Are there differences in cortical excitability between akinetic-rigid and tremor-dominant subtypes of Parkinson's disease?

OBJECTIVE To assess by transcranial magnetic stimulation (TMS) the excitability of various cortical circuits in akinetic-rigid and tremor-dominant subtypes of Parkinson's disease (PD). METHODS The study included 92 patients with PD according to UK Brain Bank criteria, with akinetic-rigid (n = 64) or tremor-dominant (n = 28) subtype. Cortical excitability study, including resting and active motor thresholds (rMT and aMT), input-output curve of motor evoked potentials, contralateral and ipsilateral silent periods (cSP and iSP), short and long-interval intracortical inhibition (SICI and LICI), and intracortical facilitation (ICF) were measured. The results obtained were compared to a control group of 30 age- and sex-matched healthy subjects. RESULTS The patients in the tremor group had significantly lower rMT and aMT compared to controls and akinetic-rigid patients and significantly shorter iSP duration compared to akinetic-rigid patients, while iSP latency tended to be longer in akinetic-rigid patients compared to controls. There were no significant differences between the two PD subgroups regarding other cortical excitability parameters, including paired-pulse TMS parameters. CONCLUSIONS Only subtle differences of cortical excitability were found between patients with akinetic-rigid vs. tremor-dominant subtype of PD. SIGNIFICANCE The clinical heterogeneity of PD patients probably has an impact on cortical excitability measures, far beyond the akinetic-rigid versus tremor-dominant profile.