A Marcus Gunn pupil, also known as a relative afferent pupillary defect (RAPD), is a non-specific sign on physical examination that indicates partial optic nerve dysfunction. It is mainly due to unilateral optic neuropathy (e.g. optic neuritis), or rarely optic chiasm or optic tract lesions.
This sign is classically elicited on the swinging light test (or Marcus Gunn test). If light is shone into the normal eye both eyes constrict normally and equally. However, in response to light input to the affected eye, both pupils do not constrict to a normal extent, and may be seen to paradoxically dilate as the light moves quickly over it. This occurs because of a decrease in afferent input reaching the pretectal pathway responsible for the pupillary light response in the midbrain via a damaged optic nerve, but efferent fibers to the affected eye are delivered via cranial nerve III.
History and etymology
Robert Marcus Gunn (1850-1909), was a Scottish ophthalmologist 2.