Transcranial electrical brain stimulation using weak direct current (tDCS) or alternating current (tACS) is being increasingly used in clinical and experimental settings to improve cognitive and motor functions in healthy subjects as well as neurological patients. This review focuses on the therapeutic value of transcranial direct current stimulation for neurorehabilitation and provides an overview of studies addressing motor and non-motor symptoms after stroke, disorders of attention and consciousness as well as Parkinson’s disease.
The past 10 years have seen an increased clinical and experimental focus on noninvasive electrical brain stimulation as an innovative therapeutic approach to support neurorehabilitation. This entails the application of either transcranial direct current stimulation (tDCS), or less commonly, transcranial alternating current stimulation (tACS). Typically, up to 0.8 A/m² is used for up to 40 min per single stimulation session . The electrical current partially penetrates the underlying structures and affects nerve cells, glia and vessels in the stimulated brain area  . Early animal experiments during the 1960s and 1970s on the effects of weak DC stimulation demonstrated an excitement-induced change of neurons lasting several hours after the end of the stimulation  . Therapeutic studies of the 1970s, at that time mainly concerning the treatment of depression, did not yield any success, which in retrospect could be attributed to the stimulation parameters used. In 2 000 key experiments by Nitsche and Paulus on polarity-related excitability changes in the human motor system after transcranial application of tDCS led to a renewed interest in the approach . The authors documented increased cortical excitability measured by the amplitude of motor-evoked potentials in healthy volunteers after anodal stimulation above the motor cortex lasting at least 9 min . Reversing the direction of stimulation (cathodal tDCS) resulted in a decrease in motor-evoked potential. In addition to the concept of pure excitability modulation, a large number of studies demonstrate modulation of neuroplasticity by tDCS in various ways, including basic scientific and mechanistic findings regarding improvement of synaptic transmission strength   , long-term influence on learning processes and behavior  , as well as a therapeutic approach to improve function in neurological and psychiatric disorders associated with altered or disturbed neuroplasticity (overview in ). In particular, simultaneous application of tDCS together with different learning paradigms, such as motor or cognitive training, appears to produce favorable effects in healthy subjects and in various patient groups  .
The following review presents the effects of tDCS on the improvement in the function of some neurological disease patterns which are regularly the focus of neurorehabilitative treatment. This especially includes stroke. In addition, we shall refer to a current database of clinical studies containing a comprehensive list of scientific and clinical studies of tDCS in the treatment of neurological and psychiatric disorders .
Post-stroke Motor Impairment
Stroke is one of the primary causes worldwide of permanent limitations of motor function and speech. Despite intensive rehabilitation efforts, approx. 50% of stroke patients remain limited in their motor and speech capabilities   . Current understanding of the mechanisms of tDCS is largely based on data documented for the human motor system. The reasons for this include the presence of direct and easily objectifiable measurement criteria (for example, motor-evoked potential, fine motor function), as well as anatomical accessibility of brain motor regions for non-invasive stimulation. Therefore, it is not surprising that the clinical syndrome of stroke with the frequent symptom of hemiparesis as a “lesion model of the pyramidal tract” received significant scientific interest with respect to researching the effects of tDCS, as evidenced by the numerous scientific publications since 2005 ([Fig. 1]). In contrast to earlier largely mechanistic studies, in the past 5 years there has been a trend toward studies addressing clinically-oriented therapeutic issues. […]