Using a high-frequency carrier does not improve comfort of transcutaneous spinal cord stimulation

Abstract

Objective. 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 using a high-frequency carrier for tSCS is more comfortable at therapeutic amplitudes, which evoke posterior root-muscle (PRM) reflexes. Approach. 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. Main Results. At PRM reflex threshold, the addition of a 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 carrier: 62.5 ± 11.1 µC). Strength-duration curves for tSCS with a high-frequency carrier had a rheobase that was 4.8X greater and a chronaxie that was 5.7X narrower than the conventional monophasic pulse, indicating that the addition of a high-frequency carrier makes stimulation was less efficient in recruiting neural activity in spinal roots. Significance. Using a high-frequency carrier for tSCS is equally as comfortable and less efficient as conventional stimulation at amplitudes required to stimulate spinal dorsal roots.

Enhanced selectivity of transcutaneous spinal cord stimulation by multielectrode configuration

“…There was a significant amount of post-activation depression of the evoked response by the pairedpulse paradigm at each optimal electrode configuration: one-sided 1-sample Wilcoxon signed rank test in RF (W = 489, p < 0.001), VL (W = 407, p = 0.004), ST (W = 527, p < 0.001), TA (W = 528, p < 0.001), MG (W = 528, p < 0.001), and SL (W = 528, p < 0.001). A significant amount of suppression was also observed in the conventional tSCS electrode (data not shown) and has been extensively reported by several groups (Dalrymple et al, 2023;Hofstoetter et al, 2021aHofstoetter et al, , 2019Oh et al, 2022). The amount of suppression for all multielectrode configurations and muscles is shown in supplementary figure 3.…”

Transcutaneous Spinal Cord Stimulation to Reduce Phantom Limb Pain in People with a Transtibial Amputation

“…Prior to tSCS neuromodulation, PRM reflex thresholds were higher in our participants with transtibial amputations (59.5 ± 6.1 µC) than in neurologically intact individuals (35.6 ± 11.6 µC; n = 12) (42). The amplitudes of the PRM reflexes at 2.5 times threshold were 0.11 ± 0.05 mV, which is over 65 times smaller than in intact individuals (> 8 mV).…”

“…The latencies of the PRM reflexes decreased significantly across the 5 days (p < 0.001; Day 1: 20.2 ± 1.1 ms; Day 5: 19.6 ± 1.1 ms; Figure 6a). The latencies of the PRM reflexes at both time points were within normal ranges (42).…”

“…We employed a high-frequency carrier of 10 kHz in our stimulation waveform because it has been proposed to be more comfortable than conventional 1 biphasic stimulation trains (45,47,92,93). However, we recently reported that the addition of a high-frequency carrier does not make tSCS more comfortable, and actually excites spinal reflex pathways less efficiently (42). Many prior studies testing tSCS to restore motor function and reduce spasticity used a conventional waveform (50,92,94).…”

“…Transcutaneous spinal cord stimulation (tSCS) is a non-invasive neuromodulation method that targets the dorsal spinal roots, similar to eSCS (42,43). To date, tSCS has been used to improve motor recovery after spinal cord injury (44)(45)(46)(47) as well as to reduce spasticity (48)(49)(50).…”