Effect of superior laryngeal nerve on vocal fold function: an in vivo canine model

Assessment of laryngeal framework surgery requires an awareness of the effect of vocal fold mass, stiffness, and position on voice production. The vibratory pattern of vocal folds during phonation depends on the subglottic pressure as well as the mass and stiffness of the folds. To assess the effect of variations in vocal fold tension with contraction of the cricothyroid muscle on phonation, eight mongrel dog larynges were studied in vivo. Photoglottography, electroglottography, and subglottic pressure were simultaneously recorded as airflow rate and superior laryngeal nerve (SLN) stimulation were varied. Stimulation of the SLN was modified by varying the frequency and voltage of the stimulating electrical signal. Multiple regression analysis of the data revealed a direct relationship between the voltage of SLN stimulation and frequency of vibration (p less than 0.001) at constant subglottic pressure. Increases in the stimulating voltage to the SLN also led to an increase in open quotient (p less than 0.001), but no statistically significant change in speed quotient, subglottic pressure, or sound intensity. Changing the frequency of SLN stimulation had only a modest effect on the frequency of vibration. These results are consistent with the reported findings of an increase in frequency and open quotient with increased tension in an in vitro canine model. The glottographic measurement open quotient appears to be an estimator of cricothyroid contraction and longitudinal vocal fold tension, and may be clinically applicable to the assessment of superior laryngeal nerve injuries and laryngeal framework procedures.