Effects of cerebellar continuous theta burst stimulation on resting tremor in Parkinson's disease

Introduction: Recent studies have suggested that the cerebellum may be involved in the pathophysiology of resting tremor in patients with Parkinson's disease (PD). The aim of the study was to investigate the effects of cerebellar continuous theta burst stimulation (cTBS) on cerebello-thalamo-cortical connectivity and resting tremor in PD patients. Methods: Thirteen PD patients and ten healthy subjects underwent two experimental sessions: (i) 'real' cTBS, delivered over the cerebellar hemisphere and (ii) 'sham' cerebellar cTBS, delivered over the neck muscles. The two sessions were performed at least one week apart. The effects of 'real' and 'sham' cerebellar cTBS were quantified as excitability changes in the contralateral primary motor cortex or as possible changes in resting tremor in the ipsilateral hand. Primary motor cortex excitability was assessed by recording the input/output curve of the motor-evoked potentials from the contralateral first dorsal interosseous muscle. Results: Resting tremor was rated clinically and objectively assessed by means of kinematic techniques. 'Real' cerebellar cTBS, though not 'sham' cerebellar cTBS, reduced the excitability in the contralateral primary motor cortex both in healthy subjects and in patients with PD. There was no significant change in rest tremor severity, as assessed by a clinical examination or kinematic techniques, after either 'real' or 'sham' cerebellar cTBS in patients. Lastly, there was no correlation between individual changes in M1 excitability and clinical or kinematic measures of resting tremor in patients. Conclusion: The cerebello-thalamo-cortical connectivity, as tested by cTBS, is not predominantly involved in the generation of resting tremor in PD.