Abstract
Paired transcranial magnetic stimulation (TMS) has been used to investigate the pathophysiology of Parkinson’s disease (PD). While previous studies reported conflicting results on short-interval intracortical inhibition (SICI, related to GABA A receptor inhibition) in PD, a detailed study found decreased SICI and increased short-interval intracortical facilitation (SICF) in the off medication state. SICI increased with dopaminergic medications. Normalization of SICF with dopaminergic medications correlated with the degree of motor improvement. The interaction between long-interval intracortical inhibition (LICI, related to GABA B receptor inhibition) and SICI, likely mediated by presynaptic inhibition, is impaired in PD and did not improve with medications. The excitability of the cerebellothalamocortical pathway is also reduced in PD. Sensorimotor integration measured with short (SAI) and long (LAI) latency afferent inhibition are decreased in PD and normalized with subthalamic nucleus deep brain stimulation (STN DBS). Cortical plasticity measured with paired-associative stimulation is decreased in PD, increased with medications in non-dyskinetic patients and is improved by STN DBS. The results of theta burst stimulation and other plasticity studies in PD may depend on the disease stage and medication status. Many studies tested non-invasive brain stimulation as a treatment for PD. A double-blinded, sham-controlled trial of anodal transcranial direct current stimulation in PD showed no effect on gait and Unified Parkinson’s Disease Rating Scale, although bradykinesia measured by a timed test was slightly improved. A meta-analysis found that high frequency (>1 Hz) repetitive TMS (rTMS) had beneficial effect while low frequency (1 Hz or lower) rTMS did not change PD motor signs. Cortical low frequency (∼1 Hz) rTMS or cerebellar rTMS are potential treatments for levodopa-induced dyskinesias. However, the published rTMS studies in PD are small and the protocols used are highly variable. The efficacy of rTMS in PD needs to be examined in a large, sham-controlled study. In summary, non-invasive brain stimulation techniques provide is useful methods to investigate the pathophysiology and effects of treatment in PD. Further studies are needed to determine whether it is a useful treatment for PD.
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