The Marcus Gunn pupil (relative afferent pupillary defect, RAPD) is detected using the swinging flashlight test. A positive RAPD indicates:

• A A bilateral symmetric optic neuropathy
• B A third nerve palsy on the ipsilateral side
• C An asymmetric afferent defect (optic nerve or extensive retinal disease) in the eye that shows paradoxical dilation when the light is swung to it ✓
• D Postganglionic sympathetic denervation

Explanation

RAPD is detected when, during the swinging flashlight test, the eye paradoxically dilates (rather than constricts) as the light swings from the normal to the affected eye — indicating that the affected eye receives less afferent light signal per unit of light than the normal eye. This indicates an asymmetric abnormality of the afferent visual pathway (optic nerve disease, severe unilateral retinal disease involving a large area). A bilateral symmetric optic neuropathy produces no RAPD (both eyes equally affected). Posterior visual pathway lesions behind the chiasm do not cause RAPD because the pupillomotor fibers have already diverged bilaterally.

Reference: Khurana Comprehensive Ophthalmology, 7th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.