Effects induced by motor cortex anodal transcranial direct current stimulation on wrist muscles in stroke patients

Transcranial direct current stimulation is used to modify noninvasively cortical brain excitability in humans. It consists in delivering a weak current trough two electrodes: one applied over the motor cortex and a second in supra-orbital position. Recently we showed in healthy subjects that anodal tDCS applied over the motor cortex can be used to modify descending inputs relayed by corticospinal tract on reciprocal inhibition pathways innervating wrist muscles [1]. We found that effects induced by anodal tDCS in wrist extensors and in wrist flexors are opposite: tDCS increases reciprocal inhibition in extensors but decreases reciprocal inhibition in flexors. It appears that modulations induced by anodal tDCS on reciprocal inhibition pathways favor the contraction in wrist extensors. Motor deficit in wrist extensors is usual in hemiplegic patients and the aim of this study is to combine spinal cord MRI and electrophysiological methods to explore effects of tDCS at wrist level in patients with impaired wrist motor control. Anodal tDCS is applied over the non-lesional hemisphere (ipsilateral motor cortex). Its effects at spinal level are explored using electrophysiological techniques based on EMG recordings and percutaneous stimulations of peripheral nerves to elicit reciprocal inhibition at wrist level. MRI study relies on diffusion tensor imaging (DTI) and magnetization transfer (MT) [2]. Preliminary results in hemiplegic patients suggest an increase of reciprocal inhibition contrary to the results found in healthy subjects. They also show a relationship between motor impairment and MRI metrics. It opens new vista for motor rehabilitation of stroke patients.