Accessory nerve neurinoma mimicking a brain stem tumor on angiography: report of a case.

A case of accessory nerve neurinoma presenting with a so-called "intraaxial mass sign" on the angiograms is reported. The tumor originated from the intracranial portion of the accessory nerve and displaced the lateral medullary segment of the posterior inferior cerebellar artery posterolaterally. The displacement of the artery can be understood by the microanatomical relations.