Epidural Cortical Stimulation as a Treatment for Poststroke Aphasia: A Systematic Review of the Literature and Underlying Neurophysiological Mechanisms

Background. Nearly 15 million people suffer from stroke every year worldwide, with about 20% of the survivors retaining chronic aphasic symptoms. Spontaneous recovery is limited to 3 to 6 months. Cortical stimulation techniques have been proposed to enhance the recovery process. Objective. The goal of this study was to evaluate the benefit of epidural cortical stimulation for the treatment of poststroke aphasia, based on a systematic review of the literature. Methods. An extensive PubMed search was performed for English language articles published from 1990 to 2014 with the keywords (cortical OR epidural) AND stimulation AND stroke AND (aphasia OR language OR speech). The criteria analyzed included the type of study, epidemiology of patients, stroke, aphasia, stimulation protocol, concurrent rehabilitation therapies, language evaluations, results observed, and follow-up. Results. Seven cases were reported to date (3 case reports, 1 randomized controlled trial). All patients experienced nonfluent aphasia following an ischemic stroke. All four studies reported encouraging effects of the stimulation with improved lexical access and fluency for all patients. The effects were specific, independent of the motor recovery or of the pain reported by the patients, and they were linked to the stimulation parameters. Conclusions. Due to the small number of existing cases in the literature, the strength of the evidence is still low. Two main hypotheses of neurobiological mechanisms have been explored: either using continuous stimulation to modify cortical perilesional inhibition or using intermittent stimulation during the speech and language therapy sessions to explore synaptic plasticity and long-term potentiation or depression. To establish the role of epidural stimulation and the relevant stimulation protocols and parameters, large randomized controlled trials are mandatory. We suggest avenues of investigation.