Transcranial magnetic stimulation and motor overflow: a systematic review in neurological disorders

Motor overflow, a neuromotor phenomenon characterized by involuntary activation of muscles during voluntary movement, reflects impairments in interhemispheric and intracortical inhibition and is commonly observed in conditions such as stroke, cerebral palsy, dystonia, and Parkinson's disease. Transcranial Magnetic Stimulation (TMS) is a non-invasive neuromodulatory technique with potential to modulate the cortical excitability underlying overflow-related dysfunctions. This systematic review aimed to evaluate the efficacy of TMS in reducing motor overflow across neurological populations. Seven randomized controlled trials (RCTs) were included, encompassing participants with stroke, cerebral palsy, Parkinson's disease, and focal hand dystonia. Protocols varied in frequency (1-10 Hz), target area (M1, SMA, PSC, PMC), and design (e.g., crossover, combined protocols with constraint-induced therapy or cerebellar stimulation; Motor overflow was directly assessed in only two studies, one involving Parkinson's disease and one involving dystonia, thereby limiting cross-study comparisons. Therefore, evidence supporting a direct effect of TMS on motor overflow remains preliminary and condition-specific. In dystonia, high-frequency rTMS (10 Hz) targeting the primary somatosensory cortex led to significant reductions in overflow, demonstrated by handwriting kinematics. In Parkinson's disease, although physiological modulation (reduced IHI) was observed after SMA stimulation, mirror movements remained unchanged. In stroke and cerebral palsy populations, overflow was not directly evaluated, though improvements in motor function, spasticity, and cortical excitability were consistently reported. Risk of bias was low in most included studies, although gaps in allocation reporting and standardization of outcome measures were noted. This review highlights the potential of TMS, particularly high-frequency protocols, to modulate motor overflow in focal dystonia. However, the lack of targeted assessment in other neurological conditions suggests a critical need for future trials with standardized protocols and specific outcome measures focused on overflow to clarify the therapeutic role of TMS in rehabilitation.