Technical aspects of the IV ventricle mapping during brainstem surgery

The modern technique of brainstem tumor surgery is based on approach through safe entry zones via the floor of the IV ventricle. These zones can be identified by electrical stimulation of the floor and evoked electromyographic (EMG) recordings of muscles of the head. In 20 patients with glioma and cavernoma of the caudal brain stem we studied technical aspects of this IV ventricle mapping. Stimulating techniques: constant current versus constant voltage; types of electrodes: bipolar forceps, monopolar probe, coaxial electrode; types of insulation of the electrode: flush tip versus bare tip were compared. EMG electrodes from muscles supplied by the VII, IX, X, XII cranial nerves of both sides were connected simultaneously to the two channel amplifier. It allowed us to find EMG response free areas of the floor of the IV ventricle, that is the safe entry zone. However a variant of mixed EMG recording does not allow to attribute any EMG response to definite motor structure of the brain stem. The results that the best technique for mapping of motor structures of the brain stem is constant current stimulation by flush tip coaxial electrode with a bipolar regime. In 14 patients we could get EMG response during stimulation of the hypoglossal and facial triangles of the rhomboid fossa from both left and right sides. In the other 6 cases identification of motor structures was partially successful. In 4 cases of them it probably depended on displacement of motor structures and in 2 cases there was severe preoperative deficit of the corresponding motor functions.