Study of cerebello-thalamocortical pathway by transcranial magnetic stimulation in Parkinson's disease.

Background: Although functional changes in the activation of the cerebellum in Parkinson's disease (PD) patients have been consistently described, it is still debated whether such altered cerebellar activation is a natural consequence of PD pathophysiology or rather it involves compensatory mechanisms.

Objective/hypothesis: We used different forms of cerebellar transcranial magnetic stimulation to evaluate the hypothesis that altered cerebello-cortical interactions can be observed in PD patients and to evaluate the role of dopaminergic treatment.

Methods: We studied the effects of a single cerebellar magnetic pulse over the excitability of the contralateral primary motor cortex tested with motor-evoked potentials (MEPs) (cerebellar-brain inhibition-CBI) in a group of 16 PD patients with (ON) and without dopaminergic treatment (OFF), and in 16 age-matched healthy controls. Moreover, we also tested the effects of cerebellar continuous theta-burst stimulation (cTBS) on MEP amplitude, short intracortical inhibition (SICI) and short intracortical facilitation (SICF) tested in the contralateral M1 in 13 PD patients in ON and OFF and in 16 age-matched healthy controls.

Results: CBI was evident in controls but not in PD patients, even when tested in both ON and OFF conditions. Similarly, cerebellar cTBS reduced MEP amplitude and SICI in controls but not in PD patients under any condition.

Conclusion(s): These results demonstrate that PD patients have deficient short-latency and long-lasting cerebellar-thalamocortical inhibitory interactions that cannot be promptly restored by standard dopaminergic medication.