Different effects of tetanic stimulation of facial nerve and ulnar nerve on transcranial electrical stimulation motor-evoked potentials.

OBJECTIVE Our objective was to examine whether prior tetanic stimulation of cranial nerves enhances the amplitudes of transcranial motor-evoked potentials (MEPs). METHODS Thirty patients undergoing elective craniotomy under propofol-fentanyl anesthesia with partial neuromuscular blockade were enrolled. Both control and posttetanic MEPs (c-MEPs and p-MEPs) monitoring were performed with a train of five pulses delivered to C3 or C4. c-MEPs were recorded from target muscles and p-MEPs were obtained 1 s after tetanic stimulation to the ulnar nerves and facial nerves. The amplitudes of paired MEPs were compared with Wilcoxon's signed rank test. RESULTS When tetanic stimulation was separately applied to the facial nerves, amplitudes of p-MEPs from abductor pollicis brevis, orbicularis oculi or oris were similar with those of c-MEPs. When tetanic stimulations were separately applied to the ulnar nerves, the amplitudes of p-MEPs from the abductor pollicis brevis but not orbicularis oculi or oris were significantly enlarged compared with c-MEP. CONCLUSIONS We found that only prior tetanic stimulation of ulnar nerve but not facial nerve could enlarge the amplitudes of trancranial hand MEPs. Augmentation of MEP amplitude via prior tetanic stimulation of peripheral nerve seems to originate from the subcortical level but not motor cortex.