Development of peripheral magnetic stimulation system to stimulate suprahyoid muscles

Abstract

Neuromuscular electrical stimulation has been widely used in patients with dysphagia, but obtaining sufficient hyoid bone movement through surface electrodes seems difficult due to pain. On the other hand, magnetic stimulation activates the nerve and the muscles without stimulating the skin nociceptors. However, the coil for magnetic stimulation is too big to stimulate smaller muscles like the suprahyoid muscles. The aim of this study was to develop the peripheral magnetic stimulation system to stimulate suprahyoid muscles. This study was approved by the institutional review board. We designed a smaller coil to stimulate suprahyoid muscles without stimulating inferior alveolar nerve that induced pain to the teeth. We also developed a jig to support the coil. Eleven healthy subjects aged 32 ± 9 years participated in this study. The new coil was placed on the submental area of the subjects and the magnetic stimulation was performed one time at a rate of 30 Hz for 2 s. The intensity level selected induces hyoid bone movement without causing intolerable pain to the subjects. The hyoid bone at rest between on- and off-magnetic stimulations of the suprahyoid muscles were identified using fluoroscopy at 30 frames/s in lateral projection and the displacements of the hyoid bone were calculated using Image J program. Pain during peripheral magnetic stimulation was assessed from 0 to 10 using the numerical rating scale (NRS). Magnetic stimulation resulted in 10.9 ± 2.8 mm forward displacement and 8.3 ± 4.1 mm upward displacement of the hyoid bone. The median NRS score during magnetic stimulation was 1. We obtained similar extent of hyoid bone movement at rest compared with normal drinking of 10-mL liquid. The peripheral magnetic stimulation can become an alternative to electrical stimulation for patients with reduced laryngeal elevation.