ObjectiveTo explore optimal current intensity for neural monitoring of vagus nerve and recurrent laryngeal nerve during the thyroid and parathyroid surgery, so that we can judge function, location, identify, and protect the nerve more effectively and more quickly. MethodA total of 100 patients who underwent thyroid or parathyroid operations by the same surgeon in West China Hospital, meanwhile accepted intraoperative neuromonitoring (IONM), and 186 nerves at risk were enrolled in this study. According to the standardized process of nerve monitoring, we stimulated the vagus nerve with the current strength of 1-5 mA, and respectively stimulated laryngeal recurrent nerve with 1-3 mA indirectly and directly, and recorded the amplitude of electromyographic signal, and changes of heart rate and blood pressure during the process. The purpose was seeking the optimum current strength for each stage of IONM. ResultsIn 186 vagus nerves being tested, when monitoring the vagus nerve outside the carotid sheath, 109 vagus nerves (58.6%) sent out signals and got stable electromyography and warning tone with 1 mA, 164 (88.2%) vagus nerves had signals with 2 mA, 177 (95.2%) vagus nerves had signals with 3 mA, 182 (97.8%) vagus nerves had signals with 5 mA. Before and after the vagus nerve stimulation, heart rate and blood pressure of patients had no significant change. When directly monitoring the vagus nerve with 1 mA, V1 signals had no response in 2 vagus nerves (1.1%), V2 signals had no response in 9 vagus nerves (4.8%). But if the current intensity of stimulation was 2 mA or 3 mA, all patients got stable electromyographic signals. When searching for the laryngeal recurrent nerve, 92 (49.5%) got signals with 1 mA, 171 (91.9%) got signals with 2 mA, 184 (98.9%) got signals with 3 mA. When identifying laryngeal recurrent nerve and others, if the intensity of current was more than 2 mA, the current might conduct around and produce illusion. However, if the intensity of stimulation current was 1 mA, there's no electromyographic signal when we put the probe onto the tissue close to the laryngeal recurrent nerve. During identification of branches of laryngeal recurrent nerve with current strength of 1 mA, each electromyographic signal could be obtained. The chief branch into the throat produced the highest amplitude. The esophagus and trachea branch emg amplitude value was similar, equalling to 1/3-1/4 of the amplitude value in chief branch. ConclusionsWe suggest using current intensity of 5 mA on the surface of the carotid sheath to monitor the vagus nerve indirectly and obtain V1 signal, as an alternative to opening the carotid sheath. If fail, dissecting the carotid sheath, and using current intensity of 3 mA to monitor the vagus nerve directly; 3 mA is the optimal current intensity to search for the laryngeal recurrent nerve, and 1 mA is the optimal current intensity to identify the laryngeal recurrent nerve and its branches of esophagus and trachea, blood vessels, and so on.