is a medical sign observed during the swinging-flashlight test[1] whereupon the patient's pupils constrict less (therefore appearing to dilate) when a bright light is swung from the unaffected eye to the affected eye.
The affected eye still senses the light and produces pupillary sphincter constriction to some degree, albeit reduced.
The most common cause of Marcus Gunn pupil is a lesion of the optic nerve (proximal to the optic chiasm) or severe retinal disease. It is named after Scottish ophthalmologist Robert Marcus Gunn.[2]
Look for an afferent pupillary defect (RAPD - also known as Marcus Gunn Pupil) by:
a. The Swinging Flash Light Test
b. The pupils must react to light in order to perform the test
c. Not a test of the pupil per se—but we do the test when we look at the pupil
d. Tests the optic nerve function
e. Relies on a difference between the two optic nerves—one must be different from each other
