The NORMAL afferent pupillary defect (RAPD/Marcus Gunn pupil) testing procedure is performed by the swinging flashlight test. A positive RAPD indicates a lesion in which part of the visual pathway?

• A Optic chiasm or posterior to chiasm involving both optic tracts
• B Occipital cortex unilaterally
• C Bilateral optic nerve disease of equal severity
• D Anterior visual pathway (optic nerve or severe retinal disease) anterior to the chiasm on the affected side ✓

Explanation

RAPD indicates an asymmetric lesion in the afferent visual pathway anterior to the lateral geniculate nucleus — specifically unilateral or asymmetric optic nerve or severe retinal disease. The affected eye has reduced afferent signal, so when light shines into it, both pupils dilate (paradoxical dilation). Chiasmal or post-chiasmal lesions cause bilateral, symmetric changes in the optic tracts and generally do not produce RAPD because each optic tract carries fibers from both eyes (nasal from contralateral, temporal from ipsilateral). Bilateral symmetric optic nerve disease also does not produce RAPD.

Reference: Khurana Comprehensive Ophthalmology, 7th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.