What is a Marcus Gunn pupil?

A Marcus Gunn pupil is a specific type of pupillary response that occurs due to a defect in the afferent pathway of the eye, typically caused by damage to the retina or optic nerve on one side. In a Marcus Gunn pupil, when light is shone in the affected eye, there is a paradoxical dilation of the pupil instead of constriction, which is the expected reaction.

The characteristic feature of a Marcus Gunn pupil is the relative afferent pupillary defect (RAPD). When compared to the unaffected eye, the pupil of the affected eye will appear to dilate when light is moved from the unaffected eye to the affected one, indicating that there is a disturbance in the sensory pathway. This abnormal response is pivotal in clinical examination as it indicates potential neurological issues or ocular disease.

Understanding this condition is critical for evaluating patients with visual disturbances, as it helps in localizing lesions within the visual pathway. Thus, recognizing a Marcus Gunn pupil signifies an afferent nerve dysfunction specifically linked to the optic nerve's inability to transmit light stimuli effectively.