Differential induction of Parieto-motor plasticity in writer's cramp and cervical dystonia

Abstract

Objectives: To investigate the plastic effects of parieto-motor (PAR-MOT) cortico-cortical paired associative paired stimulation (cc-PAS) in patients with two forms of focal dystonia, writer's cramp and cervical dystonia, compared to healthy volunteers (HVs).

Methods: We used cc-PAS to induce associative plasticity using repeated time-locked paired transcranial magnetic stimulation (TMS) pulses over the parietal and motor cortices in 16 patients with writer's cramp (WC), 13 patients with cervical dystonia (CD), and 23 healthy volunteers. We measured parieto-motor corticocortical connectivity using posterior parietal cortex (PPC) to primary motor cortex (M1) facilitation and input-output curves (IOC) of the motor-evoked potential (MEP) before and after PAR-MOT cc-PAS. The PAR-MOT cc-PAS consisted of 100 pairs of TMS pulses every 5 s, with the conditioning pulse applied to the left angular gyrus in the intraparietal sulcus and the test pulse applied to the M1 hotspot of the first dorsal interosseous muscle.

Results: The cc-PAS increased the area under the IOC by increasing its maximum level in patients with WC but not in patients with CD or healthy volunteers. The cc-PAS had no significant effect on other IOC parameters. There were no significant differences in PPC to M1 facilitation changes after PAR-MOT cc-PAS among all groups.

Conclusions: This study suggests that PAR-MOT cc-PAS abnormally increases M1 excitability in patients with WC but not in those with CD. Additionally, this increased plastic response in patients with WC does not appear to be directly linked to PPC to M1 corticocortical connectivity.

Keywords: Cervical dystonia; Parieto-motor associative plasticity; Parieto-motor connectivity; writer's cramp.

Fig. 1.

The experimental protocol. The input-output curve (IOC) and parieto-motor facilitation (PMF) were measured at the baseline and the measurements were repeated at 5–15 min, 15–25 min and 45–60 min following the parieto-motor cortico-cortical paired associative stimulation (PAR-MOT cc-PAS). Interstimulus interval (ISI) for PAR-MOT cc-PAS was chosen the ISI that showed the largest parieto-motor facilitation among 4,6 and 8 ms.

Fig. 2.

The experimental protocol. The input-output curve (IOC) and parieto-motor facilitation (PMF) were measured at the baseline and the measurements were repeated at 5–15 min, 15–25 min and 45–60 min following the parieto-motor cortico-cortical paired associative stimulation (PAR-MOT cc-PAS). Interstimulus interval (ISI) for PAR-MOT cc-PAS was chosen the ISI that showed the largest parieto-motor facilitation among 4,6 and 8 ms.

Fig. 3.

The input-output curves before and after corticocortical paired-associative stimulation (cc-PAS) at different time points. Normalized motor-evoked potentials are shown based on the increased stimulation intensities according to S50 at different time points after cc-PAS in the healthy volunteers (A), patients with writer’s cramp (B), and patients with cervical dystonia (C). T0, T1, T2 and T3 represent the periods before cc-PAS and those 5–10 min, 15–25 min, and 45–60 min after cc-PAS, respectively. Asterisks indicate significant differences among groups.

Fig. 4.

Parieto-motor facilitation (PMF) expressed as ratio of conditioning motor evoked potential (MEP) and control MEP before and after corticocortical paired-associative stimulation at each time point in the healthy volunteers (circle), patients with writer’s cramp (square), and patients with cervical dystonia (triangle).