Anisocoria in Dogs: Causes, Symptoms & Treatment Guide

Most dog owners know dogs can develop cataracts, glaucoma and other eye conditions common in humans, but anisocoria is a lesser-known canine eye issue that requires veterinary attention.

Anisocoria refers to uneven pupil sizes where one pupil is noticeably larger or smaller than the other. This difference signals an underlying problem which may affect the eye or the nervous system, and early detection is key to effective treatment.

This comprehensive guide covers what anisocoria in dogs is, the various causes, symptoms to watch for, and the treatment options to preserve your dog’s eye health and vision.

What is Anisocoria in Dogs?

Anisocoria means the pupils of a dog’s eyes are unequal in size. Normally, a dog’s pupils respond simultaneously to light, dilating in dim conditions and contracting in bright environments to protect sensitive eye structures.

When one pupil remains abnormally dilated or constricted compared to the other, this is anisocoria. It indicates potential eye damage, disease, or neurological dysfunction. Prompt veterinary evaluation is essential.

The pupils control light entry to the retina, influencing how well your dog sees. Unequal pupils can signal vision issues or discomfort and must never be ignored.

Common Causes of Anisocoria in Dogs

Anisocoria can arise from numerous underlying conditions. The affected pupil may be either smaller or larger than normal. Key causes include:

• Eye trauma or injuries such as corneal ulcers, scratches or impacts causing damage to the iris or nerves.
• Neurological disorders like Horner’s syndrome, disrupting the nerve pathways controlling pupil size and eyelid function.
• Inflammatory diseases such as uveitis causing eye pain and pupil constriction. Scar tissue from chronic inflammation can also affect pupil shape.
• Glaucoma, where increased eye pressure often causes the pupil to dilate abnormally, risking vision loss.
• Ocular cancers or tumours impacting the eye or optic nerve, often detected in advanced stages.
• Congenital or hereditary anomalies affecting the iris development.
• Retinal diseases that impair normal pupil responses.
• Head injuries resulting in nerve damage.

Anisocoria usually appears suddenly, but sometimes develops gradually. If you notice any change in your dog’s eyes, including unequal pupils or discomfort signs, see your vet immediately.

Symptoms to Recognise Anisocoria in Dogs

The hallmark symptom is one pupil being distinctly larger or smaller than the other. Additional signs may help pinpoint the cause and urgency of the problem:

• Redness or inflammation in one eye
• Discharge or excessive tearing
• Cloudy or hazy cornea
• Drooping eyelid (ptosis) or elevated third eyelid
• Squinting, pawing at the eye, reluctance to open eye
• Head tilting or difficulty focusing
• Behavioural changes such as lethargy or irritability

Always observe your dog carefully for related neurological symptoms, which may suggest more serious underlying causes.

How is Anisocoria Diagnosed in Dogs?

Your vet will perform a thorough eye examination including checking the pupillary light reflex. This tests how each pupil responds to light, determining which pupil is abnormal.

Other diagnostic steps may include:

• Fluorescein stain test to detect corneal ulcers
• Tonometry to measure intraocular pressure for glaucoma
• Neurological examination for nerve or brain disorders
• Blood tests to identify systemic infection or inflammation
• Advanced imaging like MRI or X-rays if brain or nerve tumours are suspected

Referral to a veterinary ophthalmologist may be required for complex or unclear cases to ensure accurate diagnosis and specialist care.

Treatment Options for Anisocoria in Dogs

Treatment targets the underlying cause rather than anisocoria itself, as it is a symptom, not a disease. Common treatments include:

• Antibiotic or anti-inflammatory eye drops for infections or uveitis
• Surgery to repair trauma, remove tumours, or relieve glaucoma pressure
• Long-term eye medications to manage chronic conditions
• Pain relief to increase comfort for painful eye diseases
• Specialist neurological treatments if nerve damage is involved

The prognosis depends on the cause and timely intervention. Early veterinary care greatly improves outcomes and can preserve your dog’s sight.

Preventing Eye Problems That Cause Anisocoria

While not all causes are preventable, you can reduce risk by:

• Maintaining a safe home and garden environment to avoid eye injuries
• Regular veterinary check-ups including eye exams
• Protecting your dog from toxins and harmful substances
• Promptly treating any eye discomfort or injury

Being alert to changes in your dog’s eyes and seeking early vet advice helps protect vision and wellbeing.

Recognising When to Visit the Vet

If you observe any signs such as unequal pupils, eye redness, discharge, squinting or behavioural changes, book a veterinary appointment urgently.

Eye conditions can progress rapidly. Immediate professional assessment can prevent serious permanent damage.

In the UK, veterinary ophthalmology and neurology advances in 2025 mean better diagnostics and tailored treatments are widely available.

Summary

Anisocoria in dogs is an important symptom indicating uneven pupil sizes that usually means an underlying eye or neurological problem. Causes range from injury and inflammation to glaucoma and tumours.

Knowing how to identify anisocoria, watching for additional symptoms, and seeking immediate vet care are critical steps to maintaining your dog’s eye health.

With timely diagnosis and appropriate treatment, many dogs affected by anisocoria can maintain good vision and comfort.

Responsible dog owners should prioritise preventive care and regular vet checks to safeguard their beloved companion’s sight and quality of life.

For those considering adding a dog to their family, sourcing your new pet from reputable breeders or adopting from responsible shelters helps ensure healthier pets and supports ethical ownership.