Chapter 14 - Brainstem Mapping

Abstract

This chapter discusses the concepts of brainstem mapping. Brainstem mapping is a neurophysiological method of locating the 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 of the distorting effects of a tumor 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 are electrically stimulated by the surgeon using a hand-held probe. Electromyographic responses are recorded with electrodes inserted in the muscles of the head that are innervated by cranial motor nerves. This technique is found to be useful for locating cranial nerve motor nuclei before tumor resection and enables the surgeon to avoid damaging the nuclei when entering the brainstem. Pontine tumors displace the CMN of the nerve VII around the edge of the tumor, and medullary tumors ventrally displace the low CMN. Mapping critical neural structures during surgery has become an important concept in intraoperative neurophysiology.