Laryngoscope Investigative Otolaryngology © 2019 The Authors. Laryngoscope Investigative Otolaryngology published by Wiley Periodicals, Inc. on behalf of The Triological Society.

Syntopy of in the Carotid Sheath: A Dissectional Study of 50 Cadavers

Flavio Hojaij, MD, PhD; Gabriela Rebelo, MS ; Flavia Akamatsu, PhD; Mauro Andrade, MD, PhD; Cristina Camargo, MD, PhD; Claudio Cernea, MD, PhD; Alfredo Jacomo, PhD

Background: Vagus nerve anatomical position inside the carotid sheath is not clear in the literature. Nevertheless, monitoring laryngeal nerves during thyroid surgeries may damage big vessels in the carotid sheath (; carotid artery). This gap led to an unprecedent cross sectional study of vagus syntopy using the carotid artery as anatomical mark. Methods: Fifty cadavers less than 24 hours postmortem were studied. The vagus nerve was spotted, reproducing the patterns performed in thyroidectomies. Results: On the right side, vagus nerve was posterior to the in 64% of the cases. On the left side, it was anterior, in 68% of the dissections. Comparing both sides, there was no symmetry in this syntopy. No influence of ethnic or anthropometric characteristics was observed. Conclusion: The vagus nerve is more frequently posterior to the common carotid artery on the right side and, anterior, on the left side. Key Words: Vagus nerve, carotid sheath, anatomy. Level of Evidence: 4

INTRODUCTION reports of microstructural neural injuries as consequences Located in the carotid sheath, the vagus nerve is in of surgical manipulation.5–7 the middle of two large vessels: the common carotid artery Applying neurophysiological monitoring apparatus and the .1,2 Dissections in that area, on vagus nerve ensures security, comfort, and efficiency – such as the one performed in neuromonitoring for thyroidec- for surgical steps.8 10 If the laryngeal nerves are harmed tomies, expose the vagus nerve and may affect large and during manipulation, the conduction is detected. This ini- important vessels whose injuries will lead to complications. tiative has been considered one of the most reliable ways Thyroidectomies technical precision has been espe- to analyze the integrity of laryngeal nerves.11 cially marked by the preservation of the laryngeal nerves It is easy to understand, therefore, the interest in learn- integrity.3,4 In a thyroidectomy procedure, proper tech- ing the syntopy of vagus nerve in detail to apply it when han- nique and knowledge of structural anatomy enable the dling the carotid sheath during thyroidectomies. Anatomical identification and maintenance of those branches. How- data should be provided to help surgeons approach the nerve ever, in spite of having precise technique, there are many during surgery.

This is an open access article under the terms of the Creative MATERIALS AND METHODS Commons Attribution-NonCommercial-NoDerivs License, which permits To study vagus nerve syntopy in the carotid sheath, 50 dissec- use and distribution in any medium, provided the original work is prop- erly cited, the use is non-commercial and no modifications or adaptations tions were performed in adult human cadavers of both sexes less than are made. 24 hours postmortem, by the discipline of Human Structural Study From the Department of Surgery, Laboratory of Medical Research Topography in Service of Death Verification of the Faculty of Medi- (LIM 02), Division of Human Structural Topography (F.H., G.R., F.A., cine, University of São Paulo (SVO). All the procedures were M.A., A.J.), Faculty of Medicine of the University of São Paulo (FMUSP), São Paulo, São Paulo, Brazil; Department of Surgery, Laboratory of regulamented and followed ethical standards from the University of Microsurgery and Plastic Surgery (LIM-04), Division of Plastic Surgery Sao Paulo Medical School Ethical Committee. (C.C.), Faculty of Medicine of the University of São Paulo (FMUSP), São Previous surgeries or cervical previous treatments were exclu- Paulo, São Paulo, Brazil; Department of Head and Surgery, Hospital das Clinicas (C.C.), University of Sao Paulo Medical School, São sion factors. Goiters above 50 mL were also excluded from the study. Paulo, São Paulo, Brazil Editor’s Note: This Manuscript was accepted for publication 29 April, 2019. Dissection Routine This work was presented as a poster at the AHNS 9th International 1. Inventory specimen determination of the anthropo- Conference on Head and Neck Cancer, in 2016. Conflicts of Interest: The authors declare that there is no conflict metric characteristics (brevilineal, normolineal, and of interest regarding the publication of this article. longelineal) considering subcostal angle bigger, equal, Send correspondence to Flavio Hojaij, MD, PhD, Department of or smaller than 90; Surgery, Laboratory of Medical Research (LIM 02), Division of Human Structural Topography, Faculty of Medicine of the University of São Paulo 2. Biacromial and retail incision; (FMUSP), 455, Dr. Arnaldo Av - 1st floor - Room 1302, Cerqueira Cesar, 3. Dissection of the superficial cervical , side-release Sao Paulo, SP 01246903, Brazil. Email: [email protected] of the sternocleidomastoid muscle to display the inter- DOI: 10.1002/lio2.275 section of the two parts of ;

Laryngoscope Investigative Otolaryngology 4: June 2019 Hojaij et al.: Syntopy of Vagus Nerve in the Carotid Sheath 319 Fig. 3. Scheme with three possible vagal positions relative to the left common carotid artery in a transverse section: anterior (1), medial (2), and posterior (3).

Sample Size To obtain the standard deviation (SD) and the standard error (SE) of the vagus nerve anatomical distribution, both on right and left sides, 20 cadaveric dissections participated in a Fig. 1. Left carotid sheath dissection and identification of its elements. Sternocleidomastoid muscle is deviated to the left. pilot study that revealed the left vagus nerve SD of 0.67 and SE of 0.95. The right vagus nerve presented values of 0.59 and 0.83, respectively. Sample size formula determination indicates 50 dissections 4. Bilateral dissection of the carotid sheath, 1 cm above as an ideal number for a descriptive study. the cricoid to the collarbone; 5. Determinationofvagusnervepositioninthesheathusing the common carotid artery as a reference (Figs. 1 and 2); Data Analysis 6. Paper reports from all observations. Continuous variables (numerical data) received descriptive analysis. Categorical variables (right/left, posterior/anterior) were To locate the vagus nerve, the common carotid artery was compared using Fisher’s exact test, considering significant P = .05. divided into anterior and posterior parts, considering an imaginary Also considering the weight, height, anthropometry, and transverse section. The nerve’s position was classified as anterior, the positions of right and left vagus nerve, multivariate and uni- middle, or posterior, whether it was found superficial, coincident, or variate regressions were performed for each possible anthropo- behind the coronal section, respectively (Fig. 3). metric parameter. The definitions of brevilineal, normolineal, or The internal jugular vein was not used as reference for the fol- longilineal were taken into account. lowing reasons: size variability—even between the right and left sides of the same individual—and state of distension, according to the cause of death. In contrast to an anterior position in relation to the carotid RESULTS artery, a posterior position of the vagus nerve implies in greater Description of Sample circumference of vessels to be dissected. Table I shows the characteristics of the sample.

Vagus Nerve Position As shown, the transverse section of the carotid artery was taken as a reference to establish the vagus

TABLE I. Studied Population Description.

Number of cadavers 50 Age (years) (average Æ SD) 68.51 Æ 12.7 Height (cm) (average Æ SD) 165.10 Æ 10.99 Weight (kg) (average Æ SD) 66.54 Æ 16.01 Sex Female = 46% Male = 54% Ethnicity White = 72% No White = 28% Anthropometry Brevilineal = 18% Longilineal = 26% Normolineal = 56% Fig. 2. Right carotid sheath dissection and identification of its ele- ments. Sternocleidomastoid muscle is deviated to the right. SD = standard deviation.

Laryngoscope Investigative Otolaryngology 4: June 2019 Hojaij et al.: Syntopy of Vagus Nerve in the Carotid Sheath 320 Fig. 4. Proportions of vagal nerve syntopy—anterior (1), medial (2), and posterior (3)—to the CCA in a transverse section on both right and left sides. CCA = common carotid artery; IJV = internal jugular vein. nerve syntopy in the carotid sheath: anterior, posterior, conclusion is simplistic: vagus nerve is located between or middle positions (Fig. 4). the carotid artery and jugular vein.12 Figure 5 shows the frequency of these findings, as Similar to our results presented here, Dionigi et al11 well as their statistical significance (Fisher test). found higher proportion of posterior vagus nerve location, but nothing was reported about the difference between left and right sides. This might be because they per- Symmetry and Anthropometric Characteristics formed lobectomies in more than 10% of their surgeries Comparing the presence of symmetry of vagus posi- and it is not clearly described on their paper whether the tion on both sides of the neck, in 37 out of 50 subjects, the contralateral side was approached. right vagus nerve position is different from the left’s, rep- According to our data, when the nerve is observed resenting 74% of the cases (P < .001). anterior to the carotid, neither vessels (carotid artery and When comparing anthropometric data, such as weight internal jugular vein) will have their circumferences dis- and height, in our multivariate analysis, no statistical signif- sected. When it is observed in the intermediate and posterior icance was found on the position of vagus nerve in the carotid positions, on the other hand, it will require dissection of at sheath. least part of the circumference of these vessels. In this Anterior and posterior vagal positions in the carotid unusual vascular exposure, neuromonitoring would increase sheath were analyzed separately in the brevilineal and in surgical morbidity. the longelineal groups, respectively, and there was no sta- This anatomical study shows the need of a critical think- tistical significance. ing on the dissection of the vagus nerve. It can be clearly con- cluded from our anatomical data that, when proposing to DISCUSSION dissect a right vagus nerve for instance, it should be found in Analyzing previous data in the literature concerning a posterior situation in about 2/3 of the cases, requiring the syntopy of vagus nerve in the carotid sheath, the greater surgical dissection range when compared to an ante- rior position. In addition, the symmetry comparing vagus nerve position on both sides is observed in less than 1/3 of the cases. The data presented here certainly shed light on the anatomical issue of neural monitoring in thyroidectomy. It is now up to the surgeons to choose whether or not to perform monitoring during thyroidectomy, seeking the best relationship between risk and benefit.

CONCLUSION The vagus nerve in the carotid sheath is between the common carotid artery and internal jugular vein, being posterior to the common carotid artery more often on the right side, and anterior on the left. Individual character- istics do not influence this position. Vagus nerve symme- Fig. 5. Graph showing vagal position frequencies on the right and try in the left and right carotid sheaths is seen in a on the left side of the neck. minority of cases.

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