A 45-year-old woman has an afferent pupillary defect (APD) in the right eye. When a torch is swung from the left to the right eye, the right pupil paradoxically dilates instead of maintaining constriction. Which feature of this sign confirms an optic nerve lesion rather than an anterior segment or retinal cause?
- A The lesion must produce an absolute loss of direct light reflex with intact consensual reflex
- B The APD indicates asymmetric optic nerve/retinal dysfunction — even 0.3 log unit difference is detectable ✓
- C The APD is present only in complete optic nerve transection
- D The APD reflects Edinger-Westphal nucleus dysfunction in the affected eye
Explanation
A relative afferent pupillary defect (RAPD/Marcus Gunn pupil) detected with the swinging flashlight test indicates asymmetric dysfunction of the anterior visual pathway (retina or optic nerve) — as little as 0.3 log unit asymmetry in retinal/optic nerve function creates a detectable RAPD. The swinging light test compares the afferent input of both eyes; the eye with less light input appears to receive 'less light,' so both pupils dilate (apparent paradoxical dilation). APD does not require complete optic nerve transection — even partial optic neuropathy is detectable. The Edinger-Westphal nucleus subserves the efferent pupillary pathway (accommodative/light reflex constriction); its dysfunction causes a fixed dilated pupil without RAPD. Anterior segment causes (corneal opacity, cataract) do not produce RAPD.
Reference: Khurana Comprehensive Ophthalmology, 7th ed.
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Written and medically reviewed by the StethoPrep medical team.