Regeneration of the recurrent laryngeal nerve in the guinea pig: reorganization of motoneurons after freezing injury

Functional electrical stimulation following nerve injury in a large animal model

INTRODUCTION

Controversy exists over the effects of functional electrical stimulation (FES) on reinnervation. We hypothesized that intramuscular FES would not delay reinnervation after recurrent laryngeal nerve (RLn) axonotmesis.

METHODS

RLn cryo-injury and electrode implantation in ipsilateral posterior cricoarytenoid muscle (PCA) were performed in horses. PCA was stimulated for 20 weeks in eight animals; seven served as controls. Reinnervation was monitored through muscle response to hypercapnia, electrical stimulation and exercise. Ultimately, muscle fiber type proportions and minimum fiber diameters, and RLn axon number and degree of myelination were determined.

RESULTS

Laryngeal function returned to normal in both groups within 22 weeks. FES improved muscle strength and geometry, and induced increased type I:II fiber proportion (p = 0.038) in the stimulated PCA. FES showed no deleterious effects on reinnervation.

DISCUSSION

Intramuscular electrical stimulation did not delay PCA reinnervation after axonotmesis. FES can represent a supportive treatment to promote laryngeal functional recovery after RLn injury. Muscle Nerve 59:717-725, 2019.

Recurrent laryngeal nerve regeneration using an oriented collagen scaffold containing Schwann cells

OBJECTIVES/HYPOTHESIS

Regeneration of the recurrent laryngeal nerve (RLN), which innervates the intrinsic laryngeal muscles such that they can perform complex functions, is particularly difficult to achieve. Synkinesis after RLN neogenesis leads to uncoordinated movement of laryngeal muscles. Recently, some basic research studies have used cultured Schwann cells (SCs) to repair peripheral nerve injuries. This study aimed to regenerate the RLN using an oriented collagen scaffold containing cultured SCs.

STUDY DESIGN

Preliminary animal experiment.

METHODS

A 10-mm-long autologous canine cervical ansa was harvested. The nerve tissue was scattered and subcultured on oriented collagen sheets in reduced serum medium. After verifying that the smaller cultivated cells with high nucleus-cytoplasm ratios were SCs, collagen sheets with longitudinally oriented cells were rolled and inserted into a 20-mm collagen conduit. The fabricated scaffolds containing SCs were autotransplanted to a 20-mm deficient RLN, and vocal fold movements and histological characteristics were observed.

RESULTS

Scaffolds containing cultured SCs were successfully fabricated. Immunocytochemical examination revealed that these isolated and cultured cells, identified as SCs, expressed S-100 protein and GFAP but not vimentin. The orientation of SCs matched that of the oriented collagen sheet. Two months after successful transplantation, laryngeal endoscopy revealed coordinated movement of the bilateral vocal folds by external stimulation under light general anesthesia. Hematoxylin and eosin staining showed that the regenerated RLN lacked epineurium surrounding the nerve fibers and was interspersed with collagen fibers. Myelin protein zero was expressed around many axons.

CONCLUSIONS

Partial regeneration of RLN was achieved through the use of oriented collagen scaffolding.

LEVEL OF EVIDENCE

NA Laryngoscope, 127:1622-1627, 2017.

Vocal fold paralysis: improved adductor recovery by vincristine blockade of posterior cricoarytenoid

OBJECTIVES/HYPOTHESIS

A new treatment for acute unilateral vocal-fold paralysis (UVFP) was proposed in which a drug is injected into the posterior cricoarytenoid muscle (PCA) shortly after nerve injury, before the degree of natural recovery is known, to prevent antagonistic synkinetic reinnervation. This concept was tested in a series of canine experiments using vincristine as the blocking agent.

STUDY DESIGN

Animal experiments.

METHODS

Laryngeal adductor function was measured at baseline and at 6 months following experimental recurrent laryngeal nerve (RLN) injuries, including complete transection, crush injury, and cautery. In the treatment animals, the PCA was injected with vincristine at the time of RLN injury.

RESULTS

Adductor function in the vincristine-treated hemilarynges was significantly improved compared with injury-matched noninjected controls (total n = 43). Transection/repair controls recovered 56.1% of original adductor strength; vincristine-treated hemilarynges recovered to 73.1% (P = 0.002). Cautery injuries also improved with vincristine block (60.7% vs. 88.7%; P = 0.031). Crush injuries recovered well even without vincristine (104.8% vs. 111.2%; P = 0.35).

CONCLUSION

These findings support a new paradigm of early, preemptive blockade of the antagonist muscle (PCA) to improve ultimate net adductor strength, which could potentially improve functional recovery in many UVFP patients and avoid the need for medialization procedures. Possible clinical aspects of this new approach are discussed.

Functional regeneration of recurrent laryngeal nerve injury during thyroid surgery using an asymmetrically porous nerve guide conduit in an animal model

BACKGROUND

Vocal cord paralysis (VCP) caused by recurrent laryngeal nerve (RLN) damage during thyroidectomy commonly results in serious medico-legal problems. The purpose of this study was to evaluate the usefulness of an asymmetrically porous polycaprolactone (PCL)/Pluronic F127 nerve guide conduit (NGC) for functional regeneration in a RLN injury animal model.

METHODS

A biodegradable, asymmetrically porous PCL/F127 NGC with selective permeability was fabricated for use in this study. A 10-mm segment of left RLN was resected in 28 New Zealand white rabbits, and then an asymmetrically porous NGC or a nonporous silicone tube was interposed between both stumps and securely fixed. Vocal cord mobility was endoscopically evaluated at one, four, and eight weeks postoperatively. Nerve growth through NGCs was assessed by toluidine blue staining, and thyroarytenoid (TA) muscle atrophy was evaluated by hematoxylin and eosin staining. Immunohistochemical stainings for acetylcholinesterase (AchE), anti-neurofilament (NF), and anti-S100 protein were also conducted, and transmission electron microscopy (TEM) was used to evaluate functional nerve regeneration.

RESULTS

At eight weeks postoperatively, endoscopic evaluations showed significantly better recovery from VCP in the asymmetrically porous PCL/F127 NGC group (6 of 10 rabbits) than in the silicone tube group (1 of 10 rabbits). Continued nerve growth on the damaged nerve endings was observed with time in the asymmetrically porous PCL/F127 NGC-interposed RLNs. TA muscle dimensions and AchE expressions in TA muscle were significantly greater in the asymmetrically porous PCL/F127 NGC group than in the silicone tube group. Furthermore, immunohistochemical staining revealed the expression of NF and S100 protein in the regenerated nerves in the asymmetrically porous PCL/F127 NGC group at eight weeks postoperatively, and at this time, TEM imaging showed myelinated axons in the regenerated RLNs.

CONCLUSION

The study shows that asymmetrically porous PCL/F127 NGC provides a favorable environment for RLN regeneration and that it has therapeutic potential for the regeneration of RLN damage.

Reorganization of laryngeal motoneurons after crush injury in the recurrent laryngeal nerve of the rat

Motoneurons innervating laryngeal muscles are located in the nucleus ambiguus (Amb), but there is no general agreement on the somatotopic representation and even less is known on how an injury in the recurrent laryngeal nerve (RLN) affects this pattern. This study analyzes the normal somatotopy of those motoneurons and describes its changes over time after a crush injury to the RLN. In the control group (control group 1, n = 9 rats), the posterior cricoarytenoid (PCA) and thyroarytenoid (TA) muscles were injected with cholera toxin-B. In the experimental groups the left RLN of each animal was crushed with a fine tip forceps and, after several survival periods (1, 2, 4, 8, 12 weeks; minimum six rats per time), the PCA and TA muscles were injected as described above. After each surgery, the motility of the vocal folds was evaluated. Additional control experiments were performed; the second control experiment (control group 2, n = 6 rats) was performed labeling the TA and PCA immediately prior to the section of the superior laryngeal nerve (SLN), in order to eliminate the possibility of accidental labeling of the cricothyroid (CT) muscle by spread from the injection site. The third control group (control group 3, n = 5 rats) was included to determine if there is some sprouting from the SLN into the territories of the RLN after a crush of this last nerve. One week after the crush injury of the RLN, the PCA and TA muscles were injected immediately before the section of the SLN. The results show that a single population of neurons represents each muscle with the PCA in the most rostral position followed caudalwards by the TA. One week post-RLN injury, both the somatotopy and the number of labeled motoneurons changed, where the labeled neurons were distributed randomly; in addition, an area of topographical overlap of the two populations was observed and vocal fold mobility was lost. In the rest of the survival periods, the overlapping area is larger, but the movement of the vocal folds tends to recover. After 12 weeks of survival, the disorganization within the Amb is the largest, but the number of motoneurons is similar to control, and all animals recovered the movement of the left vocal fold. Our additional controls indicate that no tracer spread to the CT muscle occurred, and that many of the labeled motoneurons from the PCA after 1 week post-RLN injury correspond to motoneurons whose axons travel in the SLN. Therefore, it seems that after RLN injury there is a collateral sprouting and collateral innervation. Although the somatotopic organization of the Amb is lost after a crush injury of the RLN and does not recover in the times studied here, the movement of the vocal folds as well as the number of neurons that supply the TA and the PCA muscles recovered within 8 weeks, indicating that the central nervous system of the rat has a great capacity of plasticity.

Somatotopy of the neurons innervating the cricothyroid, posterior cricoarytenoid, and thyroarytenoid muscles of the rat's larynx

Neurons innervating the intrinsic muscles of the larynx are located within the nucleus ambiguus but the precise distribution of the neurons for each muscle is still a matter for debate. The purpose of this study was to finely determine the position and the number of the neurons innervating the intrinsic laryngeal muscles cricothyroid, posterior cricoarytenoid, and thyroarytenoid in the rat. The study was carried out in a total of 28 Sprague Dawley rats. The B subunit of the cholera toxin was employed as a retrograde tracer to determine the locations, within the nucleus ambiguus, of the neurons of these intrinsic laryngeal muscles following intramuscular injection. The labelled neurons were found ipsilaterally in the nucleus ambiguus grouped in discrete populations with reproducible rostrocaudal and dorsoventral locations among the sample of animals. Neurons innervating the cricothyroid muscle were located the most rostral of the three populations, neurons innervating the posterior cricoarytenoid were found more caudal, though there was a region of rostrocaudal overlap between these two populations. The most caudal were the neurons innervating the thyroarytenoid muscle, presenting a variable degree of overlap with the posterior cricoarytenoid muscle. The mean number (±SD) of labelled neurons was found to be 41 ± 9 for the cricothyroid, 39 ± 10 for the posterior cricoarytenoid and 33 ± 12 for the thyroarytenoid.

Evolving concepts of laryngeal paralysis

Accepted concepts of the pathophysiology and treatment of laryngeal paralysis have changed over the years. It has long been observed that symptoms of laryngeal paralysis vary greatly, both between patients and over time. There have been various theories to explain these differences. This article reviews how these ideas have changed over time as research has produced new information. Currently, the most popular view is that the laryngeal nerve regenerates after injury, albeit incompletely and inconsistently, and that variations in symptoms and laryngeal posture can be accounted for by muscle activity.

Coexpression of erythropoietin and its receptor in endolymphatic sac tumors

Object. Von Hippel—Lindau (VHL) disease is characterized by multiple tumors in specific organs. The cell of origin and the reason for the particular organ distribution of the tumors remains unknown. Endolymphatic sac tumor (ELST) is one of the lesions associated with VHL disease. Data from previous studies of VHL disease—associated hemangioblastomas (HBs) and renal cell carcinomas (RCCs) have indicated that VHL gene deficiency causes coexpression of erythropoietin (Epo) and its receptor (Epo-R), which facilitates tumor growth.

Methods. The authors studied ELSTs from five patients with VHL germline mutations. Analysis of the five ELST samples revealed loss of the wild-type allele, consistent with Knudson's two-hit hypothesis for tumorigenesis. All five ELST specimens were characterized microscopically and by immunohistochemical analysis. Coexpression of Epo and Epo-R was found in all five tumors on immunohistochemical studies and confirmed through reverse transcription—polymerase chain reaction and Western blot analysis.

Conclusions. Expression of Epo appears to be a result of VHL gene deficiency, whereas the simultaneous coexpression of Epo-R may reflect a developmental mechanism of tumorigenesis. Coexpression of Epo and Epo-R in ELSTs together with the morphological and genetic similarities of these lesions with other VHL disease—associated tumors indicates that VHL disease—associated tumors in different organs share common pathogenetic pathways.

Neurophysiology of vocal fold paralysis

Although a tremendous volume of energy and literature has been devoted to laryngeal paralysis in the past decade, there are still substantial gaps in our understanding of fundamental issues. Oddly enough, controversy remains regarding the actual innervation pathways of the larynx and whether the paralyzed larynx is truly denervated or dysfunctionally reinnervated. An appreciation of these basic issues is prerequisite to making prudent decisions regarding the most appropriate type of intervention. The purpose of this article is to provide a brief overview of basic laryngeal anatomy and neurophysiology to prepare the reader for a subsequent discussion of futuristic research for treatment of laryngeal paralysis.A novel approach is described, which can induce selective reinnervation of individual laryngeal muscles by their original motor fibers within the recurrent laryngeal nerve.

Recurrent laryngeal nerve regeneration by tissue engineering

The recurrent laryngeal nerve (RLN) does not regenerate well after it has been cut, and no current surgical methods achieve functional regeneration. Here, we evaluate the functional regeneration of the RLN after reconstruction using a biodegradable nerve conduit or an autologous nerve graft. The nerve conduit was made of a polyglycolic acid (PGA) tube coated with collagen. A 10-mm gap in the resected nerve was bridged by a PGA tube in 6 adult beagle dogs (group 1) and by an autologous nerve graft in 3 dogs (group 2). Fiberscopic observation revealed functional regeneration of the RLN in 4 of the 6 dogs in group 1. No regeneration of the RLN was observed in any dog in group 2. We also tested for axonal transport, and measured the compound muscle action potential. The RLN can be functionally regenerated with a PGA tube, which may act as a scaffold for the growth of regenerating axons.

Laryngeal adductory pressure as a measure of post-reinnervation synkinesis

Laryngeal adductory pressure (LAP) is the pressure induced as the vocal folds squeeze on a balloon while the recurrent laryngeal nerve (RLN) is stimulated. The LAP has been shown to vary with the frequency of stimulation, with a characteristic slope. The RLN was divided and reanastomosed 4 different ways in 12 canine hemilaryngeal preparations; the 4 subgroups represented a range of expected post-reinnervation synkinesis recovery patterns. The LAP frequency-response curve was measured before surgery and at monthly intervals for 6 months after surgery. In the "best-case" group (RLN adductor and abductor trunks each divided and reanastomosed), the slope was found to return to normal. The 2 whole RLN division-reanastomosis groups (precise realignment or 180 degrees rotation) both gave results similar to those of the "worst-case" group (RLN adductor and abductor trunks divided and transposed); these 3 subgroups were all significantly different from baseline. The slope of the LAP frequency-response curve may be a useful means of indirectly quantifying laryngeal synkinesis.

Arterial supply of the human endolymphatic duct and sac

The arterial anatomy of the endolymphatic duct and sac was studied in vascular casts of methyl methacrylate of six human heads. The chief source of arterial blood supply to the endolymphatic duct and sac appeared to be the occipital artery. Arterioles entered the bone of the mastoid process. Arterioles in bone, the walls of the sigmoid sinus, and the posterior fossa dura coursed medially to supply the endolymphatic sac. The orientation of arterioles tended to be along the long axis of the endolymphatic duct and sac, whereas venules were more likely to be circumferentially oriented. Arterioles arising from dural vessels divided into deeper branches, which supplied periductal connective tissue, and superficial branches, which entered canaliculi of the vestibular aqueduct. Gross anatomic findings were confirmed by histologic examination of temporal bones.

Misdirected regeneration of injured recurrent laryngeal nerve in the cat

INTRODUCTION

Misdirected regeneration (MR) frequently occurs following injury to the recurrent laryngeal nerve (RLN) resulting in neurotmesis or axonotmesis. Physiological and anatomic parameters involved in the functional recovery of the larynx following freezing injury or neurorrhaphy of the RLN were studied. A multi-facilitated approach is undertaken to clarify the functional abnormalities caused by the MR after recurrent laryngeal nerve injury.

MATERIALS AND METHODS

Three groups of adult cats were studied. These included controls, cats with recurrent laryngeal neurorrhaphy, and cats with recurrent laryngeal nerve freeze injuries. From 2 weeks to 9 months after the nerve injury, the animals were studied endoscopically and with electromyography (EMG). Using the same animal, the number and location of motoneurons supplying the ipsilateral posterior cricoarytenoid (PCA) muscle were examined with horseradish peroxidase (HRP). Animals were subsequently sacrificed to study the pattern of reinnervation.

RESULTS

Following neurorrhaphy all cats had vocal cord paralysis. After neurorrhaphy, effective motion function did not return in the affected vocal cord and it remained fixed in the paramedian position. Although EMG of the laryngeal muscles of the affected side showed interference voltage, the pattern of activities was markedly different from that of the unaffected side, and reciprocity among the laryngeal muscles was not restored. The number of PCA motoneurons recovered to the normal range, but a considerable number of neuronal bodies were dispersed outside the normal PCA area. This indicates misdirected reinnervation to the PCA muscle by motoneurons that originally served other laryngeal muscles. In the freezing injury, effective vocal cord movement finally recovered after 6 months. At this time, EMG showed a normal pattern, although a relatively small amount of misdirected neurons was observed.

DISCUSSION

Functional recovery of vocal cord motion does not occur following neurorrhaphy. Prominently disorganized arrangement of laryngeal motor neurons was observed in the horseradish peroxidase study. This suggests that inappropriate reinnervation develops in spite of reapproximation and suturing. Altered central organization of the motor nucleus is a significant pathogenic factor in the loss of laryngeal muscular coordination following recurrent laryngeal nerve lesions. The degree of recovery is related to the mechanism of injury.

Neurite regeneration in the cat recurrent laryngeal nerve: an immunohistochemical study

The recurrent laryngeal nerve (RLN) consists of various motor, sensory and autonomic nerve fibers, although it has not been established whether different neuronal types exhibit a similar ability to regenerate. To address this question, freezing was used to injure the cat RLN fibers and the presence or absence of immunoreactivity for neuropeptides or transmitter-synthesizing enzymes was then examined as a marker to classify the fibers. In the control RLN, calcitonin gene-related peptide-immunoreactive (CGRP-IR) fibers were the highest in number and were distributed throughout the nerve fascicles. The number of substance P-immunoreactive (SP-IR) fibers was about 40% that of CGRP-IR fibers, while a portion of CGRP-IR fibers was found to contain SP immunoreactivity. Relatively low numbers of tyrosine hydroxylase-immunoreactive (TH-IR) and neuropeptide Y (NPY-IR) nerve fibers were seen which tended to form clusters. The distribution pattern of NPY-IR fibers was very similar to that of TH-IR fibers. In the regenerating RLN 1 week after the freezing injury, the fastest growing axons were CGRP-IR, while the regenerating rates of SP-IR, TH-IR and NPY-IR fibers were slower than that of CGRP-IR fibers. These results suggest that the ability for neurite regeneration varies among neuron types and that CGRP-IR fibers possess the most rapid ability to regenerate.

Laryngeal synkinesis following reinnervation in the rat. Neuroanatomic and physiologic study using retrograde fluorescent tracers and electromyography

The functional organization of laryngeal motoneurons in the nucleus ambiguous (NA) was evaluated in adult male rats before and after recurrent laryngeal nerve section and reinnervation. Using retrograde double labeling techniques with fluorescent probes, we obtained the number and position of labeled neurons by using the Bioquant 3-D imaging system. Reinnervation was documented by electromyography. In nine control animals vector analysis revealed significant (p less than .05) separation of the posterior cricoarytenoid (PCA) muscle motoneurons and the thyroarytenoid and lateral cricoarytenoid (TA/LCA) muscle motoneurons. The PCA motoneurons were positioned ventromedially in the NA, and TA/LCA motoneurons were found dorsolaterally in the NA. Rostral-caudal separation was not significant. Electromyography revealed phasic electrical activity synchronous with respiration in the PCA, and activity synchronous with deglutition in the TA/LCA. In four animals surviving 15 weeks following recurrent laryngeal nerve section and primary neurorrhaphy, functional organization within the NA was lost and phasic motor unit activity synchronous with respiration was seen in the TA/LCA muscle as well as the PCA. Vector analysis revealed the reinnervating motoneurons for both the PCA and TA/LCA to be positioned dorsolaterally, similar to the control group TA/LCA motoneurons. These findings demonstrate a shift in the topographic organization of laryngeal motoneurons within the NA following reinnervation, with random organization occurring at the neurorrhaphy site.