Ataxic hemiparesis from a midbrain mass.

A 10-year-old boy with leukemia developed ipsilateral corticospinal and cerebellar signs: the ataxic hemiparesis syndrome. Computed tomography revealed a mass lesion in the contralateral rostral midbrain. Previous reports of this syndrome placed the lesion in the basis pontis. The anatomical substrate of the ataxic hemiparesis syndrome includes the basis pontis, rostral midbrain, and possibly other areas within the central nervous system.