Surgical approach and monitoring of the external branch of the superior laryngeal nerve (EBSLN)

Abstract

In this chapter, methods of intraoperative preservation and monitoring of the external branch of the superior laryngeal nerve (EBSLN) are described in detail. Contrary to routine dissection of the recurrent laryngeal nerve (RLN), most surgeons tend to avoid rather than routinely expose and identify the EBSLN during thyroidectomy. The EBSLN is believed to be the most commonly underestimated morbidity following thyroid surgery and is at a high risk of injury during dissection of the superior thyroid pole in the course of thyroidectomy in approximately one-third of patients (Cernea type 2A and 2B nerves). The laryngeal head of the sternothyroid muscle is a robust landmark for the course of the EBSLN as it descends along the inferior constrictor to the cricothyroid muscle (CTM). In up to 20 % of cases the nerve may not be able to be visualized due to a subfascial course along the inferior constrictor muscle, and hence use of intraoperative neural monitoring (IONM) can significantly improve the identification rate of the EBSLN during thyroidectomy. CTM twitch and glottic EMG recordings are both methods of IONM which are recommended in all cases of thyroid surgery, especially those which might jeopardize the EBSLN. A technique of togging the stimulator probe between the tissue of the superior thyroid pole vessels (with negative stimulation) and the region of the laryngeal head of the sternothyroid muscle (with positive stimulation) is recommended to assure preservation of the EBSLN. Transverse division of the superior edge of sternothyroid muscle and gentle traction of the superior thyroid pole into lateral and caudal direction followed by blunt dissection within the avascular plane of sternothyroid-laryngeal triangle allow for improving exposure of the EBSLN which is usually descending parallel to superior thyroid artery and is lying on the fascia or between the fibers of the inferior constrictor muscle before its termination within CTM. Nerve stimulation can objectively identify the EBSLN, leading to a visible CTM twitch in all cases. EMG activity can currently be quantified in nearly 80 % of cases using standard EMG tubes, but in all patients using novel EMG tubes with anterior surface electrodes. The role of measuring the waveform amplitude in prognostication of EBSLN function is yet to be determined.