Electrophysiologic monitoring correlates of recurrent laryngeal nerve heat thermal injury in a porcine model

Objectives: HypothesisThermal injury to the recurrent laryngeal nerve (RLN) may not be visually apparent and may go unrecognized intraoperatively. This study aimed to investigate the heat thermal tolerance of RLN and evaluate the electrophysiologic correlates of electromyographic (EMG) signal change during an acute RLN heat damage. Study DesignProspective porcine model with continuous intraoperative neuromonitoring (CIONM). Methods:Ten pigs (20 RLNs) undergoing CIONM had their EMG tracings recorded and correlated with heated normal saline (NS) irrigation of varying temperature and duration. Results: In the initial pilot study, the EMG was without change during incremental heated NS irrigation (40/45/50/55 degrees C for 60 seconds), but adverse EMG combined events (CE) (amplitude decrease with a concordant latency increase) occurred and degraded to loss of signal (LOS) (by 17.51.3 seconds) when the temperature was elevated to 60 degrees C (n=4). Another 16 RLNs were evaluated to further compare the EMG pattern after various degrees of thermal stress (60/70 degrees C for 30/20 seconds). Electromyographic recordings showed CEs and LOS in all RLNs, and only six of eight RLNs with 60 degrees C exposure showed slight EMG amplitude recovery (16%-35%) after 20 minutes. None of the injured nerve segments were visually apparent, but all were detectable by IONM. Conclusion: Sixty degrees Celsius is a critical temperature to cause RLN thermal injury. Continuous intraoperative neuromonitoring can be used as a tool for the early detection of acute thermal stress and may guide use of energy-based devices during thyroid procedures.