What is anisocoria?

Anisocoria (say "an-eye-soh-KOR-ee-uh) is a difference in the size of your pupils. The pupil is the black area in the centre of your iris. The iris is the coloured part of your eye. The iris controls the pupil to let the right amount of light into the eye. In bright light, the pupil narrows (constricts) to let in less light. In dim light, the pupil widens (dilates) to let in more light.

When you have anisocoria, one of your pupils does not constrict or dilate as well as the other one. So the pupils look uneven.

A slight difference in the size of the pupils is normal for some people. But in other cases this condition is the result of a medical problem. Examples include an injury to the eye, an infection, or nerve damage.

What are the symptoms?

Having different-sized pupils usually doesn't cause any symptoms. And by itself, it is rarely a cause for concern.

Anisocoria is more likely to be the sign of a serious problem if it occurs along with other symptoms, such as:

• A droopy upper eyelid.
• Eye pain.
• A severe headache.
• Vision problems, such as double vision.
• Loss of vision.

Tell your doctor right away if you have any of these symptoms.

What can you expect when you have it?

An eye doctor (ophthalmologist) will do a complete eye exam. For example, the doctor will:

• See how your pupils respond to bright and dim light.
• Look at the inside of your eyes.
• Check how well your eyes focus.

The doctor might ask to see old photos to find out how long your pupils have been uneven.

You may have other tests to help rule out a serious cause of uneven pupils. For example, the doctor might check how your eyes respond to eye drops. Or you might have an imaging test, such as an MRI.

If a medical problem is causing the difference in pupil size, your treatment will depend on the cause. You will not need any treatment if your uneven pupils are not linked to a medical problem.