3External Branch of SLN1

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The External Branch of the Superior Laryngeal NerveIts Topographical Anatomy as Related to Surgery of the Neck

Antonius C. Kierner, MD; Martin Aigner, MD; Martin Burian, MD

Objective: To determine the possible courses of the ex-ternal branch of the superior laryngeal nerve (EBSLN)and its relationship to the superior thyroid artery (STA)to improve thechances of identifying andsavingthe nerveduring head and neck surgery.

Design: Anatomical analysis of the exact topography of the EBSLN.

Subjects: Thirty-one perfusion-fixed human cadavers (ie,62 preparations) of both sexes ranging in age from 50 to94 years (mean, 78 years) with neither enlarged thyroidglands nor any other signs of abnormality in this region.

Results: Four types of relationship between the EBSLN,the upper pole of the thyroid gland, and the STA were

found. In 23 preparations (42%), the EBSLN crossed theSTA more than 1 cm above the upper pole of the thyroidgland (type 1). In 15 preparations (30%), the EBSLNcrossed the STA less than 1 cm above the upper pole of the thyroid gland (type 2). In 7 preparations (14%), theEBSLN crossed the STA under cover of the upper poleof the thyroid gland (type 3). In 7 preparations (14%),theEBSLN descended dorsal to theartery andonly crossedthebranches of theSTAimmediately above theupper poleof the thyroid gland (type 4).

Conclusion: The description of the variable course of theEBSLN and its categorization may help minimize the risk

of iatrogenic lesions of the nerve during surgery.

Arch Otolaryngol Head Neck Surg. 1998;124:301-303

THE EXTERNAL branch of thesuperior laryngeal nerve(EBSLN) arisestogether withan internal branch from thesuperior laryngeal nerve

(SLN), which is one of the uppermostbranches of the 10th cranial nerve. TheEBSLN first descends dorsolaterally to thecarotid arteries, crosses them, and finally

passes to thelarynx close tothesuperior thy-roidartery (STA), always lying profound tothese vessels. The topographical relation-ship to the STA and the upper pole of thethyroid gland represents the key point foridentifying theEBSLN duringsurgery oftheneck. When these 2 landmarks are consid-ered, identification and protection of thenerve are easy. After giving off some twigsto the pharyngeal plexus and the inferiorpharyngeal constrictor, the EBSLN termi-nates mainly within the cricothyroidmuscle.1-7 Yet there is evidence that theEBSLN also contains some afferent fibers

that mediate impulses from the cricothy-roid joint and the mucouslining inside thecricothyroid membrane.8,9 Furthermore,some authors describe a supplementarymotor innervation of some of the intrinsiclaryngeal muscles by the EBSLN.10-12 How-ever, further investigations are needed onthat question.

Nevertheless, iatrogenic lesionsoftheEBSLNclearlyaremuchmorecommonthanisgenerallyrecognized.13 Sinceinjurytothis

nerve maycausetransient oreven persistentchanges either in quality of voice or in de-glutition,anattemptshouldbemadetoiden-tify the EBSLN during surgery.2,14,15 Espe-cially in partial laryngectomies, sufficientsensoryinnervationofthesupraglotticareaclearly helpsto avoidpostoperative aspira-tionanddysphagia.16,17 At present,availabledataontheexacttopographyoftheEBSLN

are sometimes ambiguous and confusing.Even thereportedproportion ofpatientsinwhich theEBSLN canactuallybe identifiedvariesfrom10%to80%.2,3,18-21 Therefore,theaims of this study were to determine thecourseof theEBSLNin a sufficient numberof subjects and to devise a method of typ-ingtherelationshipbetweentheEBSLN,theSTA,andtheupperpoleofthethyroidgland.

RESULTS

Our typing is based on the work of Cerneaet al.20,21 Because our results differ in some

respectsfromthoseofCerneaetal,itwasnec-essary to establishslightly differentcatego-ries.Onthebasisofourfindings,wedividedthetopographical relationshipbetweentheEBSLN, the STA, and the upper pole of thethyroid gland into4 categories (Figure1).

In type 1, the EBSLN crosses the STAmore than 1 cmcranial to the upper poleof thethyroidgland.Occurringin21specimens(42%),thisisclearlythemostcommontypeencounteredinourspecimens.The3 afore-

ORIGINAL ARTICLE

From the Institute of Anatomy 2, University of Vienna(Drs Kierner and Aigner),and the Department of Otolaryngology, UniversityHospital of Vienna (Dr Burian),Vienna, Austria.

ARCH OTOLARYNGOL HEAD NECK SURG/VOL 124, MAR 1998301

©1998 American Medical Association. All rights reserved. on August 16, 2009www.archoto.comDownloaded from

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mentioned preparationsin which we had difficulty in typ-ing probably also belonged to type 1.

In type 2, the EBSLN crosses the STA less than 1 cmcranial to the upper pole of the thyroid gland. In our ma-terial, 30% of the hemilarynges (15 preparations) couldclearly be classified as type 2.

In type 3, the EBSLN crosses the STA while cov-ered by the upper pole of the thyroid gland (total of 7preparations [14%] in our study).

Type 4 refers to cases in which the EBSLN does notcross the trunk of the STA at all, but runs dorsal to theartery until it has ramified. In our material, we found 7

hemilarynges of that type (14%). Although the EBSLNdoes not cross the STA in these cases, it can easily befound, since it runs parallel to the artery slightly moreprofound and dorsal to it.

Within the 52 hemilarynges we dissected, the pre-dominance of type 1 was obvious. The topographical re-lationship of the EBSLN on one side of the neck does notpredict thecourse of thenerve on theother side. Thetypeswe defined occurred independently on the 2 sides. How-ever, no other course of the EBSLN except the 4 typesdescribed above could be found.

To obtain quantitative data about the relationshipof the SLN to the common carotid artery, the distancebetween the separation of the SLN from the vagal nerve

andthe bifurcation ofthecommon carotid arterywas mea-sured (Figure 2). The distance between these 2 well-defined anatomical landmarks ranges between 2.9 and5.6 cm, with a mean value of 4.1 cm on the right sideand 4.27 cm on the left side.

COMMENT

The EBSLN innervates parts of the intralaryngeal mucousmembraneandsendssomemotor fibersto intrinsicmusclesof the larynx as well.8,10-12 Palsy of the EBSLN or an iatro-

genic lesion of the nerve might cause dysphonia and aspi-ration.16,17 The sensory deficit in the hypopharynx andsu-praglotticlarynx interfereswith thepatient’sabilityto sensesecretionandfoodparticlesinthoseareas.Especiallyincasesoftumorresection inwhichsurgery ofthe anterior triangleofthe neckiscombinedwithpartiallaryngectomyor enlargedhypopharyngeal resections, jeopardizing the SLN deterio-rates functional rehabilitation. Exact knowledge of the to-pographicalanatomyoftheSLNanditsbranchesclearlyhelps

identify and preserve this nerve during neck surgery.ThecourseoftheEBSLNhasbeendescribedbymanyauthors.1,2,4-6,11,15,18,19,22-24Most of the previous studies em-phasized identification of the EBSLN at the upper pole of thethyroidgland,wherethetopographyofthenerveshowsmuch more variability in itsrelationship to theSTA or mayeven be covered by the inferior pharyngeal constrictormuscle.2,3,11,15 Therefore,itseemseasiertoidentifytheEBSLNfurther cranially. Cernea et al20,21 triedtocategorizethedif-ferent courses of the EBSLN to facilitate identification of the nerve during surgery. Our results are similar to thoseofCerneaetal20,21; however, webelievethat ourslightly dif-ferenttyping betterreflectsthe possiblecourses ofthe nerve.In agreement with Cernea et al, we found type 1 to be the

mostcommontype.However,insteadofthe62%type1theyreported, type 1 was present in only 42% of our prepara-tions. Our types 2 and 3 correspond to types 2a and 2b of Cernea et al, respectively. Whereas the number of type 2cases (27% in our material and 11% in the examinationsby Cernea et al) are quite different, type 3 was consistentbetween the studies (13% and 14%, respectively).

In addition, we introduce a new category, which wecall type 4. This type was found in 13% of our dissectionmaterial. The question arises why previous investiga-tors2,11,15,18,20-24 did not describe that course of the EBSLN.Sincein these cases thenerve descends more dorsallythanone would expect, its identification is more difficult. Thismay be one of the reasons why Cernea et al20,21 could not

identify the EBSLN in 7% of their cases. The differencein number of each type between our material and that of Cernea et al might have resulted from the fact that weexamined almost twice as many specimens. However, theexact circumstances regarding the different distributionstill remain to be determined.

IdentificationoftheEBSLNduringsurgerycanbecom-plicated by previous operations or by preoperative radio-therapy.Weattemptedtodescribeadditionallandmarksthatguide the surgeon to the SLN, which in turn facilitates theidentificationoftheEBSLN.Oncethebifurcationofthecom-moncarotidarteryhas been identified,thebranching oftheSLN from the vagal nerve can be found an average of 4.18cm in the cranial direction (Figure 2). Since the carotid bi-

furcationand thevagal nerve areeasily accessible even un-der difficult conditions, identifying the nerve should poseno problem in anycase. Measuringthe distance to thebaseof theskull (jugular foramen,basion) might provide moreexact data.5 However, theselandmarks arecomparably dif-ficult to use in neck surgery and therefore not advisable.

Our results strongly suggest that categorization of thetopographical variations oftheEBSLN ispossible.Sincethisstudy included 62 specimens, it is unlikely that the nervewill take a course different from those covered by our clas-sification. Therefore, we believe that exact knowledge of

MATERIALS AND METHODSSixty-twohemilaryngesweretakenfrom31humanbod-ies of both sexes (20 male and 11 female) donated totheInstituteofAnatomy, Universityof Vienna,Vienna,Austria,rangingfrom50to94years(meanage,78years).The bodies were fixed as usual for anatomical dissec-tioncourses(4%phenolicacidand0.5%formaldehyde).

Onlyspecimenswithneitherenlargedthyroidglands

norany other sign of severeabnormality of that regionwere accepted. Theanterior trianglesof the neck weredissectedby1ofus(A.C.K.),whodocumentedhisfind-ingsimmediately.Resultshadbeenverifiedindependentlyby the other author (M.A.). To verify the data, cadav-ersweresubsequentlydecapitatedattheatlanto-occipitalarticulation,withtheneck’sviscerastayingwiththe head.After thisadditionalexamination,the courseandtopo-graphical relations oftheEBSLNcouldbe classifiedun-equivocallyin52hemilarynges.Within7hemilarynges,theEBSLN wasinjuredduring preparation.In 3 hemi-larynges, the type of the EBSLN was difficult to deter-mineafterall thesurrounding softtissuethatkeepsthenerve in position hadbeen dissected. These 3 prepara-tions are described separately below.










the relationshipbetween the EBSLN,the STA, andthe up-per pole of the thyroid glandas presentedherein may helpidentify the nerve during neck surgery.

Accepted for publication December 4, 1997.Reprints: Antonius C. Kierner, MD, Anatomisches

Institut, 2 Unterrichtseinheit/Waehringerstrasse 13/3, A-1090 Vienna, Austria.

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Type 1

>1 cm

CCA

TG

EBSLN

STA

<1 cm

Type 2 Type 3 Type 4

Figure 1. Classification of the possible courses of the external branch of the superior laryngeal nerve (EBSLN). In type 1, the nerve crosses the superior thyroid artery (STA) more than 1 cm above the upper pole of the thyroid gland (TG). In type 2, the nerve crosses the STA less than 1 cm above the upper pole of the TG.In type 3, the nerve crosses the STA under cover of the superior pole of the TG. In type 4, the nerve descends dorsal to the STA, crossing its branches immediately above the upper pole of the TG. CCA indicates common carotid artery.

NX

CCA

TG

EBSLN

IBSLNd

Figure 2. Topography of the superior laryngeal nerve and its branches. NX indicates 10th cranial nerve; d, the distance between the origin of the superior laryngeal nerve and the bifurcation of the common carotid artery (CCA),measured as indicated; IBSLN, internal branch of the superior laryngeal nerve; EBSLN, external branch of the superior laryngeal nerve; and TG, thyroid gland.








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