FUNCTIONAL ANATOMIC RELATIONSHIP BETWEEN BRAIN-STEM TUMORS AND CRANIAL MOTOR NUCLEI

OBJECTIVE: To identify patterns of cranial motor nuclei (CMN) displace ment in cases of intramedullary brain stem tumor, using neurophysiolog ical mapping of motor nuclei on the floor of the fourth ventricle. MET HODS: Relationships between tumors and CMN were reviewed in 18 patient s with brain stem tumors (seven pontine, nine medullary, and two ponto medullary tumors) and 2 with cervicomedullary junction spinal cord tum ors. CMN VII, IX/X, and XII were mapped by applying electrical stimuli over the surgically exposed fourth ventricular floor through a handhe ld probe and recording electrical activity in the appropriate cranial muscles. RESULTS: Tumors distorted the anatomic location of CMN in rep etitive patterns according to tumor site. Three patterns were identifi ed as follows: Type 1, CMN located around the tumor on the floor of fo urth ventricle; Type 2, one or more CMN located ventrally to the tumor ; Type 3, CMN in original anatomic position. Six of seven patients wit h pontine tumors showed the Type 1 pattern. Seven of nine patients wit h medullary tumors showed Type 2, and the other two showed Type 1. Bot h patients with pontomedullary tumors showed Type 2. One patient with a cervicomedullary junction spinal cord tumor showed Type 1 and the ot her Type 3, depending on the tumor extension into the fourth ventricle . CONCLUSION: Pontine tumors push the CMN to around the tumor edge, su ggesting that precise localization of CMN before tumor resection is ne cessary to avoid their damage. Medullary tumors grow more exophyticall y and compress the CMN ventrally. Understanding patterns of CMN displa cement can help surgeons establish the surgical plan, minimize risks, and enable safer surgery of brain stem tumors.