Epidural spinal cord stimulation (SCS) is currently regarded as a breakthrough procedure for enabling movement after spinal cord injury (SCI), yet one of its original applications was for spinal spasticity. An emergent method that activates similar target neural structures non-invasively is transcutaneous SCS. Its clinical value for spasticity control would depend on inducing carry-over effects, because the surface-electrode-based approach cannot be applied chronically. We evaluated single-session effects of transcutaneous lumbar SCS in 12 individuals with SCI by a test-battery approach, before, immediately after and 2 h after intervention. Stimulation was applied for 30 min at 50 Hz with an intensity sub-threshold for eliciting reflexes in lower extremity muscles. The tests included evaluations of stretch-induced spasticity (Modified Ashworth Scale [MAS] sum score, pendulum test, electromyography-based evaluation of tonic stretch reflexes), clonus, cutaneous-input-evoked spasms, and the timed 10 m walk test. Across participants, the MAS sum score, clonus, and spasms were significantly reduced immediately after SCS, and all spasticity measures were improved 2 h post-intervention, with large effect sizes and including clinically meaningful improvements. The effect on walking speed varied across individuals. We further conducted a single-case multi-session study over 6 weeks to explore the applicability of transcutaneous SCS as a home-based therapy. Self-application of the intervention was successful; weekly evaluations suggested progressively improving therapeutic effects during the active period and carry-over effects for 7 days. Our results suggest that transcutaneous SCS can be a viable non-pharmacological option for managing spasticity, likely working through enhancing pre- and post-synaptic spinal inhibitory mechanisms, and may additionally serve to identify responders to treatments with epidural SCS.
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