Optimal stimulation during monitored thyroid surgery: EMG response characteristics in a porcine model

Objectives: To compare electromyography (EMG) characteristics of the external branch of superior laryngeal nerve (EBSLN), recurrent laryngeal nerve (RLN), and vagus nerve (VN) evoked with different stimulation probes/dissectors during monitored thyroid surgery. Study Design: Experimental porcine model. Methods: In five piglets (10 EBSLNs/RLNs/VNs), laryngeal EMG was recorded by endotracheal tube surface electrodes with stimulation using five monopolar probes (group I), three bipolar probes (group II), and two stimulation dissectors (group III). The detectable EMG response (DER) was defined as > 100 μV and was obtained with these different probes/dissectors. Electromyography parameters, stimulus-response curve, and distance-sensitivity results were compared. Results: All stimulation probes/dissectors evoked typical EMG waveforms from the EBSLN/RLN/VN with 1 mA current. A stimulus-response curve with increasing EMG amplitude with increase in stimulating current was noted, with the maximum EMG elicited by group I/III probes/dissectors at < 1 mA and at a higher current for group II probes. All groups recorded lower evoked EMG amplitudes when the nerve was stimulated with overlying fascia or when probe/dissector to nerve distance was greater. The mean amplitude decreased by 11%/33%/13% in group I/II/III probes/dissectors when stimulating nerves covered by fascia versus nerves dissected free of overlying fascia. The rate of obtaining DER at 1- or 2-mm distance was significantly higher in group I than in group II/III probes/dissectors (P < 0.001). Latency did not change with any of the stimulation probes/dissectors or trials. Conclusion: Monopolar, bipolar probes, and newer stimulation dissectors all provided valid evoked VN/RLN/EBSLN waveforms. They have different functional sensitivity profiles and vary when stimulating with fascia and at a distance from the nerve. Selection of a stimulation probe/dissector for nerve monitoring can be based on the stimulation characteristics, the intended nerve monitoring application, and the surgeon's preference. Level of Evidence: N/A. Laryngoscope, 127:998–1005, 2017.