Anatomical and surgical considerations of the external branch of the superior laryngeal nerve: a systematic review

? The anatomical course of the external branch of the superior laryngeal nerve (EBSLN) is variable, and a consistent approach to its preservation during thyroid surgery is needed to reduce risk of post‐operative voice impairment. ? Despite agreement that careful dissection in the region of the superior thyroid pole is required, there is no accepted ‘best’ approach, nor any universal acknowledgement that location of the EBSLN is actually necessary. ? The popular cernea classification of EBSLN has limitations, including its decreased reliability with increased thyroid size and its irrelevance in cases of ‘buried’ variants. ? Recent work has identified factors such as ethnicity and stature in the prevalence of EBSLN variants. ? Consistent approaches to the post‐operative detection of EBSLN injury are needed to build an accurate picture of the incidence of surgical nerve injury. Then a standardised approach to EBSLN preservation may emerge.     

Post-thyroidectomy superior laryngeal nerve injury

Voice dysfunction after thyroidectomy may be caused by damage to laryngeal nerves or lesions to strap muscles with laryngo-tracheal movement impairment. Injury to an external branch of the superior laryngeal nerve (EBSLN) is sometimes difficult to recognize clinically and its electromyographic incidence ranges from 0% to 58%. In this study we evaluated, 12–18 months postoperatively, 45 patients who had undergone thyroid surgery (6 total lobectomy, 5 subtotal thyroidectomy, and 34 total thyroidectomy), using a subjective interview, laryngeal videostroboscopy and spectrographic analysis with a multidimensional voice program. Vocal parameters included fundamental frequency, jitter, shimmer, noise-to-harmonic-ratio (NHR) and degree of sub-harmonics. Laryngeal electromyography (LEMG) of the cricothyroid (CT) muscles was performed in 21 subjects with voice problems (35 EBSLNs) using a modified method for the CT recording. In 3 patients of this group (14%) LEMG documented a unilateral EBSLN injury. Easy voice fatigue and decreased pitch range were the most common symptoms after surgery. Average values of vocal parameters pre- and post-operatively in patients without neural damage (n = 42) were: jitter 0.64% and 0.78%, shimmer 3.25% and 3.54%, and NHR 0.12% and 0.13%, respectively (P > 0.05). Acoustic analysis revealed altered patterns in some patients with no objective evidence of damage to EBSLNs, suggesting an extralaryngeal cause of vocal dysfunction, such as laryngo-tracheal fixation or lesions to strap muscles. We conclude that laryngeal videostroboscopy and spectrographic analysis are very useful to assess voice problems after thyroidectomy, including in patients without LEMG-proven neural lesions, in order to suggest early speech rehabilitation, especially in professional voice users. Received: 7 May 2001 / Accepted: 8 June 2001     

Risk of injury to the external branch of the superior laryngeal nerve in thyroidectomy

OBJECTIVE: To establish in the Mexican population with thyroid disease the risk of injury to the external branch of the superior laryngeal nerve (EBSLN) based on its anatomical position. STUDY DESIGN: Non-randomized comparative clinical trial. METHODS: Seventy-three patients subjected to thyroidectomy because of thyroid nodule in which the EBSLN was identified and classified. The dependent variable was Cernea's classification of the EBSLN, and the independent variables were weight (in grams) of the nodule, side studied, gender, and age. For statistical analysis, chi2 test, Fisher's Exact test, and analysis of multiple variables (analysis of variance) were used. RESULTS: We studied 73 patients; 64 (87.62%) were women and 9 (12.38%) were men (average age, 39.3 years [age range, 17-73 y]; median age, 40 y; mode, 40 y; SD +/- 23.4 y). Regarding location of the EBSLN, for pathological lobes, 78.1% were located in a high-risk position and for nonpathological lobes, 72.7%. Comparative analysis between sides and relation between weight and classification revealed no statistical significance. CONCLUSION: The frequency of high-risk position for EBSLN lesion in our milieu was higher than that reported in series from other countries and races.     

The superior laryngeal nerve: function and dysfunction

Despite long-standing clinical interest in SLN dysfunction, most aspects of this entity continue to require clarification. The replacement of the laryngeal mirror by flexible fiberoptic and rigid rod-lens laryngoscopy (including stroboscopy) and the resulting improvement in laryngeal visualization and documentation of examination has not resulted in a better definition of characteristic signs. Symptoms are often vague, and most are shared with other voice disorders. Under the circumstances, there is good reason to suppose that SLN dysfunction yields a clinical picture at least as heterogeneous as recurrent laryngeal nerve injury and a good deal more subtle. Faced with significant inconsistencies in clinical presentation, the clinician is hard-pressed to draw conclusions regarding prevalence, patterns of dysfunction, natural history, treatment, and even about its overall significance. EMG. used judiciously and complemented by frequency range testing, seems to hold more promise as a means of reliable diagnosis than laryngoscopic examination and may serve to resolve some of the confusion surrounding SLN dysfunction. It is equally important that the otolaryngologist guard against falling into the easy habit of attributing vocal disturbance that cannot be otherwise explained to SLN dysfunction in the absence of EMG evidence. If ambiguities surrounding SLN paralysis and paresis are to be clarified, diagnostic rigor is essential.     

Surgical anatomy of the superior laryngeal nerve

The authors report on their anatomic findings in 42 surgical dissections of the superior laryngeal nerve. Better knowledge of this anatomy should enable a more conservative cervical surgery and help in attempts at nervous rehabilitation of laryngeal paralysis.     

Recovery of laryngeal sensation after superior laryngeal nerve anastomosis.

OBJECTIVES/HYPOTHESIS: Reliable motor reinnervation has been show in multiple laryngeal transplant studies; however, sensory reinnervation of the larynx after nerve anastomosis has yet to be demonstrated. The role of sensory nerve anastomosis in the transplanted larynx in unknown, but is thought to be necessary to provide airway protection. A canine model was developed to examine the possibility of reformation of sensory pathways in the larynx after nerve section and anastomosis. STUDY DESIGN: Randomized controlled experiment. METHODS: Ten canines were randomly assigned to two groups. Hydrochloric acid-induced laryngospasm was demonstrated in every dog. All dogs then had their necks explored, and the internal branch of the superior laryngeal nerve was identified and transected bilaterally. Following nerve section all dogs were retested for an acid-induced laryngospasm reflex. The control group had their wounds closed and were then awakened from anesthesia. The study group underwent microscopic anastomosis of their sensory nerves. Following a 6-month period the two groups of dogs were compared for the presence of the laryngospasm reflex. RESULTS: No dog in the control group had a response to the acid. All dogs in the study group had some response to the acid, although none of them had return of true laryngospasm. CONCLUSION: We concluded that sensory reinnervation does occur after nerve anastomosis, but the recovery of sensation may be incomplete or altered.     

Anatomic relation between the external branch of the superior laryngeal nerve and the thyroid gland

AimThis prospective study investigated the anatomic relations between the external branch of the superior laryngeal nerve (EBSLN), the superior thyroid artery (STA) and the thyroid gland in human cadavers.Material and MethodsTwenty-two human cadavers aged over 18 years old, less than 24 hours after death.ResultsThe mean distance between the EBSLN and the superior pole of the thyroid gland was 7.68 ±3.07 mm. A tangent to the inferior edge of the thyroid cartilage between the EBSLN and the STA measured 4.24 ±2.67 mm. A line from the intersection of the EBSLN - related to the STA - to the superior pole of the thyroid gland measured 9.53 ±4.65 mm. A line from the EBSLN to the midline of the most caudal point of the thyroid cartilage measured 19.70 ±2.82 mm. A line from the RENLS to the midline on the most cranial point of the cricoid cartilage was 18.35 ±3.66 mm.ConclusionThere is a variable proximity relation between the EBSLN and the superior pole of the thyroid gland; this distance ranges from 3.25 to 15.75 mm. There was no evidence of significant variation between the measures in the ethnic groups comprising the sample.     

Central projections of the rat superior laryngeal nerve

Laryngeal nerves contain the fibres that control the laryngeal function. On the rat, the studies on the functional components and the real origin of the fibres conveyed by the superior laryngeal nerve (SLN) are few. No one of such works were developed using biotinylated dextrane amines (BDA), a powerful tool for tracing neural pathways. The aim of our study was to identify by using BDA, in the rat, the nuclei of real origin of the fibres of the SLN, knowing in this way the functional components of this nerve. The study has been developed in 11 adult male Sprague-Dawley rats, applying the BDA into the damaged SLN. The results obtained in all the animals shown that the rat SLN carries efferent fibres originated within the ipsilateral nucleus ambiguous (NA) and dorsal nucleus of the vagus (DNV), and that afferent fibres reach the tractus solitari and the nucleus tractus solitari. So, in the rat, the SLN seems to convey efferent fibres from the NA and DNV and, probably, all the laryngeal afferent fibres.     

单侧喉返神经损伤神经修复术式探讨

目的　探讨 5种神经修复术治疗单侧喉返神经损伤声带麻痹的疗效。方法　 1993年 1月～ 2 0 0 1年 4月治疗外伤性单侧喉返神经损伤声带麻痹 38例 ,病程从损伤即刻至 2年不等。资料完整者 35例 ,其中行神经减压术 8例、颈襻主支喉返神经吻合术 16例、喉返神经端端吻合术 6例、颈襻神经肌蒂埋植术 3例、颈襻神经植入术 2例。手术前后喉镜、嗓音声学参数、肌电图检查等评价手术效果。结果　病程 4个月内神经减压 5例恢复了正常的声带内收及外展功能 ,4个月以内 1例、以上2例及颈襻主支吻合组、喉返神经端端吻合组则未恢复声带运动。但上述 3种术式均能使喉内收肌获有效的再神经支配 ,满意地恢复声带的肌张力、肌体积、声带振动对称性及正常黏膜波 ,声门闭合良好 ,嗓音恢复正常。颈襻神经肌蒂埋植术及颈襻神经植入术均能改善声嘶 ,但无恢复正常病例。结论　①单侧喉返神经损伤神经修复治疗以神经减压效果最佳 ;②颈襻主支吻合术、喉返神经端端吻合术也能有效地恢复喉的发音功能 ;③喉神经修复术式选择应根据病程、神经损伤程度、类型而定     

Laryngeal reinnervation for unilateral traumatic recurrent laryngeal nerve injuries]

OBJECTIVE: To investigate 5 procedures of laryngeal reinnervation for unilateral vocal cord paralysis induced by traumatic recurrent laryngeal nerve injury. METHODS: 35 cases were selected for our study, all patients had unilateral recurrent laryngeal nerve injury, including 8 for nerve decompression, 6 for end to end anastomosis of recurrent laryngeal nerve, 16 for main branch of ansa cervicalis anastomosis to recurrent laryngeal nerve, 3 for nerve muscular pedicle and 2 for nerve implantation. All cases have been subjected to preoperative and postoperative voice recording, acoustic analysis, videolaryngoscopy, strobscopy and electromyography. RESULTS: It is found the adductory and abductory motion of the vocal cord restored in 5 cases with less than 4 months course who received nerve decompression. Although functional motion of vocal cord was not seen in two patients who received nerve decompression with a course longer than 4 months and one less than 4 months, and in all cases who received ansa cervicalis anastomosis and end to end anastomosis of recurrent laryngeal nerve, these procedures resulted in medialization of vocal cord and the mass and tension of the reinnervated vocal cord may become much the same as the contralateral normal vocal cord, thus resuming symmetric vibration of the vocal cords and physiological phonation. Nerve muscular pedicle technique and nerve implantation enabled adductory muscles to be reinnervated, thus improving severe hoarseness, but they didn't restore normal voice. CONCLUSIONS: (1) Nerve decompression seems to be the best procedure in laryngeal reinnervation; (2) Main branch of ansa cervicalis technique raises satisfactory reinnervation of adductor muscles; (3) Selection of the laryngeal reinnervation protocols should depend on the course, severity and type of nerve injury.