Cerebral Circulation and Corticomotor Excitability After tDCS in Chronic Stroke Survivors

Background: In the past few decades, anodal transcranial direct current stimulation (tDCS) has been explored as a promising therapeutic adjuvant in stroke rehabilitation. However, observed high intra individual variability to tDCS, as identified by changes in corticomotor excitability measured with transcranial magnetic stimulation (TMS), has limited the clinical application of tDCS. Neurons are closely related to surrounding blood vessels such that changes in neural activity changes cerebral blood flow. Transcranial Doppler (TCD), a widely accepted clinical diagnostic tool, measures changes in cerebral blood flow velocity (CBFv) and is used to study neurovascular coupling during various motor and cognitive training. Hence, to understand if TCD can be a useful tool to explore changes in tDCS, we asked the following questions: Does CBFv change after tDCS in chronic stroke survivors? Does CBFv changes correlate with changes in corticomotor excitability? Can CBFv changes differentiate between Responders and Non-responders to tDCS? Methods: TMS testing was done on the lower limb motor cortex before and after 15 minutes of 1 mA anodal tDCS in 20 chronic stroke survivors to identify Responders and Non-responders. During TCD sessions, CBFv was recorded at baseline for 5 minutes and 15 minutes post anodal/sham tDCS. Results: Analysis of cortical excitability showed that only 60% of the study participants responded to tDCS. Anodal tDCS did not cause changes in CBFv at post 5 and 10 minutes and it did not correlate with changes in cortical excitability. Also, there was no significant difference in CBFv changes between responders and non-responders post tDCS. Conclusion: CBFv changes do not directly represent changes in cortical excitability after anodal tDCS. However, it should be noted that low sample size limits the interpretation of results in this study.