Objectives/hypothesis: Intraoperative neuromonitoring (IONM) is a useful adjunct for recurrent laryngeal nerve (RLN) mapping and identification in transoral endoscopic thyroidectomy vestibular approach (TOETVA). This experimental study aimed to investigate the feasibility, safety, thresholds required of an endoscopic forceps that combine the function of surgical dissection and nerve stimulation.
Study design: Prospective experimental research.
Methods: TOETVA was performed in 12 piglets, i.e., 24 RLNs and 24 vagal nerves (VN). RLNs electromyography (EMG) was recorded via endotracheal surface electrodes. Baseline EMG of VN and RLN were recorded and compared by (a) percutaneously placed monopolar stimulator probe (Group I), (b) adapted Maryland endoscopic dissector applied on nerves at its tip-end (Group II) and (c) endoscopic dissector tip-lateral applied (Group III). EMG profiles, amplitude, latency, waveform, thresholds and supra-maximal stimulation (5 mA) were analyzed.
Results: Application of the endoscopic device was feasible in all TOETVA and did not result in any morbidity. 24 RLNs and VNs were detected, stimulated and monitored. With increase of stimulation current, the amplitude of EMG increased, showing a dose-response curve. Mean VN stimulation thresholds were: Group I 0.28 mA, Group II 0.56 mA, Group III 0.58 mA (P1 = 0.00, P2 = 0.00, P3 = 0.11). Minimal current to evoked a maximal VN response was: Group I 0.65 mA, Group II 1.07 mA and Group III 1.14 mA (P1 = 0.00, P2 = 0.00, P3 = 0.48). Minimal current to evoke a RLN maximal response was Group I 0.6 mA, Group II 0.95 mA and Group III 1.05 mA (P1 = 0.00, P2 = 0.00, P3 = 0.31). Latency values were similar to each group. Repetitive (> 10 min) supra-maximal (> 5 mA) electrical stimulation was safe.
Conclusions: The application of endoscopic stimulating dissector is simple, effective and safe way to monitor both VN and RLN function during a TOETVA animal model. It provides surgeons with real-time feedback of EMG response and can be applied as a tool for RLN monitoring. Endoscopic instrument required higher current to evoke EMG response compared to hand probe stimulation. Tip-end required less current to evoke EMG response compared to tip-lateral mode of stimulation.
Keywords: Dissecting instruments; Endoscopic thyroidectomy; IONM; Neural monitoring; Recurrent laryngeal nerve; TOETVA; Transoral thyroidectomy; Vagus nerve.