Objective: To characterize intracortical facilitation and inhibition of the primary motor cortex (M1) in Parkinson’s disease (PD) using transcranial magnetic stimulation (TMS).

Background: Since introduction of paired-pulse TMS, excitability of M1 in PD has attracted much interest. Results of previous studies are, however, not congruent: some reported normal and others reduced short-interval intracortical inhibition (SICI). We hypothesized that exaggerated intracortical facilitation masked normal inhibition (Ni et al. 2013), and that a triple-pulse method (Shirota et al. 2010) is able to reveal the masked inhibition in PD.

Methods: Eleven PD patients, who had not been exposed to dopaminergic medications, were enrolled. Results were compared with those from ten age-matched healthy volunteers. TMS-elicited motor evoked potential (MEP) was measured as an index of M1 excitability. SICI, intracortical facilitation (ICF), and short-interval intracortical facilitation (SICF) were tested using the paired-pulse TMS paradigm. Conditioning intensity for the SICI and ICF was 90% of each participant’s active motor threshold (AMT), while one for the second pulse of SICF was 140% AMT, which was on average around the resting motor threshold (RMT). Furthermore, a triple-pulse protocol, SICI+SICF, was investigated using a higher conditioning intensity for SICI (120% AMT) based on our previous study. This protocol examined SICF in the presence of SICI, thereby allowing us to test net inhibitory influence on a specific component of MEP-generating mechanism known as I3 wave.

Results: SICI tended to be decreased in PD. SICF was enhanced around its second peak, compatible with Ni et al. (2013). Results of SICI+SICF were comparable between the two groups, suggesting normal inhibition of I3 wave in PD.

Conclusions: We confirmed the SICF enhancement in drug naïve PD patients. We propose that SICI of I3 waves is not affected in PD, in line with Hanajima et al. 2011. The triple-pulse method can be useful to reveal apparently masked SICI.

References: 1. Z Ni et al. Increased motor cortical facilitation and decreased inhibition in Parkinson disease. Neurology 2013;80:1746. 2. Shirota et al. Influence of short-interval intracortical inhibition on short-interval intracortical facilitation in human primary motor cortex. J Neurophysiol 2010;104:1382.

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MDS Abstracts - https://www.mdsabstracts.org/abstract/further-insights-into-excitatory-inhibitory-balance-of-the-primary-motor-cortex-in-parkinsons-disease-a-triple-pulse-stimulation-study/