Marcus Gunn

Marcus-Gunn pupil is an alternative name for a condition of the eye known as Afferent Pupillary Defect (APD) which indicates a decreased pupillary response to light in the affected eye, and pupils fail to constrict fully. A Marcus Gunn pupil is said to occur when there is a unilateral lesion in the afferent visual pathway anterior to the chiasm.

In the swinging flashlight test, a light is shone into the left and right eyes. For a normal response, there would be equal constriction of both pupils, regardless whether the light is directed at the left or right eye. This indicates an intact direct and consensual pupillary light reflex. When the test is performed in an eye with an afferent pupillary defect, light directed in the affected eye will cause only little constriction of both pupils (due to decreased response to light from the afferent defect), while light in the unaffected eye will cause a normal constriction of both pupils (due to an intact afferent path, and an intact consensual pupillary reflex). Therefore, there will be lesser pupillary constriction in the affected eye when light is shone on it, than when light is shone in the unaffected eye.

Some symptoms of a Marcus-Gunn phenomenon are that the patient has a drooping eyelid and there is brief eyelid lifting when the patient opens his mouth.