Gamma Frequency Transcranial Magnetic Stimulation in Dystonia

Objectives: The purpose of this protocol is to improve understanding of the pathophysiology of dystonia by performing an electrophysiological study using transcranial magnetic stimulation (TMS). The study may reveal new information about the role of gamma frequency oscillations in cortical facilitation in dystonia patients versus healthy volunteer subjects. The findings of this study may also help to determine if abnormal gamma frequency facilitation might become a potential diagnostic tool for dystonia. <TAB> Study population: 1. We intend to study 13 healthy volunteers and 13 patients with a diagnosis of focal hand dystonia. Thirteen subjects for each group are needed based on the power analysis, with an additional three added to the requested accrual to account for drop-outs, yielding a maximum of 16 subjects per group. Up to 15 focal hand dystonia patients and 15 healthy volunteers may be involved in the pilot experiment. The pilot study will be stopped when a trend in the data emerges, so the total number of subjects who participate in the pilot study may end up being less than the maximum requested accrual of 15. Subjects in the pilot experiment are eligible to later participate in the main experiment. Design: The subject will be seated with EMG surface electrodes over the First Dorsal Interosseous (FDI) muscle of each hand monitoring muscle activity. The TMS coil will be placed on the surface of the head and the region of motor cortex corresponding to the respective hand muscle control. The baseline motor evoked potential (MEP) for the FDI muscle will be determined by TMS stimulation and EMG monitoring. A pilot experiment will be performed first. Conditioning stimuli, consisting of repetitive subthreshold TMS stimulation with three pulses at one of eight frequencies, will be delivered. A test stimulus, consisting of a single TMS pulse, will follow the conditioning stimuli at the same interval as the conditioning train, but at a suprathreshold intensity in order to determine the MEP amplitude. After every 5 subjects the pilot data will be analyzed to check for a trend in the frequencies that lead to facilitation. Once a trend emerges, the pilot study will be stopped. The four frequencies with the greatest trend toward facilitation will then be selected and used in the main experiment. The main experiment will be performed the same way as the pilot experiment, but will use only the four frequencies found in the pilot experiment. Conditioning stimuli, consisting of repetitive subthreshold TMS stimulation with three pulses at one of the selected four frequencies, will be delivered. A test stimulus, consisting of a single TMS pulse, will follow the conditioning stimuli at the same interval as the conditioning train, but at a suprathreshold intensity in order to determine the MEP amplitude. Outcome measures: We will compare the MEP amplitude following TMS conditioning with the baseline MEP amplitude prior to conditioning. The effect on MEP will then be compared between healthy volunteers and dystonia patients.