The importance of brainstem mapping in brainstem surgical anatomy before the fourth ventricle and implication for intraoperative neurophysiological mapping
OBJECTIVE To identify patterns of cranial motor nuclei (CMN) displacement in cases of intramedullary brain stem tumor, using neurophysiological mapping of motor nuclei on the floor of the fourth ventricle. METHODS Relationships between tumors and CMN were reviewed in 18 patients with brain stem tumors (seven pontine, nine medullary, and two pontomedullary tumors) and 2 with cervicomedullary junction spinal cord tumors. CMN VII, IX/X, and XII were mapped by applying electrical stimuli over the surgically exposed fourth ventricular floor through a handheld probe and recording electrical activity in the appropriate cranial muscles. RESULTS Tumors distorted the anatomic location of CMN in repetitive patterns according to tumor site. Three patterns were identified as follows: Type 1, CMN located around the tumor on the floor of fourth ventricle; Type 2, one or more CMN located ventrally to the tumor; Type 3, CMN in original anatomic position. Six of seven patients with pontine tumors showed the Type 1 pattern. Seven of nine patients with medullary tumors showed Type 2, and the other two showed Type 1. Both patients with pontomedullary tumors showed Type 2. One patient with a cervicomedullary junction spinal cord tumor showed Type 1 and the other Type 3, depending on the tumor extension into the fourth ventricle. CONCLUSION Pontine tumors push the CMN to around the tumor edge, suggesting that precise localization of CMN before tumor resection is necessary to avoid their damage. Medullary tumors grow more exophytically and compress the CMN ventrally. Understanding patterns of CMN displacement can help surgeons establish the surgical plan, minimize risks, and enable safer surgery of brain stem tumors.