Intraoperative identification of motor areas of the rhomboid fossa using direct stimulation

Intraoperative electrical identification of motor areas within the floor of the fourth ventricle was successfully carried out in a series of 10 patients with intrinsic pontine lesions and lesions infiltrating the brain stem. Direct electrical stimulation was used to identify the facial colliculus and the hypoglossal triangle before the brain stem was entered. Multichannel electromyographic recordings documented selective stimulation effects. The surgical approach to the brain stem was varied according to the electrical localization of these structures. During removal of the lesion, functional integrity was monitored by intermittent stimulation. In lesions infiltrating the floor of the fourth ventricle, stimulation facilitated complete removal. Permanent postoperative morbidity of facial or hypoglossal nerve dysfunction was not observed. Mapping of the floor of the fourth ventricle identifies important surface structures and offers a safe corridor through intact nervous structures during surgery of brain-stem lesions. Reliable identification is particularly important in mass lesions with displacement of normal topographical anatomy.