In the optic chiasm, the specific fiber arrangement determines the nature of visual field defects. A craniopharyngioma compressing the inferior aspect of the optic chiasm first damages which fiber group?

• A Uncrossed fibers from the temporal retinae (temporal hemiretinal fibers)
• B Crossing inferior nasal fibers from both eyes (von Willebrand's knee fibers) ✓
• C Superior nasal fibers passing through the dorsal chiasm
• D Macular fibers coursing through the central chiasm

Explanation

In the optic chiasm, the crossing fibers (from the nasal retinae) occupy the central and ventral parts of the chiasm. The inferior nasal retinal fibers from each eye briefly loop anteriorly into the contralateral optic nerve before crossing — these are called von Willebrand's knee (Wilbrand's knee). A craniopharyngioma arising from the pituitary stalk compresses the chiasm from below, first damaging these inferior crossing fibers. The resulting field defect is a superior bitemporal quadrantanopia (affecting upper temporal visual field quadrants) before full bitemporal hemianopia develops. This is why early craniopharyngioma field defects are superior, unlike pituitary adenomas that cause inferior-then-full bitemporal hemianopia.

Reference: BD Chaurasia's Human Anatomy, 8th ed.

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Written and medically reviewed by the StethoPrep medical team.