Dual-hemisphere Transcranial Direct Current Stimulation on Lower Limb Motor Functions After Stroke

Stroke is a leading cause of adult motor disability. The recovery of motor function after stroke is often incomplete, despite classical rehabilitation techniques. In the beginning of the 21th century, transcranial direct current stimulation (tDCS) was introduced as a non-invasive tool to reversibly modulate brain excitability in humans in which a device sends constant low direct current (DC) delivered to the area of interest through the electrodes. These findings have opened the way to the therapeutic use of the brain stimulation for stroke. Based on the polarity-specific effects, anodal tDCS increases cortical excitability and cathodal tDCS decreases cortical excitability. To change the cortical excitability, tDCS differs from other brain stimulation techniques such as transcranial magnetic stimulation (TMS) in that it does not cause action potentials in cortical neurons, but rather induces shifts in neuronal resting membrane potential . This is considered to induce a lesser or no risk of a seizure. Given its advantages such as non-invasiveness, painlessness, safety, and possible after-effects, tDCS rapidly become a therapeutic adjuvant in neurorehabilitation especially in stroke patients with motor deficits . A number of studies in healthy subjects and stroke patients have reported that tDCS-induced excitability changes are related to increase in the performance of motor tasks and motor skills learning, mainly upper limb functions Recenty, dual-hemisphere tDCS in which, excites one hemisphere using anodal stimulation and inhibits the other by cathodal stimulation has been described in healthy subjects to greater enhance hand motor learning compared to uni-hemisphere tDCS. The corresponding tDCS-induced changes were reported in imaging study to involve interhemispheric interactions . An increase in the excitability of unaffected hemisphere and an abnormally high interhemispheric inhibition (IHI) drive from intact to lesioned hemisphere have been widely reported after unilateral stroke. Dual-tDCS has been more recent used in rehabilitation aiming to reduce the inhibition exerted by the unaffected hemisphere on the affected hemisphere and restore the normal balance of the IHI. Dual-tDCS combined with training or simultaneous occupational/physical therapy has been reported to improve motor skill learning and functions of the paretic upper limb in chronic stroke patients .

Lower-limb functions are commonly disordered after stoke, however, less study has focused the effect of tDCS on lower limb functions. A single session of anodal tDCS over the lower limb M1 has reported to acutely enhance the effect of motor practice of the paretic ankle, force production of the paretic knee extensors, and postural stability in chronic stroke patients. However, little is known about effects of dual-tDCS on lower limb in stroke patients. Only one recent study showed an improvement of walking speed immediately after a single session of dual-tDCS alone in sub-acute stroke patients .