Marcus Gunn Syndrome and Multiple Sclerosis

One test which my neurologist, ophthalmologist, and primary care doctor each conduct during every office visit is the “swinging flashlight test.” You know the one. The doctor asks you to look ahead then shines a penlight first toward one eye, then the other, alternating quickly to observe your pupils’ response to light.

I strangely enjoy this test because I know that my pupils will show something unique. Something which proves that I have damage to my optic nerve. My pupils show a Relative Afferent Pupillary Defect (RAPD) or Marcus Gunn Sign.

What does the doctor look for during the “swinging light test”?

The pupils (the black centers of the eyes which dilate or constrict in response to light) are inspected for size, equality, and regularity. Did you know that the pupils will constrict or dilate when you look at objects far or near? They do, which is kinda cool.

More importantly, each pupil should constrict quickly and equally during exposure to direct light and to light directed at the other pupil (the consensual light reflex). Using the swinging light test, the doctor can test and observe the pupillary response to consensual light in order to determine if there is a defect present.

Normally, the pupil constriction does not change as the light is swung from eye to eye. When the light is moved quickly from eye to eye, both pupils should hold their degree of constriction.

What is a Relative Afferent Pupillary Defect?

The Afferent Pupillary Defect (APD) or Relative Afferent Pupillary Defect (RAPD) is an abnormal and unequal response in the pupils of the eyes when exposed to light. It basically demonstrates that one optic nerve transmits a different message to the brain than the other one. Testing for RAPD is a good way to implicate or rule out optic nerve damage such as is caused by optic neuritis.


My temporarily blinding case of optic neuritis in 2000 left my right eye impaired. It doesn’t register light in the same way as my left eye as the optic nerve has permanent damage. When the doctor shines the light in my left eye (the “good” eye), both pupils will constrict. This is normal. When the doctor quickly moves the light to my right eye (the “bad” eye), my pupils begin to dilate since the brain thinks that less light is coming in. This shows that there is damage to the corresponding optic nerve.

Read this post in its entirety:

MS Signs and Symptoms: What is Marcus Gunn Syndrome?