The effect of surface electrical stimulation on hyolaryngeal movement in normal individuals at rest and during swallowing

Abstract

Surface electrical stimulation is currently used in therapy for swallowing problems, although little is known about its physiological effects on neck muscles or swallowing. Previously, when one surface electrode placement was used in dysphagic patients at rest, it lowered the hyolaryngeal complex. Here we examined the effects of nine other placements in normal volunteers to determine 1) whether movements induced by surface stimulation using other placements differ, and 2) whether lowering the hyolaryngeal complex by surface electrical stimulation interfered with swallowing in healthy adults. Ten bipolar surface electrode placements overlying the submental and laryngeal regions were tested. Maximum tolerated stimulation levels were applied at rest while participants held their mouths closed. Videofluoroscopic recordings were used to measure hyoid bone and subglottic air column (laryngeal) movements from resting position and while swallowing 5 ml of liquid barium, with and without stimulation. Videofluoroscopic recordings of swallows were rated blind to condition using the National Institutes of Health-Swallowing Safety Scale. Significant (P < 0.0001) laryngeal and hyoid descent occurred with stimulation at rest. During swallowing, significant (P <or= 0.01) reductions in both the larynx and hyoid bone peak elevation occurred during stimulated swallows. The stimulated swallows were also judged less safe than nonstimulated swallows using the National Institutes of Health-Swallowing Safety Scale (P = 0.0275). Because surface electrical stimulation reduced hyolaryngeal elevation during swallowing in normal volunteers, our findings suggest that surface electrical stimulation will reduce elevation during swallowing therapy for dysphagia.

Figure 1

shows the electrode positions relative to the hyoid bone, thyroid cartilage, and cricoid cartilage. The bipolar electrode pairs for each placement are connected by lead wires (dotted lines) with current flowing between the two electrodes of each pair. Placement 1, 2, 3A, and 3B have electrodes on both submental and laryngeal regions. Placements 1 superior, 2 superior, 2 inferior, 3A right, and 3B superior are individual electrode pairs. The submental-only placement has two electrode pairs above the hyoid bone on the submental region.

Figure 2

shows the placement of the measurement points including the anterior inferior point on the hyoid bone (Hy) designated by a black dot and the posterior uppermost point of the subglottal air column to indicate the position of the larynx (Lx) designated by a white dot. Also shown is the y axis designated by a straight line drawn from the anterior inferior point of the first cervical vertebra to the anterior inferior point of the third vertebra. The x axis (x) was a straight line perpendicular to the y axis. A calibration sphere was taped to the side of the neck and surface electromyographic electrodes (EMG electrodes) were taped to the side of the neck and the stimulation artifact between them was used to determine when stimulation was turned on. The position 3B with two upper electrodes (SEu) and two inferior electrodes placed in the region of the thyroid cartilage (SEi) is shown.

Figure 3

shows the distribution of change in laryngeal and hyoid bone vertical position (mm) during stimulation at rest by electrode placement using box plots. The first and second quartiles are shown in boxes with the median (line) separating them. The third and fourth quartiles are shown in lines extending from each end of the boxes (first and second quartiles). Data above zero indicate elevation and data below indicate descent, with a trend present when three quarters of the data are on one side of zero.

Figure 4

shows a change in peak elevation (mm) for laryngeal and hyoid bone vertical displacement. Data above zero indicate that the peak laryngeal or hyoid bone elevation was higher in stimulated swallows than in non-stimulated swallows. Data below zero indicate that the peak laryngeal or hyoid bone elevation was lower in stimulated swallows than in non-stimulated swallows.