Misinterpretation of parahippocampal herniation for a posterior fossa tumor: imaging and intraoperative findings.

Tumors of the hippocampal and parahippocampal gyrus are not uncommon, nor are lesions of the cerebellopontine and ambient cisterns. Lesions in these areas include astrocytomas, meningiomas, gangliogliomas, hamartomas, neurilemomas, epidermoids, and dermoids. Preoperative imaging is an important tool that can help distinguish lesions, making surgical therapy more targeted and appropriate. The authors report a case of a 41-year-old man with right-sided atypical trigeminal neuralgia and a left ambient cistern abnormality that was radiologically interpreted as a mesial temporal lobe or extramedullary ambient cistern mass lesion. Surgical exploration for biopsy revealed an abnormal posterior fossa anatomy and a parahippocampal herniation into the perimesencephalic cistern. Biopsy confirmed the finding of normal brain.