Transcutaneous recording during intraoperative neuromonitoring in thyroid surgery

Background: Recurrent laryngeal nerve (RLN) palsy remains a major source of morbidity after thyroid surgeries. Intraoperative neural monitoring (IONM) has gained increasing acceptance as an adjunct to standard practice of visual RLN identification. Endotracheal tube (ETT) surface recording electrodes systems are now widely used for IONM; however, a malpositioned ETT can cause false IONM results and requires time-consuming intraoperative verification of the ETT position and readjustment by the anesthesiologist. The aim of this experimental study was to evaluate the feasibility of the transcutaneous approach for recording evoked laryngeal electromyography (EMG) signals during IONM. Methods: A porcine model with well-established applicability in IONM research was used. Twelve piglets (24 nerve sides) were enrolled. Electrically evoked EMGs were recorded from surface electrodes on the ETT and from the adhesive pre-gelled surface electrodes on the anterior neck skin. The evoked EMG waveforms were measured and analyzed. The real-time signal stability of the electrodes during tracheal displacement and their accuracy in reflecting adverse EMG changes during RLN stress were evaluated during continuous IONM performed with automatic periodic vagus nerve (VN) stimulation. Results: In all nerves, both the ETT and neck adhesive skin electrodes successfully recorded typical evoked laryngeal EMG waveforms from the RLNs and VNs under stimulation with 1 mA. The transcutaneous electrodes recorded mean EMG amplitudes of 264 μV (±79) under RLN stimulation and 202 μV (±55) under VN stimulation. The electrodes recorded mean EMG latencies of 2.98 ms (±0.20) under RLN stimulation, 4.51 ms (±0.50) under right VN stimulation, and 8.13 ms (±0.94) under left VN stimulation, respectively. When tracheal displacement was experimentally induced, the EMG signals obtained by ETT electrodes varied significantly, but those obtained by transcutaneous electrodes did not. When RLN traction stress was experimentally induced, both ETT and transcutaneous electrodes recorded the same pattern of progressively degrading EMG amplitude with gradual recovery after release of traction. Conclusions: This study confirms the feasibility of transcutaneous recording of evoked laryngeal EMG during IONM. Although this study confirms the stability and accuracy of the transcutaneous approach, it also revealed the need for new electrode designs to improve EMG amplitudes before practical clinical application of this approach.