Quantitative Assessment of Laryngeal Muscle Morphology After Recurrent Laryngeal Nerve Injury: Right vs. Left Differences:

Augmentation and vocal fold biomechanics in a recurrent laryngeal nerve injury model

Objectives/hypothesis

Composite vocal fold (VF) biomechanical data are lacking for augmentation after recurrent laryngeal nerve (RLN) injury. We hypothesize resulting atrophy decreases VF stiffness and augmentation restores native VF biomechanics.

Methods

Sixteen Yorkshire Crossbreed swine underwent left RLN transection and were observed or underwent carboxymethylcellulose (CMC) or calcium hydroxyapatite (CaHa) augmentation at 2 weeks. Biomechanical measurements (structural stiffness, displacement, and maximum load) were measured at 4 or 12 weeks. Thyroarytenoid (TA) muscle cross‐sectional area was quantified and compared with two‐way ANOVA with Tukey's post hoc test.

Results

After 4 weeks, right greater than left structural stiffness (mean ± SE) was observed (49.6 ± 0.003 vs. 28.4 ± 0.002 mN/mm), left greater than right displacement at 6.3 mN (0.54 ± 0.01 vs. 0.46 ± 0.01 mm, p < .01) was identified, and right greater than left maximum load (72.3 ± 0.005 vs. 40.8 ± 0.003 mN) was recorded. TA muscle atrophy in the injured group without augmentations was significant compared to the noninjured side, and muscle atrophy was seen at overall muscle area and individual muscle bundles. CMC augmentation appears to maintain TA muscle structure in the first 4 weeks with atrophy present at 12 weeks.

Conclusions

VF biomechanical properties match TA muscle atrophy in this model, and both CMC and CaHa injection demonstrated improved biomechanical properties and slower TA atrophy compared to the uninjured side. Taken together, these data provide a quantifiable biomechanical basis for early injection laryngoplasty to improve dysphonia and potentially improve healing in reversible unilateral vocal fold atrophy.

Level of evidence

N/A

Electromyography of the posterior cricoarytenoid muscles: a consensus guideline

PURPOSE

Since the introduction of transcutaneous-transcricoidal needle approaches, electromyography (EMG) of the posterior cricoarytenoid muscle (PCA) became easier to perform and teach. Among the Neurolaryngology working group of the European Laryngological Society, several centers have adopted PCA EMG as part of their routine EMG workup in vocal fold immobility collectively gathering long-term experience. The purpose is to give an update and an extension to already existing guidelines on laryngeal EMG with specific regard to PCA EMG.

METHODS

Consensus of all co-authors is based on continuous exchange of ideas and on joint laryngeal EMG workshop experiences over at least 7 years. A Delphi method of consensus development was used, i.e., the manuscript was circulated among the co-authors until full agreement was achieved.

RESULTS

Step-by-step instructions on how to perform and interpret PCA EMG are provided.

CONCLUSIONS

Further research should include the establishment of normal values for PCA and thyroarytenoid muscle (TA) EMG as well as studies on the nature of some unusual activation pattern commonly seen in chronically lesioned PCA.

Crumley's Classification of Laryngeal Synkinesis: A Comparison of Laryngoscopy and Electromyography

OBJECTIVES/HYPOTHESIS

Applying the principles of misdirected nerve regeneration to the larynx, Roger Crumley in 1989 coined the term laryngeal synkinesis (LS) which he later (2000) classified into 4 types (type I - good voice, type II - involuntary twitches and poor voice, type III - adduction during inspiration, type IV - abduction during phonation). Neurophysiological data were not available for all LS patients at that time. The current study was undertaken to utilize and test the Crumley classification for a clinical interrater comparison and, secondly, compare predicted with actual laryngeal electromyography (LEMG) results.

STUDY DESIGN

Descriptive study.

METHODS

Laryngoscopic and LEMG data of patients with unilateral vocal fold paralysis (VFP) of 6 months duration or longer were combined for retrospective evaluation. Forty-five data sets were available for laryngoscopic classification by two local laryngologists and by Roger Crumley. Twenty-three data sets with complete thyroarytenoid (TA) and posterior cricoarytenoid (PCA) - EMG data were used to compare predicted with actual LEMG results.

RESULTS

Local laryngologists were able to classify 24 of 45, Crumley 30 of 45 cases into one of the 4 synkinesis types. There was substantial agreement between examiners (Cohens Kappa 0.66 [P < .001]). Comparison of predicted and actual LEMG data showed only moderate agreement. EMG sykinesis rates were lower in TA than in PCA and highest in Crumley type I cases.

CONCLUSION

The Crumley classification is helpful in describing and understanding synkinesis. It does not always correlate predictably with actual LEMG data. A complete LEMG mapping of all intrinsic muscles may improve understanding of chronic VFP.

LEVEL OF EVIDENCE

4. Laryngoscope, 131:E1605-E1610, 2021.

PCA Atrophy and Synkinesis as the Main Factors for Persistent Vocal Fold Immobility in RLN Paralysis

OBJECTIVES/HYPOTHESIS

An immobile vocal fold due to recurrent laryngeal nerve (RLN) injury usually shows no gross signs of atrophy and lies near the midline. In 1881, Felix Semon proposed that this phenomenon was due to a selective injury of nerve fibers supplying the posterior cricoarytenoid muscle (PCA) and supported this with postmortem proof of selective PCA atrophy. In recent decades, evidence has emerged that the RLN regenerates after injury but does not always result in useful motion of the vocal folds. It has been proposed that this is caused by laryngeal synkinesis. Laryngeal synkinesis describes a random distribution of regenerated nerve fibers to opposing vocal fold muscles. This study was conducted to clarify the relative contribution of these two potential pathomechanisms in our patient population.

STUDY DESIGN

Retrospective case analysis.

METHODS

Retrospective analysis of laryngeal EMG results from cases with RLN paralysis of at least 6 months duration seen at our neurolaryngology clinic.

RESULTS

Out of 118 PCA EMGs, there was not a single normal or near-normal tracing, whilst 33.3% of TA EMGs indicated normal or near normal innervation. PCA EMGs showed signs of persistent high-grade partial denervation (41.5%) as a sign of atrophy, moderate or strong synkinesis (21.2%), or a combination of both (37.3%).

CONCLUSIONS

In chronic RLN paralysis the intrinsic laryngeal muscles are affected to different extents either by atrophy or synkinesis or a combination of both. The PCA is always affected. The lesser damage to TA innervation explains the commonly seen maintenance of vocal fold muscle bulk.

LEVEL OF EVIDENCE

4 Laryngoscope, 131:E1244-E1248, 2021.

Early detection of recurrent laryngeal nerve damage using intraoperative nerve monitoring during thyroidectomy

Objective

This study aimed to investigate risk factors for early recurrent laryngeal nerve (RLN) damage during thyroid surgery with intraoperative nerve monitoring (IONM) technology to avoid RLN damage during surgery.

Methods

Data were retrospectively collected from 93 patients who underwent thyroidectomy at Beijing Hospital. All operations were performed by the same surgeon. A four-step procedure of IONM was used during the operation to determine the amplitude and latency of the RLN.

Results

The majority (51.6%) of patients who underwent surgery had thyroid carcinoma. Lymphadenectomy was carried out in 55 (59.1%) patients. A strong association was observed between temporary injury of the RLN and the extent of resection. The risk of temporary injury of the RLN during total thyroidectomy was three times that during right thyroid lobectomy (odds ratio = 3.13). The results of left lobectomy were also different from those of right lobectomy because the RLN was more likely to be damaged during left lobectomy.

Conclusions

Assessment of the amplitude and latency of the RLN can help to assess the integrity of the RLN. The extent of resection affects the functional integrity of the RLN.

Microarray Analysis Gene Expression Profiles in Laryngeal Muscle After Recurrent Laryngeal Nerve Injury

OBJECTIVES

The pathophysiology of recurrent laryngeal nerve (RLN) transection injury is rare in that it is characteristically followed by a high degree of spontaneous reinnervation, with reinnervation of the laryngeal adductor complex (AC) preceding that of the abducting posterior cricoarytenoid (PCA) muscle. Here, we aim to elucidate the differentially expressed myogenic factors following RLN injury that may be at least partially responsible for the spontaneous reinnervation.

METHODS

F344 male rats underwent RLN injury (n = 12) or sham surgery (n = 12). One week after RLN injury, larynges were harvested following euthanasia. The mRNA was extracted from PCA and AC muscles bilaterally, and microarray analysis was performed using a full rat genome array.

RESULTS

Microarray analysis of denervated AC and PCA muscles demonstrated dramatic differences in gene expression profiles, with 205 individual probes that were differentially expressed between the denervated AC and PCA muscles and only 14 genes with similar expression patterns.

CONCLUSIONS

The differential expression patterns of the AC and PCA suggest different mechanisms of reinnervation. The PCA showed the gene patterns of Wallerian degeneration, while the AC expressed the gene patterns of reinnervation by adjacent axonal sprouting. This finding may reveal important therapeutic targets applicable to RLN and other peripheral nerve injuries.

Differentiating arytenoid dislocation and recurrent laryngeal nerve paralysis by arytenoid movement in laryngoscopic video

OBJECTIVE

To present a new method of quantifying arytenoid movement during inspiration and determine if it can be used to distinguish arytenoid dislocation from vocal fold paralysis.

STUDY DESIGN

Case series with chart review.

SETTING

Retrospective study conducted in a university laboratory based on university hospital data.

SUBJECTS AND METHODS

Endoscopic videos from 8 patients with dislocation and 5 patients with vocal fold paralysis diagnosed by electromyography were included. Vector analysis measured cuneiform movement, an indirect measurement of arytenoid movement, during 1 inspiration. Measurements normalized and not normalized to vocal fold length were evaluated. Interrater reliability (2 raters) and intrarater reliability (1 rater performing the analysis twice) were evaluated using intraclass correlation coefficient (ICC) analysis. Raters were blinded to subject group during analysis.

RESULTS

Pixel-valued cuneiform movement was 81.16 ± 25.62 for dislocation and 30.22 ± 23.60 for paralysis (P = .019). Unitless cuneiform movement was 0.58 ± 0.17 for dislocation and 0.24 ± 0.18 for paralysis (P = .030). Interrater ICC was 0.942 for pixel-valued measurements and 0.962 for unitless measurements. Intrarater ICC was 0.909 for pixel-valued measurements and 0.881 for unitless measurements.

CONCLUSIONS

Both pixel-valued and unitless measures of arytenoid movement were significantly greater in arytenoid dislocation than vocal fold paralysis. Pixel-valued measurements were included to demonstrate the ability to make quantitative comparisons across subjects without precise knowledge of camera precision, provided position is approximately stable, as each measurement is inherently normalized by vocal tract length. Future studies will apply this new method of evaluating vocal fold immobility disorders on a larger scale and incorporate a more diverse group of etiologies.

Neurotrophin expression of laryngeal muscles in response to recurrent laryngeal nerve transection

OBJECTIVE/HYPOTHESIS

The recurrent laryngeal nerve (RLN) commonly regenerates after injury; however, functional motion is rarely recovered. Animal experiments have documented aberrant reinnervation after nerve transection, with motor axons reaching inappropriate muscles. More recently, experimental results suggest that lack of vocal fold motion after RLN injury is due to preferential reinnervation of adductor muscles, with inadequate reinnervation of the posterior cricoarytenoid muscle (PCA), the only abductor muscle of the larynx. Information on factors that could influence the receptiveness of these muscles to reinnervation could be useful in developing new therapeutic strategies. It is hypothesized that the thyroarytenoid muscle (TA) and the PCA differ in expression of neurotrophins in response to denervation.

STUDY DESIGN

Laboratory experiment.

METHODS

Rats were sacrificed at 3 days, 6 weeks, or 4 months after unilateral RLN injury measure expression of brain-derived nerve growth factor (BDNF), nerve growth factor (NGF), and neurotrophin 4 (NT-4) in the TA and PCA muscles, using immunohistochemistry. We also assessed nerve regeneration.

RESULTS

NGF was significantly diminished in the denervated TA muscle at 3 days after injury and increased at 6 weeks. BDNF expression was unchanged in the TA, but was diminished in both PCA muscles at 3 days and 6 weeks, returning to near-normal levels at 4 months after injury. Robust nerve regeneration of distal RLN was present at 4 months.

CONCLUSIONS

Results suggest that the TA and PCA muscles respond differently to denervation.