Afferent Pupillary Defects, Relative (Marcus Gunn Pupil)

Synonyms

Marcus Gunn pupil

Definition

The relative afferent pupillary defect (RAPD) is a clinical sign. The RAPD is caused by asymmetrical input to the Edinger-Westphal nuclei from the afferent limb of the visual pathway (e.g., retina, optic nerve, chiasm, optic tract, and pretectal neurons). Bilateral and symmetrical afferent system disorders (i.e., symmetric disease in both optic nerves) may not cause a detectable RAPD.

Etiology

Any lesion of the afferent visual pathway from the retina to the optic chiasm, optic tract, and pretectal neurons in the midbrain can result in an RAPD.

Diagnostics

The RAPD is diagnosed via the swinging flashlight test. In this test, a light is alternately shone into the left and right eyes. A normal response displays equal constriction of both pupils, regardless of which eye the light is shone in. A normal response indicates an intact direct and consensual pupillary light reflex. Observing pupil dilatation when the light is swung from the unaffected eye to...