The Effect of The Ten-Session Dual-tDCS On Lower-Limb Performance in Sub- Acute and Chronic Stroke

Stroke is a leading cause of long-term disability. The recovery of motor function after stroke is often incomplete, despite classical rehabilitation techniques. At the beginning of the 21st 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. Transcranial direct current stimulation (tDCS) has been used in neurorehabilitation to benefit ischemic stroke patients at a different stage of stroke especially during acute, sub-acute and chronic phase with positive and safety reports.

After stroke, the excitability of the lesioned hemisphere is decreased and seen like overactive of the excitability of the non-lesioned hemisphere. Abnormally high interhemispheric inhibition (IHI) drive from intact to lesioned hemisphere has been reported. The neural plasticity begins in the early stages after stroke. Prevent the imbalance IHI and increase the excitability of the lesioned hemisphere in the early phase would be beneficial for stroke rehabilitation.

Based on the polarity-specific effects, anodal tDCS increases cortical excitability and cathodal tDCS decreases cortical excitability. Transcranial direct current stimulation (tDCS) can be applied in two distinct montages: monocephalic and bi-hemispheric/dual-tDCS (applying two electrodes over both cerebral hemispheres at the same time). To induce post-stroke motor recovery, two different monocephalic montages are typically used: i) to restore excitability in the lesioned hemisphere: anode over the lesioned hemisphere and the cathode as the reference electrode placed over the contra-orbital area ii) to down-regulate the excitability of the non-lesioned hemisphere and rebalance IHI: cathode over the non-lesioned hemisphere and the anode as the reference electrode. Dual-tDCS can be also applied, permitting simultaneous coupling of excitatory and inhibitory effects on both cortices. Few evidences are showing that tDCS (monocephalic and bi-hemispheric/dual-tDCS) could improve lower limb performance at immediate and at least 3 months. However, there is still unclear effect on gait performance and muscle strength.

Recently, dual-hemisphere tDCS which excites one hemisphere using anodal stimulation and inhibits the other by cathodal stimulation has been described in healthy volunteers to greater enhance hand motor learning compared to uni-hemispheric tDCS. The corresponding tDCS-induced changes were reported in imaging study to involve interhemispheric interactions. Dual tDCS has been more recently used in rehabilitation aiming to reduce the inhibition exerted by the non-lesioned hemisphere on the lesioned 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 disabling after stroke, however, few studies have focused on the effect of tDCS on lower limb functions. A single session of anodal tDCS over the lower limb M1 has been 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. A study showed an improvement in walking speed immediately after a single session of dual-tDCS alone in sub-acute stroke patients.