title: Changes in cortical excitability with thalamic deep brain stimulation.
abstract: 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.
  
  
  CONCLUSIONS
  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.
authors.affiliation: Division of Neurology, Krembil Neuroscience Centre, Toronto Western Research Institute, University Health Network, University of Toronto, Toronto, Ontario, Canada.
authors.forename: G F
authors.initials: GF
authors.lastname: Molnar
authors.forename: A
authors.initials: A
authors.lastname: Sailer
authors.forename: C A
authors.initials: CA
authors.lastname: Gunraj
authors.forename: D I
authors.initials: DI
authors.lastname: Cunic
authors.forename: A E
authors.initials: AE
authors.lastname: Lang
authors.forename: A M
authors.initials: AM
authors.lastname: Lozano
authors.forename: E
authors.initials: E
authors.lastname: Moro
authors.forename: R
authors.initials: R
authors.lastname: Chen
mesh_terms: D000328:Adult; D000368:Aged; D001479:Basal Ganglia / Q000503:physiopathology; D002531:Cerebellum / Q000503:physiopathology; D046690:Deep Brain Stimulation / Q000379:methods*; D004576:Electromyography; D020329:Essential Tremor / Q000503:physiopathology / Q000628:therapy*; D019054:Evoked Potentials, Motor / Q000502:physiology; D005260:Female; D006225:Hand / Q000294:innervation / Q000502:physiology; D006801:Humans; D008297:Male; D008875:Middle Aged; D009044:Motor Cortex / Q000502:physiology / Q000503:physiopathology*; D009119:Muscle Contraction / Q000502:physiology; D018482:Muscle, Skeletal / Q000294:innervation / Q000502:physiology; D009433:Neural Inhibition / Q000502:physiology; D009434:Neural Pathways / Q000502:physiology / Q000503:physiopathology*; D013788:Thalamus / Q000502:physiology / Q000503:physiopathology* / Q000601:surgery; D050781:Transcranial Magnetic Stimulation; D016896:Treatment Outcome; D020651:Ventral Thalamic Nuclei / Q000502:physiology / Q000503:physiopathology / Q000601:surgery