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

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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 mo...

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. 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 action were hypothesized that when stimulation continuously affects 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. A previous study reported that the 10-consecutive sessions of tDCS were an effective treatment strategy in reducing the risk of falls and improving lower limb functions after a stroke. However, gait ability and lower limb muscle strength were not included in the outcome measures. In this study, the long-lasting effect was demonstrated at least one week to three months. There are evidences that 10 sessions for uni-hemisphere tDCS appeared to improve lower limb motor functions without serious adverse effects in patients with stroke. It is still unclear 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 present study aims to investigate the efficacy of ten 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.

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