Marcus Gunn Pupil is a highly objective and significant clinical manifestation of a relative afferent pupillary defect (RAPD), indicating decreased pupillary response.
The "swinging flashlight test" whereby a light is shone alternately back and forth between both eyes, can be used to elicit this finding. A normal response results in constriction of both pupils, which indicates intact direct and consensual pupillary light reflex. An abnormal response due to RAPD, however, results in minimal constriction of both pupils when the light in shone in the affected eye, causing one to perceive the presence of pupillary dilation.
See the attached image which contrasts a normal response (top) to an abnormal response (bottom).
Marcus Gunn Pupil is most commonly associated with lesions at the level of the optic nerve (proximal to the optic chiasm) or severe retinal disease. Associated conditions include severe glaucoma, optic nerve tumors, and ocular trauma.
