The Neurophysiological Effects of Cervical Transcutaneous Spinal Cord Stimulation With and Without a High Frequency...

Transcutaneous spinal cord stimulation (tSCS) is a promising avenue in spinal cord injury (SCI) rehabilitation; however, high currents are required to excite afferents in the spinal cord roots, which patients may not tolerate. Modulating tSCS pulses with a kHz carrier frequency (kHz-tSCS) may be used to reduce discomfort; however, the way that kHz-tSCS interacts with neural networks, compared to unmodulated pulses (conv-tSCS), is largely unknown.

Ten able-bodied participants received conv-tSCS, kHz-tSCS, and sham interventions for 20 min over the C7/T1 vertebrae. Charge delivery of both waveforms was measured. Posterior root reflexes (PRRs) and motor-evoked potentials (MEPs) were recorded from the Flexor Carpi Radialis (FCR), Extensor Carpi Radialis Longus (ECRL), Flexor Carpi Ulnaris (FCU), and Brachioradialis (BR). PRR and MEP peak-peak amplitudes were measured at baseline, 0-, 15-, and 30-min post-intervention.

The charge required to activate posterior roots with kHz-tSCS was 3.8 times higher than with conv-tSCS (p < 0.001). Differences in PRR amplitude were found in the FCR between conv-tSCS and kHz-tSCS at 0- and 15-min post-intervention (p < 0.028). PRR inhibition was found in the FCR between baseline and 30-min post-intervention with conv-tSCS and the sham intervention (p < 0.037). No change in PRR amplitudes was found for kHz-tSCS. No other muscle showed any differences in PRR responses between intervention groups. Neither intervention caused any effect in MEP responses across time or between intervention groups.

kHz-tSCS was a less efficient waveform for stimulation. Differences in effects on spinal excitability were found to be inconclusive, and conv-tSCS and kHz-tSCS had no effect on corticospinal excitability. Significant PRR inhibition in the FCR was found with this experimental setup even when no stimulation was applied, suggesting a natural reduction in spinal excitability caused by participants laying supine for an extended period. Future research should consider how participant positioning could affect neural excitability.