Intraoperative neuromonitoring of the external branch of the superior laryngeal nerve during thyroidectomy: the need for evidence-based data and perioperative technical/technological standardization.

Alberto Mangano,Dimitrios H Roukos,Georgios D Lianos,Hoon Yub Kim,Gianlorenzo Dionigi,Luigi Boni

doi:10.1155/2014/692365

Abstract

The external branch of the superior laryngeal nerve (EBSLN) is surgically relevant since its close anatomical proximity to the superior thyroid vessels. There is heterogeneity in the EBSLN anatomy and EBSLN damage produces changes in voice that are very heterogenous and difficult to diagnose. The reported prevalence of EBSLN injury widely ranges. EBSLN iatrogenic injury is considered the most commonly underestimated complication in endocrine surgery because vocal assessment underestimates such event and laryngoscopic postsurgical evaluation does not show standardized findings. In order to decrease the risk for EBSLN injury, multiple surgical approaches have been described so far. IONM provides multiple advantages in the EBSLN surgical approach. In this review, we discuss the current state of the art of the monitored approach to the EBSLN. In particular, we summarize, providing our additional remarks, the most relevant aspects of the standardized technique brilliantly described by the INMSG (International Neuromonitoring Study Group). In conclusion, in our opinion, there is currently the need for more prospective randomized trials investigating the electrophysiological and pathological aspects of the EBSLN for a better understanding of the role of IONM in the EBSLN surgery.

Highlights

Another advantage of the monitored approach is when a suture ligation or clip is applied near the external branch of the superior laryngeal nerve (EBSLN): in this case if IONM suggests a neural dysfunction, the clip or the suture must be removed in order to avoid risk of permanent neural damage [1]
If a neural mapping is performed a relevant advantage achievable only via the IONM is that the EBSLN can be definitively identified even when the nerve is not de visu detectable [1]
A damage of the EBSLN is associated with cricothyroid muscle motility impairment, altering the high tones production ability, the voice and frequency

Summary

Surgical Anatomy and EBSLN Classification

The superior laryngeal nerve (SLN) is a branch of the vagus after its exit from the skull base It usually originates at the nodose ganglion close to the jugular foramen at the level of C2 (about 4 cm cranially to the carotid artery bifurcation) and it descends posterior to the carotid arteries toward the larynx [1,2,3]. The EBSLN is dorsal to the superior thyroid artery and in a superficial location to the inferior pharyngeal constrictor muscle as it goes caudally and travels in a medial direction to innervate, on the lower portion of the cricoid cartilage, the cricothyroid muscle [1, 2]. All those aforementioned anatomical considerations are very relevant because the complete anatomical knowledge of this region (in particular to precisely evaluate the cricothyroid muscle twitch) is the conditio sine qua non and the prerequisite for an accurate surgical technique [1]

Neurophysiological and Neuropathological Aspects

Standardized Surgical Monitored Technique

Technique A EBSLN IONM

Technique B EBSLN IONM

Findings

Discussion