title: The effect of daily prefrontal repetitive transcranial magnetic stimulation over several weeks on resting motor threshold.
abstract: BACKGROUND
  The resting motor threshold (rMT) is an important factor in the selection of treatment intensity for patients receiving repetitive transcranial magnetic stimulation (rTMS). In many clinical studies to date, due to concerns about potential drift, the rMT has been routinely re-measured weekly or every fifth session.
  
  
  OBJECTIVE
  Our aim is to investigate whether ongoing treatment with rTMS affects the rMT, the degree of change, and whether frequent remeasurement is needed.
  
  
  METHODS
  Clinical data were drawn from 50 medication free patients receiving treatment for major depression with rTMS in a large U.S. NIH-sponsored multisite study. Four measurements of rMT were obtained including before and after the double blind phase, followed by weekly measurements during the open phase. Active treatment consisted of 75 four second trains of 10Hz stimulation applied over 37.5 minutes with the coil over the left DLPFC at 120% rMT.
  
  
  RESULTS
  For the group as a whole, there was no significant change in the rMT during a minimum of 2 weeks of treatment with prefrontal rTMS (p=0.911, one way ANOVA). The average within-subject coefficient of variation was 6.58%. On average the last rMT was 2.45% less than the baseline rMT (range 32.3% increase, 40.6% decrease).
  
  
  CONCLUSIONS
  Daily left prefrontal rTMS over several weeks as delivered in this trial does not cause systematic changes in rMT. While most subjects had <10% variance in rMT over time, 5 subjects had changes of approximately 20% from baseline, raising dosing and safety issues if undetected. We recommend that clinical trials of rTMS have periodic retesting of rMT, especially if the dose is at or near the edge of the TMS safety tables.
authors.affiliation: University of Washington, 325 Ninth Avenue, Seattle WA 98104-2499, USA. pzark@u.washington.edu
authors.forename: Paul
authors.initials: P
authors.lastname: Zarkowski
authors.forename: Rita
authors.initials: R
authors.lastname: Navarro
authors.forename: Martina
authors.initials: M
authors.lastname: Pavlicova
authors.forename: Mark S
authors.initials: MS
authors.lastname: George
authors.forename: David
authors.initials: D
authors.lastname: Avery
mesh_terms: D000328:Adult; D003865:Major Depressive Disorder / Q000503:physiopathology / Q000628:therapy*; D019054:Evoked Potentials, Motor; D005260:Female; D006801:Humans; D008297:Male; D008875:Middle Aged; D017397:Prefrontal Cortex / Q000502:physiology*; D050781:Transcranial Magnetic Stimulation / Q000379:methods*; D016896:Treatment Outcome