A patient with a left optic tract lesion has a right homonymous hemianopia. When a narrow beam of light is projected onto the LEFT half of each retina (stimulating the right visual field), the pupils do NOT constrict, whereas light on the right halves of the retinae produces brisk constriction. This sign is called:
- A Tonic pupillary response
- B Marcus Gunn pupillary phenomenon
- C Argyll Robertson reaction
- D Wernicke's hemianopic pupil reaction ✓
Explanation
Wernicke's hemianopic pupil is elicited by shining a fine beam onto the hemiretina corresponding to the blind field. Absence of constriction means the pupillary afferent fibres have already been damaged, placing the lesion anterior to the lateral geniculate body. Since postgeniculate lesions such as those of the optic radiations and visual cortex spare the pupillary fibres, this sign localises the hemianopia to the optic tract or distal optic nerve, distinguishing tract lesions from retrogeniculate ones. It requires careful slit lamp technique with a narrow beam to avoid scattering light onto seeing retinae.
Reference: Kanski's Clinical Ophthalmology, 9th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.