Surgical anatomy of the external branch of the superior laryngeal nerve

Background: The variations in the anatomy of the external branch of the superior laryngeal nerve (EBSLN) are generally classified according to the relationship of the nerve to the superior thyroid artery, or the superior pole of the thyroid. Both artery and superior pole are themselves variable landmarks, and therefore are not consistent between subjects. We sought to examine EBSLN anatomy in relation to alternate, more consistent surgical landmarks. Methods: Fifteen hemi‐larynges from 20 embalmed human cadavers were dissected anatomically. Each nerve was categorized using established classification systems, and terminal branching patterns were also noted. Nerve location was recorded in relation to three different constant anatomical structures: the laryngeal prominence, midline junction of the cricothyroid muscles and ipsilateral cricothyroid joint. Results: All cadavers were of European descent. The EBSLN had two branches to the cricothyroid muscle in 34% of cases. The EBSLN anatomical types found were mainly Cernea type 1 (80%), with 7% type 2a and 13% type Ni. An EBSLN was more likely to lie in an ‘at risk’ location if the subject was less than 160 cm tall. The EBSLN entered the crico‐thyroid muscle at a median distance of 14 mm lateral from the laryngeal prominence and 8 mm inferiorly. The median distance from the medial‐most point of the cricothyroid muscle junction was 14 mm laterally and 14 mm superiorly, and from the cricothyroid joint the entry lay a median distance of 10 mm superiorly and 11 mm medially. Conclusions: The variability of EBSLN anatomy is again evident, as is the need for careful and knowledgeable surgical technique. New surgical landmarks for the relations of the insertion of the EBSLN reveal a deployment range for each, but one of more of these landmarks may prove useful in thyroid surgery.     

Surgical anatomy of the external branch of the superior laryngeal nerve.

Iatrogenic lesions of the external branch of the superior laryngeal nerve (EBSLN) during thyroidectomies are not infrequent due to the possibility of anatomic variations of the relationships of this nerve with the superior thyroid vessels. Therefore, based on an anatomic analysis of 30 superior thyroid poles from 15 fresh cadavers, a new classification of the EBSLN was proposed, considering the jeopardy during a thyroidectomy. Thirty-seven percent of the nerves were type 2, ie, crossing the superior thyroid pedicle less than 1 cm above the superior thyroid pole. It is notable that 20% were type 2b, ie, crossing the vessels below the upper border of the pole, having been considered "high risk." This incidence was comparable with other series, which found dangerous anatomic variations of the EBSLN in the range of 15% to 68%, confirming that a significant proportion of these nerves might be at risk during surgery on the superior thyroid pole.     

Surgical anatomy of the external branch of the superior laryngeal nerve: clinical-anatomical investigation

On the basis of an anatomical (60 observations) and clinical material (160 patients with different diseases of the thyroid gland) the authors specified topography of the external branch of the superior laryngeal nerve, proposed supplements to the classification of variants of its passage, estimated the risk of traumatizing this structure during operation, determined the zone of most probable injury. Anatomical substantiation is given to the places of most convenient ligation of the main branches of the superior thyroid vessels.     

Identification of the external branch of the superior laryngeal nerve during thyroidectomy.

Seventy-six patients underwent preoperative vocal evaluation and were randomized into 3 groups: (1) those with the superior thyroid pole dissected by the first author, with the external branch of the superior laryngeal nerve (EBSLN) identified by means of a nerve stimulator; (2) those patients whose dissection was executed by a resident, with no nerve search; and (3) those whose dissection was undertaken by the first author, without any nerve search. Postoperative analysis consisted of voice evaluation and electromyography of the cricothyroid muscle. No lesion occurred in patients in group 1. Twenty-eight percent of patients in group 2 and 12% in group 3 experienced a complete lesion of the EBSLN (p = 0.0123). When the patients in group 1 were compared with the patients with 62 nerves corresponding to nonoperated thyroid lobes, patients in group 1 exhibited no increased risk, whereas a significantly increased hazard was evident in both groups 2 (p = 0.0002776) and 3 (p = 0.0346393). In this study, effective prevention of iatrogenic EBSLN lesions during thyroidectomies was achieved only by the intraoperative identification of the nerve with the nerve stimulator.     

Surgical anatomy of the internal branch of the superior laryngeal nerve

The internal branch of the superior laryngeal nerve (ibSLN) may be injured during anterior approaches to the cervical spine, resulting in loss of laryngeal cough reflex, and, in turn, the risk of aspiration pneumonia. Such a risk dictates the knowledge regarding anatomical details of this nerve. In this study, 24 ibSLN of 12 formaldehyde fixed adult male cadavers were used. Linear and angular parameters were measured using a Vernier caliper, with a sensitivity of 0.1 mm, and a 1° goniometer. The diameter and the length of the ibSLN were measured as 2.1±0.2 mm and 57.2±7.7 mm, respectively. The ibSLN originates from the vagus nerve at the C1 level in 5 cases (20.83%), at the C2 level in 14 cases (58.34%), and at the C2–3 intervertebral disc level in 5 cases (20.83%) of the specimens. The distance between the origin of ibSLN and the bifurcation of carotid artery was 35.2±12.9 mm. The distance between the ibSLN and midline was 24.2±3.3 mm, 20.2±3.6 mm, and 15.9±4.3 mm at the level of C2–3, C3–4, and at the C4–5 intervertebral disc level, respectively. The angles of ibSLN were mean 19.6±2.6° medially with sagittal plane, and 23.6±2.6° anteriorly with coronal plane. At the area between the thyroid cartilage and the hyoid bone the ibSLN is the only nerve which traverses lateral to medial. It is accompanied by the superior laryngeal artery, a branch of the superior thyroid artery. The ibSLN is under the risk of injury as a result of cutting or compression of the blades of the retractor at this level. The morphometric data regarding the ibSLN, information regarding the distances between the nerve, and the other consistent structures may help us identify this nerve, and to avoid the nerve injury.     

External branch of the superior laryngeal nerve: applied surgical anatomy and implications in thyroid surgery

The external branch of the superior laryngeal nerve (EBSLN) is the only motor supply to the cricothyroid muscle and has an important role during phonation in high frequencies. Iatrogenic injury of the EBSLN, most commonly during thyroid surgery, is associated with varying levels of alterations in phonation, which may have an impact on a patient's life, especially when his or her career depends on the full range of voice. EBSLN injury incidence after thyroid surgery ranges widely in the literature (0 to 58%). Despite this wide variation, it appears that EBSLN injury is a not uncommon, and frequently overlooked, complication of thyroid surgery. An in-depth knowledge of the surgical anatomy of the EBSLN is therefore required from the part of the operating surgeon to protect this nerve during thyroid surgery.     

神经监测在喉上神经外支临床解剖分型探讨中的应用

目的 探讨在甲状腺手术中应用神经监测技术确定喉上神经外支解剖分型的临床意义.方法 2011年3-10月,在70例甲状腺手术患者术中以喉上神经外支受电流刺激后可产生环甲肌收缩活动作为阳性反应定位喉上神经外支的走行并判明其与甲状腺上极、咽下缩肌的关系.结果 手术共定位108条喉上神经外支(99.1％),解剖显露42条(38.9％),神经与甲状腺上动脉的交叉点距离甲状腺上极＞1 cm者55条(50.9％),＜1 cm者53条.＜0.5 cm或者位于腺体背面者29条(26.9％),为喉上神经外支高风险分型,＞0.5 cm＜1 cm者24条(22.2％),甲状腺上下径＞5 cm者中＜0.5 cm或者位于腺体背面比例显著增加.手术前后均行喉镜检查,一例术后出现发音低沉等喉上神经外支损伤表现.结论 甲状腺手术中神经监测技术可以定位喉上神经外支的走行及其变异并指导手术者规避神经损伤风险.     

Neuromonitoring of the external branch of the superior laryngeal nerve during thyroid surgery

Changes of the voice caused by injury of the external branch of the superior laryngeal nerve (EBSLN) during thyroid surgery may have essential consequences for a patient's life. Therefore, intraoperative effort has to be made to prevent EBSLN damage. Neuromonitoring has already been described as helpful to improve the identification rate of the recurrent laryngeal nerve. We describe our surgical procedure of upper thyroid pole preparation using neuromonitoring to avoid intraoperative damage of the superior laryngeal nerve. Neuromonitoring allowed safe preparation of the upper thyroid pole without injury of the ESBLN in 108 consecutive patients.     

Applied anatomy of the superior laryngeal nerve and surgical prophylactics of its lesions

The article gives a brief historical reference on the discovery and investigation of the superior laryngeal nerve, the anatomical and clinical exploration of its topography, physiology and pathophysiology, as well as main variants of the correlations of the external branch of the nerve and the superior thyroid artery. The importance of superior laryngeal nerve is shown in the voice-forming function of the larynx. The incidence and main methods of prevention of intraoperative lesions of the nerve are described. The authors consider the potentials of clinical and instrumental diagnostics, as well as the effectiveness of treatment of traumas of the exterior branch of the superior laryngeal nerve during surgery for diseases of the thyroid gland.     

喉上神经监测在腔镜辅助小切口甲状腺手术中的应用

目的 探讨在腔镜辅助小切口甲状腺手术中,应用神经监测技术避免喉上神经外支损伤的可行性.方法 2011年2-9月间36例腔镜辅助小切口甲状腺手术患者,术中以电流刺激下环甲肌收缩活动作为阳性反应,定位喉上神经外支的走行,远离神经处理上极血管.手术前后VHI-10评分评价声音质量变化、喉镜检查观察声带活动情况.结果 手术共定位56侧喉上神经外支(100％),神经与甲状腺上动脉的交叉点距离甲状腺上极＞1 cm者26侧(46.4％),＜1 cm者30侧(53.6％),而在甲状腺上下径＞5 cm患者中,则73％患者此距离＜1.0 cm,P=0.006,手术前后VHI-10评分差异无统计学意义(P＞0.05).结论 腔镜辅助小切口甲状腺手术中喉上神经监测可定位喉上神经外支走行,有利于预防喉上神经外支损伤.     

Functional anatomy of the recurrent and superior laryngeal nerve

Background and aims The purpose of this study was to present the current topographic and anatomical knowledge in neurolaryngology, with special regard to laryngeal paralyses as a major complication in thyroid surgery. Patients and methods Microscopic anatomical preparation of 22 human hemilarynges was accomplished. Results Due to their neuroanatomical courses, the following extralaryngeal nerves may be at risk in thyroid surgery: the external branch of the superior laryngeal nerve, the paralaryngeal part of the vagal nerve, the Ansa Galeni, the trunk of the recurrent laryngeal nerve (RLN) and the delicate branches of the RLN to the posterior cricoarytaenoid muscle. The anterior and posterior branches of the RLN (antRLN and postRLN) are less endangered by thyroid surgery because they are covered by the thyroid cartilage and posterior cricoarytaenoid muscle (PCA), respectively. In contrast, the antRLN is vulnerable if a ventilation tube is dislocated, with cuff-induced pressure to the glottic level. Conclusion The increased knowledge in neurolaryngology provides the basis for a selective neuromonitoring to lower the risk of laryngeal paralyses after thyroid surgery. Presented at the International Symposium “Modern Technologies in Thyroid Surgery”, 10–11 February 2006, Halle/Saale, Germany