A 42-year-old woman with bitemporal visual field defects undergoes MRI, which shows a pituitary macroadenoma compressing the optic chiasm from below. Which fibres are compressed to produce this pattern?

• A Uncrossed temporal retinal fibres in the lateral chiasm
• B Crossing nasal retinal fibres from both eyes in the central chiasm ✓
• C Wilbrand knee fibres looping into the opposite optic nerve
• D Macular fibres decussating in the posterior chiasm

Explanation

Nasal retinal fibres, which receive input from the temporal visual fields, cross in the central portion of the chiasm. B tumour compressing the chiasm from below strikes these crossing fibres first, producing bitemporal hemianopia. Uncrossed temporal retinal fibres lie laterally and would be hit by a lesion pushing from the side, as with an internal carotid artery aneurysm, giving a nasal field defect. Macular fibres are relatively spared early because they sit posteroinferiorly.

Reference: Snell's Clinical Neuroanatomy, 8th ed.

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Written and medically reviewed by the StethoPrep medical team.