Heterogeneous Recruitment Curve Responses to Anodal Transcranial Direct Current Stimulation in Children With Unilateral Spastic Cerebral Palsy

Background: Children with unilateral spastic cerebral palsy (USCP) exhibit altered corticospinal organization that may influence responsiveness to treatment. Transcranial direct current stimulation (tDCS) may modulate corticospinal excitability, though responses vary widely. Recruitment curves derived from transcranial magnetic stimulation (TMS) can quantify excitability changes. Objective: To evaluate the feasibility, safety, and preliminary effects of combined TMS and anodal tDCS on recruitment curve-derived corticospinal excitability in children with USCP and explore predictors of responsiveness. Methods: In a single-group pre/post study, 20 children with USCP completed TMS-evoked recruitment curve testing before and after 20 minutes of 2mA anodal tDCS. Primary outcomes included changes in S50 (half-maximal motor-evoked potential [MEP] intensity) and slope at the curve inflection point. Secondary outcomes included MEP amplitude-based and model-derived recruitment curve metrics. Safety and tolerability were assessed using a standardized symptom screen. Results: The protocol was well-tolerated: no serious adverse events occurred, 80% reported no symptoms, and the most common symptom was mild scalp itching. No significant group-level pre/post differences were observed in this preliminary study. Responses were heterogeneous: 11/20 children met the responder definition (ΔS50 < 0). Exploratory analyses identified no consistent clinical or neurophysiologic predictors, although a potential responder pattern was observed in participants with bilateral corticospinal tract patterns. Conclusions: Although uniform group-level excitability changes were not observed following a single session of 2mA anodal tDCS, recruitment curve metrics were feasible, well-tolerated, and sensitive to interindividual differences. These findings support the use of recruitment curves as feasible tools to characterize variability and inform individualized neuromodulation strategies. Trial registration number : NCT03402854