According to the previous studies, it is reported that the use of sevoflurane in the maintenance of anesthesia may lead to delayed occurrence of positive electromyographic (EMG) signal and lower obtained signal amplitude. In this study, the investigators aimed to investigate the anesthetic effect of sevoflurane combined with dexmedetomidine for neuromonitoring during thyroid surgery. The investigators hypothesize that sevoflurane combined with dexmedetomidine can reduce the movement or spontaneous activity of vocal cords in patients and without affecting the EMG signals.

Recurrent laryngeal nerve (RLN) injury is one of the most serious common complications in thyroid operation. Compared with the traditional operation under direct eye vision alone, application of intraoperative neural monitoring (IONM) is able to help surgeon in identifying the RLN during thyroid dissection, and enabling continuous monitoring of the neurological function during operation, thus reducing the incidence of intraoperative RLN injury. Sevoflurane is a common inhaled anesthetic in clinical practice, with the effect of prolonging the action time of muscle relaxants. According to the previous studies, it is reported that the use of sevoflurane in the maintenance of anesthesia may lead to delayed occurrence of positive electromyographic (EMG) signal and lower obtained signal amplitude, which may affect the surgeon's judgment on neurological function during the operation. In this study, the investigators aimed to investigate the anesthetic effect of sevoflurane combined with dexmedetomidine for neuromonitoring during thyroid surgery. The investigators hypothesize that sevoflurane combined with dexmedetomidine can reduce movement or spontaneous activity of vocal cords in patients without affecting the IONM signal.

Inclusion Criteria:

• patients scheduled for thyroidectomy under general anesthesia
• American Society of Anesthesiologists (ASA) grade of I or II
• aged 18 to 65 years
• body mass index (BMI) < 32 kg/m2.

Exclusion Criteria:

• patients with RLN or vagus nerve injury were identified preoperatively
• severe cardiovascular, pulmonary, liver and kidney diseases
• history of allergy to narcotic include rocuronium -pregnancy or lactation-
• history or current usage of medication which may interfere neuromuscular transmission;
• family or personal history of malignant hyperthermia
• myasthenia gravis
• potential risk of airway difficulty
• unable to cooperate.

Six months after the completion of the study, original data can be obtained from the study's corresponding author.