Surgical landmarks for identification and preservation of the internal branch of the superior laryngeal nerve

doi:10.21203/rs.3.rs-2227437/v1

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Surgical landmarks for identification and preservation of the internal branch of the superior laryngeal nerve

https://doi.org/10.21203/rs.3.rs-2227437/v1

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published 30 Dec, 2022

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Objective: To determine the topographical anatomic features of the internal branch of the superior laryngeal nerve (ibSLN) at the thyrohyoid membrane entrance area in relation to certain consistent anatomical structures.

Material-Method: Twenty-two fresh adult head cadavers (9 male, 13 female; age range 52- 95 years) with no signs of abnormality in the neck were dissected to determine the anatomic relationship of ibSLN and superior border of thyroid cartilage, thyroid notch, carotis bifurcation, hyoid corpus and hyoid greater cornus.

Result: The topographical relationship between ibSLN and superior border of thyroid cartilage, thyroid notch, carotis bifurcation, hyoid corpus and hyoid greater cornus was identified bilaterally in all cadavers.

Conclusion: We provided the surgical anatomy and important landmarks for determining the internal branch of superior laryngeal nerve in the thyrohyoid membrane entrance region to avoid surgical damage during surgeries of this region.

superior laryngeal nerve

internal branch

surgical landmarks

anatomy

Internal branch of superior laryngeal nerve (ibSLN) provides sensory innervation for the major part of the laryngeal mucosa above the vocal cords [1, 2]. The risk of damaging of ibSLN is high in laryngeal surgeries, neck dissections and thyroid surgeries, cervical spine surgeries [3–7]. For providing the healty glottic closure reflex during coughing, vomiting and swallowing that protects us from aspiration of secretions, preservartion of ibSLN is very important. Especially during supraglottic and supracricoid partial laryngectomies, neck dissections and mass excisions from this region, identification and preservation of this nevre should be done in order to maintain the protective function of larynx.

The topographical anatomic features should be determined so as to identify and prevent damage of the nerve. In this study we aimed to investigate the relationship between the ibSLN at the point of penetrating the thyrohyoid membrane and certain consistent anatomical landmarks that are superior border of thyroid cartilage, thyroid notch, carotis bifurcation, hyoid corpus and hyoid greater cornus.

The research protocol was approved by the local ethics committee (protocol no:523; 25.05.2021) and was conducted in accordance with the Declaration of Helsinki. Between June 2021 and December 2021, 22 fresh human head cadavers were used to determine the topography of ibSLN. The cadavers were placed in supine position, and bilateral dissection (44 sides in total) was performed under a Zeiss Extaro 300 stereoscopic dissection microscope.

The laryngeal region was carefully dissected and thyrohyoid membrane was preserved to fully demonstrate the entrance of the ibSLN and its relationship between other consistent structures. The sternocleidomastoid muscle was laterally deviated and carotid sheet and then the vagus nerve were identified. By following the tracing of the vagus nerve, ibSLN and its penetrating point in the thyrohyoid membrane was identified. Five different distances were measured via using caliper and recorded from where the nerve pierced the membrane. The distance between penetrating point of the ibSLN to the membrane and corpus of the hyoid bone, the greater cornus of the hyoid bone, carotis bifurcation, vertical distance to superior border of thyroid cartilage, horizontal distance to thyroid notch were all measured by means of ruler and noted (Fig. 1–2).

Statistical Analyses

Data were analyzed by means of SPSS v. 25 software. All data is presented as mean ± standard deviation. Continuous variables were visually assessed for normality using histograms and Q-Q plots. Independent sample Student’s t-test was used to compare the mean values of parameters between men and women. Paired sample test was applied to compare right and left side measurements of cadavers. A p-value of < 0.05 was considered statistically significant.

Of the 22 fresh human head cadavers, 9 were male and 13 were female with an age range 52–95 years. Demographical features of the cadavers were noted on Table 1. The ibSLN were dissected bilaterally in each cadaver and the most inferior ramus of each ibSLN at its entry point to thyrohyoid membrane was selected for measurements. The measured parameters of all cadavers were listed in Table 2, and the parameters according to the gender were listed in Table 3.

Vertical distance between the penetration point of ibSLN to thyrohyoid membrane and the superior border of thyroid cartilage (TS) was 0.60 ± 0.14 SD cm for all measurements. There was no statistically difference between male and female cadavers (Table 3).

We draw a vertical line from the penetrating point of the ibSLN to the superior border of the thyroid cartilage. The horizontal distance between the midpoint of the thyroid notch and the intersecting point of the line we draw at the superior border of the thyroid cartilage (TN) was measured. The distance was 1.97 ± 0.29 SD cm for all cadavers. It was statistically longer in male cadavers. The mean distance was 2.10 ± 0.19 SD cm in male cadavers and it was 1.88 ± 0.24 SD cm in female cadavers (p = 0.034) (Table 3).

The distance from the midpoint of hyoid cornus (HC) to the penetration point of the ibSLN was 2.04 ± 0.30 SD cm for all cadavers. There was no statistically difference between genders (Table 3).

We found the distance between greater horn of the hyoid (HH) and the penetration point of the ibSLN as 1.65 ± 0.38 SD cm. There was no statistically difference between genders (Table 3).

The distance between the carotis bifurcation (CB) and the penetration point of ibSLN was 1.67 ± 0.32 SD cm. It was also found to be longer in male cadavers that was statistically different (p = 0.012). The mean distance was 1.94 ± 0.26 SD cm in male cadavers, and 1.56 ± 0.37 SD cm in female cadavers (Table 3).

In all cadavers we observe the penetrance point of the SLA and ibSLN were common at the thyrohyoid membrane area.

It has been reported that inferior branch of ibSLN serves as principal sensory nerve in the region above the level of the vocal cords and sends fibers to posterior glottis, hypopharynx and subglottic mucosa [8–10].

The preservation of ibSLN is mandatory for providing sensory innervation from most of the laryngeal mucosa. During swallowing, vomiting and coughing the glottic closure reflex mechanism protect us from aspiration. Damage or disfunction of ibSLN may cause insufficient mucosal sensation and lack of glottic closure reflex that may lead aspiration of secretions/foreign body to lungs [8, 11–13].

It is possible to damage the ibSLN during surgical procedures like neck dissections, partial laryngectomies, cervical spine surgery or carotid endarterectomies [6–7]. In order to prevent the nerve during the surgeries of this region, knowledge about the topographic anatomy of ibSLN becomes important. More accurate documentation of the topographic anatomy of the nerve and its relation to certain anatomical landmarks should be identified thus promoting safer handling during surgery.

Superior laryngeal nerve (SLN) arises from vagus nerve and deep into the carotid artery it splits into internal and external branches at the level of lesser cornu of hyoid [8, 14, 15]. The ibSLN courses medially and accompanied by the superior thyroid artery [15, 16]. Entrance point of the ibSLN to the larynx is through thyrohyoid membrane [13]. While some authors reported that superior laryngeal artery (SPA) and ibSLN enters the thyrohyoid membrane through a common opening, others reported that SLA pierces the membrane below ibSLN from a different foramen [8, 13, 17, 18]. In the present study we observe the penetrance point of the SLA and ibSLN were common at the thyrohyoid membrane area.

After the penetration of thyrohyoid membrane, ibSLN was shown to be divided into two branches or three branches [3, 8, 19, 20]. Mucosa of the epiglottis and a some part of vallecula is inervated by superior branch and the aryepiglottic folds are innervated by the middle branch. Interarytenoid muscles receives branches from the inferior branches [9, 10, 21, 22].

Knowledge about the relationship between the topographic anatomy of ibSLN and certain anatomical landmarks is very limited in the literature. The diameter of the ibSLN is thin in the thyrohyoid membrane region and thus vulnerable for damage around this area [3, 20, 21]. In the present study we demonstrated its topographic anatomy at the entrance point through the thyrohyoid membrane in relation to superior border of thyroid cartilage, thyroid notch, carotis bifurcation, hyoid corpus and hyoid greater cornus. During the surgical procedures of this region (either dissection or the placement of retractor blades), these anatomical landmarks are frequently encountered structures.

In the present study we found the distance between the carotis bifurcation (CB) and the penetration point of ibSLN as 1.67 ± 0.32 SD cm. It was also longer in male cadavers that was statistically different (p = 0.012). During exposure the relation between the nerve and carotis bifurcation and the difference between genders should be taken into consideration.

Prior to piercing the thyrohyoid membrane, the ibSLN crosses the greater cornu of the hyoid bone. We found the distance between greater horn of the hyoid (HH) and the penetration point of the ibSLN as 1.65 ± 0.38 SD cm. Also the distance from the midpoint of HC to the penetration point of the ibSLN was 2.04 ± 0.30 SD cm for all cadavers.

We found that vertical distance between the penetration point of ibSLN to thyrohyoid membrane and the superior border of thyroid cartilage (TS) was 0.60 ± 0.14 SD cm. In another study the authors had found the same distance ranging from 0 to 1.8 cm. They had noted that in 87.5% of the cases the distance ranged from 0.1 to 1.2 cm [23]. During the incisions to thyrohyoid membrane for partial laryngectomies or laryngocele surgeries, the distance between superior border of the thyroid bone and the penetration point of the nerve should be kept in consideration.

In our study, the horizontal distance between the midpoint of the thyroid notch and the intersecting point of the line we draw at the superior border of the thyroid cartilage (TN) was 1.97 ± 0.29 SD cm. It was statistically longer in male cadavers (p = 0.034). As a result we can conclude that the data should be remembered for male and female candidates.

In clinical practice, by means of knowledge of the topographical anatomy, danger zone for damaging the ibSLN can be predicted and damage risk can be minimized. The area that should be preserved can be identified.

There were some limitations for our study. Firstly, the races of the cadavers were unknown, therefore the measurements may vary in different races. Secondly the number of the cadavers the number of cadavers may not be sufficient to identify anatomical variants.

We provided the surgical anatomy and important landmarks for determining the internal branch of superior laryngeal nerve in the thyrohyoid membrane entrance region. With the use of this knowledge, the nerve can be identified and/or preserved during surgical interventions such as partial laryngectomies, neck dissections and cervical spine surgeries.

Acknowledgement

The authors sincerely thank those who donated their bodies to science so that anatomical research could be performed. The authors wish also to thank Prof. Dr. Ufuk Şakul for excellent logistic and equipment support during the study.

Ethical Approval

The research protocol was approved by the local ethics committee (protocol no:523; 25.05.2021) and was conducted in accordance with the Declaration of Helsinki.

Competing Interests

The authors declared no potential conflict of interest with respect to the research, authorship, and/or publication of this article.

Authors’ contributions

Concept and Design – Bilge Türk, Buket Canda, Kadriye Betül Pençe, Neslihan Yüzbaşıoğlu, Suat Turgut ; Supervision – Bilge Türk, Neslihan Yüzbaşıoğlu, Suat Turgut ; Materials, Data collection &/or processing – Bilge Türk, Buket Canda, Kadriye Betül Pençe, Neslihan Yüzbaşıoğlu, Suat Turgut ; Analysis and/or interpretation – Bilge Türk, Buket Canda, Kadriye Betül Pençe, Neslihan Yüzbaşıoğlu, Suat Turgut ; Literature search – Bilge Türk, Buket Canda, Kadriye Betül Pençe, Neslihan Yüzbaşıoğlu, Suat Turgut ; Writing– Bilge Türk, Kadriye Betül Pençe, Suat Turgut; Critical review – Bilge Türk, Suat Turgut

Funding

This research received a grant from the Anatomy Department of the Medipol University and the Faculty of Medicine Research Fund (Grant No: 2021/18) of Medipol University, Turkey.

Availability of data and materials

The data that support the findings of this study are available from the corresponding author, Buket Canda, upon reasonable request.

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Table 1

Specimen	Gender (F=female M=male)	Age (year-old)	Height (cm)	Weight (kg)	BMI
1	F	72	157	86	34.7
2	F	72	162	45	17.2
3	F	64	157	98	39.7
4	M	63	165	56	20.8
5	F	66	166	95	34.4
6	F	64	180	104	34.0
7	F	79	162	63	24.0
8	M	77	182	40	12.2
9	M	72	180	81	25.1
10	M	94	177	61	19.4
11	F	57	162	67	25.6
12	F	66	152	40	17.6
13	F	95	157	45	18.3
14	M	72	175	61	19.9
15	F	68	162	48	18.4
16	F	71	149	40	17.8
17	F	93	137	61	32.5
18	F	85	144	40	16.2
19	M	72	160	56	22.1
20	M	92	170	45	15.7
21	M	52	177	81	25.8
22	M	54	182	42	12.7

Table 2

Parameters (Certain landmarks)	Mean (cm) (n=44)	SD	Minimum (cm)	Maximum (cm)
Hyoid major cornus	1.65	0.38	0.80	2.50
Hyoid corpus	2.04	0.30	1.30	2.50
Carotis bifurcation	1.67	0.32	1.00	2.50
Thyroid notch	1.97	0.29	1.00	2.50
Thyroid superior border	0.60	0.14	0.30	0.90

Table 3

Parameters (Certain landmarks)	Female		Male		p value
Parameters (Certain landmarks)	Mean (cm)	SD	Mean (cm)	SD	p value
Hyoid major cornus	1.66	0.42	1.73	0.38	0.718
Hyoid corpus	2.02	0.35	2.10	0.25	0.560
Carotis bifurcation	1.56	0.37	1.94	0.26	*0.012*
Thyroid notch	1.88	0.24	2.10	0.19	*0.034*
Thyroid superior border	0.65	0.13	0.55	0.12	0.098

No competing interests reported.

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Journal Publication

published 30 Dec, 2022

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Editorial decision: Major revision
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Reviews received at journal
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Reviewers agreed at journal
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You are reading this latest preprint version

Surgical landmarks for identification and preservation of the internal branch of the superior laryngeal nerve

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Journal Publication

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Abstract

Figures

Introduction

Materials And Methods

Statistical Analyses

Results

Discussion

Conclusion

Declarations

References

Tables

Additional Declarations

Status:

Journal Publication

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