Marcus gunn pupil lesion

(A) Cortex# Marcus Gunn pupil is a medical sign observed during the swinging-flashlight test whereupon the patient's pupils constrict less (therefore appearing to dilate) when the light swings from the unaffected eye to the affected eye.> The affected eye still senses light and produces pupillary constriction to some degree, albeit reduced.> The most common cause of Marcus Gunn pupil is a lesion of the optic nerve (before the optic chiasm) or severe retinal disease.> The Marcus Gunn phenomenon is a relative afferent pupillary defect indicating a decreased pupillary response to light in the affected eye.> In the swinging flashlight test, a light source is alternately shone into the left and right eyes.> A normal response would be equal constriction of both pupils, regardless of which eye the light is directed at. 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 mild 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).