The first purpose of this study was to determine the physiological effects of surface electrical stimulation on the position of the hyoid and larynx in the neck. We had predicted that when both the submental and laryngeal electrode pairs were stimulating at the participants’ maximal tolerated levels, that the hyoid bone would be pulled downward, most likely due to stimulation of the sternohyoid muscle. The data supported this hypothesis; all but two of the participants had depression of the hyoid bone by as much as 5 to 10 mm during stimulation at rest (). We also predicted that the hyoid bone might be pulled posteriorly, however, limited anterior-posterior movement occurred in the hyoid bone. Three participants had hyoid anterior movement, by as much as 5 mm in one case, while the others had minimal movement in the posterior direction. Whereas minimal ascending movement (2-3 mm) occurred in the larynx in two participants, none of the other participants experienced any appreciable laryngeal movement () and the 2-3 mm changes were potentially due to measurement variation. To summarize these findings, the only appreciable motoric effects of surface electrical stimulation was to cause the hyoid bone to descend in the neck, producing movement in the opposite direction from that required for swallowing.
These results suggest that when surface stimulation was applied to the neck at rest, stimulation was either too weak or not deep enough to stimulate axons innervating the muscles that produce hyoid and laryngeal elevation such as the mylohyoid and the thyrohyoid muscles respectively. No change in laryngeal position was observed with surface stimulation at rest.
The second purpose of this study was to determine the immediate effects of surface stimulation on swallowing in participants with chronic pharyngeal dysphagia. Based on previous use of sensory stimulation in the oral and pharyngeal cavities to augment patients’ volitional control of swallowing [3
], we compared sensory levels of electrical stimulation just above the participants’ sensory threshold for detecting a tingling sensation on the skin, and found a significant improvement during swallowing on the NIH-SSS scale but no change on the Pen-Asp scale (). The improvement on the NIH-SSS tended to be related to higher initial scores; that is the more severely affected patients were those who had the greatest improvement with stimulation. Because the NIH-SSS captures pharyngeal pooling and failed esophageal entry in contrast with the Pen-Asp scale, which only measures aspiration and penetration, sensory stimulation may be somewhat helpful in those patients who have reduced ability to clear the bolus from the airway.
Based on the expected lowering of the hyoid with motor levels of stimulation, we hypothesized that the group would have increased penetration and aspiration during swallowing with motor stimulation. No group change in aspiration was noted on either scale with motor levels of stimulation. When the degree of improvement on the Pen-Asp scale with motor levels of stimulation was examined relative to the degree of hyoid depression, we found an unexpected relationship indicating that patients with the greatest hyoid depression during motor levels of stimulation at rest had the greatest improvement during swallowing with the same levels of stimulation. When the hyoid was depressed with stimulation, a patient probably experienced a greater resistance to hyo-laryngeal elevation during swallowing. Perhaps those patients who felt a greater downward pull on the hyoid, when stimulation was turned on at maximal levels, made a greater effort to elevate the hyo-laryngeal complex when swallowing in an attempt to overcome the effects of the stimulation. It could also be the case that those patients who had greater residual power in their hyo-laryngeal muscles would have not only experienced greater hyoid descent with stimulation but could also have greater residual power that they could recruit for hyo-laryngeal elevation to counteract the stimulation induced descent during swallowing.
This study did not address whether or not surface electrical stimulation aids dysphagia therapy in patients’ at risk of aspiration. A controlled clinical trial with random assignment to two groups with equal contact time to compare traditional therapy techniques and therapy using surface electrical stimulation with blinded outcome ratings is needed to answer this question. This study addressed the immediate physiological effects of the use of surface electrical stimulation at rest and during swallowing. However, this study suggests that electrical stimulation should be used judiciously dependent upon a patient’s type and degree of difficulty with swallowing. In those patients who already have some ability to raise the hyo-laryngeal complex, hyoid depression with stimulation may serve as “resistance” during therapy. On the other hand, if a patient is unable to produce any hyo-laryngeal elevation, and therefore would not be able to resist the hyoid depression induced by stimulation, stimulation might put such a patient at greater risk of aspiration as the hyo-laryngeal complex is held down during swallowing. This may have occurred in some of the more severely affected patients who increased in severity on the Pen-Asp and NIH-SSS with motor levels of stimulation, while those less impaired did not change ().
In this study both submental and laryngeal pairs of electrodes were used simultaneously as is recommended for VitalStim® Therapy. It is likely that the simultaneous stimulation resulted in hyoid lowering because the stronger stimulation to the more superficial and larger sternohyoid and sternothyroid muscles overcame any action that might have been induced by stimulation of the mylohyoid muscle in the submental region or the thyrohyoid muscle beneath the sternohyoid in the throat region. Some have proposed using submental stimulation alone to activate the anterior belly of the digastric and the mylohyoid muscles to pull the hyoid bone upward. However, elevation of the hyoid bone without simultaneous stimulation of the thyrohyoid to raise the larynx would leave the larynx down resulting in further opening of the vestibule and increased risk of aspiration. Only if the mylohyoid and thyrohyoid muscles are activated together, without contraction of the sternohyoid, would both the hyoid and larynx be raised together as has previously been shown with intramuscular stimulation [12
]. This cannot be achieved using surface stimulation, because the larger sternohyoid muscle overlies the thyrohyoid and pulls the hyoid downward.
The finding that the group as a whole improved with sensory levels of stimulation alone on the Pen-Asp scale was somewhat unexpected. Previous research has shown that stimulation of the anterior and posterior faucial pillars was most effective stimulation for eliciting a swallow reflex in normal persons[13
]. Although not studied physiologically, stroking the throat region is known to assist with the spontaneous elicitation of swallowing in infants and some mammals. Stimulation of either the glossopharyngeal or the superior laryngeal nerves has been shown to elicit swallowing in animals [14
] and bilateral chemical blockade of the superior laryngeal nerves disrupts swallowing in normal humans [15
]. It has not been observed that sensory stimulation to the surface of the throat would reflexively trigger a swallow in adults; however, sensory levels of electrical stimulation on the skin in the throat may facilitate volitional triggering of swallowing in dysphagia. These preliminary results suggest that low levels of electrical stimulation on the skin might be beneficial in some patients. Because such low levels of electrical stimulation were not observed to induce hyoid depression, we posit that none of the patients would be put at increased risk for aspiration using lower sensory levels of stimulation. Careful systematic study is needed, however, before such low levels of stimulation could be considered as an additional tool for dysphagia therapy.
This study has several limitations. Only one or two trials were evaluated in each patient in each condition because of the need to limit radiation exposure for research purposes in patients who were already receiving radiation exposure for clinical purposes. In addition, the NIH-SSS had limited reliability when first used. This tool improved once rules for judging each of the categories were developed; however, further assessment of the individual items on this scale is needed. The results differed when using the Pen Asp scale and the NIH-SSS. Further research is needed to examine how these scales differ and what they reflect. However, the NIH-SSS scale was more useful for the particular group of severely affected patients with chronic pharyngeal dysphagia who were studied here. The assessment of pooling and esophageal entry was useful for assessing the particular deficits of patients who were on enteric feeding and had not ingested food for six months or more.
In spite of these limitations the results presented here may be helpful as a first step in developing physiological understanding of the immediate effects of surface electrical stimulation in dysphagia. Before such a tool is used in therapy, improved understanding of its immediate effects should be gained in the presence of specific types of swallowing difficulties before it is applied widely to a variety of patients regardless of their risk of aspiration with hyoid lowering.
Before surface electrical stimulation is used, the patients should be carefully screened to determine whether they would be placed at increased risk of aspiration with a procedure that lowers the hyoid. Our results indicate that in some patients with dysphagia this form of stimulation could interfere with hyo-laryngeal elevation required for airway protection during swallowing.