Intramedullar stimulation of the facial and hypoglossal nerves: Estimationof the stimulated site

Aim. To determine the stimulation site of both facial and hypoglossal nerve s after transcranial magnetic stimulation. Methods. After surgical exposure of the brainstem in 22 patients with intri nsic pontine (n=9) or medullary (n=13) tumors, the facial colliculus and th e hypoglossal triangle were electrically stimulated. The EMG responses were recorded with flexible wire electrodes from the orbicularis oculi/orbicula ris oris muscles, and genioglossal muscles. Patients had no preoperative de ficit of the nerves. Results. The EMG mean latencies of the unaffected facial nerve were 5.2+/-0 .6 ms for the orbicularis oculi, and 5.2+/-0.5 ms for the orbicularis oris muscle. After the stimulation of 18 possibly affected facial nerves, the EM G mean latencies were 5.3+/-0.3 ms for the orbicularis oculi (p=0.539, unpa ired Student's t-test), and 5.4+/-0.2 ms for the orbicularis oris (p=0.122) . The EMG mean latency of the unaffected hypoglossal nerve was 4.1+/-0.6 ms for the genioglossal muscle. After the stimulation of 26 possibly affected hypoglossal nerves, the EMG mean latency for the genioglossal muscle was 5 .3+/-0.3 ms. There was a significant difference (p<0.001) in latency for ge nioglossal EMG responses between the patients with pontine and those with m edullary tumors. Conclusion. Shorter EMG mean latencies of unaffected facial nerves obtained after direct stimulation of the facial colliculi confirm that magnetic sti mulation is most likely to occur closer to the nerve's exit from the brains tem than to its entrance into the internal auditory meatus. The hypoglossal nerve seems to have the site of excitation at the axon hillock of the hypo glossal motor neurons.