29. Pupil Light Reflex
In an afferent pupillary defect there is a decreased direct response caused by decreased visual function (CN II) in one eye. However, there is spared pupillary constriction (CN III) when elicited through the consensual response. This can be demonstrated with the swinging flashlight test, in which the light is moved back and forth between the eyes every 2 to 3 seconds. The afferent pupillary defect becomes obvious when the flashlight is moved from the normal to the affected eye and the affected pupil dilates instead of constricting in response to light. Brief oscillations of pupillary size called hippus occur normally in response to light and should not be confused with an afferent pupillary defect.