, can be effectively managed. Long-term management may be necessary for conditions like uveitis and glaucoma, with anisocoria potentially resolving alongside appropriate disease management.

Some conditions, like iris atrophy or certain cancers, may not require treatment. Iris atrophy is often benign and doesn’t necessitate intervention, whereas treatment for specific cancers like brain tumors may not be feasible if it compromises the dog’s quality of life.

Veterinarians will discuss treatment options following a diagnosis, emphasizing that anisocoria is a symptom of an underlying condition rather than a standalone disease. Thus, treatment focuses on addressing the root cause of the pupil size discrepancy.

Living and Management

The recovery and management of anisocoria in dogs hinge upon the underlying cause. In cases where iris atrophy is the culprit, especially in small-breed senior dogs, the prognosis for a normal life is typically excellent. Anisocoria resulting from nerve or eye injuries usually resolves with recovery from the injury itself.

Some dogs may necessitate lifelong treatment, such as the consistent use of eye drops, if an underlying eye condition like glaucoma is diagnosed. Regular veterinary check-ups are essential to monitor eye pressure throughout the dog’s life.

Permanent vision loss may occur if the condition leads to irreversible damage. Although this outcome is rare for most causes of anisocoria, it can happen if conditions remain untreated for prolonged periods. Therefore, prompt consultation with a veterinarian is crucial upon noticing any discrepancies in your dog’s pupil sizes.

Anisocoria in Dogs FAQS

What constitutes temporary anisocoria in dogs?

Temporary anisocoria occurs when a dog’s pupils are uneven for a brief period before returning to normal. This may result from eye inflammation or inflammation affecting the nerves responsible for pupil size regulation. If the inflammation subsides quickly and the pupils return to their usual size, it qualifies as temporary anisocoria.

Can anisocoria in dogs resolve without treatment?

While anisocoria may resolve spontaneously, it’s crucial to promptly consult a veterinarian to rule out potentially serious underlying causes, such as trauma.

Why do my dog’s pupils differ in size?

Anisocoria refers to the condition of having unequal pupil sizes in dogs. It can arise from various underlying factors, ranging from minor issues to potentially life-threatening ones. When a dog’s pupils differ in size, it indicates inflammation, compression, or injury affecting the eye, the nerves linked to the eye, or even the brain.

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