Novel Mechanisms Underlying Inhibitory and Facilitatory Transcranial Magnetic Stimulation Abnormalities in Parkinsonʹs Disease

Background and Aims tor-evoked potential (MEP) induced by transcranial magnetic stimulation (TMS), its recruitment and the conditioning effects of weak stimuli in Parkinsonʹs disease (PD) have shown contradictory results. The aim of this study is to definitively establish the influences of PD on the TMS-evoked MEP. s estigated resting and active motor thresholds, resting and active recruitment curves, and short interval intracortical inhibition (ICI) and facilitation (ICF) in 39 PD patients and 40 age-matched healthy controls. The Unified Parkinsonʹs Disease Rating Scale (UPDRS) motor score was used as a clinical measure. s o single pulses were slightly, but significantly, larger in the PD patients at rest, but increased much less than controls with voluntary muscle activation. PD patients also showed clearly and consistently less ICI and ICF by the conditioning pulse. Both ICI and ICF correlated with MEP threshold in healthy subjects, but not in PD patients. None of the TMS measures correlated with the UPDRS. sions tudy strongly supports the view that PD-related MEPs reflect a disturbed signal-noise ratio of pyramidal neuron responses. Such disturbance may be due to a complex combination of altered presynaptic and surround inhibition that results in unbalanced excitatory/inhibitory input at the brain motor cortex level.