P155 Fluctuation of motor evoked potentials and resting motor threshold over time

Abstract

Introduction MEP amplitude and RMT are measures of corticospinal excitability (CSE) that are often used as baseline characteristics and outcome measures for various TMS protocols. However, marked variation in the amplitude of MEPs to consecutive stimuli reduces the reliability of these estimates. Variation in CSE over time may cause values of RMT and MEP amplitude to deviate from their initial estimates. Objectives Our aim was to assess the factors contributing to fluctuations of CSE, namely dispersion of the corticospinal volley and coil positioning. We tested the hypothesis that (i) trial-to-trial variation of MEP area is less than that of amplitude and (ii) significant changes in average MEP amplitude and RMT occur due to displacements of the stimulating coil. Materials & methods To assess fluctuations of CSE, single-pulse TMS was applied to the motor area of the dominant hemisphere and EMG recorded from the contralateral relaxed FDI muscle. RMT was defined as stimulation intensity required to evoke a response of 0.2 mV and was determined by threshold tracking. Subsequently, RMT was tracked continuously. MEPs were recorded using stimulation intensity of 120% RMT. For each protocol, ten 20-min recordings were obtained from healthy subjects. Pulses were applied every 4.6 ± 0.5 s. A navigation system was used to record coil displacement during the experiments. Results Trial-to-trial variation of MEP area was as great as that of amplitude (median normalised mean consecutive difference 0.48 (range 0.5) for both). Whilst RMT and average MEP amplitude were found to fluctuate substantially over the course of the experiment, this was not attributable to coil displacement. Conclusions Our data suggest that coil displacement and response analysis (MEP area vs amplitude) do not contribute substantially to the variability of CSE parameters. Variation in MEP responses and RMT over time has implications for TMS protocols which use estimates of the average MEP amplitude and RMT. Studies such as paired pulse and rTMS should not use a single estimate to define parameters for a long duration. Further investigations into the sources of variation will be useful to improve its control.