P 31. Effects of transcutaneous spinal Direct Current Stimulation (tsDCS) on the conduction along the human corticospinal system

Abstract

Objective There are to date few studies investigating the effects of spinal direct current stimulation (tsDCS) on spinal ascending tract and reflexes. Whether tsDCS interferes with conduction along pyramidal tract at a spinal level is unknown. Methods Eleven healthy subjects underwent anodal and cathodal tsDCS of the thoracic spinal cord, at the T10–T12 level (2.5 mA, 20 min). The two conditions were tested in random order and at least 1 week elapsed between sessions. The subjects were blinded about the polarity of conditioning stimulation. MEPs elicited by transcranial magnetic stimulation (TMS) were recorded from abductor hallucis (AH) and abductor digiti minimi (ADM) before tsDCS and after the current offset. We also evaluated AH and ADM F-waves and soleus H-reflex. Results Baseline neurophysiological variables (MEPs threshold, area and latency; H-reflex threshold, latency, amplitude and Hmax/Mmax ratio; F-wave mean latency, minimal latency, amplitude and chronodispersion) from upper and lower limb were comparable in anodal and cathodal sessions. After tsDCS, whereas cathodal tsDCS left MEP threshold unchanged ( p > 0.1), anodal tsDCS increased significantly MEP threshold (two-way ANOVA as “time” and “stimulation” as factors: F (2,20) = 5.01, p = 0.017). Similarly, lower limb MEPs area significantly decreased after anodal tsDCS, increasing however after cathodal stimulation (two-way ANOVA as “time” and “stimulation” as factors: F (2,20) = 10.2, p = 0.0009). Both anodal and cathodal tsDCS failed to affect lower limb MEP latency, upper limb MEPs threshold, area and latency, H-reflex and F-wave. Discussion tsDCS modulates the function of human corticospinal pathways. The effects occur selectively in muscles controlled by corticospinal axons running below the tsDCS electrode over the lower thoracic spinal cord without concomitant effect on both upper limb muscles and motorneuronal excitability. Overall these findings suggest a direct effect of tsDCS on the corticospinal system below the stimulating electrode: among the possible explanations, anodal tsDCS could block impulse conduction along corticospinal axons. A further possibility is that changes of corticospinal function in the brain after tsDCS could be indirectly elicited by its effect on tonic resting activity of ascending spinal pathways-an important input to motor areas–ultimately influencing the excitability of corticospinal neurons at cortical level. Conclusions tsDCS over the dorsal spinal cord can modulate the corticospinal input to sacral motoneurons. The possibility of modulating corticospinal input to motorneurons has potential clinical implications.