Anatomical relationship between external branch of the superior laryngeal nerve and thyroid upper pole with intraoperative nerve monitoring in Turkish population

Objective: Detection of the injury to the external branch of superior laryngeal nerve (EBSLN) is often missed after thyroidectomy, except in patients who use their voice professionally. Injury of external branch of superior laryngeal nerve may cause serious voice disorders especially in patients who use their voice professionally. Intraoperative nerve monitoring is recommended to prevent these complications. The aim of this study was to determine the rate of anatomic variations of EBSLN branch in Turkish and to determine the relationship between the anatomic localization of the nerve and post-thyroidectomy functional disorders by using electromyographic analysis and voice disorder index. Patients and Methods: A total of 118 patients were included in the study after exclusion of the patients with secondary intervention and missing data. The EBSLN's were classified as Type 1, Type 2A and Type 2B according to Cernea classification. Cricothyroid muscle contraction and electromyographic activity on the monitor were recorded when the nerves were visually identifed and stimulated with the probe. In nerves that were not identified visually, nerve mapping was achieved by the guidance of nerve monitoring and presence of cricothyroid muscle contraction. The superior thyroid vessels were ligated individually and at a safe level inferior to the EBSLN after nerve mapping. The EMG data obtained from the EBSLN branch prior to and after lobectomy were defined as S1 and S2. Student's t test, chi-square test, paired samples T test were used for statistical analysis. The results were evaluated with a confidence interval of 95% and p <0.05 was accepted as significant. Results: A total of 120 surgical interventions were performed in 118 patients. Of the 118 patients, 91 (76%) underwent total thyroidectomy and 29 (24%) unilateral lobectomy. In 120 operations 211 EBSLN's were defined as nerves at risk (NAR). Of 211 NAR, 11 (5% ) could not be identified visually or by nerve monitoring. One hundred and fifty-four (73%) nerves were identified visually and EMG data were obtained by monitoring. Forty-six nerves could not be visualized and mapped by nerve monitoring with evoked cricothyorid muscle contraction. According to Cernea classification, the rate of type1,2 A and 2 B nerves were 58% , 36.5% 5.5%, respectively. The mean S1 amplitude values of the Cernea Type 1, 2A and 2B before lobectomy were found to be 198.6 ± 317 µV, 144 ± 83 µV and 102 ± 49 µV, respectively (p = 0.1). The mean S1 latency in Cernea Type 1, 2A and 2B were 1.6 ± 0.5 ms, 1.7 ± 0.3 ms and 1.85 ± 0.6 ms, respectively (p = 0.3). The mean S1 and S2 amplitude and latency Cernea Type 1, 2A and 2B groups showed no significant difference. There was no moderate or severe voice disturbance in any patient at the 1th and 3rd month according to Voice Disorder Index - 5. The ratio of patients with total voice index score of 0 (normal) in the preoperative period was 77%, while this rate was 37% in the first postoperative month (p = 0.0001). However, the ratio of patients with a total sound index of 0 (normal) at the 3rd postoperative month was 76%, reaching preoperative value. Conclusions: EBSLN is not visually detectable in approximately 20% of thyroidectomy cases. The use of nerve monitoring during thyroid upper pole dissection may decrease the voice disturbances that may occur after thyroidectomy by identifying the EBSLN and careful dissection of the superior pole vessels below the level of EBSLN. In the early postoperative period, the sound quality equivalent to preoperative sound quality can be achieved in patients in whom thyroidectomy was performed by identification of EBSLN and meticulous superior pole dissection by avoidance of EBSLN.