title: Navigated transcranial magnetic stimulation does not decrease the variability of motor-evoked potentials.
abstract: BACKGROUND
  One major attribute of transcranial magnetic stimulation (TMS) is the variability of motor-evoked potential (MEP) amplitudes, to which variations of coil positioning may contribute. Navigated TMS allows the investigator to retrieve a stimulation site with an accuracy of 2.5 mm and to retain coil position with low spatial divergence during stimulation.
  
  
  OBJECTIVE
  The purpose of this study was to investigate whether increased spatial constancy of the coil using a navigational system decreases the variability of MEP amplitudes and increases their reproducibility between different points in time of investigation.
  
  
  METHODS
  We investigated eight healthy subjects (mean age 23.8 +/- 1.2 years, range 22-25, four women, four men) at three different points in time with and without an optically tracked frameless navigational device, respectively. Input-output curves, motor threshold, and MEP amplitudes were recorded. We calculated the coefficient of variation as statistical parameter of variability. Reproducibility between different sessions was assessed via the MEP amplitude.
  
  
  RESULTS
  The coefficient of variance of MEP amplitudes did not show a distinct difference between navigated and non-navigated TMS in input-output curves. MEP amplitudes, indicating reproducibility, did not significantly differ between sessions with and without navigated TMS, either.
  
  
  CONCLUSIONS
  Our results do not support the hypothesis that increased spatial constancy using a navigational system improves variability and reproducibility of MEP amplitudes. Variability of MEPs might mainly be due to not influenceable neurophysiologic factors such as undulant cortical excitability and spinal desynchronization.
authors.affiliation: Department of Neuropaediatrics and Muscular Disorders, University Medical Center Freiburg, Germany.
authors.forename: Nikolai H
authors.initials: NH
authors.lastname: Jung
authors.forename: Igor
authors.initials: I
authors.lastname: Delvendahl
authors.forename: Nicola G
authors.initials: NG
authors.lastname: Kuhnke
authors.forename: Dieter
authors.initials: D
authors.lastname: Hauschke
authors.forename: Sabine
authors.initials: S
authors.lastname: Stolle
authors.forename: Volker
authors.initials: V
authors.lastname: Mall
mesh_terms: D000328:Adult; D004576:Electromyography; D019054:Evoked Potentials, Motor / Q000502:physiology*; D005260:Female; D006801:Humans; D008297:Male; D050781:Transcranial Magnetic Stimulation / Q000295:instrumentation / Q000379:methods*; D055815:Young Adult