Electrical stimulation of the motor cortex enhances treatment outcome in post-stroke aphasia

Transcranial direct current stimulation has shown promise to improve recovery in patients with post-stroke aphasia, but previous studies have only assessed stimulation effects on impairment parameters, and evidence for long-term maintenance of transcranial direct current stimulation effects from randomized, controlled trials is lacking. Moreover, due to the variability of lesions and functional language network reorganization after stroke, recent studies have used advanced functional imaging or current modelling to determine optimal stimulation sites in individual patients. However, such approaches are expensive, time consuming and may not be feasible outside of specialized research centres, which complicates incorporation of transcranial direct current stimulation in day-to-day clinical practice. Stimulation of an ancillary system that is functionally connected to the residual language network, namely the primary motor system, would be more easily applicable, but effectiveness of such an approach has not been explored systematically. We conducted a randomized, parallel group, sham-controlled, double-blind clinical trial and 26 patients with chronic aphasia received a highly intensive naming therapy over 2 weeks (8 days, 2 x 1.5 h/day). Concurrently, anodal-transcranial direct current stimulation was administered to the left primary motor cortex twice daily at the beginning of each training session. Naming ability for trained items (n = 60 pictures that could not be named during repeated baseline assessments), transfer to untrained items (n = 284 pictures) and generalization to everyday communication were assessed immediately post-intervention and 6 months later. Naming ability for trained items was significantly improved immediately after the end of the intervention in both the anodal (Cohen's d = 3.67) and sham-transcranial direct current stimulation groups (d = 2.10), with a trend for larger gains in the anodal-transcranial direct current stimulation group (d = 0.71). Treatment effects for trained items were significantly better maintained in the anodal-transcranial direct current stimulation group 6 months later (d = 1.19). Transfer to untrained items was significantly larger in the anodal-transcranial direct current stimulation group after the training (d = 1.49) and during the 6 month follow-up assessment (d = 3.12). Transfer effects were only maintained in the anodal-transcranial direct current stimulation group. Functional communication was significantly more improved in the anodal-transcranial direct current stimulation group at both time points compared to patients treated with sham-transcranial direct current stimulation (d = 0.75-0.99). Our results provide the first evidence from a randomized, controlled trial that transcranial direct current stimulation can improve both function and activity-related outcomes in chronic aphasia, with medium to large effect sizes, and that these effects are maintained over extended periods of time. These effects were achieved with an easy-to-implement and thus clinically feasible motor-cortex montage that may represent a promising 'backdoor' approach to improve language recovery after stroke.