Navigated transcranial magnetic stimulation does not decrease the variability of motor-evoked potentials
Received 6 July 2009; received in revised form 7 October 2009; accepted 7 October 2009. published online 02 November 2009.
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.
aDepartment of Neuropaediatrics and Muscular Disorders, Department of Paediatrics and Adolescent Medicine, University Medical Center Freiburg, Germany
bDepartment of Medical Biometry and Medical Informatics, University Medical Center Freiburg, Germany
Correspondence: Prof. Dr. med. Volker Mall, Division of Neuropaediatrics and Muscular Disorders, Department of Paediatrics and Adolescent Medicine, University Medical Center Freiburg, Germany.
The study was funded by the German Research Council (DFG, MA 3306/1-1).
ABSTRACT