Effects of asymmetric superior laryngeal nerve stimulation on glottic posture, acoustics, vibration

Abstract

Objectives/hypothesis: Evaluate the effects of asymmetric superior laryngeal nerve stimulation on the vibratory phase, laryngeal posture, and acoustics.

Study design: Basic science study using an in vivo canine model.

Methods: The superior laryngeal nerves were symmetrically and asymmetrically stimulated over eight activation levels to mimic laryngeal asymmetries representing various levels of superior laryngeal nerve paresis and paralysis conditions. Glottal posture change, vocal fold speed, and vibration of these 64 distinct laryngeal-activation conditions were evaluated by high speed video and concurrent acoustic and aerodynamic recordings. Assessments were made at phonation onset.

Results: Vibratory phase was symmetric in all symmetric activation conditions, but consistent phase asymmetry toward the vocal fold with higher superior laryngeal-nerve activation was observed. Superior laryngeal nerve paresis and paralysis conditions had reduced vocal fold strain and fundamental frequency. Superior laryngeal nerve activation increased vocal fold closure speed, but this effect was more pronounced for the ipsilateral vocal fold. Increasing asymmetry led to aperiodic and chaotic vibration.

Conclusions: This study directly links vocal-fold tension asymmetry with vibratory phase asymmetry, in particular the side with greater tension leads in the opening phase. The clinical observations of vocal fold lag, reduced vocal range, and aperiodic voice in superior laryngeal paresis and paralysis is also supported.

Keywords: Superior laryngeal nerve; acoustics; high speed video; laryngeal asymmetry; vibration; voice production.

Figure 1

Muscle Activation Plot (PLOT) for left vocal fold closure speed as a function of graded levels of left and right SLN activation. The left vocal fold closure speed increased with activation of either CT muscle but the speed increased more when the left SLN activation level was greater. The right vocal fold mirrored this finding.

Figure 2

Muscle Activation Plot (MAP) illustrating the side with greater relative increase in vocal fold closure speed as a function of graded levels of left and right SLN activation. SLN activation led to greater increase in ipsilateral vocal fold closure speed than the contralateral one. This finding is particularly consistent unilateral SLN paralysis conditions (L SLN levels 2-7 at R SLN level 0; R SLN levels 1-7 at L SLN level 0).

Figure 3

Muscle Activation Plot (MAP) for vocal fold strain as a function of graded levels of left and right SLN activation. The greatest positive strain (+25%) was seen with the highest symmetric bilateral SLN activation and the greatest negative strain (−18%) was seen at bilateral zero SLN activation. Unilateral SLN activation levels resulted in maximal strain that was in between these values (+5.8% for R SLN paralysis; +5.3 for L SLN paralysis).

Figure 4

Glottal opening phase demonstrating vocal fold lateral excursion and mucosal wave on the superior vocal fold surface. (A) Condition L SLN level 4, R SLN level 0. The left vocal fold opened earlier and a robust mucosal wave is visible on the left vocal fold. (B) Condition L SLN level 4 and R SLN level 4. Mucosal waves are equally robust and symmetric. (C) Condition L SLN 0, R SLN 4, the right vocal fold opened earlier and a robust mucosal wave is present on the right vocal fold surface. (See text for full discussion.)

Figure 5

Digital kymograms (DKG) from three illustrative cases as shown in figure 4. Enlargements of one glottal cycle from the corresponding condition are in the right subpanel next to its DKG. In the DKG figure top represents the right vocal fold and the bottom represents the left vocal fold. (A) Condition L SLN level 4, R SLN level 0. The vibration is chaotic with phase lead to the left. (B) Condition L SLN and R SLN level 4. Vibration is periodic and glottal opening phase is symmetric. (C) Condition L SLN 0, R SLN 4. Vibration is aperiodic and phase lead is to the right.

Figure 6

Muscle Activation Plot (MAP) for phase lead as a function of graded levels of left and right SLN activation. There is a preponderance of phase lead to the left with left dominant SLN asymmetry and phase lead to the right with right dominant SLN asymmetry.

Figure 7

Muscle Activation Plot (MAP) for fundamental frequency (F0) at phonation onset, as a function of graded levels of left and right SLN activation. The highest F0 is reached with high bilateral activation levels. SLN paralysis conditions show contracted vocal range. (See text for full discussion.)