Peduncular hemiplegia following removal of large cerebellopontine angle tumors: discussion of a mechanism of brain stem injury.

Large cerebellopontine angle tumors distort and displace the brain stem. Two cases are reported in which ipsilateral hemiparesis ensued postoperatively. Compression of the contralateral cerebral peduncle against the free edge of the tentorium cerebelli appears to be the mechanism leading to this mesencephalic dysfunction. Review of the literature failed to reveal a discussion of this mechanism of brain stem injury. Extension of tumor through the tentorial hiatus appears to make the mesencephalon particularly vulnerable. Incision of the tentorium prior to removal of a tumor when it invades the incisural hiatus may have merit.