Using transcranial direct current stimulation (tDCS) to improve language recovery in post-stroke aphasia

Abstract

Stroke is the fourth leading cause of death and the leading cause adult disability in Hong Kong. Approximately 25,000 strokes occur each year, including both new and recurrent cases. Approximately 3,000 (i.e., 12%) of patients die within 1 year following stroke. Given the increasing average lifespan in Hong Kong, the incidence and prevalence of patients with stroke will only increase in the future. One of the most common neurological deficits among stroke survivors is aphasia, commonly defined as impairment or loss of language functions (e.g., production and comprehension), which has detrimental effects on patient autonomy and health-related quality of life. The current standard treatment for post-stroke chronic aphasia is the conventional speech and language therapy (SLT). However, despite its general effectiveness, treatment effect sizes are only low to moderate. Therefore, there is a pressing need to explore new adjuvant therapies to enhance the treatment efficacy of conventional speech and language therapy. Transcranial direct current stimulation (tDCS) is an emerging noninvasive brain stimulation technique to induce polarity-dependent changes on the excitability of cerebral cortex by applying weak electrical currents to the head via electrodes that are placed on the scalp. Anodal tDCS pushes neural resting membrane potentials closer to the activation threshold and therefore increases neuronal excitability, while cathodal tDCS inhibits cell firing and decreases excitability. Previous research has shown that increasing the excitability of impaired primary motor cortex by anodal tDCS in conjunction with motor training therapy was more effective in restoring the paretic hand motor function compared to motor training therapy alone. We hypothesize that increasing the excitability of impaired language- relevant cortical areas with anodal tDCS in conjunction with intensive speech language therapy (tDCS-SLT) will improve language recovery outcomes in stroke-induced aphasia compared to SLT alone. We will also measure effects of tDCS-SLT on the neural plasticity using magnetic resonance imaging (MRI).