An experimental study of needle recording electrodes placed on the thyroid cartilage for neuralmonitoring during thyroid surgery

Background: Needle electrodes placed on the thyroid cartilage (TC) are an alternative to endotracheal tube (ET) electrodes to assess the function of the recurrent laryngeal nerve (RLN) during thyroid surgery. This study assessed the use of needle electrodes on the TC to obtain an electromyography (EMG) signal profile of the RLN in an experimental porcine model. Methods: Continuous intraoperative neuromonitoring (CIONM) was used to record the electromyogram. Each TC side was delineated into nine areas in order to find the optimal placement of the electrode. Needle electrode area, depth, and orientation were evaluated for optimal EMG amplitudes. RLN root locations were stimulated at four locations: vagus nerve distal to the CIONM electrode; and most proximal; middle; and laryngeal entry point of the RLN. A nerve retraction injury model was adapted to compare RLN monitoring by TC and ET electrodes. Results: One of the nine areas was identified as the optimal site for placement of needle electrodes. The electromyograms obtained from the various needle insertion depths and orientations were similar. The latencies obtained from the TC and ET electrodes were similar. The amplitude profile of the TC electrodes responded earlier than did the ET electrodes to RLN injury (0.88 ± 1.25 s). Amplitude and drop to loss-of-signal were also registered earlier (3.88 ± 1.89 and 5.25 ± 1.98 s, respectively). Conclusion: EMG amplitudes obtained through TC electrodes were higher, and recorded RLN injury earlier relative to ET electrodes.