INTRAOPERATIVE IDENTIFICATION OF MOTOR AREAS OF THE RHOMBOID FOSSA USING DIRECT STIMULATION

Intraoperative electrical identification of motor areas within the flo or of the fourth ventricle was successfully carried out in a series of 10 patients with intrinsic pontine lesions and lesions infiltrating t he brain stem. Direct electrical stimulation was used to identify the facial colliculus and the hypoglossal triangle before the brain stem w as entered. Multichannel electromyographic recordings documented selec tive stimulation effects. The surgical approach to the brain stem was varied according to the electrical localization of these structures. D uring removal of the lesion, functional integrity was monitored by int ermittent stimulation. In lesions infiltrating the floor of the fourth ventricle, stimulation facilitated complete removal. Permanent postop erative morbidity of facial or hypoglossal nerve dysfunction was not o bserved. Mapping of the floor of the fourth ventricle identifies impor tant surface structures and offers a safe corridor through intact nerv ous structures during surgery of brain-stem lesions. Reliable identifi cation is particularly important in mass lesions with displacement of normal topographical anatomy.