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From the Divisions of Neurology (G.F. Molnar, C.A. Gunraj, and Drs. Sailer, Lang, Moro, and Chen) and Neurosurgery (Dr. Lozano), Krembil Neuroscience Centre and Toronto Western Research Institute, University Health Network, University of Toronto, Toronto, Ontario, Canada.
Address correspondence and reprint requests to Dr. Robert Chen, Division of Neurology, Room 7MC411, Toronto Western Hospital, 399 Bathurst Street, Toronto, Ontario M5T 1S8, Canada; e-mail: robert.chen{at}uhn.on.ca
Background: Deep brain stimulation (DBS) is an effective treatment for several movement disorders. However, its mechanism of action is largely unknown. Both lesioning and DBS of the ventralis intermedius (VIM) nucleus of thalamus improve essential tremor. Although DBS was initially thought to inhibit the target neurons, recent studies suggest that DBS activates neurons.
Objective: To test the hypothesis that thalamic DBS activates the target area in patients with essential tremor.
Methods: Cortical excitability was assessed in seven unmedicated patients with essential tremor using unilateral stimulators implanted in the VIM of the dominant hemisphere and in 11 healthy controls using transcranial magnetic stimulation (TMS). Patients were studied during optimal DBS (ON condition), half the optimal frequency (HALF), and with DBS off (OFF) in random order. Tremor was assessed after a change in DBS setting. Electromyography was recorded from the dominant hand, and TMS was applied over the contralateral motor cortex using single and paired pulses to elicit motor evoked potentials (MEPs). MEP recruitment was determined using stimulus intensities from 100% to 150% of motor threshold.
Results: Tremor scores were significantly improved with DBS ON. Analysis of variance showed a significant interaction between condition (ON, HALF, OFF, Normal) and stimulus intensity on MEP amplitude. During DBS ON MEP amplitudes were significantly higher compared with controls at high but not at low TMS intensities.
Conclusion: Because the ventralis intermedius (VIM) projects directly to the motor cortex, the high motor evoked potential amplitude with deep brain stimulation ON suggests that VIM DBS activates rather than inhibits the target area.
Supported by the Canadian Institutes of Health Research (G.F.M. and R.C.), the Canada Foundation for Innovation, the Ontario Innovation Trust, and the University Health Network Krembil Family Chair in Neurology.
Dr. Lang received research grant support from Medtronic Inc. in excess of $10,000. Dr. Lozano received consulting fees and research grant support from Medtronic Inc. in excess of $10,000. Dr. Moro received honorarium for speaking from Medtronic Inc.
Received November 3, 2004. Accepted in final form March 1, 2005.
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