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

Objective To study the effects of subthalamic nucleus (STN) stimulation on motor cortex excitability in Parkinson’s disease (PD). Methods Fifteen patients were evaluated under off-medication, OFF- and ON-STN conditions, using the UPDRS. We studied the effects of STN stimulation on motor cortex excitability under three conditions (OFF, ON with chronic therapeutic parameters, ON HighV with a voltage set 10% under permanent side effects threshold), using transcranial magnetic stimulation. We measured the central motor conduction time (CMCT), rest and active motor threshold (MT), F-wave persistence and amplitude, silent period (SP), intracortical inhibition and facilitation at 3, 5 and 15 ms interstimulus intervals (ISI). Results The UPDRS motor score decreased from 44.4 ± 14.0 under OFF, to 15.2 ± 8.0 under ON-STN condition. The CMCT and MT did not change across STN conditions. The F-wave parameters were within normal range under STN stimulation. STN stimulation lengthened the SP and induced facilitation at 15 ms ISI compared to the OFF-STN condition. Conclusion Therapeutic STN stimulation modulates cortical excitability in PD. High voltage STN stimulation could also act on spinal excitability. Significance At the motor cortical level, STN stimulation may have a direct effect on intracortical interneurons, modifying the balance between excitation and inhibition.