The Identification, Preservation and Classification of the External Branch of the Superior Laryngeal Nerve in Thyroidectomy.

Background: Avoiding injury to the external branch of the superior laryngeal nerve is one of the major challenges during thyroid operation. The aim of this study was to propose a practical classification of the external branch of the superior laryngeal nerve.

Methods: A retrospective study of total thyroidectomy was performed. Totally 240 patients were included, with 480 external branches of the superior laryngeal nerves explored by intraoperative neuromonitoring. The classification of the external branch of the superior laryngeal nerve was determined by the distance between the upper edge of the superior thyroid pole and the lowest point of the nerve when the thyroid lobe was retracted in the lateral and inferior direction. Multinomial logistic regression analysis was run to predict the type of the nerve from several variables.

Results: The identification rate of the external branch of the superior laryngeal nerve was 98.54% (473 of 480 nerves). Higher ratio of longitudinal size of the thyroid lobe to ipsilateral neck length increased the likelihood of that both the type 2 and 3 nerve with respect to type 1 nerve, with OR 2.72, 95% CI = 1.21-6.12 and OR 5.30, 95% CI = 2.09-13.44, respectively. (1a) The nerve whose lowest point (entry into the muscle) was located more than 1 cm above the horizontal plane passing the upper border of superior thyroid pole. (1b) The nerve whose lowest point (the point right above the superior thyroid pole) was located more than 1 cm above the plane. (2a) The nerve whose lowest point (entry into the muscle) was located within 1 cm above the plane. (2b) The nerve whose lowest point (the point right above the superior thyroid pole) was located within 1 cm above the plane. (3a) The nerve whose lowest point (entry into the muscle) was located below the plane. (3b) The nerve whose lowest point (the point right below the superior thyroid pole) was located anterior to the gland. (3c) The nerve whose lowest point (the point right below the superior thyroid pole) was located posterior to the gland.

Conclusions: Identification rate of the external branch of the superior laryngeal nerve by intraoperative neuromonitoring was significantly high. Understanding the new practical classification of the nerve allows for better identification and function preservation of this nerve during thyroidectomy.