ACNS Guideline: Transcranial Electrical Stimulation Motor Evoked Potential Monitoring.

Motor evoked potentials (MEPs) are electrical signals recorded from neural tissue or muscle after activation of central motor pathways. They complement other clinical neurophysiology techniques, such as somatosensory evoked potentials (SEPs), in the assessment of the nervous system, especially during intraoperative neurophysiologic monitoring (IONM). Somatosensory evoked potentials directly assess only a part of the spinal cord, the dorsal columns (Emerson, 1988), and also the medial lemniscus, the thalamocortical radiations, and somatosensory cortex. Because they provide indirect surveillance of the motor tracts, their use has been shown to improve neurologic outcomes during spinal surgery (Nuwer et al., 1995). However, SEPs can fail to detect damage to the spinal cord motor pathways when the dorsal columns are spared (Ben-David et al., 1987; Ginsburg et al., 1985; Jones et al., 2003; Krieger et al., 1992; Legatt et al., 2014; Zornow et al., 1990); this led to the development of techniques for directly monitoring the central motor pathways. Most often, this is accomplished using transcranial electrical stimulation (TES) of the brain and recording of evoked neural or myogenic activity caudal to the area that is at risk during surgery (Legatt, 2002). During TES, high-intensity stimuli must be delivered to the scalp to stimulate the brain through the intact skull, with stimulus voltage and current levels far above those used to elicit SEPs. If a craniotomy permits direct stimulation of motor cortex by electrodes placed on the brain surface, low-intensity direct cortical stimulation can also be used to elicit MEPs for IONM (Szelényi et al., 2007b; Taniguchi et al., 1993). Direct cortical stimulation is outside the scope of this guideline, but the recommendations herein for the recording of the MEPs that are elicited by transcranial electrical brain stimulation would also apply to recording of MEPs elicited by direct cortical stimulation. Transcranial magnetic stimulation has also been used to elicit MEPs by inducing electrical current flows within the brain tissue without passing large amounts of current through the scalp. This reduces stimulation of pain fibers in the scalp, skull, and meninges and makes it a practical technique for MEP studies in awake subjects (Chen et al., 2008). However, transcranial magnetic stimulation is not the optimal MEP technique for IONM because of the anesthetic suppression of transcranial magnetic stimulation– MEPs which are generated mainly by eliciting I-waves (see section on Definitions and Physiology, below) and difficulties in maintaining a constant position of the coil relative to the patient’s head (Legatt, 2004). Neither TES with single stimulus pulses nor transcranial magnetic stimulation consistently produces robust myogenic MEPs suitable for IONM. The commercial availability of stimulators that can deliver trains of high-intensity electrical pulses has made reliable MEP monitoring using TES possible in most patients. At this time, the techniques for recording and interpreting TES-MEPs have become sufficiently well established to warrant the formulation of these guidelines. Personnel performing TES-MEP monitoring must be cognizant of the technical challenges and risks of the technique.