In a patient with a left optic tract lesion producing a right homonymous hemianopia, swinging the flashlight test reveals a relative afferent pupillary defect in the LEFT eye. This finding, called the Wernicke hemianopic pupil, occurs because:
- A The optic tract carries efferent parasympathetic fibres to the ipsilateral pupil
- B The crossed nasal retinal fibres outnumber the uncrossed temporal fibres, so the eye contralateral to the tract lesion contributes more afferent input ✓
- C Decussating fibres are damaged twice, once in the chiasm and again in the tract
- D The ipsilateral eye loses both its nasal and temporal retinal input
Correct answer: B. The crossed nasal retinal fibres outnumber the uncrossed temporal fibres, so the eye contralateral to the tract lesion contributes more afferent input
Explanation
About 53 percent of retinal ganglion cell axons cross at the chiasm, so the nasal retina of the contralateral eye contributes slightly more fibres to each optic tract than the ipsilateral temporal retina. B left tract lesion therefore leaves a mild relative afferent defect in the left eye, the eye whose hemifield is blind. This asymmetry helps localise a homonymous hemianopia to the tract rather than to the retrogeniculate pathway, where pupillary afferents have already synapsed.
Reference: Khurana's Comprehensive Ophthalmology, 8th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.