Transcranial direct current stimulation in stroke rehabilitation: a review of recent advancements

Abstract

Transcranial direct current stimulation (tDCS) is a promising technique to treat a wide range of neurological conditions including stroke. The pathological processes following stroke may provide an exemplary system to investigate how tDCS promotes neuronal plasticity and functional recovery. Changes in synaptic function after stroke, such as reduced excitability, formation of aberrant connections, and deregulated plastic modifications, have been postulated to impede recovery from stroke. However, if tDCS could counteract these negative changes by influencing the system's neurophysiology, it would contribute to the formation of functionally meaningful connections and the maintenance of existing pathways. This paper is aimed at providing a review of underlying mechanisms of tDCS and its application to stroke. In addition, to maximize the effectiveness of tDCS in stroke rehabilitation, future research needs to determine the optimal stimulation protocols and parameters. We discuss how stimulation parameters could be optimized based on electrophysiological activity. In particular, we propose that cortical synchrony may represent a biomarker of tDCS efficacy to indicate communication between affected areas. Understanding the mechanisms by which tDCS affects the neural substrate after stroke and finding ways to optimize tDCS for each patient are key to effective rehabilitation approaches.

Figure 1

Physiological effects of tDCS. (a) Illustration of the typical placement of the anode (red square) and cathode (blue square) during stimulation of the primary motor cortex. The direction of stimulation causes differential effects on neuronal activation and plasticity. (b) Illustration of anodal (red) and cathodal (blue) transcranial direct current stimulation on spike activity in animals (modified from [39]). Anodal stimulation increased subsequent spike activity by lowering the membrane potential, whereas cathodal stimulation reduced subsequent spike activity in the stimulated area by increasing the membrane potential. (c) DCS promotes LTP in motor cortical slices. The sample of fEPSPs showing a 2-hour time course after DCS (vertical gray line) (from [40]).

Figure 2

Interhemispheric competition following a stroke. The model suggests that the contralesional (unaffected) motor region exerts an excessive inhibitory influence on the ipsilesional (affected) motor cortex which might limitpoststroke motor recovery. The model provides a hypothetical framework for developing therapeutic strategies. (a) Upregulation of neural excitability of the intact regions of the ipsilesional (affected) motor cortex by anodal tDCS. (b) Downregulation of excitability of the contralesional (unaffected) motor cortex by cathodal tDCS.

Figure 3

Using brain activity as a “biomarker”. We hypothesize that choosing tDCS parameters to maximize synchrony between cortical areas could lead to improved communication between the affected areas and thus result in more effective stroke rehabilitation.