The mechanism of repetitive transcranial magnetic stimulation for central post-stroke pain

We have previously reported the efficacy of 5 Hz repetitive transcranial magnetic stimulation (rTMS) of the primary motor cortex (M1) for the relief of intractable neuropathic pain. The objective of this study was to assess the changes of cortical excitability induced by rTMS and relations between rTMS efficacy and the condition of subcortical fiber connection in the patients with central post-stroke pain (CPSP), which might give an insight to the mechanism of pain relief with rTMS. In 16 patients with CPSP in a hand and five healthy controls, the cortical excitability and visual analogue scale (VAS) were evaluated before and after 5 HzrTMS of M1 corresponding to the painful hand. Resting motor threshold, amplitude of motor evoked potential, cortical silent period, short interval intracortical inhibition and intracortical facilitation (ICF)were evaluated as parameters of cortical excitability with single-or pairedpulse TMS method. Fiber tracking of the corticospinal tract and the thalamocortical tract was investigated in 13 patients with CPSP, who underwent 5 Hz-rTMS of M1. The patients who showed 30% pain reduction in VAS after rTMS were classified as "good responders", the others were classified as "poor responders". Significant reduction of ICF in the good responders was observed compared with that in the controls before rTMS (p=0.010) and ICF in the good responders significantly increased after rTMS (p=0.035). There were no significant differences and changes in the other parameters. The good responders had higher delineation ratios of the corticospinal tract and the thalamocortical tract than the poor responders (p=0.02, p=0.005, respectively). Our findings suggested that the change of ICF in M1 and the subcortical network around the sensorimotor cortex play a role in pain relief by rTMS of M1.