High-Frequency Stimulation Does Not Improve Comfort of Transcutaneous Spinal Cord Stimulation

Abstract

Spinal cord neuromodulation has gained much attention for demonstrating improved motor recovery in people with spinal cord injury, motivating the development of clinically applicable technologies. Among them, transcutaneous spinal cord stimulation (tSCS) is attractive because of its non-invasive profile. Many tSCS studies employ a high-frequency (10 kHz) carrier, which has been reported to reduce stimulation discomfort. However, these claims have come under scrutiny in recent years. The purpose of this study was to determine whether high-frequency tSCS is more comfortable at therapeutic amplitudes, which evoke posterior root-muscle (PRM) reflexes. In 16 neurologically intact participants, tSCS was delivered using a 1-ms long monophasic pulse with and without a high-frequency carrier. Stimulation amplitude and pulse duration were varied and PRM reflexes were recorded from the soleus, gastrocnemius, and tibialis anterior muscles. Participants rated their discomfort during stimulation from 0-10 at PRM reflex threshold. At PRM reflex threshold, high-frequency stimulation (0.87 ± 0.2) was equally comfortable as conventional stimulation (1.03 ± 0.18) but required approximately double the charge to evoke the PRM reflex (conventional: 32.4 ± 9.2 μC; high-frequency: 62.5 ± 11.1 μC). Strength-duration curves for high-frequency stimulation had a rheobase that was 4.8X greater and a chronaxie that was 5.7X narrower than the conventional monophasic pulse, indicating that high-frequency stimulation was less efficient in recruiting neural activity in spinal roots. High-frequency tSCS is equally as comfortable as conventional stimulation at amplitudes required to stimulate spinal dorsal roots.

Reflex modulation and functional improvements following spinal cord stimulation for sensory restoration after lower-limb amputation

“…Participant 3 had painful diabetic neuropathy prior to their amputation. In our experience, Hreflexes are difficult, if not impossible, to evoke in people with a lower-limb amputation, regardless of whether a neuropathy is present or not [49]. SCS targets the dorsal spinal roots, exciting large diameter afferent fibers [15,50].…”

“…Furthermore, RDD of the PRM reflexes was present in all participants, including during walking, demonstrating that stimulation was not directly activating motor neurons in the ventral spinal cord or roots. We have previously shown that PRM reflexes can be evoked using transcutaneous spinal cord stimulation (tSCS) in people with transtibial amputations with and without diabetic neuropathy [49]. tSCS is a noninvasive stimulation technique that uses electrodes placed on or adjacent to the spine [18,52].…”