Effects of subthalamic nucleus stimulation on motor cortex excitability in Parkinson’s disease

Background Transcranial magnetic stimulation (TMS) studies have found abnormalities in several excitatory and inhibitory circuits in the motor cortex in PD. These include motor evoked potential (MEP) recruitment curve, silent period duration (SP), short-interval intracortical inhibition (SICI), intracortical facilitation (ICF), and long-interval intracortical inhibition (LICI). Methods The authors studied the effects of subthalamic nucleus (STN) deep brain stimulation (DBS) on these circuits in 12 patients with PD treated with STN DBS. Data from nine patients who completed the study were analyzed. Patients remained on their usual medications. The stimulators were set at the optimal parameters (ON), half the optimal amplitude (HALF), and switched off (OFF) in random order. Results The Unified PD Rating Scale motor scores were significantly lower in the ON compared to the HALF and OFF conditions. Resting SICI, studied with paired-pulse TMS at interstimulus interval of 2 ms, was reduced in the OFF and HALF conditions compared to normal subjects. STN stimulation restored SICI to normal levels. STN stimulation had no effect on motor threshold, MEP recruitment curve, SP, active SICI, ICF, and LICI. Conclusions Although restoration of short-interval intracortical inhibition by STN stimulation is similar to the effects of dopaminergic drugs, it has no effect on silent period duration and long-interval intracortical inhibition, which are also influenced by dopaminergic drugs. A previous study found that internal globus pallidus (GPi) stimulation reduced SP but did not change SICI. Thus, GPi and STN stimulation may affect different circuits in the motor cortex.