Intraoperative neurophysiologic monitoring: basic principles and recent update

Abstract

The recent developments of new devices and advances in anesthesiology have greatly improved the utility and accuracy of intraoperative neurophysiological monitoring (IOM). Herein, we review the basic principles of the electrophysiological methods employed under IOM in the operating room. These include motor evoked potentials, somatosensory evoked potentials, electroencephalography, electromyography, brainstem auditory evoked potentials, and visual evoked potentials. Most of these techniques have certain limitations and their utility is still being debated. In this review, we also discuss the optimal stimulation/recording method for each of these modalities during individual surgeries as well as the diverse criteria for alarm signs.

Keywords: Evoked Potential; Guideline; Intraoperative Neurophysiologic Monitoring.

Fig. 1

Waves of median nerve somstosensory evoked potential (MNSEP) and posterior tibial nerve somatosensory evoked potential (PTSEP). Electrical stimulation at the median nerve and posterior tibial nerve can evoke generation of the cortical waves of N20 and P37, respectively.

Fig. 2

Halogenated anesthetics can abolish the generation of motor evoked potential (MEP). MEP is generated well in a patient underwent surgery with total venous anesthesia (A). The MEP in this patient completely disappears on using halogenated gas anesthetics (B). This patient had no post-operative neural deficit.

Fig. 3

Train of four (TOF) can monitor the degree of neuromuscular blockage in IOM with muscle motor evoked potential (mMEP). Both number and amplitude of muscle twitch, obtained with four consecutive electrical stimulation at the motor nerve (TOF), represent the degree of neuromuscular blockage. No muscle twitch is observed with the TOF and no mMEP are generated (A). When 4 muscle twitches were observed with TOF but the amplitude fourth muscle twitch (T4) were relative small, compared with that of first muscle twitch (T1), mMEP with small amplitude are generated (B). When four muscle twitches with constant amplitude are observed with TOF, mMEP with large amplitude are observed (C).

Fig. 4

Monitoring for the recurrent laryngeal nerve injury in thyroid surgery. During thyroidectomy, compound muscle action potential (CMAP) is recorded at the vocal cord muscles, using continuous nerve stimulation technique (A). The CMAP is abruptly lost during surgery and does not recover at the end of surgery (B). This patient experienced post-operative complication of the vocal cord paralysis.