Homolateral hemiparesis as an early sign of cerebellar mass effect.

A patient with Wallenberg's syndrome and an inferior cerebellar infarction developed progressive hemiplegia ipsilateral to the infarction as cerebellar edema emerged. An MRI showed diagonal displacement of the medulla with impaction of the pyramids against the clivus; the hemiplegia resolved after posterior fossa decompression. In the pathologic specimen, the pyramids were flattened and showed small subpial ischemic lesions. Progressive ipsilateral hemiparesis in the setting of cerebellar infarction is an early sign of posterior fossa mass effect similar to the Kernohan's notch phenomenon.