Noninvasive spinal cord stimulation by low-intensity ultrasound

Abstract

Stimulation of the spinal cord is used to alleviate chronic pain and manage motor-related neurologic disorders. The typical approach is to deliver a mild electric current precisely at a targeted spinal segment. Despite the practical benefits, the method requires permanent fixation of the electrodes in epidural space and periodic surgical intervention to replace the system power supply. Recently, low-intensity ultrasound has been utilized to modulate neural circuit with high precision in a noninvasive fashion. Previous studies have shown the ability of ultrasound to excite and suppress brain network and stimulate the abducens and sciatic nerves, but not on the spinal cord. In this work, we investigate the ability of low-intensity ultrasound as a noninvasive tool to stimulate the spinal cord. A portable, custom-made transducer with a fundamental frequency of 450 kHz was attached on mouse paraspinal muscle above the intact T13 spinal vertebra. Acoustic stimulation was delivered to the L3 segment and verified by measuring electromyography (EMG) from the associated hind limb through induced motor responses. The stimulation parameters (1 kHz of pulse-repetition frequency, 0.5 ms of tone-burst duration, and 300 ms of sonication duration) were selected as excitatory protocol from our preliminary trials and based on other published resources. The ultrasound-based spinal cord stimulation was able to elicit the robust twitch of hind limbs with a delay of approximately 23 ms from the stimulation onset, while identical acoustic stimulation targeting muscle did not show any motor responses. The results verified the ability of low-intensity ultrasound to stimulate neurons at the spinal cord. Ultrasonic spinal cord stimulation as a promising application of therapeutic ultrasound may provide new hope for patients with chronic pain, locomotion deficit, and other disorders of the spinal cord.