Reflexes 2

A relative afferent pupillary defect (RAPD) (Marcus-Gunn pupil) indicates disease anterior to the optic chiasm, usually in the optic nerve or retina. It is identified by the swinging light test, conducted in a dimmed environment.

Normally, pupillary size is dependent on the total perceived light reaching the brain. If, when rapidly moving a bright light between the two eyes, one (abnormal) eye transmits less light to the brain, which reacts by dilating both pupils, as shown in the following links.

A normal reaction

Flash a bright light first into one eye for 2-3 seconds; both pupils should constrict. Then rapidly move to the other eye for a similar time. Normally, both pupils should remain constricted.

An abnormal reaction (RAPD in left eye)

If, once the light is moved from one side to the other, the pupils dilate, this is abnormal and represents a relative afferent pupillary defect in this case, pathology in the left eye, or optic nerve.

Learning bite

A relative afferent pupillary defect indicates retinal or optic nerve pathology.