Motor innervation of the canine arytenoid muscle]

Dual motor innervation by the bilateral recurrent laryngeal nerves (RLNs) has been demonstrated in the human arytenoid muscle (AR). Whether AR of the dog receives dual motor innervation remains to be cleared yet, although the canine larynx is frequently used in experimental studies. To answer this question, the author observed the muscular structure in detail and anastomotic nerve branch between the bilateral RLNs, and then carried out glycogen depletion experiments on AR of dog compared with typical unpaired ARs of monkey and of guinea pig. 1) Muscular structure AR of the dog consisted of three parts of muscle bundles: the transverse arytenoid muscle (TVA), ventricular muscle (VT) and anonymous small bundle provisionally named the smaller interarytenoid muscle (IAm). While TVA and VT were paired type, IAm was unpaired type and lay horizontally on the dorsal aspect of the sesamoid cartilage around the midline. So the canine AR displayed a trigastric muscle as a whole. 2) Anastomotic nerve branch By the vital staining with methylene blue, the arytenoid branch of canine RLN ramified in three directions: anteriorly to the bellies of TVA and VT, medially to the anastomotic branch and superomedially to IAm. By the silver impregnation method of Barker and Ip, the bilateral IAm ramuli were found to form collateral anastomoses and terminate disorderly on the individual fibers. 3) Glycogen depletion experiments When an electrical stimulation was applied to the unilateral RLN in the monkey and the guinea pig, about one half of AR fibers were unstained with PAS staining and, in turn, these unstained fibers were known to be innervated by the ipsilateral RLN. While these unpaired ARs receive dual motor innervation as a whole muscle, every individual fiber is innervated by the unilateral RLN. In the canine VT and TVA, almost 90% of fibers were depleted of glycogen on the belly of the stimulated side, while the reverse was on the nonstimulated side. This finding suggests that most fibers of canine VT and TVA are ipsilaterally and the remaining fibers are contralaterally innervated. About one half of fibers of IAm were unstained and the others were stained. This pattern was similar to that observed in the monkey and the guinea pig. Therefore, IAm receives dual motor innervation from both RLNs as a whole muscle.     

Restoring vocal fold movement after transection and immediate suturing of the recurrent laryngeal nerve with local application of basic fibroblast growth factor: an experimental study in the rat

OBJECTIVE: To evaluate the effects of basic fibro-blast growth factor (bFGF) on the recovery of vocal fold movement and the attenuation of laryngeal muscle atrophy after transection of the recurrent laryngeal nerve (RLN). STUDY DESIGN: Quantitative assessment of vocal fold movement using the video cassette recorder (VCR) image-analysis method and histologic examination of the laryngeal muscle. METHODS: Fifty-eight Wistar rats underwent RLN transection and one of the following three procedures: 1) transection of the RLN alone (transection group, n = 18), 2) suture of the nerve stumps followed by local administration of phosphate-buffered saline (PBS) solution using an osmotic pump (PBS group, n =20), or 3) suture of the nerve stumps followed by local administration of bFGF (FGF group, n = 20). Vocal fold movements were recorded with VCR by way of a rigid endoscope, and the VCR images were analyzed on a computer. Histologic changes in the thyroarytenoid (TA) muscle were evaluated by measuring the cross-sectional area of the muscle and average size of muscle fibers. RESULTS: In the transection group, vocal fold movement did not recover, and atrophy of the TA muscle gradually progressed after sectioning the nerve. In contrast, vocal fold movement as assessed by VCR image-analysis recovered in some cases in the immediate suturing groups, more markedly in the FGF group (34.1 +/- 29.1%) than in the PBS group (5.5 +/- 7.9%) (P <.05). Histologically, atrophy of the laryngeal muscle was significantly attenuated by the local administration of bFGF. CONCLUSION: bFGF facilitates regeneration of the transected RLN and attenuation of intrinsic laryngeal muscle atrophy, thereby restoring laryngeal function.     

Surgical anatomy of the recurrent laryngeal nerve: implications for laryngeal reinnervation

Functional laryngeal reinnervation depends upon the precise reinnervation of the laryngeal abductor and adductor muscle groups. While simple end-to-end anastomosis of the recurrent laryngeal nerve (RLN) main trunk results in synkinesis, functional reinnervation can be achieved by selective anastomosis of the abductor and adductor RLN divisions. Few previous studies have examined the intralaryngeal anatomy of the RLN to ascertain the characteristics that may lend themselves to laryngeal reinnervation. Ten human larynges without known laryngeal disorders were obtained from human cadavers for RLN microdissection. The bilateral intralaryngeal RLN branching patterns were determined, and the diameters and lengths of the abductor and adductor divisions were measured. The mean diameters of the abductor and adductor divisions were 0.8 and 0.7 mm, while their mean lengths were 5.7 and 6.1 mm, respectively. The abductor division usually consisted of one branch to the posterior cricoarytenoid muscle; however, in cases in which multiple branches were seen, at least one dominant branch could usually be identified. We conclude that the abductor and adductor divisions of the human RLN can be readily identified by an extralaryngeal approach. Several key landmarks aid in the identification of the branches to individual muscles. These data also indicate the feasibility of selective laryngeal reinnervation in patients who might be candidates for laryngeal transplantation after total laryngectomy.     

喉返神经损伤后环杓关节变化的观察

为观察喉返神经（ＲＬＮ）损伤后杓状软骨及环杓关节的变化，将１８只犬分为３组。第１单侧ＲＬＮ不完全损伤，第２组为单侧ＲＬＮ完全损伤，第３组为双侧ＲＬＮ损伤。结果第１组随着ＲＬＮ的自行修复，杓状软骨恢复正常无环杓关节纤维化；第２组随着时间的推移，出现杓状软固定，环杓关节纤维化亦加重；第３组较时时间就出现杓状软骨固定，且随时间推移，环杓关节纤维化程度亦较第２组严重     

Laryngeal synkinesis: its significance to the laryngologist

Basic research and surgical cases have shown that the injured recurrent laryngeal nerve (RLN) may regenerate axons to the larynx that inappropriately innervate both vocal cord adductors and abductors. Innervation of vocal cord adductor muscles by those axons that depolarize during inspiration is particularly devastating to laryngeal function, since it produces medial vocal cord movement during inspiration. Many patients thought to have clinical bilateral vocal cord paralysis can be found to have synkinesis on at least one side. This will make the glottic airway smaller, particularly during inspiration, than would true paralysis of all the intrinsic laryngeal muscles. Patients with bilateral vocal cord paralysis should undergo laryngeal electromyography. If inspiratory innervation of the adductor muscles is present, simple reinnervation of the posterior cricoarytenoid muscle will fail. The adductor muscles also must be denervated by transection of the adductor division of the regenerated RLN.     

Variability in nerve patterns of the adductor muscle group supplied by the recurrent laryngeal nerve

INTRODUCTION: Accurate knowledge of the nerve supply of each individual muscle is needed to achieve a successful selective reinnervation of the larynx. The aim of the present work was to study the nerve supply of the adductor laryngeal muscles supplied by the recurrent laryngeal nerve. STUDY DESIGN: Morphologic study of human larynges. METHODS: The muscular nerve supply was studied in a total sample of 75 human larynges obtained from necropsies (47 males and 28 females, age range from 41-95 years) and examined by careful dissection using a surgical microscope. RESULTS: The arytenoid muscle received one branch from each recurrent nerve. In 88% of cases, this branch arose in a common trunk with the upper branch of the posterior cricoarytenoid muscle. In 8% of cases, the nerve for the arytenoid muscle also had a branch going to the lateral cricoarytenoid muscle. The arytenoid muscle also received from one to three pairs of branches from the posterior division of the internal laryngeal nerve; these were interconnected ipsi- and contralaterally and were also connected to the two branches coming from the recurrent laryngeal nerve. The lateral cricoarytenoid muscle received from one to six branches from the recurrent nerve, but in 5.8% of cases, it also received a twig from a connecting branch between the recurrent nerve and the external (5.6%) or the internal laryngeal nerves (0.2%). The thyroarytenoid muscle received from one to four branches from the recurrent nerve, but in 5.6% of cases, it also received a twig from a connecting branch between the recurrent nerve with the external (4.6%) or the internal (1%) laryngeal nerves. CONCLUSION: No abductor or adductor division of the recurrent laryngeal nerve was found in the present study. In 88% of cases, the nerve supply to the arytenoid muscle (adductor) and the posterior cricoarytenoid muscle (abductor) arose from a common trunk, which in 8% of cases, also had a branch to the lateral cricoarytenoid muscle. Furthermore, the high incidence of branches innervating the adductor muscles from connections between the recurrent laryngeal nerve and the internal and external laryngeal nerves led us to reconsider the contribution of these nerves in the supply to this muscle group.     

Innervation of the paralyzed laryngeal muscles by phrenic motoneurons. A quantitative study by light and electron microscopy.

In the cat, inspiratory opening of the paralyzed glottis recovered after unilateral or bilateral reinnervation of the posterior cricoarytenoid (PCA) muscles by phrenic axons. The morphometric analysis of the regenerated recurrent laryngeal nerves (RLNs), showed that proliferation was abundant; 4 months after the nerve anastomosis, more than 500 myelinated axonal branches repopulated the RLNs. The mean diameter of motor axons (3.5 to 5.0 microns) was lower than in normal phrenic and RLN (8 to 10 microns), and the mean internode length was about half that of the normal RLN. Histochemical examination of the PCA muscle revealed that muscle fiber composition (44% type I and 56% type II muscle fiber) was fairly similar to that of normal PCA. The contraction time of the reinnervated muscles was as long as 60 msec at the time of movement recovery, but it shortened to 25 to 30 msec when the reinnervation time increased. These anatomical and functional results support the choice of the phrenic nerve for laryngeal reinnervation.     

Measurement of adductory force of individual laryngeal muscles in an in vivo canine model

In this experiment, the adductory properties of three intrinsic laryngeal muscles (the thyroarytenoid [TA], lateral cricoarytenoid [LCA], and interarytenoid [IA]) were studied and quantified. Using an in vivo canine laryngeal model, a recently developed “tensionometer” was used to measure the adductory force produced by each of these muscles at the vocal process of the arytenoid. Isolated muscle activation was obtained by stimulating selective terminal branches of the anterior division of the recurrent laryngeal nerve. Results indicate that the LCA is the strongest adductory muscle, followed by the TA and the IA Videolaryngoscopy revealed that LCA contraction causes adduction of the vocal fold and vocal process, with the predominant effect on the process. TA stimulation leads primarily to adduction of vocal fold, and the IA adducts mainly the vocal process. Implications of these findings are discussed.     

Objective Measures of Laryngeal Function After Reinnervation of the Anterior and Posterior Recurrent Laryngeal Nerve Branches

Previous research indicates that separate reinnervation of the anterior and posterior branches of the recurrent laryngeal nerve (RLN) can provide purposeful motion of the larynx, even after transplantation. This canine study was undertaken to better determine the results of RLN reinnervation after nerve transection distal to its bifurcation. This approximates ideal conditions for transplantation, because potential rejection and nerve branch mismatch are eliminated. Eight months after nerve repair, video, electromyographic, mechanical, and histologic data were collected on four canines. Results show return of appropriate motion without synkinesis, including purposeful abduction on endotracheal tube occlusion. Abductory function was weaker on the reinnervated side, but adduction was equal or stronger on the reinnervated vocal cord. These results indicate that this method of RLN reinnervation produces consistent, strong physiologic motion in the denervated larynx.     

Thyroarytenoid muscle responses to air pressure stimulation of the laryngeal mucosa in humans

Others have observed glottic adduction in response to air puff stimuli and suggested that this is a reliable indicator of laryngeal sensation. We undertook to determine whether the same thresholds are found if one uses either thyroarytenoid (TA) muscle responses or subjects' reports of laryngeal sensation. We also studied the characteristics of TA responses to unilateral air pressure stimulation of the mucosa overlying the arytenoid cartilages. Ten normal volunteers provided button press responses to air pressure stimuli during bilateral TA electromyography. Similar thresholds were determined by reports of sensation as by electromyographic responses (p < .0005). The early TA responses occurred either around 80 ms or around 125 ms after onset of the air puff, with equal frequency on the ipsilateral and contralateral sides. The TA muscle responses to air pressure stimulation differ in physiological characteristics from the laryngeal adductor reflex that occurs in response to electrical stimulation of the superior laryngeal nerve.     

Vagus nerve stimulation: quality control in thyroid and parathyroid surgery

This paper describes the use of the Neurosign 100 Nerve Monitor and vagus nerve stimulation in the identification and assessment of the integrity of the recurrent laryngeal nerve (RLN) during thyroid and parathyroid surgery. Vocal fold function was assessed pre- and post-operatively in all patients undergoing thyroid and parathyroid surgery. The nerve monitor, used in association with endotracheal electrodes, was used to confirm correct RLN identification and demonstrate its integrity at the completion of surgery. There were 21 unilateral and 19 bilateral neck explorations. In these 40 patients, 57 of 59 RLNs were identified. The nerve monitor demonstrated RLN continuity in all but one case (equipment failure: electrode misplacement) after initial identification. Vagus nerve stimulation was performed in 21 patients without adverse sequelae. Damage to the RLN was identified in one of these patients, in whom direct RLN stimulation close to the larynx had failed to indicate discontinuity. Post-operatively this patient had a transient unilateral vocal fold palsy. The use of the Neurosign 100 Nerve Monitor is no substitute for meticulous surgery. Stimulation of the vagus nerve may be a more sensitive means of assessing RLN integrity during thyroid and parathyroid surgery than stimulation of the RLN itself. Confirmation of RLN integrity allows the surgeon to proceed with confidence to the contralateral side of the neck during hazardous bilateral explorations.     

An anatomical study of anastomoses between the laryngeal nerves.

OBJECTIVE: To systematize the anatomy of the connecting branches between laryngeal nerves. METHODS: Microdissection of 90 larynges obtained from necropsies (57 men and 33 women; age range, 41-95 y). RESULTS: Anastomoses between the internal and recurrent nerves appeared in four different patterns: 1) Galen's anastomosis, as the connection between the dorsal branches of both nerves (100%); 2) arytenoid plexus, as the connection between the arytenoid branches of both nerves, in relation with the arytenoid muscle, and divided in a deep part (100%) and a superficial part (86%); 3) cricoid anastomosis, previously only described in cows, located in the front of the cricoid lamina (6/10 cases); and 4) thyroarytenoid anastomosis, as the connection of a descending branch of the internal laryngeal nerve and an ascending branch of the recurrent nerve (14%). Anastomosis between the internal laryngeal and the external laryngeal nerves appeared as a connecting branch throughout the foramen thyroideum (21%). Anastomosis between the external laryngeal and recurrent nerves appeared as a connecting branch throughout the cricothyroid muscle (68%). CONCLUSION: At least two anastomoses (Galen's anastomosis and arytenoid plexus) appeared in 21% of hemilarynges, and 79% of cases had three or more anastomoses between the laryngeal nerves. The different prevalence of this complex anastomotic pattern suggests functional differences in the sensory and motor innervation of individual subjects.     

Laryngeal reinnervation surgery — results of a selective approach in an animal study

Laryngeal reinnervation surgery is a difficult subject due to the fact that the recurrent laryngeal nerve (RLN) is responsible for both adductor and abductor laryngeal activity. Non-selective reinnervation procedures will result in laryngeal synkinesis with restoration of tonicity. Restoration of mobility requires selective reinnervation of the adductor and abductor branches with nerves with similar activity patterns as the initial abductor and adductor branches of the RLN.     

Spontaneous laryngeal reinnervation after recurrent laryngeal or vagus nerve injury

OBJECTIVES: The status of innervation in patients with laryngeal paralysis is somewhat controversial. Electromyographic activity has been frequently documented in the laryngeal muscles of patients with laryngeal paralysis, and animal experiments report a strong propensity for reinnervation after laryngeal nerve injury. However, a study of intraoperative electromyography performed in patients during reinnervation surgery failed to document activity with stimulation of the recurrent laryngeal nerve (RLN). Noting the long-observed differences in the symptoms of patients with vagus nerve injury and those with RLN injury, I hypothesized that reinnervation is influenced by the site of nerve injury. METHODS: Cats were sacrificed at various intervals after resection of 1 cm of either the RLN or the vagus nerve, without any attempt to repair the nerve. RESULTS: Four months after RLN resection, distal nerve biopsy revealed unmyelinated axons scattered through fibrous tissue. By 6 months, myelinated axons were organized, and electromyographic and histologic examination showed preferential reinnervation of the thyroarytenoid muscle. After vagotomy, the RLN was fibrotic and no axons were present. Both the thyroarytenoid and posterior cricoarytenoid muscles were fibrotic and had no electromyographic activity. CONCLUSIONS: The results confirm the strong propensity for laryngeal reinnervation after RLN injury, but not after vagus nerve injury. Preferential reinnervation of adductor muscles may account for a medial position of the paralyzed vocal fold.     

Pediatric laryngeal paralysis: a new proposed surgical therapy

OBJECTIVES: The cricothyroid muscle (CTM) has a separate innervation from that of intrinsic laryngeal muscles; therefore, its action may contribute to airflow resistance in children with laryngeal paralysis (LP) secondary to recurrent laryngeal nerve (RLN) palsy. We proposed removal of the CTM as a means of indirectly widening the paralyzed neonatal glottis. METHODS: A prospective study was conducted using a piglet animal model to simulate LP and evaluate the proposed treatment's outcome. LP was induced via bilateral RLN sectioning in seven piglets. The CTMs were then removed. Animals acted as their own controls. Outcome measures consisted of serial inspiratory and expiratory airflow resistance measurements taken (1) with no intervention, (2) after RLN sectioning, and (3) after CTM removal. Several animals were awakened to assess their clinical responses to the interventions. The paired Student's t-test was used for statistical analysis. RESULTS: Inspiratory airflow resistance was significantly increased by RLN sectioning (p = .0062) and then significantly decreased after subsequent CTM removal (p = .0005). Clinical responses to the interventions mirrored the measured findings. CONCLUSIONS: Removal of the CTM significantly decreases inspiratory airway resistance in piglets with induced LP. This proposed surgical therapy for pediatric bilateral LP warrants further investigation.     

Recovery of inspiratory abduction of the paralyzed vocal cords after bilateral reinnervation of the cricoarytenoid muscles by one single branch of the phrenic nerve

The aim of this study was to provide the bilateral reinnervation of the posterior cricoarytenoid (PCA) muscles by the superior root of the right phrenic nerve. In six adult cats, the right phrenic root was anastomosed to the distal stump of the transected recurrent laryngeal nerve (RLN) on the same side. The RLN adductor branch was then cut and anastomosed to a nerve graft whose end was carried contralaterally and sutured to the left RLN or to the left PCA muscle. The phrenic fibers regrowing along the RLN abductor branch reinnervated the right PCA muscle and restored the inspiratory abduction of the right vocal cord in all the animals. In five of the six cats, the fibers regenerated through the RLN adductor branch and the graft reached the left PCA muscle and also restored the inspiratory opening of the larynx on the left side. Histological nerve examination revealed a fairly symmetrical distribution of the regenerated phrenic axons to the right and left PCA muscles.     

Laryngeal features of external superior laryngeal nerve denervation: revisiting a century-old controversy

A long-standing controversy exists regarding the laryngoscopic features associated with unilateral denervation of the external superior laryngeal nerve (ESLN). Recently, we modeled acute unilateral cricothyroid muscle paralysis by blocking the ipsilateral ESLN with lidocaine hydrochloride, and identified epiglottic petiole deviation to the side of paralysis during high-pitched voice production as a possible diagnostic sign. This study provides preliminary clinical evidence supporting the presence of petiole deviation in cases of ESLN denervation. Epiglottic petiole deviation to the side of weakness was present in electromyographically confirmed cases of unilateral partial or complete ESLN denervation, in isolation or in combination with denervation of other branches of the vagus nerve. In addition, a case of complete ESLN and recurrent laryngeal nerve (RLN) denervation showed return of the petiole to the midline 6 months after surgical reinnervation of the ESLN and RLN. Finally, petiole deviation was not present in isolated RLN paralysis--a finding suggesting that the diagnostic sign is uniquely associated with ESLN denervation. We concluded that deviation of the petiole to the side of cricothyroid muscle weakness during high-pitched voice production represents a potential diagnostic sign of unilateral ESLN denervation. Further research is necessary to determine factors that influence the expression and detection of this sign, as well as its diagnostic precision.