What Does It Mean When a Dog's Pupils Are Different Sizes?

When diagnosing anisocoria, your veterinarian may recommend several tests to evaluate the eye.
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The pupil is the round hole in the middle of each eye that constricts or dilates in response to the amount of light present. Your dog’s pupils should normally constrict (grow smaller) in bright light and dilate (grow larger) in dim light. "Anisocoria" is the word used to describe unequal pupil size between the two eyes, and can be caused by a number of conditions.


Anisocoria is caused by a problem in the brain or nerves that control the pupils, or a condition of the eye itself that prevents the pupil from responding appropriately to light. Only one of the pupils is abnormal in this condition, so it is important to determine which pupil is in the inappropriate position. Common neurological reasons for anisocoria are tumors or injury of the brain, the eye or the nerves affecting the eye. Anisocoria that develops due to a problem with the eye itself most commonly results from glaucoma, uveitis (inflammation of the eye), injury to the cornea or iris atrophy. Iris atrophy occurs in older dogs when the iris degenerates, making the pupil wider.


The most obvious symptom is a difference in pupil size between the two eyes. Anisocoria, however, is just a symptom of the underlying condition. Depending on the cause of the anisocoria, you may notice other signs such as red or inflamed eyes, a bluish tint to the cornea or, in the case of brain disease, neurological symptoms such as seizures or confusion.


It is important to seek veterinary attention immediately if you notice anisocoria in your dog so the cause of the condition can be determined. Your veterinarian will examine your dog’s eye structures, perform tests that evaluate his response to light and look for other signs of neurological disease. Blood work may be performed to rule out any systemic illness, and additional testing may be recommended. If a brain or nerve lesion is suspected, your veterinarian may recommend a CT scan or an MRI. Your veterinarian may even recommend evaluation by a veterinary ophthalmologist for further evaluation, testing and treatment.


The treatment for anisocoria centers on the cause of the condition. Certain problems with the eye may require treatment with medications or surgical procedures, while neurological disease requires a different type of treatment plan. The chance of your dog’s pupil function returning to normal also depends on the underlying cause, as some diseases will respond more favorably to treatment. Your veterinarian will guide you through the treatment process and provide the best recommendations for your dog’s specific diagnosis, allowing the best chance for a full recovery.