A patient has a complete left homonymous hemianopia. When light is shone specifically onto the blind halves of both retinas there is no pupillary constriction, while light on the seeing halves constricts both pupils normally. This sign, which localises the lesion, is called:
- A Marcus Gunn pupil, indicating an optic nerve lesion
- B Wernicke's hemianopic pupil, indicating an optic tract lesion ✓
- C Argyll Robertson pupil, indicating a midbrain lesion
- D Hutchinson pupil, indicating uncal herniation
Correct answer: B. Wernicke's hemianopic pupil, indicating an optic tract lesion
Explanation
The hemianopic reaction of the pupil occurs when the lesion interrupts the optic tract itself, because afferent pupillary fibres leave the visual pathway at the pretectal nuclei just before the lateral geniculate body. Fibres from each half of the retina still travel together in the tract, so illuminating the blind hemiretina fails to drive the light reflex. Lesions behind the lateral geniculate body spare this sign because pupillary fibres have already exited.
Reference: Parsons' Diseases of the Eye, 22nd ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
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