Variable Frequency Stimulation of Subthalamic Nucleus for Freezing of Gait in Parkinson's Disease

This is the first study on the effect of the variable frequency stimulation (VFS) on Freezing of gait (FOG) in Parkinson's disease (PD) patients with STN DBS. FOG has responded poorly to High frequency stimulation (HFS) but can be alleviated by relatively VFS. This study would have significant clinical implications in the management of Freezing of gait (FOG) in these PD patients with subthalamic nucleus (STN) DBS. The investigators hypothesize that VFS could have better effect on FOG than HFS and low frequency stimulation (LFS).