What are normal quantitative parameters of intraoperative neuromonitoring (IONM) in thyroid surgery?

PURPOSE: This study aimed at definition of normal quantitative parameters in intraoperative neuromonitoring during thyroid surgery. Only few and single center studies described quantitative data of intraoperative neuromonitoring. Definition of normal parameters in intraoperative neuromonitoring is believed to be a prerequisite for interpretation of results and intraoperative findings when using this method. Moreover, these parameters seem important in regard to the prognostic impact of the method on postoperative vocal cord function.
MATERIAL AND METHODS: In a prospective multicenter study, quantitative analysis of vagal nerve stimulation pre- and postresection was performed in thyroid lobectomies. A standardized protocol determined set up and installation of neuromonitoring and defined assessment of quantitative parameters. Data of intraoperative neuromonitoring were respectively print-documented and centrally analyzed.
RESULTS: In six participating centers a total of 1,289 patients with 1,996 nerves at risk underwent surgery for benign and malignant thyroid disease. Median amplitude was significantly larger for the right vs. left vagal nerve, latency was significantly longer for left vs. right vagal nerve and duration of the left vs. right vagal nerve significantly longer. Age disparities were only present in form of significantly higher amplitude in patients below 40 years; however, there is no continuous increase with age. Regarding gender, there was significantly higher amplitude and smaller latency in women compared to men. Duration of surgery revealed a reduction of amplitude with operative time; contrarily, latency and signal duration remained stable. The type of underlying thyroid disease showed no influence on quantitative parameters of intraoperative neuromonitoring.
CONCLUSIONS: Systematic data of multicenter evaluation on quantitative intraoperative neuromonitoring parameters revealed differences between left and right vagal nerves in regard to amplitude, latency and duration of signal, gender, and age. The nature of thyroid disease showed no significant influence on quantitative parameters of intraoperative neuromonitoring. This study presents for the first time collective data of a large series of nerves at risk in a multicenter setting. It seems that definitions of "normal" parameters are prerequisite for the interpretation of quantitative changes of intraoperative neuromonitoring during thyroid surgery to enable interpretation of influence on surgical strategy and prediction of postoperative vocal cord function.