WS5.4. EMG Monitoring: Free-Run and Triggered

Abstract

The recording of compound muscle action potentials (CMAPs) in response to spontaneous or electrically stimulation of the cranial, spinal, peripheral nerves or nerve root activation is known as intraoperative electromyography (EMG). EMG monitoring is widely used in surgeries when the aforementioned neural structures might be at risk. It allows early detection of surgery- related nerve injury so that proper intervention can be accordingly adopted; it also allows confirmation of the functional status of the nerve. The intraoperative monitoring team must thoroughly understand the nervous pathways that require monitoring and mapping in order to develop optimal stimulation and recording strategies for each surgical procedure. Intraoperative EMG testing includes triggered EMG (tEMG) for mapping and free-run EMG (fEMG) for monitoring purposes. Triggered EMG is used to identify the nerves and nerve root, to define a safe entry pathway to access the lesion, or to assess the functional integrity of the nerve so that the surgical plan can be modified accordingly. Clinical applications of tEMG also include confirmation of the placement of pedicle screws during spine surgery and guiding the sectioning of the lumbosacral rootlets during selective dorsal rhizotomy. The following three kinds of stimulation electrodes are available: monopolar, bipolor, and tripolar electrode. Each of the aforementioned electrodes are associated with certain strengths and weakness; thus, they can be used under different circumstances and meets most of our clinical needs. fEMG involves recording spontaneous or evoked muscle activity without interfering with the surgery. It is used as a monitoring tool to detect surgically driven mechanical irritation of the peripheral nerves and motor cranial nerves, hopefully before permanent damage occurs. It also can be applied to detect muscle activity evoked by mechanical irritation of the corticospinal tract and/or alpha motor neurons during brain tumor or intramedullary spinal cord tumor resection. There are three types of fEMG activity, referred to as the A, B and C train. Among them, the A train has been more associated with poor postoperative outcomes, although false-positive and false-negative findings do exist. Thus, the concomitant use of somatosensory and motor evoked potentials will certainly increase the sensitivity and specificity of fEMG.