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

Abstract

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.

Keywords: Motor-evoked potentials; abductor pollicis brevis; facial nerve; orbicularis oculi; orbicularis oris; tetanic stimulation; ulnar nerve.

Figure 1

The schematic graphic of tetanic stimulation prior to transcranial electric stimulation and motor evoked potentials (MEPs) recording. A: Tetanic stimulation of bilateral ulnar nerves before transcranial electric stimulation and MEPs were unilaterally recorded in target muscles (abductor pollicis brevis, orbicularis oculi or oris). B: Tetanic stimulation of bilateral rami temporales nervi facialis or rami buccales nervi facialis before transcranial electric stimulation and MEPs were unilaterally recorded in target muscles (abductor pollicis brevis, orbicularis oculi or oris).

Figure 2

Amplitudes of control motor-evoked potentials (c-MEPs) and post-tetanic motor-evoked potentials (p-MEPs) from orbicularis oculi (OOC), orbicularis oris (OOR) and abductor pollicis brevis (APB). The Wilcoxon’s signed rank test is used to compare the amplitudes of paired MEPs. A: Tetanic stimulation of the ipsilateral rami temporales nervi facialis; B: Tetanic stimulation of the ipsilateral rami buccales nervi facialis; C: Tetanic stimulation of the contralateral rami temporales nervi facialis; D: Tetanic stimulation of the contralateral rami buccales nervi facialis; E: Tetanic stimulation of the ipsilateral ulnar nerve; F: Tetanic stimulation of the contralateral ulnar nerve.

Figure 3

The representative waveforms of paired motor-evoked potentials (c-MEPs and p-MEPs) recorded from target muscles. A: The waveforms of paired MEPs recorded from orbicularis oculi with prior tetanic stimulation of the ipsilateral (1-2 channels) and contralateral (3-4 channels) rami temporales nervi facialis in the same patient; B: The waveforms of paired MEPs recorded from abductor pollicis brevis with prior tetanic stimulation of the ipsilateral (1-2 channels) and contralateral (3-4 channels) rami temporales nervi facialis in the same patient; C: The waveforms of paired MEPs recorded from orbicularis oculi with prior tetanic stimulation of the ipsilateral (1-2 channels) and contralateral (3-4 channels) ulnar nerve in the same patient; D: The waveforms of paired MEPs recorded from abductor pollicis brevis with prior tetanic stimulation of the ipsilateral (1-2 channels) and contralateral (3-4 channels) ulnar nerve in the same patient.