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Anisocoria in Cats: Symptoms, Causes, and Care (Vet Answer)

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By Dr. Lindsay Bisset

anisocoria in cat

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Dr. Lindsay Bisset

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Cats have some of the most beautiful and distinctive eyes in the animal kingdom. One characteristic that make their eyes particularly unique is the shape of their pupils. A cat’s pupils change from vertical slits to completely round, depending on their mood and the ambient lighting.

The pupil is the opening within the iris (the colored part of the eye) that allows light to pass through the eye to the retina. The iris controls the size of the pupil, based on the body’s needs, by dilating or enlarging to increase the amount of light that enters the eye, or by constricting or narrowing to allow less light in.

What is Anisocoria?

Normal pupils work in unison—when one constricts or dilates, so should the other. Normal pupils are, therefore, the same size. Anisocoria is a condition where the pupils of a cat’s eyes are different sizes. In some cases, the larger pupil is the abnormal one, while in other cases, it is the smaller pupil that is abnormal.

a cat with anisocoria (unequal pupil size) condition
Image Credit: ArlanJace, Shutterstock

What Are the Symptoms of Anisocoria?

Cats with anisocoria have pupils that are two different sizes. Depending on the underlying cause of anisocoria, affected cats may show other symptoms too. If anisocoria is caused by an ocular (eye) disease, there may be other symptoms present, such as redness of the eye, a cloudy cornea (the clear part of the eye), eye discharge, or blinking and squinting from pain. If the cause of anisocoria is neurological, other symptoms, such as a head tilt, abnormal behavior, altered consciousness, drooping of the upper eyelid, and a protruding third eyelid may be seen.

What Are the Causes of Anisocoria?

Anisocoria may be the result of either an ocular or a neurological disease.

Common underlying conditions that may cause a cat to develop anisocoria include the following:
  • Iris atrophy: a thinning of the tissue of the iris, which leads to the appearance of ‘holes’. This condition is usually due to age-related degeneration of the iris, but can also be caused by trauma, glaucoma, or chronic uveitis.
  • Iris hypoplasia: a condition in which the iris does not develop properly
  • Glaucoma: a disease in which the pressure within the eye is elevated
  • Uveitis: inflammation of the middle layer of the eye, including the iris. This is a severely painful disease.
  • Corneal ulcers: a painful defect or wound on the surface of the cornea (the clear part of the eye)
  • Posterior synechiae: strands of tissue that adhere to the iris and the lens capsule of the eye. The pupil appears larger and may be misshapen. This condition may develop following uveitis.
  • Retinal disease: such as one-sided retinal detachment
  • Head trauma: trauma to the head, which may result in hemorrhage and increased pressure within the skull, leading to anisocoria
  • Brain tumors: brain tumors can cause compression, leading to anisocoria
  • Feline spastic pupil syndrome: a condition that may be seen in cats infected with Feline Leukemia Virus
  • Other infectious diseases such as Feline Immunodeficiency Virus (FIV), and toxoplasmosis
  • Horner’s Syndrome: a neurological disorder affecting the eye and facial muscles. Some causes for Horner’s Syndrome include a middle ear infection, trauma, and tumors

How Do I Care for a Cat with Anisocoria?

If your cat develops anisocoria, it’s important that you seek veterinary care as soon as possible. It’s best not to wait and see if the condition improves on its own, as some diseases causing anisocoria are extremely painful and can result in permanent blindness, if left untreated. Others conditions may be life threatening without appropriate veterinary care.

At the clinic, your veterinarian will perform a physical examination on your cat, including an eye and neurological exam, in order to determine the underlying cause of anisocoria. It may also be necessary to do blood tests to check for underlying systemic diseases that may be responsible, such as Feline Leukemia Virus (FeLV) and Feline Immunodeficiency Virus (FIV). In some cases, it may be necessary to refer your cat to a veterinary ophthalmologist for further testing.

Once your veterinarian has made a diagnosis, he or she will discuss a treatment plan with you. The treatment depends on the underlying cause of the anisocoria. Some causes of anisocoria, such as iris atrophy and iris hypoplasia, don’t require treatment at all, while other conditions may require treatment with eye drops, pain medication, or antibiotics.

In order to ensure the best outcome for your cat, be sure to follow the treatment plan closely. It’s important to administer the prescribed medication at the correct times, and to take your cat for regular follow-ups as advised by your veterinarian.

Frequently Asked Questions (FAQs)

Can Anisocoria Cause Blindness?

Certain conditions causing anisocoria such as glaucoma, corneal ulcers, and uveitis, may result in blindness if left untreated.

Is Anisocoria Painful?

Anisocoria itself is not painful, however certain diseases causing anisocoria, such as trauma, uveitis, glaucoma, and corneal ulcers can be painful.

What is the Prognosis for Cats with Anisocoria?

The prognosis depends on the underlying cause of anisocoria. Some diseases are treatable, with affected cats making a full recovery, while other diseases require life-long management. Unfortunately, some conditions causing anisocoria are life-threatening and carry a poor prognosis.


Anisocoria may be caused by many different underlying conditions, ranging from minor to life-threatening. Without knowing the underlying cause of anisocoria, it’s impossible to know whether it’s an emergency or not.

For this reason, it’s best to err on the side of caution, and to seek urgent veterinary care if you notice that your cat’s pupils are different sizes.

Featured Image Credit: HNP, Shutterstock

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