Optic chiasm compression — MRCPsych Paper A MCQ
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Correct answer: E — Inferior optic chiasm
The correct answer is E, the inferior optic chiasm. Each temporal visual field projects onto the nasal retina, whose axons cross in the optic chiasm. Fibres carrying the superior temporal fields run inferiorly in the chiasm and are therefore affected early when a pituitary macroadenoma expands upwards from the sella. A left optic tract lesion would cause a right homonymous hemianopia, while a right optic nerve lesion would produce monocular right-sided visual loss. Bilateral inferior optic-radiation lesions would cause superior homonymous, rather than bitemporal, defects. A lesion of the right superior calcarine bank would cause a left inferior homonymous quadrantanopia. The bilateral superior temporal pattern therefore specifically localises the lesion to the inferior chiasm.
Reference: Neuro-ophthalmic evaluation and management of pituitary disease, section/figure on visual-field defects associated with chiasmal compression, 2024. https://pubmed.ncbi.nlm.nih.gov/39039214/