Is MS Making My Pupils Different Sizes?

Optic neuritis from MS can cause afferent pupillary defect (APD)

Mirror image of young man watching himself. Credit: Westend61 / Getty Images

For at least 15 years before I was diagnosed with multiple sclerosis (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, such as 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.

Much later, I realized this was a symptom of my MS. 

Afferent Pupillary Defect (APD) in Multiple Sclerosis 

An abnormal response to light in the pupils of the eyes is a phenomenon called afferent pupillary defect (APD) or relative afferent pupillary defect (APD), also referred to as Marcus Gunn Pupil. It occurs when the nerve pathways from the affected eye to the brain fail to transmit messages properly.

In people with multiple sclerosis (MS), APD usually happens because the person has had optic neuritis and suffered optic nerve damage in one eye, even if the episode was so mild that they were not aware that it occurred. However, APD can also occur with other conditions, including ischaemic optic neuropathy, optic nerve compression, trauma, asymmetric glaucoma, and several other lesson common causes. 

How Is APD Diagnosed?

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

When a light is shone in the unaffected eye, both pupils constrict (get smaller) at the same time, which is the normal response. However, when a light is quickly shone in the affected eye of a patient with an APD, the pupils of both eyes dilate (gets larger) rather than constrict. This abnormal response signifies the brain is not perceiving the light properly and thinks that less light is coming in, due to demyelination of the optic nerve.


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

Broadway, David C. Community Eye Health. How to test for a relative afferent pupillary defect (RAPD). 2012; 25(79-80): 58–59. 


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