Post-tetanic transcranial motor evoked potentials augment the amplitude of compound muscle action potentials recorded from innervated and non-innervated muscles.

Abstract

Transcranial electrical stimulation used to produce motor evoked potentials (TES-MEPs) and subsequent compound muscle action potential (CMAP) recording is widely used to monitor motor function during surgery when there is risk of damaging the spinal cord. Nonetheless, some muscles do not produce CMAP amplitudes sufficient for intraoperative monitoring. This study aimed to investigate the utility of tetanic stimulation at single and multiple peripheral nerve sites for augmenting CMAP amplitudes recorded from innervated and non-innervated muscles. A retrospective study was carried out. The study sample comprised 24 patients with cervical myelopathy who underwent decompression surgery at our department between November 2005 and March 2007. Compound muscle action potential amplitude was a physiological measure. We used two patterns of tetanic peripheral nerve stimulation for each patient. The first pattern consisted of tetanic stimulation of the left tibial nerve only (Pattern 1), and the second pattern consisted of tetanic stimulation of the bilateral median nerves and left tibial nerve (Pattern 2). Compound muscle action potential amplitudes from all muscles were augmented by both tetanic stimulation patterns compared with conventional TES-MEP recording; however, Pattern 2 elicited the greatest augmentation of CMAP amplitudes, especially for CMAPs recorded from the bilateral abductor pollicis brevis muscles. Although tetanic stimulation of a single peripheral nerve increased CMAP amplitudes recorded from both innervated and non-innervated muscles, CMAP amplitudes were best augmented when the corresponding nerve received tetanic stimulation. Additionally, tetanic stimulation of multiple nerves rather than a single nerve appears to provide better augmentation.