Neural correlates of balance performance in chronic stroke: A repetitive transcranial magnetic stimulation study

Purpose: The primary motor cortex (M1) and the cerebellum are both crucial for balance control and can be viable targets for non-invasive brain stimulation aiming to improve balance performance. However, the optimal stimulation target for balance improvement in individuals post stroke has yet to be determined. The purpose of this study was to determine the role of two neural loci (M1 and cerebellum) in modulating balance performance in individuals with chronic stroke using repetitive transcranial magnetic stimulation (rTMS). Methods: Sixteen individuals with chronic stroke (81% male; mean age = 58.1 ± 10 years; chronicity mean = 63.1 ± 41.6 months) participated in the study. A single session of 5Hz rTMS was applied to M1 and the cerebellum in a cross-over randomized manner with a washout period of approximately seven days. Before and after each rTMS session, single pulse transcranial magnetic stimulation (TMS) was used to measure the motor evoked potential (MEP) amplitude of the affected lower limb muscles to quantify corticospinal excitability. The Limits of Stability (LOS) time, LOS overall sway angle, and the modified Clinical Test of Sensory Interaction on Balance (mCTSIB) sway index were used to assess anticipatory and reactive balance control respectively, before and after each rTMS session. Repeated measures ANOVA and Spearman’s correlation analysis were performed. Results: M1 rTMS improved the LOS overall sway angle to a greater extent compared to cerebellar rTMS (p < .001), whereas LOS time was not affected by stimulation target. Sway iv index on the mCTSIB was mediated by stimulation target, proprioception, vision, and time (p = .006). There was no significant effect of rTMS application on mCTSIB sway index under the eyes open conditions. Under eyes closed conditions, M1 rTMS resulted in a significantly improved sway index on the firm surface condition (p = .002), whereas cerebellar rTMS improved sway index on the foam surface condition (p = .001). There was a weak non- significant correlation between change in corticospinal excitability after rTMS and change in balance performance. Conclusion: Both M1 and the cerebellum are viable targets for rTMS and have unique roles in mediating balance performance in individuals with stroke. This information could assist with the development of targeted non-invasive brain stimulation to enhance balance recovery after stroke.