Afferent pupillary defect, also known as Marcus Gunn pupil or relative afferent pupillary defect (RAPD), is a clinical finding in which the pupil of one eye responds differently from the other eye when exposed to light. This condition is caused by a dysfunction in the afferent pathway of the pupillary light reflex, which carries information from the retina to the brain.
In normal conditions, both pupils constrict equally when light is shone into either eye. However, in the presence of an afferent pupillary defect, the affected pupil dilates or fails to constrict normally while the other pupil constricts as expected. This discrepancy occurs because there is a disruption in the transmission of light signals from the affected eye to the brain.
Afferent pupillary defects are often a result of optic nerve damage or severe retinal dysfunction, which may be caused by conditions such as optic neuritis, glaucoma, or retinal detachment. In some cases, afferent pupillary defects can occur due to brain-related issues, including tumors or strokes affecting the visual pathway.
To diagnose an afferent pupillary defect, a clinician will examine the response of the pupils using a swinging flashlight test. The test involves repeatedly shining a light back and forth between the two eyes while observing how the pupils react. The presence of an afferent pupillary defect is indicated by a dilation or reduced constriction response in the affected eye.
Afferent pupillary defects can provide important clinical information about the integrity of the visual pathway and are commonly used as a diagnostic tool to detect underlying eye or neurological disorders.
