A patient with a midbrain lesion has ipsilateral CN III palsy with contralateral hemiplegia. This is called Weber syndrome and involves damage to which specific structure alongside the corticospinal fibers?

• A Red nucleus
• B Substantia nigra
• C Superior colliculus
• D CN III fascicles passing through the crus cerebri ✓

Explanation

Weber syndrome results from ventral midbrain infarction affecting the crus cerebri (corticospinal/corticobulbar fibers) and CN III fascicles that pass through the medial portion of the midbrain tegmentum before exiting the interpeduncular fossa. The ipsilateral CN III palsy (ptosis, mydriasis, downward-outward gaze) combined with contralateral hemiplegia (due to corticospinal fiber damage before decussation) is the hallmark. The red nucleus is dorsal and when damaged produces Benedikt syndrome (ipsilateral CN III + contralateral tremor/chorea).

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

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Written and medically reviewed by the StethoPrep medical team.