Neurophysiology of the Superior Laryngeal Nerve in an In Vivo Rat Model

OBJECTIVE

The superior laryngeal nerve (SLN) is fundamental in laryngeal sensation, cough reflex, and pitch control. SLN injury has substantial consequences including altered sensation, aspiration, and dysphonia. To date, in vivo measurement of the SLN remains elusive. The purpose of this study was to assess the feasibility of recording motor and sensory evoked potentials in a rat SLN model.

METHODS

Twenty-two rat hemi-laryngeal preparations (n = 11) were obtained from 4-month-old Sprague-Dawley rats and included in this study. Compound motor action potentials (CMAPs) and motor unit number estimation (MUNE) were calculated by stimulating the SLN at the point of medial extension near the carotid artery and by placing a recording electrode on the cricothyroid muscle. Sensory response was determined through stimulation of the SLN and laryngoscopic visualization of a laryngeal adductor reflex (LAR). SLN and cricothyroid muscle cross-sections were stained and histologic morphometrics were quantified.

RESULTS

Laryngeal evoked potentials were successfully obtained in all trials. Mean CMAP latency and negative durations were 0.99 ± 0.57 ms and 1.49 ± 0.57 ms, respectively. The median MUNE was 2.06 (IQR 1.88, 3.51). LAR was induced with a mean intensity of 0.69 ± 0.20 mV. Mean axon count, myelin thickness, and g-ratio were 681 ± 192.2, 1.72 ± 0.26, and 0.45 ± 0.04, respectively.

CONCLUSIONS

This study demonstrates the feasibility of recording evoked response potentials following SLN stimulation. We hypothesize that this work will provide a tractable animal model to study changes in laryngeal sensation and cricothyroid motor function with aging, neurodegenerative disease, aspiration, or nerve injury.

LEVEL OF EVIDENCE

NA Laryngoscope, 134:1778-1784, 2024.

Development and Application of Automated Vocal Fold Tracking Software in a Rat Surgical Model

OBJECTIVE

The rat is a widely used model for studying vocal fold (VF) function after recurrent laryngeal nerve injury, but common techniques for evaluating rat VF motion remain subjective and imprecise. To address this, we developed a software package, called RatVocalTracker1.0 (RVT1.0), to quantify VF motion and tested it on rats with iatrogenic unilateral vocal fold paralysis (VFP).

METHODS

A deep neural network was trained to identify the positions of the VFs and arytenoid cartilages (ACs) in transoral laryngoscope videos of the rat glottis. Software was developed to estimate glottic midline, VF displacement, VF velocity, and AC angle. The software was applied to laryngoscope videos of adult rats before and after right recurrent and superior laryngeal nerve transection (N = 15; 6M, 9F). All software calculated metrics were compared before and after injury and validated against manually calculated metrics.

RESULTS

RVT1.0 accurately tracked and quantified VF displacement, VF velocity, and AC angle. Significant differences were found before and after surgery for all RVT1.0 calculated metrics. There was strong agreement between programmatically and manually calculated measures. Automated analysis was also more efficient than nearly all manual methods.

CONCLUSION

This approach provides fast, accurate assessment of VF motion in rats with minimal labor and allows for quantitative comparison of lateral differences in movement. Through this novel analysis method, we can differentiate healthy movement from unilateral VFP. RVT1.0 is open-source and will be a valuable tool for researchers using the rat model for laryngology research.

LEVEL OF EVIDENCE

NA Laryngoscope, 134:340-346, 2024.

Fentanyl causes naloxone-resistant vocal cord closure: A platform for testing opioid overdose treatments

BACKGROUND

High doses of the synthetic opioid fentanyl cause rapid and sustained vocal cord closure (VCC) leading to airway obstruction that prevents overdose victims from breathing. This airway effect is not caused by morphine-derived opiates (e.g. heroin), is distinct from respiratory depression, resistant to naloxone, and can be lethal. However, VCC has not been previously included in animal models of opioid overdose.

METHODS

Video laryngoscopy was used to monitor vocal cord movement in anesthetized Sprague-Dawley rats. Rats were administered saline, fentanyl (5, 25, or 50 μg/kg) or morphine (5 mg/kg) in an intravenous (IV) bolus delivered over a 10 s period. The mu opioid receptor (MOR) antagonist naloxone was administered as a pre-treatment (1 mg/kg, IV) 5 min prior to fentanyl (25 μg/kg) or a post-treatment (1 and 2 mg/kg) 1 min after fentanyl (25 μg/kg).

RESULTS

Fentanyl (25 and 50 μg/kg) caused sustained and lethal VCC within 10 s. Morphine (5 mg/kg) and fentanyl (5 μg/kg) caused only brief laryngospasm with full recovery. Pre-treatment with naloxone (1 mg/kg) prevented fentanyl-induced VCC, but naloxone (1 and 2 mg/kg) was unable to reverse VCC when administered after fentanyl.

CONCLUSIONS

These results indicate sustained VCC is a lethal physiological reaction, specific to fentanyl and resistant to naloxone treatment. While pre-treatment with naloxone prevented fentanyl-induced VCC, naloxone was unable to reverse the effect, suggesting a non-opioid receptor-mediated mechanism. These findings demonstrate the necessity of VCC inclusion in animal models of synthetic opioid overdose and the urgent need for more effective treatments for fentanyl-related overdoses.

Mechanisms and prevention of acid reflux induced laryngospasm in seizing rats

OBJECTIVE

Recent animal work and limited clinical data have suggested that laryngospasm may be involved in the cardiorespiratory collapse seen in sudden unexpected death in epilepsy (SUDEP). In previous work, we demonstrated in an animal model of seizures that laryngospasm and sudden death were always preceded by acid reflux into the esophagus. Here, we expand on that work by testing several techniques to prevent the acid reflux or the subsequent laryngospasm.

METHODS

In urethane anesthetized Long Evans rats, we used systemic kainic acid to acutely induce seizure activity. We recorded pH in the esophagus, respiration, electrocorticography activity, and measured the liquid volume in the stomach postmortem. We performed the following three interventions to attempt to prevent acid reflux or laryngospasm and gain insights into mechanisms: fasting animals for 12 h, severing the gastric nerve, and electrical stimulation of either the gastric nerve or the recurrent laryngeal nerve.

RESULTS

Seizing animals had significantly more liquid in their stomach. Severing the gastric nerve and fasting animals significantly reduced stomach liquid volume, subsequent acid reflux, and sudden death. Laryngeal nerve stimulation can reverse laryngospasm on demand. Seizing animals are more susceptible to death from stomach acid-induced laryngospasm than nonseizing animals are to artificial acid-induced laryngospasm.

SIGNIFICANCE

These results provide insight into the mechanism of acid production and sudden obstructive apnea in this model. These techniques may have clinical relevance if this model is shown to be similar to human SUDEP.

Laryngospasm, central and obstructive apnea during seizures: Defining pathophysiology for sudden death in a rat model

Seizure spread into the autonomic nervous system can result in life-threatening cardiovascular and respiratory dysfunction. Here we report on a less-studied consequence of such autonomic derangements-the possibility of laryngospasm and upper-airway occlusion. We used parenteral kainic acid to induce recurring seizures in urethane-anesthetized Sprague Dawley rats. EEG recordings and combinations of cardiopulmonary monitoring, including video laryngoscopy, were performed during multi-unit recordings of recurrent laryngeal nerve (RLN) activity or head-out plethysmography with or without endotracheal intubation. Controlled occlusions of a tracheal tube were used to study the kinetics of cardiac and respiratory changes after sudden obstruction. Seizure activity caused significant firing increases in the RLN that were associated with abnormal, high-frequency movements of the vocal folds. Partial airway obstruction from laryngospasm was evident in plethysmograms and was prevented by intubation. Complete glottic closure (confirmed by laryngoscopy) occurred in a subset of non-intubated animals in association with the largest increases in RLN activity, and cessation of airflow was followed in all obstructed animals within tens of seconds by ST-segment elevation, bradycardia, and death. Periods of central apnea occurred in both intubated and non-intubated rats during seizures for periods up to 33s and were associated with modestly increased RLN activity, minimal cardiac derangements, and an open airway on laryngoscopy. In controlled complete airway occlusions, respiratory effort to inspire progressively increased, then ceased, usually in less than 1min. Respiratory arrest was associated with left ventricular dilatation and eventual asystole, an elevation of systemic blood pressure, and complete glottic closure. Severe laryngospasm contributed to the seizure- and hypoxemia-induced conditions that resulted in sudden death in our rat model, and we suggest that this mechanism could contribute to sudden death in epilepsy.

Predictors for Permanent Medialization Laryngoplasty in Unilateral Vocal Fold Paralysis

OBJECTIVE

Early differentiation of patients with unilateral vocal fold paralysis (VFP) who recover from those who do not and consequently require permanent medialization laryngoplasty (ML) remains a challenge. The goal of this study is to identify factors that predict the need for ML.

STUDY DESIGN

Case series with chart review.

SETTING

Academic center.

SUBJECTS AND METHODS

A total of 507 records of patients with VFP were analyzed, of which 252 met criteria and were stratified according to whether or not they obtained ML. Demographic information and clinical features were analyzed to determine predictors of ML. A nomogram was generated according to the significance and utility of these parameters.

RESULTS

Of 252 patients, 86 underwent ML, and 166 did not. No differences in age or sex were observed between the ML and non-ML patients (P = .27 and P = .35, respectively). The most common cause of VFP was iatrogenic injury (62.79%, ML; 49.40%, non-ML). ML correlated with VFP secondary to neoplastic disease (odds ratio [OR], 2.14; 95% confidence interval [95% CI], 1.01-4.53) and iatrogenic injury (OR 1.73; 95% CI 1.01-2.94). ML had an inverse correlation with idiopathic VFP (OR, 0.40; 95% CI, 0.20-0.79). Patients in the ML group were more likely to have left-sided VFP, to have a history of aspiration, and to present ≥90 days from onset and less likely to have had temporary injection augmentation.

CONCLUSION

Clinical features may be used to predict the likelihood of a patient obtaining ML. Nomograms may be useful to counsel patients who would benefit from early definitive surgery.