Self-triggered functional electrical stimulation during swallowing

Abstract

Hyolaryngeal elevation is essential for airway protection during swallowing and is mainly a reflexive response to oropharyngeal sensory stimulation. Targeted intramuscular electrical stimulation can elevate the resting larynx and, if applied during swallowing, may improve airway protection in dysphagic patients with inadequate hyolaryngeal motion. To be beneficial, patients must synchronize functional electrical stimulation (FES) with their reflexive swallowing and not adapt to FES by reducing the amplitude or duration of their own muscle activity. We evaluated the ability of nine healthy adults to manually synchronize FES with hyolaryngeal muscle activity during discrete swallows, and tested for motor adaptation. Hooked-wire electrodes were placed into the mylo- and thyrohyoid muscles to record electromyographic activity from one side of the neck and deliver monopolar FES for hyolaryngeal elevation to the other side. After performing baseline swallows, volunteers were instructed to trigger FES with a thumb switch in synchrony with their swallows for a series of trials. An experimenter surreptitiously disabled the thumb switch during the final attempt, creating a foil. From the outset, volunteers synchronized FES with the onset of swallow-related thyrohyoid activity (approximately 225 ms after mylohyoid activity onset), preserving the normal sequence of muscle activation. A comparison between average baseline and foil swallows failed to show significant adaptive changes in the amplitude, duration, or relative timing of activity for either muscle, indicating that the central pattern generator for hyolaryngeal elevation is immutable with short term stimulation that augments laryngeal elevation during the reflexive, pharyngeal phase of swallowing.

Figures

FIG. 1

An example of electromyographic (EMG) recordings of the thyrohyoid and mylohyoid muscles contralateral to the side of stimulation, piezoelectric laryngeal motion signals, and TTL pulses showing the time of stimulation onset from 1 volunteer during a baseline swallow (no stimulation, A), synchronization swallow during which the volunteer attempted to trigger functional electrical stimulation onset in time with a discrete swallow (B), and foil trial during which the volunteer anticipated stimulation but it was surreptitiously withheld by the experimenter (C). Additional bursts in trial iii after offset are not typical and may represent the volunteer’s reaction to a lack of stimulation during the trial. Horizontal lines represent 500 μV on the electromyographic signals. The horizontal lines on the piezoelectric signal (i and iii) shows the onset of laryngeal movement.

FIG. 2

Dot plot of muscle activity onsets relative to FES onset for each volunteer. —, the average onset time for each volunteer. A: synchronization of FES with mylohyoid. B: synchronization with thyrohyoid. Positive values represent muscle activity onsets that occur after FES onset, and negative values represent muscle onsets that occur before FES onset. + and *, volunteers who differed significantly from 1 another.

FIG. 3

Dot plot of muscle activity onsets relative to FES onset for each trial. —, the volunteers’ average onset time for each trial. A: synchronization of FES with mylohyoid. B: synchronization with thyrohyoid. Positive values represent muscle activity onsets that occur after FES onset, and negative values represent muscle onsets that occur before FES onset.

FIG. 4

Line plots showing the mean amplitude (A) and duration (B) across average baseline and foil trials for mylohyoid (•) and thyrohyoid (▴) muscles in all volunteers.

FIG. 5

Line plots displaying the interval in seconds between the onset of mylohyoid activity and the onset of thyrohyoid activity for average baseline and foil swallows. Negative values represent trials in which mylohyoid onset preceded thyrohyoid onset.