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].