Relative afferent pupillary defect (RAPD), Marcus Gunn pupil


A relative afferent pupillary defect (RAPD), also known as a Marcus Gunn pupil, is a medical sign observed during the swinging-flashlight test.



It  is a relative afferent pupillary defect indicating a decreased pupillary response to light in the affected eye.



With the RAPD the patient’s pupils dilate when a bright light is swung from the unaffected eye to the affected eye. 



The affected eye still senses the light and produces pupillary sphincter constriction to some degree, albeit reduced.



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.



The presence of a Marcus Gunn pupil indicates an afferent defect, usually at the level of the retina or optic nerve. 



The most common cause of Marcus Gunn pupil is a lesion of the optic nerve due to glaucoma, or severe retinal disease, or due to multiple sclerosis. 



A 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.



Mild RAPD presents as a weak pupil constriction initially, after which dilation continues to happen.



When RAPD is moderate, pupil size will remain, after which it dilates



When RAPD is severe, the pupil will dilate quickly



Normally in the swinging flashlight test, a light is alternately shone into the left and right eyes, with an equal constriction of both pupils, regardless of which eye the light is directed at. 



This normal response indicates an intact direct and consensual pupillary light reflex. 



The swinging light 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.



The 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.



The light shone in the affected eye will produce less pupillary constriction than light shone in the unaffected eye.



Anisocoria is absent. 



A Marcus Gunn pupil is seen with: optic neuritis, and with retrobulbar optic neuritis due to multiple sclerosis,  but only for 3–4 weeks, until the visual acuity begins to improve in 1–2 weeks and may return to normal.



A total CN II lesion, in which the affected eye perceives no light, is very similar to a Marcus Gunn pupil. 



To distinguish the difference: with a CNII total lesion shining the light in the affected eye produces zero dilation nor constriction.



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