An infarction of which brain region typically causes ipsilateral oculomotor nerve palsy and contralateral weakness?

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Multiple Choice

An infarction of which brain region typically causes ipsilateral oculomotor nerve palsy and contralateral weakness?

A ventral midbrain infarct affecting the cerebral peduncle explains this pattern. The oculomotor nerve fibers exit the midbrain here, so a small infarct can cause ipsilateral oculomotor nerve palsy with classic eye findings (ptosis and “down and out,” sometimes with pupil involvement depending on the exact fibers hit). The nearby descending corticospinal tract in the crus cerebri is also damaged, producing contralateral weakness. This combination is classic for a ventral midbrain lesion, often described as Weber syndrome. The other regions don’t typically produce this exact mix: a pons infarct involves different cranial nerves and tracts localized there; basilar artery infarction tends to cause broader brainstem failure; a posterior cerebral artery infarct mainly causes cortical or visual deficits and does not usually yield the ipsilateral CN III palsy with contralateral hemiparesis seen here.

A ventral midbrain infarct affecting the cerebral peduncle explains this pattern. The oculomotor nerve fibers exit the midbrain here, so a small infarct can cause ipsilateral oculomotor nerve palsy with classic eye findings (ptosis and “down and out,” sometimes with pupil involvement depending on the exact fibers hit). The nearby descending corticospinal tract in the crus cerebri is also damaged, producing contralateral weakness. This combination is classic for a ventral midbrain lesion, often described as Weber syndrome. The other regions don’t typically produce this exact mix: a pons infarct involves different cranial nerves and tracts localized there; basilar artery infarction tends to cause broader brainstem failure; a posterior cerebral artery infarct mainly causes cortical or visual deficits and does not usually yield the ipsilateral CN III palsy with contralateral hemiparesis seen here.

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