Discussion:
NRLN is a rare anatomic condition. It was first described by Stedman in 1823. Its incidence is reported to range from 0.6 to 1.3 % on the right side [3], although some reports showed it more frequent when detected by systematic intraoperative neuromonitoring (IONM) [4]. It is exceptionally located on the left side, only in case of dextrocardia or in situs inversus; its incidence is 0.04% in this situation [3].
This anomaly originates from a malformation of the aortic arch and it is associated with an ARSA [2]. During the embryological development and as the heart descends, inferior laryngeal nerves assume their recurrent course hooking around the sixth branchial arch. On the left side, the 6th aortic arch remains until birth forming the ductus arteriosus and later ligamentum arteriosum, the left inferior laryngeal nerve keeps its recurrent path in the mediastinum. In the right side however, the 5thand the distal part of the 6th aortic archs disappear, the right laryngeal nerve ascends to the larynx as high as the 4th aortic arch. The right 4thaortic arch gives birth to the initial segment of the right subclavian artery. An embryological anomaly consisting in the obliteration of the right 4th aortic arch can be seen, the right subclavian artery takes off below the left subclavian artery crossing the midline to irrigate the right arm. Thus, the right laryngeal nerve arises from the vagus in the cervical region passing directly to the larynx without any recurrent path [2].
The higher incidence of nerve injury, estimated to 12.9% on NRLN compared to 1.8% on recurrent laryngeal nerve [3, 5], and the absence of reliable clinical signs of a NRLN[6] exhorted many authors to look for ARSA preoperatively to predict a NRLN.
There are different methods to identify an ARSA.
MRI and CT scan can find it, although it can be missed falsely for technical considerations [6, 7, and 8] or not mentioned by the radiologist in the final report[7]. The right subclavian artery can sometimes be oppressed dorsally by the thyroid tumor mimicking an ARSA[ 8]. Furthermore, MRI and CT scan are not recommended for all patients who will undergo a thyroid or parathyroid surgery [9].
Some recent reports suggest the use of ultrasonography (US) as a useful tool to predict a NRLN [7, 9, 10-14]. It is a simple, rapid, non-invasive, reliable and cost-effective method[7, 14], it is also included in the preoperative assessment before thyroid surgery [7]. Its sensibility and sensitivity varies between 99-100% and 41-100%[1].
For Devèze and al, it tooks 5mn on ultrasonography of the brachiocephalic trunk, to prove an arteria lusoria on patients with known NRLN. The absence of the brachiocephalic artery and the direct origin of the right common carotid artery from the aorta arch were assessed [14]. In the controlled trial of Iacobone and al, the surgeon performed preoperatively an ultrasonography of brachiocephalic trunk in one group and none in the control one. The examination aimed to visualize the presence of the division of the brachiocephalic artery into the right common carotid artery and the right subclavian artery (“Y sign”). When the division of the brachiocephalic artery and the subclavian artery was not immediately evident, the course of the right common carotid artery was traced in order to identify its possible origin directly from the aortic arch. The absence of the “Y sign” indicates the presence of a NRLN. Results from this study proved that absence of “Y sign” predicted NRLN with an accuracy of 100%, and showed that mean time to identify laryngeal nerve in group with preoperative ultrasonography was shorter[7]. Frequency of laryngeal nerve palsy was significatively lower in predicted NRLN group (0/5) compared to NRLN discovered per operatively (3/4) [7].
IONM can also predict the presence of a NRLN by showing negative electromyographic signals from the lower portion (inferior border of the fourth tracheal ring) but positive responses from the upper portion of the vagus nerve (superior border of the thyroid cartilage) after its stimulation [6, 15].