Neuromonitored Thyroid Surgery: Optimal Stimulation Based on Intraoperative EMG Response Features

Objectives: To evaluate/compare normative electrophysiologic electromyography (EMG) response characteristics of recurrent laryngeal, vagus, and external branch of superior laryngeal nerve evoked with different stimulators used in neuromonitored thyroid surgery. Study Design: Prospective crossover study. Methods: EMG responses obtained via endotracheal tube surface electrodes in 11 patients undergoing thyroid surgery were recorded when stimulated with four stimulators: two monopolar (Prass standard and ball tip), one bipolar, and one dissecting instrument. Normative mean EMG results including latency, amplitude, threshold, saturation currents, and distance-sensitivity were compared. Results: The Prass standard stimulator had shorter latency time when nerve was not covered with fascia (P =.04). The bipolar, dissecting instrument, and ball tip demonstrated similar latency times with and without nerve fascia. Pooled mean latency increased significantly from 1.86 ms to 2.16 ms when comparing nerves without fascia and nerves with fascia (P <.05). The Prass standard monopolar stimulator had the lowest mean threshold at 0.40 mA, with the dissecting instrument having the highest threshold at 0.89 mA for dissected nerve. Pooled mean threshold and saturation increased from 0.6 mA to 1.7 mA (P <.0001) and 1.57 mA to 4.15 mA (P <.001) with fascia covering nerve, respectively. The mean depolarization rate was 100% for monopolar and bipolar electrodes and 81% for dissecting instrument at 1 mA. Only 9% of monopolar electrodes generated an EMG response when stimulated from 2 mm away. Conclusion: Monopolar stimulators are more sensitive for neural mapping, whereas bipolar instruments are more specific, thus reducing false positive stimulation. Dissecting instruments share many features of monopolar stimulators while being more specific, and thus are a viable alternative. Level of Evidence: 2b Laryngoscope, 2020.