The evolution and progress of intraoperative monitoring of the external branch of the superior laryngeal nerve in thyroid surgery

Hypoparathyroidism and recurrent laryngeal nerve (RLN) damage are the most frequent reported morbidity in thyroid surgery whereas damage of the external branch of the superior laryngeal nerve (EBSLN) has been considered to be the most neglected sequel. EBSLN injury results in subtle changes in voice as a result of dysfunction of the cricothyroid muscle (CTM): changed basic voice frequency, deterioration of voice quality in the production of high frequency sounds and reduced voice projection. Symptoms of this dysfunction are usually more notable for professional voice users and can be difficult to diagnose. EBSLN injury can happen during dissection of the vessels of the upper thyroid pole due to proximity between the nerve and these vessels. Several maneuvers can minimize the risk of injury to the EBSLN including visual identification of the nerve before ligation of the upper thyroid pole vessels and use of either a nerve stimulator or intraoperative neuromonitoring (IONM) for neuromapping and evidence of the EBSLN identification. The EBSLN monitoring is relied on evaluation of CTM twitch (present in all patients) and electromyographic curve traced by the monitor using surface tube electrodes within the vocal folds which present in the majority but not all individuals (70–80% of patients using standard EMG tubes) or approaching to 100% of patients (using NIM TriVantage tubes). IONM has a potential to increase the rate of the EBSLNs identification, diminish prevalence of neural damage, and decrease prevalence of postoperative voice impairment which has been repeatedly supported by recently published data. It is advised that at the end of upper thyroid lobe dissection the EBSLN should be stimulated at the most cranial arc of dissection to confirm not only anatomical but what is even more important functional preservation of intact neural function.