The potential dual role of transcallosal inhibition in post-stroke motor recovery

Up to now, the mechanism of motor impairment and recovery after stroke has been thought to be based on the interhemispheric competition model. According to this model, which assumes that suppressing the excitability of contralesional hemisphere will enhance recovery by reducing transcallosal inhibition (TCI) of the stroke hemisphere, many clinical trials used non-invasive brain stimulation to improve motor function. Despite some positive findings, meta-analysis shows an important source of variability in the results, questioning whether the interhemispheric competition model would be exhaustive enough to explain the positive results or whether other mechanisms could explain the motor effects of inhibitory stimulation in the contralesional hemisphere. The goal of this study was to review the relationship between increased TCI and motor impairment after stroke.A systematic review of clinical studies investigating TCI through transcranial magnetic stimulation (TMS) in stroke patients and the relationship of this metric with motor recovery was then performed. After a literary search in PubMed eleven articles were included. The potential role of several covariates was examined and discussed.Overall, the importance of TCI as a putative mechanism for stimulation of the contralesional hemisphere seems to depend on the baseline motor function. In other words, from evidence coming mostly from chronic patients, modulation of abnormal TCI seems to be useful for patients with good motor function and less important in patients with poor motor function. TCI seems to be negatively correlated with mirror movements of the paretic hand. It can be inferred that suppressing the activity of the contralesional hemisphere could be beneficial for patients with good residual motor function and strong TCI, but not for those with poor motor function and weak TCI. Baseline motor function and measure of TCI should be taken into account for stratification of patients in clinical trials and for the design of customized treatment.