Marcus Gunn Pupil

Marcus Gunn pupil (MGP) is the term given to an abnormal pupil showing aberrant pupillary response in certain ocular disorders. In literature, the term is often used synonymously with Marcus Gunn phenomenon or relative afferent pupillary defect (RAPD). After exposure to bright light, a normal pupil constricts.A Marcus Gunn pupil, on the other hand, has a relative weakness of the afferent limb of the pupillary light reflex compared to the other eye because of which when light is rapidly transferred from the normal eye to the eye with MGP, the MGP dilates instead of constricting. It is named after Scottish Ophthalmologist Robert Marcus Gunn. Hirschberg first noted this phenomenon in the case of unilateral retrobulbar optic neuritis. The presence of a relative afferent pupil defect (RAPD) is the hallmark of a unilateral afferent sensory abnormality or bilateral asymmetric visual loss. An afferent pupillary response is characterized by: 1. Constriction of pupils of both eyes when the light stimulus is applied to the normal eye.       2. Dilatation of pupils of both eyes when the light stimulus is rapidly transferred from the normal eye (after brief light exposure to the normal eye) to the affected eye.