Feasibility of intraoperative neuromonitoring during thyroid surgery using transcartilage surface recording electrodes

Background: Intraoperative neural monitoring (IONM) has gained widespread acceptance as an adjunct to the gold standard of visual identification of the recurrent laryngeal nerve (RLN) during thyroid surgery. Currently, laryngeal electromyography (EMG) recording during IONM is almost always performed using endotracheal tube (ETT) surface electrodes placed adjacent to vocal folds originating from the inner surface of the thyroid cartilage (TC). Therefore, we hypothesized that surface recording electrodes placed on the outer surface of the TC should enable access to the EMG response of the vocal folds during IONM. The aims of this experimental study were to evaluate the feasibility of the transcartilage approach for laryngeal EMG recording during IONM. Methods: A porcine model (12 pigs and 24 RLN sides) with well-established applicability in IONM research was used for the experiments. Both ETT electrodes adjacent to vocal folds and adhesive pre-gelled electrodes on the TC were used for EMG recording during IONM. Electrically evoked EMG signals detected by both electrode types were recorded and analyzed. EMG changes during tracheal displacement and RLN traction injury were compared. Results: Both the ETT and TC electrodes recorded typical laryngeal EMG waveforms evoked by a 1 mA stimulus current applied on both sides of the RLNs and vagus nerves (VNs). Under RLN stimulation, the mean EMG amplitudes recorded with the ETT and TC electrodes were 973μV (±179) and 695μV (±150), respectively. Under VN stimulation, the mean amplitudes were 841μV (± 163) and 607μV (± 162), respectively. When upward displacement of the trachea was experimentally induced, the TC electrodes showed less variation in recorded EMG signals compared to ETT electrodes. When RLN traction stress was experimentally induced, both the ETT and TC electrodes accurately recorded the typical EMG pattern of progressively degrading amplitude and gradual recovery after release of traction. Conclusions: This study confirmed the feasibility of using transcartilage surface electrodes for recording of laryngeal EMG signals evoked during IONM in an animal model. Before practical application of this approach in clinical thyroid surgery, however, further studies are needed to improve electrode designs by optimizing their shapes and sizes, and increasing their adhesive stability and sensitivity.