Brain stem mapping

Brain stem mapping is a neurophysiological method of locating cranial nerve motor nuclei (CMN) on the floor of the fourth ventricle. The motor nuclei of the cranial nerves are usually located in the vicinity of specific anatomical landmarks on the floor of the fourth ventricle. Because a tumor can cause distorting effects on the local anatomy, these landmarks are not evident in most patients. Even in patients without a tumor, specific anatomical landmarks are often not visible. Different points of the surgically exposed floor of the fourth ventricle were electrically stimulated by the surgeon using a handheld probe. Electromyographic responses were recorded with electrodes inserted in the face muscles innervated by cranial motor nerves. This technique was found to be useful for locating CMN before tumor resection and enabled the surgeon to avoid damaging the nuclei when entering the brain stem. Furthermore, intraoperative neurophysiological localization of CMN showed specific patterns of displacement by brain stem tumors. Pontine tumors displaced CMN of nerve VII around the edge of the tumor, and medullary tumors ventrally displaced low CMN. Understanding the patterns of CMN displacement can help in establishing a surgical plan that minimizes the risk of damaging CMN and allows for safer surgery for brain stem tumors.