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A Marcus Gunn pupil is seen, among other conditions, in unilateral optic neuritis. Thus, light shone in the affected eye will produce less pupillary constriction than light shone in the unaffected eye. 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 efferent path, and an intact consensual pupillary reflex). This indicates an intact direct and consensual pupillary light reflex. A normal response would be equal constriction of both pupils, regardless of which eye the light is directed at. In the swinging flashlight test, a light is alternately shone into the left and right eyes. The Marcus Gunn pupil is a relative afferent pupillary defect indicating a decreased pupillary response to light in the affected eye. Ī second common cause of Marcus Gunn pupil is a contralateral optic tract lesion, due to the different contributions of the intact nasal and temporal hemifields. It is named after Scottish ophthalmologist Robert Marcus Gunn. The most common cause of Marcus Gunn pupil is a lesion of the optic nerve (between the retina and the optic chiasm) due to glaucoma, or severe retinal disease, or due to multiple sclerosis. When RAPD is severe, the pupil will dilate quickly When RAPD is moderate, pupil size will remain, after which it dilates Mild RAPD will presents as a weak pupil constriction initially, after which dilation continues to happen. The affected eye still senses the light and produces pupillary sphincter constriction to some degree, albeit reduced.ĭepending on severity, different symptoms may appear during the swinging flash light test: Moving a bright light from the unaffected eye to the affected eye would cause both eyes to dilate, because the ability to perceive the bright light is diminished.Ī relative afferent pupillary defect ( RAPD), also known as a Marcus Gunn pupil, is a medical sign observed during the swinging-flashlight test whereupon the patient's pupils dilate when a bright light is swung from the unaffected eye to the affected eye. A Marcus Gunn pupil indicates an afferent defect, usually at the level of the retina or optic nerve. The left optic nerve and the optic tracts. Medical condition Relative afferent pupillary defect