Aim: This study aimed at describing the anatomical variations of the external branch of the superior laryngeal nerve and at estimating the frequency of nerves at risk during the thyroid surgery. Methodology: We realized a forward-looking study from September, 2016 in May 31 st, 2018 in the laboratory of anatomy of the Faculty of Medicine and Odontostomatology of Bamako in Mali. All the fresh anatomical subjects not carrying trauma and or a scar at the level of the previous region of the neck were held. The anatomical subjects were not included presenting a traumatic lesion and\or a scar of the previous region of the neck. Results: We realized 34 dissections of the external branch of the superior laryngeal nerve to 17 deathly subjects (11 men and 6 women with a sex-ratio of 1.8). The average age of the subjects was of 42 years (extremes: 18 and 70 years). Our study allowed highlighting in 100% of the cases, the external branch of the superior laryngeal nerve and the superior thyroid artery so to the right as to the left. On 34 dissected external branch of the superior laryngeal nerve, none had a previous route. However we found 28 nerves (82.4%) having a later route, stuck to the external face of the lower constrictor of the pharynx. These were not mixed with the superior thyroid artery and its branch of division and were situated outside the thyroid capsule. In 17.6% of the cases (6 cases), the nerve had a route mixed in the branch of the superior thyroid artery. These were found inside of the capsule (11.8% adhered to the artery and 5.8% crossed its branch of division). Conclusion: The risk of injury of the external laryngeal nerve during thyroid surgery procedure is never zero. It is more important on the left side.
The external branch of the superior laryngeal nerve ( BENLS), is a mixed nerve, hail, assuring the innervation of the cricothyroid muscle (muscle tensor of vocal cords) [
This study aimed at describing the anatomical variations of the external branch of the superior laryngeal nerve and at estimating the frequency of nerves at risk during the thyroid surgery.
We realized a forward-looking study from September, 2016 in May 31st, 2018 in the laboratory of anatomy of the Faculty of Medicine and Odontostomatology of Bamako in Mali. All the fresh anatomical subjects not carrying trauma and or a scar at the level of the previous region of the neck were held. Were not included the anatomical subjects presenting a traumatic lesion and\or a scar of the previous region of the neck. We proceeded to the dissection of the various superficial, muscular and vascular plans of the previous cervical region. The dissections concerned only the previous region of the neck. The corpse was put in dorsal decubitus with the neck in extension by means of a transverse block inserted under scapula and nape of the neck. The cutaneous sections were the followingones:
・ a first horizontal section akin to the mandible going of the protuberance or the eminence chin strap to the angle of the chin;
・ the second also horizontal, slightly convex section towards the arm below, going of the manubrium sternal to the collarbone;
・ the third vertical section, which leaves the eminence chin strap to the manubrium sternal.
The superficial blade of the cervical fascia was vertically made an incision.
We proceeded then to a wide cutaneous unsticking in the blade of bistoury and by drive going until the outside of the insertions sternal and clavicular below; then mastoid at the top of the sternocleidomastoid. The sternocleidomastoid muscle was transversely split and its two extremities pulled down at the top and below. The omohyoid muscle was split in his turn, so giving access to the vascular plan. Sternocleidohyoid muscles were reclined to have a good access to the thyroid changing room.
We proceeded to a ligature upstream and downstream to the third way of the internal jugular vein which was cut so giving access to the common carotid artery. The superior thyroid artery and the external branch of the superior laryngeal nerve were spotted and dissected.
The following parameters were studied: the origin, the route of the nerve, its reports with the superior thyroid artery and the superior pole of the thyroid gland.
Epidemiological characteristics of the anatomical subjects
We realized 34 dissections of the external branch of the superior laryngeal nerve to 17 deathly subjects (11 men and 6 women with a sex-ratio of 1.8). The average age of the subjects was of 42 years (extremes: 18 and 70 years).
Origin
Our study allowed to highlight in 100% of the cases the external branch of the superior laryngeal nerve and the superior thyroid artery so to the right as to the left. 34 dissected external branch of the superior laryngeal nerve was born at the level of the big horn of the bone hyoid. The superior thyroid arteries approached the side lobe of the thyroid gland at the level of its cranial pole.
Routes and reports
On 34 dissected external branch of the superior laryngeal nerve, none had a previous route (Tables 1-3; Figures 1-6). However we found 28 nerves (that is 82.4%) having a later route, stuck to the external face of the lower constrictor of the pharynx. These were not mixed with the superior thyroid artery and its branch of division and were situated outside the thyroid capsule.
In 17.6% of the cases (6 BENLS), the nerve had a route mixed in the branch of the superior thyroid artery. These were found inside of the capsule (11.8% adhered to the artery and 5.8% crossed its branch of division).
In our study, the nerve is distant from the artery in 31.25% of the cases between 0 in 0.5 cm. This constitutes a distance at risk.
| Route of the BENLS | Left | Right | Total | Percentage |
|---|---|---|---|---|
| Stuck to the external face of the lower constrictor of the pharynx | 13 | 15 | 28 | 82.4% |
| Member in the superior thyroid artery (ATS) | 2 | 0 | 2 | 5.8% |
| Crossing the branch of the superior thyroid artery | 2 | 2 | 4 | 11.8% |
| Total | 17 | 17 | 34 | 100% |
BENLS: external branch of the superior laryngeal nerve; ATS: superior thyroid artery.
| Arrangement of the BENLS with regard to the ATS | Left | Right | Total | Percentage |
|---|---|---|---|---|
| Mixed | 4 | 2 | 6 | 17.6% |
| Not mixed | 13 | 15 | 28 | 82.4% |
| Total | 17 | 17 | 34 | 100% |
| Route of the external branch of the superior laryngeal nerve | Authors | ||
|---|---|---|---|
| Our study | N’guessan [ | Page [ | |
| Stuck to the external face of the constrictor of the pharynx | 82.4% | 75% | 80% |
| Adhering to the superior thyroid artery | 11.8% | 18.75% | 20% |
| Crossing the branches of the superior thyroid artery | 5.8% | 6.25% | |
Numerous works realized on the external branch of the superior laryngeal nerve allowed describing multiple anatomical forms with variations of its origin, its route and these anatomical reports.
Origin
The external branch of the superior laryngeal nerve is classically considered as a branch of division or a collateral branch of the superior laryngeal nerve.
In our study, the dissections did not interest particularly its precise origin but it was in a macroscopic manner a branch stemming from the superior laryngeal nerve which was born at the level of the big horn of the bone hyoid in every case. This origin at the level of the big horn of the bone hyoid is the one brought back by most of the authors. However certain authors describe different origins or above big horn of the bone hyoid as Laux [
Route
During our study carrying 34 dissected nerves, in 82.4% of the cases the external branch of the superior laryngeal nerve was mixed in the superior thyroid artery and had a posterior route with regard to this one. It was stuck to the external face of the muscle lower constrictor of the pharynx. These anatomical measures offer more advantages to the surgeon because this one will have external branch of the superior laryngeal nerve not only distant from the superior thyroid artery but also from the superior thyroid capsule (18.75% of the cases). The risk of hurting the external branch of the superior laryngeal nerve was real in 17.6% of the cases.
These results are almost identical to those of the series of Mossman and Dweese [
In our study, the nerve has at the same time a route later and not mixed in the superior thyroid artery in 87.5% of the cases to the right and in 62.6% of the cases to the left.
Page et al. [
The risks of lesion of the external branch of the superior laryngeal nerve are defined by:
・ an adhesion or an intermingling of the nerve with the superior thyroid vessels;
・ a situation distal superficial of the nerve at the level of the cricothyroid muscle;
・ the presence of a satellite vascular pedicle;
・ the situation distal of the nerve between the larynx and the face interns of the thyroid lobe.
From these criteria, we estimated in our study that risks to hurt the external branch of the superior laryngeal nerve were 12.5% to the right and 37.5% to the left.
These risks were estimated by several authors whose results are presented in the (
Concerning the distance between the external branch of the superior laryngeal nerve and the superior thyroid artery, we found in 31, 25% of the cases a distance at risk (between 0 in 0, 5 cms) whereas Naidoo et al. [
This study allowed us to estimate the risk of lesion of the BENLS. This risk is 25 % whatever the type of thyroid surgery is. This risk is higher to the left than to the right.
The surgeon has to proceed to a meticulous and attentive dissection and recognize elements to be bound and to be cut by having in mind the possible anatomical variations [
We, authors of this article declare that there is no conflict of interests.
Kanté, A., Sidibé, Y., Ba, B., Bengaly, B., Coulibaly, B., Traoré, D., Ouattara, D., Daou, M., Touré, T., Diakité, S., Koné, M., Diallo, S., Diallo, C.T., Touré, O.I., Togola, B. and Ongoïba, N. (2018) Anatomical Dissection of the External Branch of the Superior Laryngeal Nerve to the Laboratory of Anatomy of the Faculty of Medicine of Bamako (Mali). Forensic Medicine and Anatomy Research, 6, 47-54. https://doi.org/10.4236/fmar.2018.64005