Effect on laryngeal adductor function of vincristine block of posterior cricoarytenoid muscle 3 to 5 months after recurrent laryngeal nerve injury

Current landscape in motoneuron regeneration and reconstruction for motor cranial nerve injuries

The intricate anatomy and physiology of cranial nerves have inspired clinicians and scientists to study their roles in the nervous system. Damage to motor cranial nerves may result from a variety of organic or iatrogenic insults and causes devastating functional impairment and disfigurement. Surgical innovations directed towards restoring function to injured motor cranial nerves and their associated organs have evolved to include nerve repair, grafting, substitution, and muscle transposition. In parallel with this progress, research on tissue-engineered constructs, development of bioelectrical interfaces, and modulation of the regenerative milieu through cellular, immunomodulatory, or neurotrophic mechanisms has proliferated to enhance the available repertoire of clinically applicable reconstructive options. Despite these advances, patients continue to suffer from functional limitations relating to inadequate cranial nerve regeneration, aberrant reinnervation, or incomplete recovery of neuromuscular function. These shortfalls have profound quality of life ramifications and provide an impetus to further elucidate mechanisms underlying cranial nerve denervation and to improve repair. In this review, we summarize the literature on reconstruction and regeneration of motor cranial nerves following various injury patterns. We focus on seven cranial nerves with predominantly efferent functions and highlight shared patterns of injuries and clinical manifestations. We also present an overview of the existing reconstructive approaches, from facial reanimation, laryngeal reinnervation, to variations of interposition nerve grafts for reconstruction. We discuss ongoing endeavors to promote nerve regeneration and to suppress aberrant reinnervation and the development of synkinesis. Insights from these studies will shed light on recent progress and new horizons in understanding the biomechanics of peripheral nerve neurobiology, with emphasis on promising strategies for optimizing neural regeneration and identifying future directions in the field of motor cranial neuron research.

Comparison of Myelin-Associated Glycoprotein With Vincristine for Facial Nerve Inhibition After Bilateral Axotomy in a Transgenic Thy1-Gfp Rat Model

IMPORTANCE

Aberrant synkinetic movement after facial nerve injury can lead to prominent facial asymmetry and resultant psychological distress. The current practices of neuroinhibition to promote greater facial symmetry are often temporary in nature and require repeated procedures.

OBJECTIVE

To determine whether myelin-associated glycoprotein (MAG), a specific neuroinhibitor, can prevent neuroregeneration with efficacy comparable with that of vincristine, a well-established neurotoxin.

DESIGN, SETTING, AND PARTICIPANTS

Rats transgenic for Thy-1 cell surface antigen-green fluorescent protein (Thy1-Gfp) were randomized into 3 groups. Each rat received bilateral crush axotomy injuries to the buccal and marginal mandibular branches of the facial nerves. The animals received intraneural injection of saline, MAG, or vincristine.

MAIN OUTCOMES AND MEASURES

The animals were imaged via fluorescent microscopy at weeks 1, 3, 4, and 5 after surgery. Quantitative fluorescent data were generated as mean intensities of nerve segments proximal and distal to the axotomy site. Electrophysiological analysis, via measurement of compound muscle action potentials, was performed at weeks 0, 3, 4, and 5 after surgery.

RESULTS

A total of 12 rats were included in the study. Administration of MAG significantly reduced fluorescent intensity of the distal nerve in comparison with the control group at week 3 (mean [SD], MAG group: 94 [11] intensity units vs control group: 130 [11] intensity units; P < .001), week 4 (MAG group: 81 [19] intensity units vs control group: 103 [9] intensity units; P = .004), and week 5 (MAG group: 76 [10] intensity units vs control group: 94 [10] intensity units; P < .001). In addition, rats treated with MAG had greater fluorescent intensity than those treated with vincristine at week 3 (mean [SD], MAG group: 94 [11] intensity units vs vincristine group: 76 [6] intensity units; P = .03), although there was no significant difference for weeks 4 and 5. At week 5, both MAG and vincristine demonstrated lower distal nerve to proximal nerve intensity ratios than the control group (control group, 0.94; vs MAG group, 0.82; P = .01; vs vincristine group; 0.77; P < .001). There was no significant difference in amplitude between the experimental groups at week 5 of electrophysiological testing.

CONCLUSIONS AND RELEVANCE

Lower facial asymmetry and synkinesis are common persistent concerns to patients after facial nerve injury. Using the Thy1-Gfp rat, this study demonstrates effective inhibition of neuroregeneration via intraneural application of MAG in a crush axotomy model, comparable with results with vincristine. By potentially avoiding systemic toxic effects of vincristine, MAG demonstrates potential as an inhibitor of neural regeneration for patients with synkinesis.

LEVEL OF EVIDENCE

NA.

Toxicity trial of canine posterior cricoarytenoid intramuscular vincristine injections

OBJECTIVES/HYPOTHESIS

In animal studies, intramuscular vincristine injections have been shown to block reinnervation of the denervated target muscle. This application could be used selectively to influence recovery patterns following injury of recurrent laryngeal nerves (RLNs). However, vincristine is currently Food and Drug Administration approved only for intravenous use. A formal toxicity trial of intramuscular injections was performed.

STUDY DESIGN

Animal study.

METHODS

Sixteen female canines underwent direct laryngoscopy with injection of moderate- (0.4 mg, n = 8) or high-dose (0.6 mg, n = 8) vincristine into the posterior cricoarytenoid (PCA) muscles. Plasma samples were collected at various time points postinjection and vincristine levels determined. At 24 hours (n = 7) or 14 days (n = 9) postinjection, animals were anesthetized and videolaryngoscopy documented vocal fold mobility and mucosal appearance. Adductor function was measured during stimulation of the RLN. Larynges were processed for histology.

RESULTS

Fifteen minutes after injection, plasma vincristine levels averaged 10.2% ± 6.7% of the intravenous maximum, suggesting about 90% of the vincristine remained within the PCA muscle. Plasma levels were usually below detectable limits within 24 hours. At the end points, all animals had grossly normal-appearing mucosa and full range of motion. Laryngeal adductor strength was normal in all cases. Histology showed moderate to severe acute inflammation in the submucosa only in the high-dose group at 24 hours. There was no necrosis of muscle or mucosa.

CONCLUSIONS

Intramuscular vincristine injections into the canine PCA muscles resulted in no significant local toxicity, even at the maximum dose. It would be reasonable to evaluate this treatment strategy in a phase I human trial.

LEVEL OF EVIDENCE

NA Laryngoscope, 128:E247-E250, 2018.

Gene Therapy for Recurrent Laryngeal Nerve Injury

Recurrent laryngeal nerve (RLN) injury has considerable clinical implications, including voice and swallowing dysfunction, which may considerably impair the patient’s quality of life. Recovery of vocal fold movement is an essential novel treatment option for RLN injury. The potential of gene therapy for addressing this issue is highly promising. The target sites for RLN gene therapy are the central nervous system, nerve fibers, laryngeal muscles, and vocal cord mucosa. Gene transduction has been reported in each site using viral or non-viral methods. The major issues ensuing after RLN injury are loss of motoneurons in the nucleus ambiguus, degeneration and poor regeneration of nerve fibers and motor end plates, and laryngeal muscle atrophy. Gene therapy using neurotrophic factors has been assessed for most of these issues, and its efficacy has been reported. Another important matter for functional vocal fold movement recovery is misdirected regeneration, in which the wrong neurons may innervate other laryngeal muscles, where even if innervation is reestablished, proper motor function is not restored. Novel strategies involving gene therapy bear promise for overcoming this issue and further investigations are underway.

Paclitaxel inhibits post-traumatic recurrent laryngeal nerve regeneration into the posterior cricoarytenoid muscle in a canine model

OBJECTIVES/HYPOTHESIS

To investigate the efficacy of paclitaxel, a potent microtubule inhibitor with a more favorable therapeutic index as compared with vincristine, in preventing post-traumatic nerve regeneration of the recurrent laryngeal nerve into the posterior cricoarytenoid muscle in a canine laryngeal model.

STUDY DESIGN

Experimental animal study.

METHODS

Forty-nine canine hemilaryngeal specimens were divided into five experimental groups. Under general anesthesia, a tracheostomy, recurrent laryngeal nerve (RLN) transection and repair, and laryngeal adductory pressures (LAP) were measured pre-RLN injury. The approach to the posterior cricoarytenoid (PCA) muscle for neurotoxin injection was transoral or open transcervical, at 0 or 3 months. At 6 months, postinjury LAPs were measured and the animals were sacrificed at 6 months to allow for laryngeal harvesting and analysis.

RESULTS

Paclitaxel demonstrated increased mean laryngeal adductory pressures (70.6%) as compared with saline control (55.5%). The effect of paclitaxel was the same as observed with vincristine at 0 months and with a delayed injection at 3 months. There was no difference between transoral or open injection groups.

CONCLUSIONS

PCA muscle injection with paclitaxel resulted in improved strength of laryngeal adduction. This effect was similar to that of vincristine at both 0 and 3 months following nerve injury. A single intramuscular injection of paclitaxel was well tolerated. Additional human studies are needed to determine the degree of clinical benefit of this intervention.

LEVEL OF EVIDENCE

NA Laryngoscope, 127:651-655, 2017.

Recurrent laryngeal nerve recovery patterns assessed by serial electromyography

OBJECTIVES/HYPOTHESIS

Following acute injury to the recurrent laryngeal nerve (RLN), laryngeal electromyography (LEMG) is increasingly being used to determine prognosis for recovery. The LEMG findings change during the recovery process, but the timing of these changes is not well described. In this canine study, LEMGs were obtained serially following model RLN injuries.

STUDY DESIGN

Animal Study.

METHODS

Thirty-six canine RLNs underwent crush (n = 6), complete transection with reanastomosis (n = 6), half-transection half-crush (n = 5), cautery (n = 5), stretch (n = 5), inferior crush (n = 4), or inferior transection with reanastomosis (n = 5) injuries. Injuries were performed 5 cm from cricoid or were 5 cm further inferior. Under light sedation, LEMG of thyroarytenoid muscles was performed monthly for 6 months following injury. At 6 months, spontaneous and induced vocal fold motion was assessed.

RESULTS

Except for the stretch injury, the remaining groups showed very similar recovery patterns. Fibrillation potentials (FPs) and/or positive sharp waves (PSWs; signs of bad prognosis) were seen in all cases at 1 month and lasted on average for 2.26 months (range = 1-4 months). Motor unit potentials of at least 2+ (scale = 0-4+; signs of good prognosis) were seen beginning at 3.61 months (range = 2-6 months). The stretch injury was less severe, with 3 of 5 showing no FPs/PSWs at 1 month; all recovered full mobility. Ten of the 36 thyroarytenoid muscles (27.8%) had 1 electromyograph showing both bad prognosis and good prognosis signs simultaneously at 2 to 4 months postinjury.

CONCLUSIONS

LEMG can be used to predict RNL recovery, but timing is important and LEMG results earlier than 3 months may overestimate a negative prognosis.

LEVEL OF EVIDENCE

NA Laryngoscope, 126:651-656, 2016.

Prevention of post-traumatic reinnervation with microtubule inhibitors

OBJECTIVES/HYPOTHESIS

Functional recovery after a recurrent laryngeal nerve or facial nerve injury may be impaired due to aberrant reinnervation. Previous work in a rat peripheral nerve injury model found vincristine to be a potent inhibitor of reinnervation, and it has since been used to effectively block neural regeneration in other animal models. However, vincristine's narrow therapeutic index may limit its utility; therefore, another microtubule inhibitor, paclitaxel, which has a higher therapeutic index, was tested.

STUDY DESIGN

Animal (rat) study.

METHODS

After controlled injury to the rat posterior tibial (PT) nerve, the gastrocnemius/soleus complex was injected with saline (control, n = 14), vincristine (n = 30), or paclitaxel (n = 20). Injections without a crush injury were performed using saline (n = 5) or paclitaxel (n = 9). The functional recovery (FR) of the PT nerve was assessed using walking track analysis.

RESULTS

At 6 weeks, controls had already recovered to baseline (FR = 1.0), whereas the paclitaxel group had FR = 0.724 ± 0.064 and the vincristine group had FR = 0.709 ± 0.078. At 6 months, the paclitaxel rats had FR = 0.798 ± 0.167 and the vincristine rats had FR = 0.754 ± 0.240. These differences were significantly different from baseline, but the two agents were not different from each other. Paclitaxel did not affect the FR in the absence of a nerve injury.

CONCLUSIONS

Intramuscular paclitaxel and vincristine both significantly inhibit regeneration of the PT nerve after crush injury for at least 6 months. Potential clinical uses of inhibition of reinnervation are discussed.

LEVEL OF EVIDENCE

NA