Does navigated transcranial magnetic stimulation (TMS) decrease the variability of MEP-amplitudes?

Introduction: One major attribute of transcranial magnetic stimulation (TMS) is the variability of the MEP-amplitudes. Navigated TMS allows the investigator to retrieve the stimulation site with an accuracy of 2.5 millimetres (Schönfeldt-Lecuona et al., 2005). The purpose of this study was to investigate the variability and reproducibility of MEP-amplitudes with and without navigated TMS at one point in time as well as the reproducibility between different points in time.

Methods: We investigated n=8 healthy subjects (w: n=4, m: n=4; age: 22 to 25; mean age 23.8±1.24) at three different points in time (interval between investigation was at least 24 hours) with and without an optically tracked frameless navigational device (Brainview, Fraunhofer Institut (IPA), Stuttgart, Germany). Dose-recruitment curves (-5% to +30% stimulator output of resting motor threshold (rMT)), MEP-amplitudes of 1 mV (SI_1mV) and of 120% rMT (SI_120MT) were recorded. Parameters of statistical spread were the standard deviation (SD), range and the coefficient of variation (CV).

Results: SD, range and CV of the MEP-amplitudes did not show a significant difference between navigated and not-navigated TMS in dose-recruitment curves, SI_1mV and SI_120MT. This was the case for variability at one point in time as well as for reproducibility at different points in time. In dose-recruitment curves, the CV showed a trend towards a decrease in the navigated sessions.

Discussion: Our results do not support the hypothesis that spatial accuracy reduces the variability and heightened the reproducibility of MEP-amplitudes. Other factors such as spinal desynchronisation might be responsible for this effect.