Non-invasive brain stimulation (NIBS) and motor recovery after stroke

Recovery of motor function after stroke occurs largely on the basis of a sustained capacity of the adult brain for plastic changes. This brain plasticity has been validated by functional imaging and electrophysiological studies. Various concepts of how to enhance beneficial plasticity and in turn improve functional recovery are emerging based on the concept of functional interhemispheric balance between the two motor cortices. Besides conventional rehabilitation interventions and the most recent neuropharmacological approaches, non-invasive brain stimulation (NIBS) has recently been proposed as an add-on method to promote motor function recovery after stroke. Several methods can be used based either on transcranial magnetic stimulation (repetitive mode: rTMS, TBS) via a coil, or small electric current via larges electrodes placed on the scalp, (transcranial direct current stimulation tDCS). Depending on the different electrophysiological parameters of stimulation used, NIBS can induce a transient modulation of the excitability of the stimulated motor cortex (facilitation or inhibition) via a probable LTP-LTD-like mechanism. Several small studies have shown feasible and positive treatment effects for most of these strategies and their potential clinical relevance to help restoring the disruption of interhemispheric imbalance after stroke. Results of these studies are encouraging but many questions remain unsolved: what are the optimal stimulation parameters? What is the best NIBS intervention? Which cortex, injured or intact, should be stimulated? What is the best window of intervention? Is there a special subgroup of stroke patients who could strongly benefit from these interventions? Finally is it possible to boost NIBS treatment effect by motor training of the paretic hand or by additional neuropharmacological interventions? There is clearly a need for large-scale, controlled, multicenter trials to answer these questions before proposing their routine use in the management of stroke patients.