A patient with a left temporal lobe tumor is found to have a right superior homonymous quadrantanopia. Which structure has been interrupted?

• A Fibres of the optic tract serving the upper quadrant
• B Superior bundle of the optic radiation in the parietal lobe
• C Meyer's loop in the temporal lobe ✓
• D Calcarine cortex above the calcarine sulcus

Explanation

After synapsing in the lateral geniculate body, fibres from the inferior retinal quadrants sweep forward into the temporal lobe as Meyer's loop before turning back to the occipital cortex. Damage to Meyer's loop therefore removes the contralateral superior visual field quadrant, classically producing a pie-in-the-sky defect. The parietal radiation bundle carries superior field fibres, so its lesion gives an inferior quadrantanopia, and occipital lesions give congruous hemianopias with macular sparing rather than a pure quadrant defect.

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

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Written and medically reviewed by the StethoPrep medical team.