What type of response is absent in an afferent pupillary injury?

In the context of an afferent pupillary injury, the direct response to light is the one that is absent. Afferent pupillary defects, as seen in conditions like optic nerve damage or severe retinal issues, impede the transmission of sensory signals from the retina to the brain. Consequently, when light is shone into the affected eye, the brain does not receive any signaling from that eye, resulting in the absence of a direct pupillary constriction response.

While there may still be preserved consensual responses in the non-affected eye when light is directed into the unaffected eye, the critical aspect of afferent pupillary defects is that they specifically impact the ability of the affected eye to constrict in response to light. This is due to the failure of sensory input to reach the central nervous system, which is necessary for initiating the direct response.

Understanding that in cases of afferent pupillary defects, the consensual response may still be present when light is directed into the unaffected eye helps clarify why the direct response remains absent rather than both responses or any other combination.