A 48-year-old woman with bitemporal visual field loss from a pituitary macroadenoma is asked why her upper temporal fields were affected before the lower temporal fields. The correct explanation is that the tumour compresses, first:

• A The inferior nasal retinal fibres, which lie inferomedially in the chiasm ✓
• B The superior nasal retinal fibres, which decussate posteriorly in the chiasm
• C The macular fibres, which cross in the centre of the chiasm
• D The uncrossed temporal retinal fibres, which pass laterally in the chiasm

Explanation

Fibres from the inferior nasal retina, which subserve the upper temporal visual field, sweep anteroinferiorly in the chiasm. An enlarging pituitary tumour compresses the chiasm from below, so these fibres are affected first, producing an upper temporal quadrantic defect that progresses to complete bitemporal hemianopia. Option C is tempting because macular fibres do cross centrally, but they are involved late, explaining the relative preservation of central acuity until the tumour is large.

Reference: Parsons' Diseases of the Eye, 23rd ed.

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Written and medically reviewed by the StethoPrep medical team.