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Sometimes my pupils are different sizes. What causes this?


Updated June 26, 2014

Question: Sometimes my pupils are different sizes. What causes this?
For at least 15 years before I was diagnosed with MS, people would point out that my pupils were different sizes at times. It seems like this happened most often when the room itself was dim, but there was a source of light nearby, like a window. Because my eyes are light blue, I guess it was really noticeable, although every time I ran to a mirror to look, I really couldn't see what anyone was talking about.
Answer: This is a phenomenon called afferent pupillary defect (APD), also referred to as Marcus Gunn Pupil. It occurs when the nerve pathways from the eyes to the brain fail to transmit messages properly. In people with multiple sclerosis (MS), it usually happens because the person has had optic neuritis, even if the episode was so mild that they were not aware that it occurred.

APD is identified by examining the eyes with a bright light. During a neurological exam, this is called the swinging flashlight test, when the doctor shines the flashlight in one eye and then the other.

In normal circumstances, when a light is shone in one eye, both pupils constrict (get smaller) at the same time. However, when a light is shone in the affected eye of a patient with an APD, the pupil of the affected eye dilates (gets larger) rather than constricts. This abnormal response signifies the brain is not receiving the message properly, due to demyelination of the optic nerve.

APD itself does not affect vision in any way, and often the person is unaware that they have it until they look in a mirror or it is mentioned by somebody.


Turkington, Carol. The A to Z of Multiple Sclerosis. New York: Checkmark Books. 2005.

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