Hypoglossal nerve transfer for laryngeal reinnervation: a preliminary study

Selective reinnervation of the posterior cricoarytenoid and interarytenoid muscles: an anatomical study

OBJECTIVES/HYPOTHESIS

Selective reinnervation for bilateral vocal fold paralysis has been successful in animal models and shows promise in humans, but detailed, surgically relevant measurements for performing this in the human larynx are not readily available.

STUDY DESIGN

Anatomical study describing the anatomy and gender differences of the recurrent laryngeal nerve, with specific attention to the distance between the posterior cricoarytenoid (PCA) branch and the interarytenoid (IA) branch.

METHODS

Dissection of 20 human cadaveric larynges.

RESULTS

The mean distance between PCA and IA branches on the left side in male specimens was 4.9 +/- 2.7 mm; on the right side 5.4 +/- 2.5 mm. The mean distance between PCA and IA branches on the left side in female specimens was 4.9 +/- 2.0 mm; on the right side 5.5 +/- 2.6 mm. A thyroid cartilage notch was required to be able to achieve sufficient access for neurorrhaphy in 57.1% of male specimens on either side and in 69.2% of female specimens on either side. The mean size of the thyroid cartilage notch required in male specimens was 39.55 +/- 19.67 mm(2), and in female specimens 47.61 +/- 12.98 mm(2).

CONCLUSIONS

This study provides new insight into laryngeal anatomy and further data for developing a reliable surgical approach.

Effects of long-term denervation on the rat thyroarytenoid muscle

OBJECTIVES/HYPOTHESIS

To determine the effects of long-term denervation on the rat thyroarytenoid (TA) muscle and neuromuscular junctions.

STUDY DESIGN

A quantitative histologic assessment of the TA muscle after long-term denervation.

METHODS

Thirty Wistar rats were euthanized 10, 18, 26, 42, and 58 weeks after left recurrent laryngeal nerve resection. The areas of the entire muscle and individual muscle fibers were evaluated using hematoxylin-eosin staining, and neuromuscular junctions were detected by immunohistochemistry. Changes after denervation were evaluated by comparing the treated (T) and untreated (U) sides (T/U ratio). The ratio of the number of nerve terminals (NTs) to that of acetylcholine receptors (AChRs) (NT/AChR ratio) was also assessed.

RESULTS

The average T/U ratio for the entire muscle area of the denervation groups ranged between 61.1% and 72.5% and did not differ significantly. Similarly, the T/U ratios for the individual muscle fiber area ranged between 45.0% and 51.9%, and the differences were not significant. The T/U ratio of AChRs at 58 weeks (35.3 +/- 20.2%) was significantly lower than that at 10 weeks (76.3 +/- 9.0%; P < .01). The NT/AChR ratios ranged between 30.3% and 35.6% and did not differ significantly among the denervation groups.

CONCLUSIONS

The entire TA muscle area, individual muscle fiber area, and NT/AChR ratio did not decrease with long-term denervation. Thus, the TA muscle may retain an ability to receive regenerating nerve axons. However, the ability of the TA muscle to receive nerve axons may deteriorate after an excessively long denervation period because the T/U ratio of AChRs decreased with long-term denervation.

Intrinsic Laryngeal Muscle Reinnervation Using the Muscle-Nerve-Muscle Technique

Objectives:

This study was performed to investigate the muscle-nerve-muscle reinnervation technique in the larynx, in which a nerve conduit implanted into an innervated muscle conducts axonal sprouting into a denervated muscle while maintaining function of the donor muscle.

Methods:

In this study, the muscle-nerve-muscle technique was used to direct superior laryngeal nerve axons to reinnervate intrinsic laryngeal muscles by implanting the recurrent laryngeal nerve stump into the cricothyroid muscle in 8 dogs. In 4 of the dogs, the recurrent laryngeal nerve trunk to the adductor muscles was divided so that all axonal sprouting was directed to the posterior cricoarytenoid muscle. Six-month electromyography data were obtained from 6 of the 8 dogs.

Results:

All 6 dogs showed evidence of successful reinnervation of the thyroarytenoid or posterior cricoarytenoid muscles with action potentials that corresponded to spontaneous respiratory efforts, while the donor cricothyroid muscles retained their phasic contraction. These responses were obliterated when the recurrent laryngeal nerve conduit was divided. Histologic examination of the intrinsic laryngeal muscles demonstrated successful reinnervation.

Conclusions:

The results confirm that intrinsic laryngeal muscles may be successfully reinnervated by the superior laryngeal nerve with the muscle-nerve-muscle technique, without sacrifice of function of the cricothyroid muscle. This method offers an alternative source of appropriately firing axons for laryngeal reinnervation procedures.

Vocal fold paresis and paralysis: what the thyroid surgeon should know

The thyroid surgeon must have a thorough understanding of laryngeal neuroanatomy and be able to recognize symptoms of vocal fold paresis and paralysis. Neuropraxia may occur even with excellent surgical technique. Patients should be counseled appropriately, particularly if they are professional voice users. Preoperative or early postoperative changes in voice, swallowing, and airway function should prompt immediate referral to an otolaryngologist. Early recognition and treatment may avoid the development of complications and improve patient quality of life.

Vocal fold paresis and paralysis

Diagnosis and treatment of the immobile or hypomobile vocal fold are challenging for the otolaryngologist. True paralysis and paresis result from vocal fold denervation secondary to injury to the laryngeal or vagus nerve. Vocal fold paresis or paralysis may be unilateral or bilateral, central or peripheral, and it may involve the recurrent laryngeal nerve, superior laryngeal nerve, or both. The physician's first responsibility in any case of vocal fold paresis or paralysis is to confirm the diagnosis and be certain that the laryngeal motion impairment is not caused by arytenoid cartilage dislocation or subluxation, cricoarytenoid arthritis or ankylosis, neoplasm, or other mechanical causes. Strobovideolaryngoscopy, endoscopy, radiologic and laboratory studies, and electromyography are all useful diagnostic tools.

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.

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.

Thyroarytenoid muscle maintains normal contractile force in chronic vocal fold immobility

BACKGROUND

Denervation of skeletal muscle typically results in irreversible denervation atrophy over time. This finding has generated controversy as to the efficacy of reinnervation procedures for chronic vocal fold immobility related to recurrent laryngeal nerve injury.

OBJECTIVE

To test the hypothesis that chronic vocal fold immobility after recurrent laryngeal nerve injury does not result in diminished maximal isometric force generation in the thyroarytenoid muscle.

STUDY DESIGN

Adult random-bred cats underwent either unilateral laryngeal denervation (n = 6) or sham surgery (n = 6). After 6 months, videolaryngoscopy was performed followed by in vitro measurement of maximal isometric tetanic force produced by the thyroarytenoid muscle.

RESULTS

All animals in the denervation group showed right vocal fold paralysis after the initial denervation operation; none of these animals had return of appropriately phased movement with respiration. Four had intermittent disorganized twitching movements. One had these movements plus an occasional weak adduction, and one had no movement. Normal vocal fold mobility was observed in 6 of 6 animals undergoing sham surgery. The maximal isometric tetanic force measured from the thyroarytenoid muscle in the sham group was 438 mN (+/-92 mN standard deviation [SD]). The maximal isometric tetanic force measured from the thyroarytenoid muscle in the chronically immobile group was 405 mN (+/-107 mN SD). Differences were not statistically significant.

CONCLUSION

Maximal isometric force in the thyroarytenoid muscle is not diminished in chronic vocal fold immobility after recurrent laryngeal nerve injury. We conclude that the possibility for restoration of contractile force to the chronically immobile thyroarytenoid muscle exists. This finding supports the pursuit of reinnervation procedures in the treatment of chronic vocal fold immobility.

Laryngeal reinnervation with the hypoglossal nerve. I. Physiology, histochemistry, electromyography, and retrograde labeling in a canine model

This study was performed to determine whether the hypoglossal nerve (cranial nerve XI [XII]) would serve as a useful donor for laryngeal reinnervation by anastomosis to the recurrent laryngeal nerve (RLN). Twenty hemilarynges in 10 dogs were studied prospectively after XII-RLN anastomosis (group A; n = 5), split XII-RLN anastomosis (group B; n = 3), XII-RLN anastomosis with a 2-cm interposition graft (group C; n = 2), no treatment (group D; n = 5), RLN section (group E; n = 2), or ansa cervicalis-RLN anastomosis (group F; n = 3). Spontaneous activity was observed monthly by infraglottic examination through permanent tracheostomies and was recorded by electromyography. Laryngeal adductory pressure and induced phonation were obtained by stimulating the RLN while passing a pressure transducer balloon or humidified air through the glottis. At sacrifice, the laryngeal muscles were stained for adenosine triphosphatase to determine the ratio of type I to type II fibers. Retrograde labeling of the brain stem was performed with horseradish peroxidase. Infraglottic examination at 6 months showed a full range of adductory motion in groups A and B during the swallow reflex, comparable with that in group D. Groups C and F showed good bulk and tone, but little spontaneous motion. Group E remained paralyzed. Stimulation of the transferred nerves caused more activity in groups A and B than in the other groups; groups C and F partially adducted at high levels. The laryngeal adductory pressure responses of groups A and B were similar to those of group D. The XII-reinnervated larynges were capable of producing normal induced phonation. Retrograde labeling of the RLN showed that the reinnervating axons originated only in the hypoglossal nucleus. Electromyography of the reinnervated adductor muscles confirmed spontaneous activity in the dogs (awake). Histochemical analysis confirmed slow-to-fast transformation of both the posterior and lateral cricoarytenoid muscles, indicating that significant reinnervation occurred. We conclude that the hypoglossal nerve functions well as a donor for adductory reinnervation of the larynx.

Cardiac adrenergic innervation within the first 3 months after acute myocardial infarction

It is widely accepted that myocardial infarction results in adrenergic denervation of the infarcted and peri-infarcted myocardium. On the contrary, the concept of re-innervation of adrenergic nerve fibres is less well established. Although there is evidence of partial re-innervation occuring several months after myocardial infarction, the extent and time scale of re-innervation are only poorly known. In this study we investigated changes in cardiac adrenergic innervation and myocardial perfusion during the early convalescence period (the first 3 months) after an acute myocardial infarction. Single-photon emission computed tomographic imaging was conducted in 15 men 1 week and 3 months after an acute myocardial infarction with I123-metaiodobentzylguanidine (MIBG) and Tc99m-sestamibi (MIBI) to determine the extent of adrenergic denervation and impaired perfusion, respectively. A MIBG and MIBI defect was determined as regional uptake </=30% of maximal myocardial activity. The size of the MIBG defect calculated as a percentage of left ventricular mass remained unchanged between 1 week and 3 months after myocardial infarction (31.1 +/- 17.3% vs. 30. 5 +/- 16.8%, respectively). Accordingly, MIBG activity of the infarct and peri-infarct zones (expressed as a percentage of MIBG activity of the myocardium with normal perfusion) showed no significant change (23.7 +/- 10.0% vs. 25.3 +/- 11.0% and 39.0 +/- 11.3% vs. 40.8 +/- 12.8%, respectively) during the follow-up. On the other hand, the size of MIBI defect decreased significantly during the follow-up (14.2 +/- 11.5% vs. 11.4 +/- 9.7%, P<0.05, respectively) indicating improved myocardial perfusion. The results demonstrate that cardiac adrenergic re-innervation is a slow process; despite a significant increase in myocardial perfusion we found no evidence of adrenergic re-innervation during the first 3 months after acute myocardial infarction.

Laryngeal reinnervation with the hypoglossal nerve: II. Clinical evaluation and early patient experience

OBJECTIVES/HYPOTHESIS

To determine whether the hypoglossal nerve (XII) can serve as a suitable donor for human laryngeal reinnervation.

STUDY DESIGN

Prospective, nonrandomized.

METHODS

Measurements were made on patients undergoing open neck procedures to determine the length of XII available and that required to perform XII-recurrent laryngeal nerve (RLN) anastomosis. The morbidity of combined XII and RLN injuries was studied using temporary lidocaine block of the ipsilateral XII in patients with unilateral vocal fold paralysis (UVFP). A pilot series of patients with UVFP who underwent XII-RLN reinnervation was evaluated for morbidity of the procedure, and for improvement in voice and swallowing.

RESULTS

In 89 necks the average available length of XII was 2 cm less than that needed to reach the larynx, indicating the RLN stump must be at least 3 cm to allow tension-free anastomosis. Twenty-five patients with untreated UVFP underwent temporary lidocaine block of XII; 8 had slight changes in their speech, none had increased aspiration. Nine patients underwent XII-RLN reinnervation. Postoperative speech analysis correlated well with the findings of the temporary lidocaine block of XII. One-year follow-up of five patients showed excellent voice quality, resolution of any preoperative aspiration, and minimal morbidity. Slight adductory movement of the reinnervated vocal fold was seen during tongue thrust. Electromyography confirmed substantial polyphasic action potentials in the thyroarytenoid muscle.

CONCLUSIONS

The hypoglossal nerve is a very suitable donor for reinnervation of patients with UVFP. There should be enough length for primary XII-RLN anastomosis in most patients. Donor site morbidity is acceptable. Preoperative lidocaine block of XII is a good predictor of actual donor site morbidity and could be used to assess patients undergoing facial-hypoglossal anastomosis as well.