Discussion
pain
ADVERSE EVENTS
The purpose of this study was to investigate the mechanism of the rehabilitative effects of the sequential 4-channel NMES compared with that of the conventional 2-channel NMES, through kinematic analysis of hyoid bone movement. In this study, we observed differences in the spatial and temporal parameters of hyoid bone movement in the 4- and 2-channel NMES groups. Only the sequential 4-channel NMES group showed significant increase in the horizontal distance of A–B and significant decrease in the total time when swallowing thin fluid. Furthermore, we found that the horizontal distance of A–B and A–C, and the vertical distance of A–C in soft and regular food swallowing were significantly increased. According to our previous studyIn a previous study, 2-channel conventional NMES improved pharyngeal peristalsis and cricopharyngeal functions at the esophageal entry but did not cause elevation of the hyolaryngeal complexHowever, the stimulation algorithm of the sequential 4-channel NMES is based on a normal contractile sequence. In the electromyography analysis, the suprahyoid muscles are activated about 150 ms and 350 ms earlier that of thyrohyoid and other infrahyoid (sternohyoid, sternothyroid, and omohyoid) musclesThe horizontal A–B distance (thin fluid) was significantly increased in the 4-channel NMES group compared with that in the 2-channel NMES group (Table The main shortcoming of the 4-channel NMES is that it is more cumbersome to attach electrodes compared to the 2-channel NMES. Additionally, the 4-channel NMES can have similar adverse events to conventional NMES, including discomfort, pain, burning sensation, and skin irritationThis study has some limitations. First, data from only a small number of subjects of the original randomized controlled trial could be analyzed. This is probably because the clinical settings of VFSS were different between the rehabilitation centers, and in particular, the VFSS images from Jeju University Hospital were on a camcorder, so two-dimensional kinematic analysis could not be performed on this data. We also excluded some recordings from the Daegu-Patima General Hospital because recording was done at 10 frames/s. As a result, the number of VFSS images that could be kinematically analyzed was smaller in the control group containing more these hospitals. The present study was a multicenter prospective study, and the sample size could not be further increased. In the future, for an exact comparison according to the food form or fluid amount and more reliable results, it will be necessary to recruit sufficient patients calculated for the sample size. Second, the kinematic analysis procedure may have led to some measurement errors. The axes and variables can change with the patient's posture and neck movements which were not accounted for. However, a previous study stated that the correlation between automatic and manual analysis of swallowing was in the range of 0.972–0.996 (0.986 ± 0.010) for the X-axis and 0.939–0.998 (0.985 ± 0.018) for the Y-axis