External Branch of the Superior Laryngeal Nerve Mediated Glottic Closing Force in the Porcine Model.

OBJECTIVES: Based on our laboratory's newly confirmed motor pathway for glottic closure, we measured the glottic closing force (GCF) during isolated stimulation of the external branch of the superior laryngeal nerve (eSLN) in the porcine model. Glottic closure is 1 of the primary mechanisms for prevention of aspiration during deglutition. METHODS: The recurrent laryngeal nerve (RLN) and eSLN were identified bilaterally in 4 porcine necks. Subsequently, GCF was measured with a pressure transducer as the distal ends of individual nerves were stimulated in 4 animals. The RLN mediated GCF was measured first, followed by isolated eSLN mediated GCF, followed by transection of the RLN and repeat measurement of the eSLN GCF. Ultimately, the cricothyroid (CT) muscle attachment was released and the GCF measured. RESULTS: The GCF during isolated eSLN stimulation before and after RLN transection is approximately 89% of the RLN mediated GCF in each animal. The GCF after CT release is approximately 84% of the RLN perceived GCF. Transection of the RLN did not alter the eSLN observed GCF. CONCLUSIONS: The GCF obtained during isolated eSLN stimulation is adequate for delivery of an appropriate laryngeal protective response and may be considered a target motor nerve for augmenting GCF in selected rehab settings.

Thyroarytenoid cross-innervation by the external branch of the superior laryngeal nerve in the porcine model.

OBJECTIVES/HYPOTHESIS: Cross-innervation patterns to the thyroarytenoid (TA) muscle have long been sought after. We have identified in the porcine model, cross-innervation by way of the external branch of the superior laryngeal nerve (eSLN). STUDY DESIGN: Experimental study. METHODS: TA contraction was electromyographically recorded when electrically stimulating the eSLN in six porcine necks. The recurrent laryngeal nerve (RLN) was subsequently transected. The insertion of the cricothyroid (CT) muscle on the cricoid was then subsequently removed as well. RESULTS: Stimulation of the eSLN rendered a response from the TA muscle in 6/6 subject necks, with a mean latency of 2.76 msec. TA muscle contraction by way of eSLN stimulation persisted after the RLN was transected and after CT insertion release. CONCLUSIONS: The TA muscle is directly cross-innervated by a branch of the eSLN in the porcine model. This finding may have implications regarding possible future laryngeal pacing strategies and could be a target nerve for rehabilitation.

The pharyngeal plexus-mediated glottic closure response and associated neural connections of the plexus.

IMPORTANCE: There continues to be a paucity of data regarding the pharyngeal plexus (PP) and its interconnectivity with the laryngeal nerves and function. OBJECTIVE: To identify the specific neural pathways involved in the glottic closure reflex (GCR)-like pathway of the PP and other pathways to the thyroarytenoid (TA) muscle in the porcine model. DESIGN, SETTING, AND ANIMAL SUBJECTS: Animal experimental study from September 2013 to June 2014 conducted in a tertiary academic medical center on male Yorkshire pigs. INTERVENTIONS: Contraction of the TA was detected with electromyography (EMG) during electrical stimulation of the PP in 7 porcine necks. Subsequently, the external branch of the superior laryngeal nerve (eSLN), communicating nerve of Galen (NG), and the recurrent laryngeal nerve (RLN) were sequentially transected to help elucidate the path of neural conduction. MAIN OUTCOMES AND MEASURES: Confirmation of TA muscle contraction by EMG. RESULTS: Stimulation of the PP evoked a response from the TA muscle in 6 of 7 subject animals. In 3 of 7 subjects, a long latency response (mean, 14.62 milliseconds) was identified, which was eliminated only after transection of the RLN. In 3 of 7 subjects, a short latency response (mean 3.05 milliseconds) was identified, which disappeared in 1 subject each by eSLN, RLN, and NG transection. CONCLUSIONS AND RELEVANCE: We identified the specific neural pathway involved in the PP's GCR-like pathway. We also noted a variable direct pattern of innervation to the TA.

Neuromuscular basis for ventricular fold function.

OBJECTIVES: We sought to examine the neuromuscular basis for ventricular fold function. The primary function of the ventricular folds is to assist in the regulation of intra-abdominal and intrathoracic pressure. They also influence phonation in the setting of vocal fold paralysis or ventricular dysphonia, or after partial laryngectomy. The neuromuscular basis of true vocal fold function has been well studied; however, its neuromuscular correlates in the ventricular folds are ambiguous. The literature is unclear as to whether ventricular fold contraction is passive or active. The musculature and innervation responsible for this action also have not been well defined. The aim of this study was to provide clarity in regard to these mechanisms. METHODS: We examined a whole-organ section of a human larynx from a patient with unilateral vocal fold paralysis. The region of the ventricular folds was compared on both the paralyzed and normal sides. Electrophysiological examination was performed in a porcine model. The superior laryngeal nerve was stimulated, and recording electrodes in both ventricular folds measured the electrical activity. The recurrent laryngeal nerve was then severed, and the experiment was repeated. RESULTS: The histologic slides from the patient with unilateral vocal fold paralysis demonstrated atrophied ventricularis and thyroarytenoid muscles on the paralyzed side. On the unaffected side, these muscles were of normal size. The electrophysiological examination in the porcine model demonstrated findings consistent with innervation of the ventricularis muscle by the recurrent laryngeal nerve. An association of ventricularis muscle activity with ventricular fold contraction also was demonstrated. CONCLUSIONS: Ventricular fold adduction appears to be a result of ventricularis muscle contraction that is mediated by the recurrent laryngeal nerve.

Effect of altered core body temperature on glottal closing force.

OBJECTIVES: A basic function of the larynx is to provide sphincteric protection of the lower airway, initiated by a brain stem-mediated glottal closure reflex. Glottal closing force is defined as the measured pressure generated between the vocal folds during glottal closure. One of the factors thought to affect the glottal closure reflex is a variation in core body temperature. METHODS: Four adult male Yorkshire pigs were used in this study. The subjects were studied under control conditions (37 degreesC), hyperthermic conditions (38 degrees C to 41 degrees C), and hypothermic conditions (36 degrees C to 34 degrees C). RESULTS: We demonstrated that the glottal closing force increased significantly with an increase in core body temperature and also decreased significantly with decreased core body temperature. These results are supported by neurophysiological changes demonstrated by other studies in pups and adult dogs in response to altered core body temperatures. The mechanism for these responses is thought to reside centrally, rather than in the peripheral nervous system. CONCLUSIONS: We hope that a better understanding of these aspects of glottal closure will alter the care of many patients with postanesthesia hypothermia and many sedated inmates and will also further enhance preventive measures needed to decrease the incidence of sudden infant death syndrome in overheated or febrile infants.

Modulating effects of hypoxia and hypercarbia on glottic closing force.

OBJECTIVES: Aspiration has been identified as one of the independent risk factors for development of respiratory tract infections, the incidence of which varies from 10% to 65% in patients in intensive care units. The primary defense mechanism for protection of the lower airway is the glottic closure reflex (GCR), elicited by stimulation of the internal branch of the superior laryngeal nerve. This reflex, once considered highly stable, is now considered vulnerable to a growing number of clinical factors. This study was designed to explore the biomechanical effects of hypoxia and hypercarbia, common occurrences among critically ill patients, on the GCR. METHODS: Five adult male Yorkshire pigs were used in the study. Both internal superior laryngeal nerves were simultaneously stimulated with bipolar platinum-iridium electrodes. The glottic closing force (GCF) was then measured by placing a pressure transducer between the adducting vocal folds under 3 different protocols: protocol 1 (control), protocol 2 (hypoxia: partial pressure of arterial oxygen [PaO2] levels of 90, 70, and 50 mm Hg), and protocol 3 (hypercarbia: partial pressure of arterial carbon dioxide [PaCO2] levels of 60 and 70 mm Hg). Six readings were recorded under each experimental paradigm, and Student's t-test was applied to calculate the statistical significance against the control. RESULTS: Hypoxia reduced the GCF to 75%, 40%, and 29% of control for PaO2 levels of 90, 70, and 50 mm Hg, respectively, and hypercarbia reduced the GCF to 40% and 27% of control for PaCO2 levels of 60 and 70 mm Hg, respectively. CONCLUSIONS: This is the first study that highlights the biomechanical impact of hypoxia and hypercarbia on the GCR, providing a unified explanation for the increased incidence of life-threatening aspiration in critically ill patients with such alterations.