Experiments in laryngeal reinnervation

Vocal cord palsy as a sequela of paediatric cardiac surgery - a review

BACKGROUND

Vocal cord palsy is one of the recognised complications of complex cardiac surgery in the paediatric population. While there is an abundance of literature highlighting the presence of this complication, there is a scarcity of research focusing on the pathophysiology, presentation, diagnosis, and treatment options available for children affected by vocal cord palsy.

MATERIALS AND METHODS

Electronic searches were conducted using the search terms: "Vocal Cord Palsy," "VCP," "Vocal Cord Injury," "Paediatric Heart Surgery," "Congenital Heart Surgery," "Pediatric Heart Surgery," "Vocal Fold Movement Impairment," "VFMI," "Vocal Fold Palsy," "PDA Ligation." The inclusion criteria were any articles discussing the outcomes of vocal cord palsy following paediatric cardiac surgery.

RESULTS

The two main populations affected by vocal cord palsy are children undergoing aortic arch surgery or those undergoing PDA ligation. There is paucity of prospective follow-up studies; it is therefore difficult to reliably assess the current approaches and the long-term implications of management options.

CONCLUSION

Vocal cord palsy can be a devastating complication following cardiac surgery, which if left untreated, could potentially result in debilitation of quality of life and in severe circumstances could even lead to death. Currently, there is not enough high-quality evidence in the literature to aid recognition, diagnosis, and management leaving clinicians to extrapolate evidence from adult studies to make clinical judgements. Future research with a focus on the paediatric perspective is necessary in providing evidence for good standards of care.

Dual laryngeal reinnervation in bilateral vocal fold paralysis: anatomical pitfalls

BACKGROUND

Bilateral laryngeal reinnervation can be a promising procedure for reanimation of laryngeal muscles, but currently not yet standardized. Besides patient conditions some intraoperative anatomical pitfalls need to be solved.

METHODS

Twelve human head and neck specimens (24 sides) have been studied using microdissection and histological serial sections of the nerves. The surgical anatomy of the dual reinnervation procedure according to JP Marie was investigated notably the branching pattern of the phrenic nerve (PN), the Ansa cervicalis (AC) and the recurrent laryngeal nerve (RLN).

RESULTS

Despite variations of the AC, a prominent inferior common trunk for sterno-hyoid and sterno-thyroid muscles can be used in more than 90% of the specimens. If the AC is missing because of previous surgery, the tiny nerve of the thyro-hyoid muscle can be used preferred. The PN display a double roots pattern from C3 to C4 cervical plexus in 50% of the specimens. A single root pattern can be found and an end-to-lateral neurorraphy can be used. Intra-laryngeal nerves pattern of the RLN display tiny collaterals which cannot be selected for abduction-adduction activity. Direct implantation of the Y-shape great auricular nerve within the posterior crico-arytenoid muscles can be a reliable method leading to challenging mechanical and functional conditions.

CONCLUSION

Several anatomical pitfalls, including intra-operative choices and variants of the donor nerves, but also the challenging intra-laryngeal dissection of the inferior laryngeal nerve need to be solved. A successful laryngeal reinnervation still needs further studies for a simplified procedure.

Development of a Closed-Loop Stimulator for Laryngeal Reanimation: Part 2. Device Testing in the Canine Model of Laryngeal Paralysis

OBJECTIVE:

Laryngeal paralysis of central or peripheral origin can potentially be treated using functional electrical stimulation (FES) of laryngeal muscles. Experiments in canines (dogs) were performed using implant prototypes capable of closed-loop FES to refine engineering designs and specifications, test surgical approaches for implantation, and better understand the in vivo effects of laryngeal muscle stimulation on short- and long-term glottic function.

STUDY DESIGN:

Prospective, laboratory.

METHODS:

We designed and tested a series of microprocessor-based implantable devices that can stimulate glottic opening or closing based on input from physiological control signals (real-time processing of electromyographic [EMG] signals). After acute device testing experiments, 2 dogs were implanted for 8 and 24 months, with periodic testing of closed-loop laryngeal muscle stimulation triggered from EMG signals. In total, 5 dogs were tested for the effects of laryngeal muscle stimulation on vocal fold (VF) posturing in larynges with nerve supplies that were intact (7 VFs), synkinetically reinnervated (2 VFs), or chronically denervated (1 VF). In 3 cases, the stimulation was combined with airflow-driven phonation to study the consequent modulation of phonatory parameters.

RESULTS:

Initial device prototypes used inductive coupling for power and communication, while later iterations used battery power and infrared light communication (detailed descriptions are provided in the Part 1 companion paper). Two animals were successfully implanted with the inductively powered units, which operated until removed at 8 months in 1 animal or for more than 16 months in the second animal. Surgically, the encapsulated implants were well tolerated, and procedures for placing, attaching, and connecting the devices were developed. To simulate EMG control signals in anesthetized animals, we created 2 types of nerve/muscle signal sources. In one approach, a neck muscle had a cuff electrode placed on its motor nerve that was connected to transdermal electrical connection ports for periodic testing. In the second approach, the recurrent laryngeal nerve on one side of the larynx was stimulated to generate a VF EMG signal, which was then used to trigger FES of the paralyzed contralateral side (eg, restoring VF movement symmetry). Implant testing identified effective stimulation parameters and closed-loop stimulation artifact rejection techniques for FES of both healthy and paralyzed VFs. Stimulation levels effective for VF adduction did not cause signs of discomfort during awake testing.

CONCLUSION:

Our inductive and battery-powered prototypes performed effectively during in vivo testing, and the 2 units that were implanted for long-term evaluation held up well. As a proof of concept, we demonstrated that elicited neck strap muscle or laryngeal EMG potentials could be used as a control signal for closed-loop stimulation of laryngeal adduction and vocal pitch modulation, depending on electrode positioning, and that VFs were stimulable in the presence of synkinetic reinnervation or chronic denervation.

Dual innervation may occur in a partially denervated muscle

INTRODUCTION

With a view to simplifying surgical techniques for selective laryngeal reinnervation, we addressed the question of whether it is feasible to receive additional innervation by a partially denervated muscle using an infrahyoid muscle model.

METHODS

In 90 rats (6 groups of 15), phrenic nerve transfer was used to reinnervate the sternothyroid muscle. In some cases, residual innervation by the original nerve was present. Three months later we performed electromyographic studies, contraction strength measurements, histologic assessment, and retrograde labeling.

RESULTS

Muscles reinnervated by the phrenic nerve had a greater "dual-response" rate (in terms of nerve latency, contraction strength, and retrograde labeling) than muscles in the control groups.

DISCUSSION

The phrenic nerve can impart its inspiratory properties to an initially denervated strap muscle-even when residual innervation is present. The preservation of contractile potential confirmed the feasibility of dual innervation in a previously injured muscle. Muscle Nerve 59:108-115, 2019.

Role of reinnervation in the management of recurrent laryngeal nerve injury: current state and advances

PURPOSE OF REVIEW

To present the current state of knowledge concerning different laryngeal reinnervation procedures for unilateral and bilateral vocal palsy.

RECENT FINDINGS

Recent reports show positive outcomes on both unilateral and bilateral reinnervations. The phrenic nerve is the most commonly used donor for bilateral vocal palsy, but use of the superior laryngeal nerve has also been suggested.

SUMMARY

Reinnervation of the larynx is a complex undertaking that can be performed by ENT surgeons with skills in microsurgery. Advances in this this field represent a paradigm shift in laryngeal rehabilitation and a prerequisite for laryngeal transplantation. Advances in basic understanding of nerve regeneration and in particular the need to surgically manage competitive reinnervation make the results of laryngeal reinnervation more predictable.

Muscle reinnervation with nerve-muscle-endplate band grafting technique: correlation between force recovery and axonal regeneration

BACKGROUND

This study was designed to determine the correlation between functional recovery and the extent of axonal regeneration after muscle reinnervation with our recently developed nerve-muscle-endplate band grafting (NMEG) technique in a rat model.

MATERIALS AND METHODS

The right experimentally paralyzed sternomastoid (SM) muscle by nerve transection was immediately reinnervated with an NMEG pedicle harvested from a neighboring sternohyoid muscle. The NMEG pedicle contained a muscle block (6 × 6 × 3 mm), a donor nerve branch with nerve terminals, and a motor endplate band. Three months after surgery, the tetanic force of the SM muscle was measured and the regenerated axons in the muscle were detected using neurofilament immunohistochemistry.

RESULTS

The results showed that the maximal tetanic force (a measure of muscle functional recovery) of the NMEG-reinnervated SM muscle reached up to 66.0% of the normal control. The wet weight of the reinnervated SM muscle (a measure of muscle mass recovery) was 87.2% of the control. The area fraction of the regenerating axons visualized with neurofilament staining within the NMEG-reinnervated SM muscle (a measure of muscle reinnervation) was 42.3%. A positive correlation was revealed between the extent of muscle reinnervation and maximal muscle force.

CONCLUSIONS

Our newly developed NMEG technique results in satisfactory functional outcomes and nerve regeneration. Further improvement in the functional recovery after NMEG reinnervation could be achieved by refining the surgical procedure and creating an ideal environment that favors axon-endplate connections and accelerates axonal growth and sprouting.

The evolution of laryngeal reinnervation, the current state of science and thoughts for future treatments

The treatment of unilateral vocal fold palsy (UVFP) or bilateral vocal fold palsy (BVFP) has been the subject of debate and experiment for 150 years. To date, dozens of different surgical methods have been described to reinnervate this most complex of organs, the larynx. As yet, there is no consensus on the most functionally effective method of reinnervation. However, it is a rapidly expanding area of research and remains an area of controversy. Indications for reinnervation for both UVFP and BVFP are still evolving and our understanding of the neuromuscular supply to the larynx continues to expand. What may have been considered unacceptable results from previous studies with one pathology may actually be of benefit in patients with different pathologies. This uncertainty of treatment options and potential outcomes can be confusing. In addition alternative techniques have been postulated as mainstays or adjuncts of treatment to the stalwart of reinnervation, neurorrhaphy. Determining what the correct treatment for an individual patient should be is still a gray area. With this in mind, this article reviews the evolution of laryngeal reinnervation, reviews the current state of the science, and suggests directions in which it might move in the future.

Office-based injection laryngoplasty for the management of unilateral vocal fold paralysis

OBJECTIVE

Office-based injection laryngoplasty (OBIL) is a common method of addressing glottal insufficiency. This retrospective chart review identifies the demongraphics, laterality, technique, success rate, injectates, and complications of OBIL performed over a 3-year period at a single institution.

STUDY DESIGN

Retrospective chart review.

METHODS

All OBILs performed for the management of UVFP by the senior author over 3 years (2007-2009) were identified from billing records. The age, gender, laterality, underlying disease process, augmentation material, route of injection, and complications were recorded.

RESULTS

Eighty-two OBILs were attempted on 57 patients. The most common route of access was transoral (85.6%). All OBILs were able to be completed. Injectates used were hyaluronic acid derivatives (57.3%), calcium hydroxyapatite (16%), and Cymmetra (16.5%). Three complications (3.7%) occurred. Thirty percent of patients ultimately elected for thyroplasty or ansa reinnervation, 22% found their condition to self-resolve, 14% died, and 25% were lost to follow-up.

CONCLUSIONS

Using a variety of approaches, OBIL is possible in almost all patients. The single surgeon transoral route using a rigid angled telescope and curved injection needle was the most commonly used approach. Multiple injectates can be used and have good safety records. The final disposition of patients may be variable and warrants further investigation.

Reinnervation of bilateral posterior cricoarytenoid muscles using the left phrenic nerve in patients with bilateral vocal fold paralysis

Objective

To evaluate the feasibility, effectiveness, and safety of reinnervation of the bilateral posterior cricoarytenoid (PCA) muscles using the left phrenic nerve in patients with bilateral vocal fold paralysis.

Methods

Forty-four patients with bilateral vocal fold paralysis who underwent reinnervation of the bilateral PCA muscles using the left phrenic nerve were enrolled in this study. Videostroboscopy, perceptual evaluation, acoustic analysis, maximum phonation time, pulmonary function testing, and laryngeal electromyography were performed preoperatively and postoperatively. Patients were followed-up for at least 1 year after surgery.

Results

Videostroboscopy showed that within 1 year after reinnervation, abductive movement could be observed in the left vocal folds of 87% of patients and the right vocal folds of 72% of patients. Abductive excursion on the left side was significantly larger than that on the right side (P < 0.05); most of the vocal function parameters were improved postoperatively compared with the preoperative parameters, albeit without a significant difference (P > 0.05). No patients developed immediate dyspnea after surgery, and the pulmonary function parameters recovered to normal reference value levels within 1 year. Postoperative laryngeal electromyography confirmed successful reinnervation of the bilateral PCA muscles. Eighty-seven percent of patients in this series were decannulated and did not show obvious dyspnea after physical activity. Those who were decannulated after subsequent arytenoidectomy were not included in calculating the success rate of decannulation.

Conclusions

Reinnervation of the bilateral PCA muscles using the left phrenic nerve can restore inspiratory vocal fold abduction to a physiologically satisfactory extent while preserving phonatory function at the preoperative level without evident morbidity.

Comparison of muscle force after immediate and delayed reinnervation using nerve-muscle-endplate band grafting

BACKGROUND

Because of poor functional outcomes of currently used reinnervation methods, we developed novel treatment strategy for the restoration of paralyzed muscles-the nerve-muscle-endplate band grafting (NMEG) technique. The graft was obtained from the sternohyoid muscle (donor) and implanted into the ipsilateral paralyzed sternomastoid (SM) muscle (recipient).

METHODS

Rats were subjected to immediate or delayed (1 or 3 mo) reinnervation of the experimentally paralyzed SM muscles using the NMEG technique or the conventionally used nerve end-to-end anastomosis. The SM muscle at the opposite side served as a normal control.

RESULTS

NMEG produced better recovery of muscle force as compared with end-to-end anastomosis. A larger force produced by NMEG was most evident for small stimulation currents.

CONCLUSIONS

The NMEG technique holds great potential for successful muscle reinnervation. We hypothesize that even better muscle reinnervation and functional recovery could be achieved with further improvement of the environment that favors axon-end plate connections and accelerates axonal growth and sprouting.

Short-term laryngeal electromyography and histopathological findings after primary reconstruction of the inferior laryngeal nerve in rabbits: prospective study

INTRODUCTION

The recurrent laryngeal nerve can be injured during surgery. This study investigated recurrent laryngeal nerve reinnervation.

OBJECTIVE

To study the short-term effects of primary anastomosis of the recurrent laryngeal nerve, by laryngeal electromyography and histopathological analysis, in a rabbit model.

METHOD

Twenty Zealand rabbits underwent either right recurrent laryngeal nerve (1) transection with excision of 1 cm or (2) transection and end-to-end primary anastomosis. Vocal fold movements, laryngeal electromyography results and histological changes were recorded.

RESULTS

Vocal fold analysis showed a paramedian vocal fold in both groups, with perceptible vibratory movements in group two. Electromyography revealed total denervation potentials in group one, but denervation and regeneration signs in group two. Histopathologically, hyperkeratosis and parakeratosis of the vocal fold mucosa were seen in group one, and signs of parakeratosis and hyperplasia in group two.

CONCLUSION

Even under ideal conditions for primary recurrent laryngeal nerve anastomosis, a return to normal muscle function is unlikely. However, such anastomosis prevents muscle atrophy, and should be performed as soon as possible. The degree of nerve recovery is associated with the number, amplitude and myelination level of fibrils returning to the original motor end-plaque.

Laryngeal reinnervation for bilateral vocal fold paralysis

PURPOSE OF REVIEW

Laryngeal reinnervation for bilateral vocal fold paralysis (BVFP) patients is a promising technique to achieve good airway, although preserving a good quality of voice. On the other hand, the procedure is not simple. This review explores the recent literature on surgical technique and factors that may contribute to the success.

RECENT FINDINGS

Research and literature in this area are limited due to variability and complexity of the nerve supply. The posterior cricoarytenoid (PCA) muscle also receives nerve supply from the interarytenoid branch. Transection of this nerve at the point between interarytenoid and PCA branch may prevent aberrant reinnervation of adductor nerve axons to the PCA muscle. A varying degree of regeneration of injured recurrent laryngeal nerves (RLN) in humans of more than 6 months confirms subclinical reinnervation, which may prevent denervation-induced atrophy.

SUMMARY

Several promising surgical techniques have been developed for bilateral selective reinnervation for BVFP patients. This involves reinnervation of the abductor and adductor laryngeal muscles. The surgical technique aims at reinnervating the PCA muscle to trigger abduction during the respiratory cycle and preservation of good voice by strengthening the adductor muscles as well as prevention of laryngeal synkinesis.

Nerve-muscle-endplate band grafting: a new technique for muscle reinnervation

BACKGROUND

Because currently existing reinnervation methods result in poor functional recovery, there is a great need to develop new treatment strategies.

OBJECTIVE

To investigate the efficacy of our recently developed nerve-muscle-endplate band grafting (NMEG) technique for muscle reinnervation.

METHODS

Twenty-five adult rats were used. Sternohyoid (SH) and sternomastoid (SM) muscles served as donor and recipient muscle, respectively. Neural organization of the SH and SM muscles and surgical feasibility of the NMEG technique were determined. An NMEG contained a muscle block, a nerve branch with nerve terminals, and a motor endplate band with numerous neuromuscular junctions. After a 3-month recovery period, the degree of functional recovery was evaluated with a maximal tetanic force measurement. Retrograde horseradish peroxidase tracing was used to track the origin of the motor innervation of the reinnervated muscles. The reinnervated muscles were examined morphohistologically and immunohistochemically to assess the extent of axonal regeneration.

RESULTS

Nerve supply patterns and locations of the motor endplate bands in the SH and SM muscles were documented. The results demonstrated that the reinnervated SM muscles gained motor control from the SH motoneurons. The NMEG technique yielded extensive axonal regeneration and significant recovery of SM muscle force-generating capacity (67% of control). The mean wet weight of the NMEG-reinnervated muscles (87% of control) was greater than that of the denervated SM muscles (36% of control).

CONCLUSION

The NMEG technique resulted in successful muscle reinnervation and functional recovery. This technique holds promise in the treatment of muscle paralysis.

The central projections of the laryngeal nerves in the rat

The larynx serves respiratory, protective, and phonatory functions. The motor and sensory innervation to the larynx controlling these functions is provided by the superior laryngeal nerve (SLN) and the recurrent laryngeal nerve (RLN). Classical studies state that the SLN innervates the cricothyroid muscle and provides sensory innervation to the supraglottic cavity, whereas the RLN supplies motor innervation to the remaining intrinsic laryngeal muscles and sensory innervation to the infraglottic cavity, but recent data suggest a more complex anatomical and functional organisation. The current neuroanatomical tracing study was undertaken to provide a comprehensive description of the central brainstem connections of the axons within the SLN and the RLN, including those neurons that innervate the larynx. The study has been carried out in 41 adult male Sprague-Dawley rats. The central projections of the laryngeal nerves were labelled following application of biotinylated dextran amines onto the SLN, the RLN or both. The most remarkable result of the study is that in the rat the RLN does not contain any afferent axons from the larynx, in contrast to the pattern observed in many other species including man. The RLN supplied only special visceromotor innervation to the intrinsic muscles of the larynx from motoneurons in the nucleus ambiguus (Amb). All the afferent axons innervating the larynx are contained within the SLN, and reach the nucleus of the solitary tract. The SLN also contained secretomotor efferents originating from motoneurons in the dorsal motor nucleus of the vagus, and special visceral efferent fibres from the Amb. In conclusion, the present study shows that in the rat the innervation of the larynx differs in significant ways from that described in other species.

Locations of the motor endplate band and motoneurons innervating the sternomastoid muscle in the rat

Sternocleidomastoid (SCM) is a long muscle with two bellies, sternomastoid (SM) and cleidomastoid (CM) in the lateral side of the neck. It has been widely used as muscle and myocutaneous flap for reconstruction of oral cavity and facial defects and as a candidate for reinnervation studies. Therefore, exact neuroanatomy of the SCM is critical for guiding reinnervation procedures. In this study, SM in rats were investigated to document banding pattern of motor endplates (MEPs) using whole-mount acetylcholinesterase (AChE) staining and to determine locations of the motoneurons innervating the muscle using retrograde horseradish peroxidase (HRP) tracing technique. The results showed that the MEPs in the SM and CM were organized into a single band which was located in the middle portion of the muscle. After HRP injections into the MEP band of the SM, ipsilaterally labeled motoneurons were identified in the caudal medulla oblongata (MO), C1, and C2. The SM motoneurons were found to form a single column in lower MO and dorsomedial (DM) nucleus in C1. In contrast, the labeled SM motoneurons in C2 formed either one (DM nucleus), two [DM and ventrolateral (VL) nuclei], or three [DM, VL, and ventromedial (VM)] columns. These findings are important not only for understanding the neural control of the muscle but also for evaluating the success rate of a given reinnervation procedure when the SM is chosen as a target muscle.

Arytenoid Abduction: Indications and Limitations

Objectives

I report further experience with arytenoid abduction (AAb), a procedure that enlarges the glottis by external rotation of the arytenoid cartilage and thus moves the vocal process laterally and rostrally, but does not preclude adduction for phonation. Therefore, AAb has the potential to preserve voice in patients with bilateral abductor laryngeal paralysis.

Methods

I performed a retrospective review of AAb in 11 patients with bilateral laryngeal paralysis and 3 patients with other neurologic causes of glottal airway compromise, ie, adductor breathing dystonia, frequent laryngospasm, and progressive laryngeal breathing dysfunction.

Results

Seven of the 11 patients with bilateral paralysis had dramatic airway improvement. One patient required a tracheotomy after AAb, and 3 patients with an existing tracheotomy could not be decannulated. Arytenoid abduction relieved airway obstruction in the patient with recurrent laryngospasm and in the child with progressive laryngeal breathing dysfunction, but the patient with adductor breathing dystonia has persistent stridor. The factors associated with a poor airway outcome included prolonged tracheotomy, electromyographic evidence of inspiratory activity of adductor muscles, chronic obstructive pulmonary disease, sleep apnea, and prior cordotomy or arytenoidectomy.

Conclusions

Arytenoid abduction is most effective in patients with bilateral laryngeal paralysis of less than 1 year's duration who do not have unfavorable laryngeal adductor activity.

Arytenoid Abduction: Indications and Limitations

Objectives

I report further experience with arytenoid abduction (AAb), a procedure that enlarges the glottis by external rotation of the arytenoid cartilage and thus moves the vocal process laterally and rostrally, but does not preclude adduction for phonation. Therefore, AAb has the potential to preserve voice in patients with bilateral abductor laryngeal paralysis.

Methods

I performed a retrospective review of AAb in 11 patients with bilateral laryngeal paralysis and 3 patients with other neurologic causes of glottal airway compromise, ie, adductor breathing dystonia, frequent laryngospasm, and progressive laryngeal breathing dysfunction.

Results

Seven of the 11 patients with bilateral paralysis had dramatic airway improvement. One patient required a tracheotomy after AAb, and 3 patients with an existing tracheotomy could not be decannulated. Arytenoid abduction relieved airway obstruction in the patient with recurrent laryngospasm and in the child with progressive laryngeal breathing dysfunction, but the patient with adductor breathing dystonia has persistent stridor. The factors associated with a poor airway outcome included prolonged tracheotomy, electromyographic evidence of inspiratory activity of adductor muscles, chronic obstructive pulmonary disease, sleep apnea, and prior cordotomy or arytenoidectomy.

Conclusions

Arytenoid abduction is most effective in patients with bilateral laryngeal paralysis of less than 1 year's duration who do not have unfavorable laryngeal adductor activity.

Characteristics of tetanic force produced by the sternomastoid muscle of the rat

The sternomastoid (SM) muscle plays an important role in supporting breathing. It also has unique anatomical advantages that allow its wide use in head and neck tissue reconstruction and muscle reinnervation. However, little is known about its contractile properties. The experiments were run on rats and designed to determine in vivo the relationship between muscle force (active muscle contraction to electrical stimulation) with passive tension (passive force changing muscle length) and two parameters (intensity and frequency) of electrical stimulation. The threshold current for initiating noticeable muscle contraction was 0.03 mA. Maximal muscle force (0.94 N) was produced by using moderate muscle length/tension (28 mm/0.08 N), 0.2 mA stimulation current, and 150 Hz stimulation frequency. These data are important not only to better understand the contractile properties of the rat SM muscle, but also to provide normative values which are critical to reliably assess the extent of functional recovery following muscle reinnervation.

Nerve-muscle pedicle implantation facilitates re-innervation of long-term denervated thyroarytenoid muscle in rats

CONCLUSIONS

Nerve-muscle pedicle (NMP) implantation was effective in the recovery from atrophic changes in long-term denervated thyroarytenoid (TA) muscle. Re-innervation occurred via the transferred nerve. However, the effectiveness of the NMP method may decline with increasing duration of denervation.

OBJECTIVES

To evaluate the effects of NMP implantation on long-term denervated rat TA muscle.

MATERIALS AND METHODS

Wistar rats (n=105) were divided into two groups in which the left recurrent laryngeal nerve (RLN) was transected without (DNV group) or with (NMP group) subsequent NMP implantation, and subgroups of each group were formed depending on the period after RLN transection (immediate to 48 weeks). In the DNV subgroups, we histologically assessed the area of muscle and the number of neuromuscular junctions. In the NMP subgroups, we performed electromyographic, videolaryngoscopic, and histologic assessments. The muscle area and muscle action potentials were evaluated by comparing the treated and untreated sides. The ratio of the number of nerve terminals to that of acetylcholine receptors was also assessed.

RESULTS

The TA muscle area was significantly larger in most of the NMP subgroups compared with the DNV subgroups. Muscle action potentials were present in all NMP animals. All histologic and physiologic assessments revealed degradation as the denervation period in the five NMP subgroups.

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.

Diaphragmatic effects of selective resection of the upper phrenic nerve root in dogs

The aim of this study was to evaluate the effects on the diaphragm of upper phrenic nerve root resections in dogs. During laryngeal reinnervation, selective resections of the upper phrenic nerve root (C5) were performed unilaterally (right side, n=7; Group A) and bilaterally (n=6; Group B) and compared to non denervated animals (n=5). After 8 months, a diaphragmatic evaluation was performed: X-ray, EMG, transdiaphragmatic pressure (Pdi) after ipsi- and bilateral tetanic stimulation of the phrenic nerves and a bilateral histological study of five hemidiaphragmatic regions. EMG alterations were significantly more severe in Group B than in Group A, for the left (p<0.05) and right hemidiaphragms (p<0.01). No differences in the X-rays were noted between the three groups. The Pdi of the three groups after occlusion and phrenic nerve stimulations (unilateral and bilateral) were not statistically different. Histological data demonstrated that there were no differences in fibre irregularity, predominant fibre type or fibrosis between the three groups. Macroscopic and microscopic atrophy, which was mainly present on the anterior regions of the hemidiaphragms, was significantly higher in Group B than in Group A and undenervated dogs (p<0.05). In conclusion, resection of the upper phrenic nerve root of one phrenic nerve (right side) have limited effect on the diaphragm in dogs. However, resection of the upper phrenic nerve root on both sides resulted in a significant effect on the EMGs and histology of the entire diaphragm without any significant consequences on transdiaphragmatic pressure.

Effects of a Nerve-Muscle Pedicle on the Denervated Rat Thyroarytenoid Muscle

OBJECTIVES

To evaluate the effects of the nerve-muscle pedicle (NMP) method on the rat thyroarytenoid (TA) muscle after transection of the recurrent laryngeal nerve (RLN).

STUDY DESIGN

Quantitative histologic assessment of the TA muscle after NMP implantation.

METHODS

Thirty-six Wistar rats were divided into two groups: animals subjected to transection of the left RLN alone (DNV group) and animals subjected to transection of the left RLN followed by immediate transplantation of a NMP flap containing the sternohyoid (SH) muscle and ansa cervicalis nerve branch (NMP group). Animals were killed 2, 4, and 10 weeks after the treatments. The TA muscle stained with hematoxylin-eosin was evaluated quantitatively. The pre- and postsynaptic structures of the neuromuscular junction (NMJ) in the TA muscle were analyzed histochemically. The myosin heavy chain (MyHC) isoforms were evaluated using immunohistochemistry and reverse-transcriptase polymerase chain reaction (RT-PCR).

RESULTS

In the NMP group, the TA muscle fiber recovered to almost normal at 10 weeks, and the ratio of the number of synaptophysin-positive nerve terminals to that of alpha-bungarotoxin-positive acetylcholine receptors recovered to 79.8 +/- 11.8% (P < .05, compared with the control). Immunohistochemistry and the RT-PCR method after laser capture microdissection revealed the expression of MyHC isoforms type 2B and type 2A; the latter was detected in the SH muscle but not in the normal or denervated TA muscle.

CONCLUSION

The NMP method was effective for recovering from the atrophic changes of the TA muscle after transection of the RLN. This was attributed to successful reinnervation by reconstruction of the NMJ, which might change MyHC isoform expression.

Intrinsic laryngeal muscle reinnervation with nerve-muscle pedicle

OBJECTIVE

To test the application of the nerve-muscle pedicle (NMP) technique for selective reinnervation of previously denervated posterior cricoarytenoid (PCA) muscle.

METHODS

The left recurrent laryngeal nerve (RLN) was severed in 5 mongrel dogs, and an ansa cervicalis-sternohyoid muscle pedicle was sutured to the left PCA muscle. Three dogs underwent a sham operation. Videolaryngoscopy was performed, and electromyographic data were collected after 1 year on average. Finally, histologic analysis of the NMP was performed.

RESULTS

The video records showed the return of mobility of the PCA muscle reinnervated by the NMP. EMG data as to show evoked polyphasic potentials showed also evidence of reinnervation of the PCA muscle. With immunohistochemical reaction (antineurofilament antibody+biotin) we could show neurofilaments and motor endplates in both sides in all 5 animals.

CONCLUSIONS

The NMP technique could eliminate the need for arytenoidectomy and laterofixation in patients with unilateral or bilateral vocal fold paralysis. The quality of life and voice may be improved.

Laryngeal reinnervation

Laryngeal reinnervation refers to any of a number of surgical procedures intended to restore neural connections to the larynx, which have usually been lost from some type of trauma (eg, surgical). The nerve function(s) to be restored may be those of the recurrent laryngeal nerve or its subdivisions, those of the superior laryngeal nerve, or both, and they may be motor or sensory. Several different donor nerves are available and have been described. The technique used may be direct end-to-end anastomosis (neurorrhaphy), direct implantation of a nerve ending into a muscle, the nerve-muscle pedicle technique, or muscle-nerve-muscle methods. These nerves and techniques may be combined in many ways. A number of new techniques have been reported in animal studies; however, the animal studies do not always predict the results of analogous surgeries in human patients. The historical and current perspectives on these techniques are discussed in this article.

Granulomatous Inflammation and Nerve Necrosis in a Case of Apparent Idiopathic Vocal Fold Paralysis: Report of a Case

OBJECTIVES

We will present clinical and pathologic findings of the recurrent laryngeal nerve of a 54-year-old man with apparent idiopathic vocal fold paralysis and discuss clinical and scientific implications of these findings.

STUDY DESIGN

Our design is in the format of a case report with emphasis on the clinical, intraoperative, histologic, and neuropathologic findings, followed by a discussion of contemporary theories of idiopathic unilateral vocal fold immobility and the implications of our findings.

METHODS

A 54-year-old man presented with a 2-year history of unilateral vocal fold motion impairment (VFMI) after an upper respiratory infection. Radiographic and laboratory data revealed no organic cause. The patient clinically appeared to have a classical unilateral vocal fold paralysis and had previously undergone collagen injection and medialization thyroplasty, each with only temporary improvement of voice. We performed ansa cervicalis-recurrent laryngeal nerve anastomosis.

RESULTS

Intraoperative dissection revealed an indurated and thickened section of nerve approximately 15 mm in length, located at the beginning of the intralaryngeal segment, near the cricothyroid joint. Segments of this nerve were submitted for histologic evaluation, revealing necrosis with granulomatous inflammation. Postoperative electromyography and videostrobolaryngoscopy were consistent with successful reinnervation from the ansa procedure.

CONCLUSIONS

The diagnosis of "idiopathic VFMI" likely represents a number of distinct pathologic entities. This case highlights our lack of understanding of idiopathic VFMI and raises many important questions regarding our current theories of this diagnosis. The clinical and pathologic implications are discussed, and continued investigation is recommended.

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.

Variability of the nerve supply patterns of the human posterior cricoarytenoid muscle

OBJECTIVES/HYPOTHESIS

To achieve a successful selective reinnervation of the larynx, an accurate knowledge of the nerve supply of each individual muscle is required. The posterior cricoarytenoid muscle, the only abductor or respiratory muscle of the larynx, plays a vital role in cases of recurrent palsy and orthotopic transplantation. Descriptions of the posterior cricoarytenoid muscle nerve supply pattern vary considerably. The goal of the present study was to establish an accurate morphological description of the posterior cricoarytenoid muscle nerve supply in a large sample of human larynges.

STUDY DESIGN

Morphologic study of human larynges.

METHODS

The posterior cricoarytenoid muscle nerve supply was studied in a total sample of 75 human larynges obtained from necropsies (47 male and 28 female samples; age range, 41-95 y) and examined by careful dissection using a surgical microscope.

RESULTS

The posterior cricoarytenoid muscle nerve supply in all cases (100%) came from the anterior division of the recurrent nerve. However, in six cases (4%) a small branch also arose from the ramus anastomoticus. The number of branches coming from the anterior division varied, ranging from one to six. The two-branch pattern was the most frequent (42.7%), followed by the three-branch pattern (34%) and the one-branch pattern (7.3%). The remaining 16% of cases showed patterns of four, five, or six branches. When two or more branches were present, a connection between them was observed in 64% of cases. Five different types of origin of the various branches were observed along the course of the recurrent nerve in relation to the cricothyroid joint: type a, vertical segment below the cricothyroid joint (7.5%); type b) vertical segment behind the cricothyroid joint (40.5%); type c) vertical segment just above the cricothyroid joint (16%); type ) from the genu, in common with the arytenoid branch, above the cricothyroid joint and just below the cricoarytenoid joint (34%); and type e) oblique segment (2%).

CONCLUSION

Despite the variability of the innervation of the posterior cricoarytenoid muscle and its strong connection with the interarytenoid nerve, this should not preclude successful reinnervation.

Complications of thyroid and parathyroid surgery

Today most complications of thyroid and parathyroid surgery are related to either metabolic derangements or injury to the recurrent laryngeal nerves. Other complications include superior laryngeal nerve injury, infection, airway compromise, and bleeding. Although the principal goal of thyroid and parathyroid surgery is the prevention of these complications, prompt recognition and intervention will minimize morbidity and provide the patient with the best chance of a satisfactory outcome.

Model for experimental revascularized laryngeal allotransplantation

BACKGROUND

Although a human laryngeal transplant has been undertaken successfully, important questions remain that require a suitable animal model.

METHODS

A pig model for allotransplantation has been developed. Organ perfusion was studied in nine animals before four transplants were performed in congenic (unrecovered) animals and eight in unmatched (recovered) animals. Larynges were regularly examined endoscopically until death at 14 days. Immunosuppression included the use of tacrolimus. Revascularization was achieved by anastomosing the donor right cervical vascular tree to the recipient common carotid. In recovered animals, four allografts were placed orthotopically and four heterotopically.

RESULTS

The pig larynx was perfused adequately via the right cervical vascular tree and congenic grafts were well tolerated. Of eight allografts, seven were well tolerated and remained healthy for the duration of the study (14 days). One allograft became infected between days 4 and 7 after operation. Median operating time was 6 h, with a median cold ischaemia time of 3 h.

CONCLUSION

Revascularized allotransplants of the larynx can be undertaken reliably in pigs and this provides a preclinical model for studies of laryngeal transplantation.

Long-term monitoring of respiration with a mediastinal pressure sensor in dogs

The ability of an implanted mediastinal pressure sensor to produce a stable respiratory signal that could be used to trigger stimulation of upper airway muscles was examined. In 6 dogs, a pressure sensor was secured to the manubrium (4 by wires and 2 by transmanubrial placement). In 6 other dogs, the pressure sensor was placed in the upper anterior mediastinum. The animals were monitored for a minimum of 8 weeks (2 transmanubrial sensors for 12 months). Sensors that were able to maintain a midline position, high in the mediastinum, had the best signals. A caudal sensor position or abutment against an intrathoracic structure caused signal inversion (unusable signals). Transmanubrial placement resulted in a stable signal for 1 year. We conclude that long-term monitoring of respiration with a mediastinal pressure sensor can be successfully performed in dogs, providing an adequate signal for nerve-muscle stimulation. Separation from cardiovascular structures improves signal quality.

Neurolaryngology: past, present, and future

Neurolaryngology is the study and management of disorders that impair neural control of the larynx and pharynx in breathing, swallowing, and speech. Advances in functional endoscopy and fluoroscopy and increased understanding of neurophysiology have greatly facilitated the development of this discipline. The empiric observations of effective therapies have been equally important, however. In comparison to other fields of medicine, neurolaryngology is a relatively young discipline, and much remains to be discovered and developed.

The evolution of the assessment and treatment of paralytic dysphonia

Vocal fold paralysis has been an omnipresent disorder throughout the annals of laryngologic history since the origin of the specialty in 1858. The attendant complexity of laryngoscopic presentation, physiologic dysfunction, and surgical rehabilitation have led to a rich heritage of scientific investigations that can be traced through the past 140 years. The following historical summary provides the reader with a working knowledge of past experiences that connect to current initiatives and portend future progress.

Laryngeal synkinesis revisited

First described in 1982, laryngeal synkinesis continues to play an important diagnostic and therapeutic role following recurrent laryngeal nerve (RLN) injury. Vocal fold motion impairment (formerly called "vocal cord paralysis"), hyperadducted and hyperabducted vocal folds, and certain laryngeal spasmodic and tremor disorders are often best explained by synkinesis. A closer look at these mechanisms confirms that following RLN injury, immobile vocal folds may be nearly normally functional (favorable), or spastic, hyperadducted, or hyperabducted (unfavorable). This has resulted in a functional classification of laryngeal synkinesis as follows: type I laryngeal synkinesis, with satisfactory voice and airway (vocal fold poorly mobile, or immobile); type II synkinesis, with spasmodic vocal folds and an unsatisfactory voice and/or airway; type III synkinesis, with hyperadducted vocal folds and airway compromise; and type IV synkinesis, with hyperabducted vocal folds, poor voice, and possible aspiration. This classification facilitates the understanding of laryngeal pathophysiology following RLN injuries and promotes a more scientific basis for management.

Experimental reinnervation of a strap muscle with a few roots of the phrenic nerve in rabbits

In order to compare application of the roots of the phrenic nerve to the ansa hypoglossi for laryngeal muscle neurotization, 1 or more roots from the phrenic nerve were implanted into the right sternothyroid (RST) muscle of rabbits (n = 36). Controls were intact animals (in which RST innervation is provided by the ansa; n = 6) and denervated ones (n = 6). At 66 +/- 2 days (mean +/- SE) after neurotization, during quiet breathing, inspiratory electromyographic activity and isometric contraction force were observed in all reinnervated RST muscles (n = 24). During maximal inspiratory effort, electromyographic activity and force increased. In animals reinnervated by the C4 root alone, forces (46.22 +/- 7.8 g) were significantly higher than in intact animals (10.83 +/- 5.0 g). Retrograde labeling proved the phrenic origin of the neurotization. Electromyography of the diaphragm was recorded. We conclude that in rabbits, neurotization of a strap muscle by 1 or 2 roots of the phrenic nerve allows inspiratory contraction, even during quiet breathing. Such inspiratory activity is not observed in sternothyroid muscles of intact animals innervated by the ansa hypoglossi.

Cervical anatomy of phrenic nerve roots in the rabbit. European Group for Research on the Larynx

The cervical anatomy of the different nerve contributions that constitute the phrenic nerve (phrenic nerve roots and accessory phrenic nerve) were studied in rabbits. In 55 dissections, 6 main root arrangement types were observed. The roots that issued from the fourth and fifth cervical nerves (C4 and C5 roots) were constant. The C4 root was either short or long. The C6 root was at times absent, or sometimes double. An accessory phrenic nerve was present in 43% of the right and 28% of the left dissections. The distribution of the phrenic nerve roots often displayed left-right asymmetry. We conclude that a better knowledge of the cervical anatomy of the phrenic nerve is useful both in physiological studies involving diaphragm denervation and in experimental laryngeal reinnervation.

An experimental comparison of different kinds of laryngeal muscle reinnervation

In this study, we attempted to determine which method was the best for reinnervating the laryngeal adductor muscles by comparing nerve suture, nerve implantation, and nerve-muscular pedicle (NMP) transfer, as well as the length of time that could elapse after denervation and still allow for successful reinnervation with the ansa cervicalis. Reinnervation was performed in 36 dogs, at 6-, 8-, 10-, 12- and 18-month intervals after denervation via the three methods of muscle reinnervation described above. We noted some return of adduction in the cases using nerve suture before a 10-month interval after denervation, and with nerve implantation and NMP transfer before the 8-month intervals. The variable adduction was caused by reinnervation of the adductor muscles from the ansa cervicalis, as demonstrated by laryngeal spontaneous and evoked electromyography, the strength of muscle contraction, and histologic findings. Adduction was not observed in the cases after the above-mentioned intervals but partial improvement of the bulk and strength of the reinnervated vocal cord was still achieved. An analysis of the experimental results showed that nerve suture was superior to nerve implantation and the NMP technique. Little difference was noted between nerve implantation and the NMP technique.

Posterior glottic stenosis: a canine model

Posterior glottic stenosis is a disabling disease in which the vocal folds are fixed near the midline. This allows adequate vocal fold adduction for voicing, but does not permit useful abduction for ventilation. The most common cause is prolonged endotracheal intubation for mechanical ventilation, and the incidence is estimated at 4% for intubations between 5 and 10 days. Currently, our understanding and treatment modalities are based on retrospective reviews of small, nonrandomized clinical experiences. The purpose of this project was to develop an animal model that would improve our understanding of histologic changes and allow future prospective randomized trials for therapeutic intervention. Twelve dogs, 15 to 25 kg, were randomly divided into three groups. Animals in group I had a superficial injury produced in the tissue over the right cricoarytenoid joint; animals in group 2 had a deep soft tissue injury produced; and animals in group 3 underwent joint opening. The animals were allowed to recover for 2 months. Morphometric analysis of the harvested larynges demonstrated clinically significant limitation in motion in the animals with deep soft tissue injury and in animals with joint disruption. Histologic analysis revealed various degrees of injury, from loss of subepithelial soft tissue to cartilaginous resorption and fusion of the arytenoid to the cricoid. These findings were directly related to the depth of the initial injury. It is possible to produce posterior glottic stenosis in the canine species. This will serve as a reliable animal model for future study.

Long-term excitability and fine tuning of nerve pedicles reinnervating strap muscles in the dog

Contraction of paralyzed striated muscles has been restored by stimulating reinnervating pedicles with currents of low intensity. In order to allow clinical application, stable, long-term excitability must emulate the parameters necessary for the stimulation of normal motor nerves. In 6 dogs, the ansa hypoglossi nerve was implanted into the contralateral denervated sternohyoid muscle and surrounded with a bipolar cuff electrode. Three of the reinnervating pedicles were chronically paced with a Medtronic Itrel II Multiprogrammable Pulse Generator (0.5 V, 0.2 second on [30 pulses per second, 0.21-millisecond pulse width], 2.9 seconds off). At reexploration after 8 months (6 months for 1 dog), frank contraction confirmed by electromyography tracings occurred in all animals with currents in the range of 0.1 to 0.5 mA. Muscle force was further manipulated by selective release of blocking currents (600 Hz, 1.7 to 0.4 mA) superimposed over regular stimulation (50 Hz, 0.3 to 1.7 mA). Nerve and muscle vitality were histologically confirmed. Long-term, low-intensity conduction capabilities, fine tuning, good tolerance of implanted electrodes, and lack of fatigue suggest that reinnervating pedicles may be successfully used for pacing when clinically indicated.

Motor nerve transplantation

The motor nerve transplantation (MNT) technique is used to transfer an intact nerve into a denervated muscle by harvesting a neurovascular pedicle of muscle containing motor endplates from the motor endplate zone of a donor muscle and implanting it into a denervated muscle. Thirty-six adult New Zealand White rabbits underwent reinnervation of the left long peroneal (LP) muscle (fast twitch) with a motor nerve graft from the soleus muscle (slow twitch). The right LP muscle served as a control. Reinnervation was assessed using microstimulatory single-fiber electromyography (SFEMG), alterations in muscle fiber typing and grouping, and isometric response curves. Neurofilament antibody was used for axon staining. The neurofilament studies provided direct evidence of nerve growth from the motor nerve graft into the adjacent denervated muscle. Median motor endplate jitter was 13 microsec preoperatively, and 26 microsec at 2 months, 29.5 microsec at 4 months, and 14 microsec at 6 months postoperatively (p < 0.001). Isometric tetanic tension studies showed a progressive functional recovery in the reinnervated muscle over 6 months. There was no histological evidence of aberrant reinnervation from any source outside the nerve pedicle. Isometric twitch responses and adenosine triphosphatase studies confirmed the conversion of the reinnervated LP muscle to a slow-type muscle. Acetylcholinesterase studies confirmed the presence of functioning motor endplates beneath the insertion of the motor nerve graft. It is concluded that the MNT technique achieves motor reinnervation by growth of new nerve fibers across the pedicle graft into the recipient muscle.