This is an important part of the neurological examination of the eye. The Relative Afferent Pupillary Defect (RAPD), is also called the Marcus Gunn pupil, after the ophthalmologist that described the defect in 1883. The sign causes much confusion, but in reality is a very simple sign. In summary, a light is swung between both eyes. If all is normal, both pupils will remain constricted. If there is an optic nerve lesion, both pupils will ‘paradoxically’ dilate, when the light is shone into the affected side . It is really a test for decreased visual acuity. The side of decreased acuity will see a decreased afferent feed to the brain. The RAPD is a partial lesion and thus there is still some response. If the CNII lesion were complete, there would be no reaction in the eye. Other causes of RAPD include retinal detachment and central retinal artery occlusion.
HERE IS HOW IT WORKS.
The RAPD can only be demonstrated when the swinging light test is used. This is because the dynamic dilatation of both eyes gives the diagnosis. When the light being shone into the normal eye. The afferent fibres are intact and so the information goes to the brain and returns as a direct response of pupillary constriction and a consensual response(opposite eye) When the light being shone into the affected eye. Because there is impairment of the afferent fibres, the information is not adequately fed back to the brain, so minimal information is returned back to the eyes. The result is a much reduced direct and consensual response. This in effect gives the impression of the pupils dilating.
Light in the normal eye causes constriction of both pupils, ie., direct and consensual response. Light into the affected eye – Because the afferent fibres aren’t able to transmit the same amount of information to the brain, there is no time for a direct and consensual response, so both pupils appear to dilate.

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