Spinal Direct Current Stimulation Modulates Short Intracortical Inhibition

Objective: Transcutaneous spinal direct current stimulation (tsDCS) is a new and safe technique for modulating spinal cord excitability. We assessed changes in intracortical excitability following tsDCS by evaluating changes in cortical silent period (cSP), paired-pulse short intracortical inhibition (SICI), and intracortical facilitation (ICF). Materials and Methods: Healthy subjects were studied before (T0) and at different intervals (T1 and T2) after anodal, cathodal, and sham tsDCS (20', 2.0 mA) applied over the thoracic spinal cord (T10-T12). We assessed changes in cSP, SICI (interstimulus interval, ISI = 3 ms) and ICF (ISI = 10 ms). Motor-evoked potentials (MEPs) were recorded from first digital interosseus (FDI) and tibialis anterior (TA) muscles. Results: Cathodal tsDCS increased MEP amplitudes at interstimulus interval of 3 ms, while anodal one elicited opposite effects (FDI: p = 0.0023; TA: p = 0.0004); conversely, tsDCS left MEP amplitudes unchanged at ISI of 10 ms (FDI: p = 0.39; TA: p = 0.45). No significant change in cSP duration was found from upper limb (p = 0.81) and lower limb (p = 0.33). Conclusion: tsDCS modulates inhibitory GABA(A) ergic drive, as assessed by SICI, without interfering with cSP and ICF. The possibility to interfere with cortical processing makes tsDCS a useful approach to modulate spinal drive through nonspinal mechanisms. tsDCS could also represent an early rehabilitation strategy in patients with acute brain lesions, when other noninvasive brain stimulation (NIBS) tools are not indicated due to safety concerns, as well as in the treatment of spinal diseases or pain syndromes.