The Effect of The Five-Session Dual-tDCS On Lower-Limb Performance in Sub-Acute Stroke

Stroke is a leading cause of long term disability. The recovery of motor function after stroke is often incomplete, despite classical rehabilitation techniques. In 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 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 non-lesioned hemisphere. Abnormally high interhemispheric inhibition (IHI) drive from intact to lesioned hemisphere has been reported. The neural plasticity begins in early stages after stroke. Prevent the imbalance IHI and increase the excitability of the lesioned hemisphere in the early phase would be a 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 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. There are few evidences 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, in 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 recent 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 stoke, however, less studies have 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. A study showed an improvement of walking speed immediately after a single session of dual-tDCS alone in sub-acute stroke patients .

Based on the previous study, 1-2 mA current intensity of tDCS is usually used for modulating brain activity. The proper current density delivered is between 0.029-0.008 mA/cm 2. The higher current density, the longer lasting, stronger, and deeper cortical neuron stimulation. Therefore, 2 mA current intensity of tDCS was applied in stroke patients for lower limb performance improvement. Following homologous brain regions, the motor area of lower extremities is in depth and previous studies demonstrated that current intensity 2 mA of tDCS could pass through the lower extremities area of the brain.

Moreover, Tahtis et al, 2013 found that using 2 mA of dual-tDCS could improve walking speed immediately after a single session in sub-acute stroke and Klomjai et al, 2018 found that a single session of dual-tDCS 2 mA with physical therapy in sub-acute stroke immediately improved sit to stand performance greater than physical therapy alone. The mechanisms of actions was hypothesized that when stimulation continuously has effect on neuronal plasticity changes. Therefore, they suggested that further study shall implement more sessions of dual-tDCS combined with physical therapy to improve lower limb performance and determine the long lasting-after effect.

The effect of 5-consecutive sessions dual-tDCS during physical therapy on upper limb performance showed significant improvement of upper limb motor functions and maintained at least 1 week. In this study, long-lasting effect was demonstrated at least one week to three months. There are evidences that 5 consecutive days of uni-hemisphere tDCS appeared to improve lower limb motor functions without serious adverse effects in patients with stroke.It is still unclear for the effect of multiple sessions of dual-tDCS on gait performance and muscle strength as well as the long lasting after-effect of multiple sessions of dual-tDCS.

Therefore, the aim of the present study is to investigate the efficacy of 5 consecutive sessions of dual-tDCS combined with conventional physical therapy on the lower limb functions after stroke in sub-acute. Clinical outcomes for lower limb performance evaluations will consist of muscle strength assessed by hand-held dynamometer, the Time up and go test for lower limb functional performance, Five times sit to stand test for dynamic balance and muscle strength, and the Zebris Force distribution measurement (FDM) for gait analysis.