||The impact of high-frequency magnetic stimulation of peripheral nerves：muscle hardness, venous blood flow, and motor function of upper extremity in healthy subjects
奥寺, 良弥OKUDERA, Yoshihiko
The purpose of this study was to investigate the impact of high-frequency peripheral nerve magnetic stimulation on the upper limb function. Twenty-five healthy adults (16 men and 9 women) participated in this study. The radial nerve of the non-dominant hand was stimulated by high-frequency magnetic stimulation device. A total of 600 impulses were applied at a frequency of 20 Hz and intensity of 1.2 resting motor threshold (rMT). At three time points (before, immediately after, and 15 min after stimulation), muscle hardness of the extensor digitorum muscle on the stimulated side was measured using a mechanical tissue hardness meter and a shear wave imaging device, cephalic venous blood flow on the stimulated side was measured using an ultrasound system, and the Box and Block test (BBT) was performed. Mechanical tissue hardness results did not show any significant differences between before, immediately after, and 15 min after stimulation. Measurements via shear wave imaging showed that muscle hardness significantly decreased both immediately and 15 min after stimulation compared to before stimulation (P<0.05). Peripheral venous blood flow and BBT score significantly increased both immediately and 15 min after stimulation compared to before stimulation (P<0.01). High-frequency peripheral nerve magnetic stimulation can achieve effects similar to electrical stimulation in a less invasive manner, and may therefore become an important element in next-generation rehabilitation.In a pathological state in which the muscle cannot contract voluntarily due to uppermotor neuron disorders such as stroke and spinal cord injury, it is possible to contract the paralyzed muscle by directly applying electrical stimulation to the lower motor neuron and its innervating muscles (21). Functional electrical stimulation (FES) can be applied to such motor paralysis; specifically, using an FES system, targeted movements can be recovered by applying programmed movement stimuli to the paralyzed limb via multiple stimulation electrodes (22). Moreover, a recent report using fMRI demonstrated afferent effects in the motor cortex after FES (20), and FES is therefore anticipated for future development in the field of rehabilitation.The electrodes utilized in electrical stimulation are generally classified into surface electrodes or implanted electrodes. Some of the disadvantages of surface electrodes include: they induce pain and have risk of burn due to Joules heat caused by a concentration of current that is dependent on the non-uniform distribution of electrode contact impedance; they can only stimulate the shallow muscle layer and cannot selectively apply stimuli; and they require time for attachment and detachment (9). On the other hand, while implanted electrodes have the advantage over surface electrodes of having the capability to selectively stimulate the deep muscle layer, they have several disadvantages including a necessity for an invasive surgical operation for insertion, risk of infection, and breakage of electrodes (9).Magnetic stimulation is a method in which the nerve is stimulated through an eddy current arising from a magnetic field that occurs perpendicular to the plane of the stimulation coil when current passes through the coil (2). Unlike electrical stimulation, magnetic stimulation is a non-contact method with very few occurrences of pain caused by skin impedance, and has an advantage of being able to stimulate deep tissue areas noninvasively (14).There have been many clinical reports on the improvement in motor function of a paralyzed limb by transcranial magnetic stimulation (13, 24). Transcranial magnetic stimulation in stroke patients with hemiplegia leads to a decrease in excitability of the unaffected motor cortex with low-frequency stimulation, and an enhanced motor training effect on the paralyzed limb through the activation of the pyramidal tract function and augmentation of cerebral cortex plasticity with high-frequency stimulation (24). Magnetic stimulation of the peripheral nerves has been primarily performed at high frequencies, and has demonstrated preventative effects against muscle atrophy in animals (18). In humans, effects at the central nervous system level have been reported with high-frequency peripheral nerve magnetic stimulation, which was shown to induce the activation of frontal and parietal lobes of magnetic stimulation on motor function in humans (23). However, in contrast to the clinical reports on transcranial magnetic stimulation, no reports to our knowledge have investigated the effects of high-frequency peripheral nerve magnetic stimulation on motor function in humans.The purpose of this study is to investigate the impact of high-frequency magnetic stimulation of peripheral nerves on the upper limb function.