Role of Transcranial direct current stimulation in Neurological Disorders

Abstract

Transcranial Direct Current Stimulation (tDCS) has been utilized for the modulation of cortical excitability ( Fregni et al., 2005; Dieckhöfer et al., 2006; Nitsche et al., 2007) in various neurological diseases, such as stroke, chronic neuropathic pain, Alzheimer disease, and Parkinson’s disease (Hansen et al., 2010; Antal and Paulus 2011;DaSilva et al., 2012&Knotkova et al., 2012). tDCS consists of applying direct current over the scalp using electrodes that are enclosed in perforated sponge pockets soaked with a saline solution or a rubber electrode with conductive gel (DaSilva et al., 2011). It effects depend on the following factors: the size, polarity and position of the electrodes, the applied current intensity, the density and duration of stimulation, and the properties of the tissue in the stimulated area. This technique can induce long-lasting and polarity-specific changes in the excitability of the motor cortex in humans (Nitsche and Paulus, 2001& Lang et al., 2004). Depending on the current flow, it can increase or decrease neuronal excitability. The mechanisms are electrode-dependent and involve either (1) membrane depolarization (increased spontaneous firing and excitability of the cortical neurons for anodal stimulation) or (2) membrane hyperpolarization (decreased neuronal firing and excitability for cathodal stimulation. In the most commonly used procedure ,one electrode is placed over a specific site while the other reference electrode is placed over another location to complete the circuit of current flow. The electrode positioning is critical for determining the direction and spatial distribution of the current flow and, ultimately, the effectiveness of the treatment (Utz et al., 2010). The exact pathways involved in the effects of tDCS are not fully understood (Wagner et al., 2007; Utz et al., 2010; Stagg and Nitsche, 2011). tDCS is new compatable and safe non-invasive brain stimulation technique for treatment and rehabilitation of serious neurological disorders: Activation of the left frontal cortex through the application by tDCS suggests that tDCS can modulate linguistic performances in both healthy and neurological patients, with results typically demonstrating that language and motor skills processing can be improved by applying A-tDCS to the left hemisphere However(Gross 2007) , recent work challenges such a simple interpretation, in which C-tDCS (2mA; 10-min) applied to Broca’s and Motor areas resulted in an improved ability to name pictures in eight patients with chronic nonfluent aphasia; no effects were noted following A-tDCS or sham (placebo-like) tDCS . whereas other studies that found effects following A-tDCS, coupled the stimulation with a relevant task to engage the brain area.4,6,10 Therefore, while the main goal of the present studies determine that A-tDCS would improve naming accuracy in aphasic patients and motor rehabilitation when applied to the left frontal cortex(Miranda et al.,2006).