Klinische Neurophysiologie 2010; 41 - ID114
DOI: 10.1055/s-0030-1250943

Influence of current direction in transcranial magnetic stimulation (TMS) on MEP amplitude and latency – opportunity of I wave specific evaluation and stimulation

N Jung 1, I Delvendahl 1, A Pechmann 1, V Mall 1
  • 1Zentrum für Kinder- und Jugendmedizin, Neuropädiatrie, Freiburg, Deutschland

Introduction: It has been shown from invasive epidural recordings that in transcranial magnetic stimulation (TMS) a current in the brain flowing from a posterior to an anterior (p.a.) direction mainly evokes I1, I2 and I3 waves whereas a current flowing from an anterior to a posterior direction mainly evokes I3 waves. This leads to the hypotheses that in p.a. current direction the motor threshold (MT) is lower, motor evoked potentials (MEPs) are higher and latency is shorter than in a.p. direction. The aim of this study was to re-evaluate the inconsistent results in literature by means of a stimulation technique that enabled us to change the current direction without turning the coil.

Methods: We investigated n=23 healthy subjects (female: n=12; mean age: 26.2±1.9 years). We recorded the MT, MEP amplitude to target 400µV (SI400µV), MT adapted IO curves and maximum stimulator output (MSO) adapted IO curves in a.p. and p.a. current direction (monophasic waveform) with the P-Stim 160 stimulator (Mag and More, Germany).

Results: MT was significantly lower (p=0.003), MEP amplitudes (SI400µV) were significantly higher (p<0.001) and latencies significantly shorter (p<0.001) in p.a. current direction. RmANOVA revealed a significant influence of current direction in MSO adapted IO curves on both, MEP amplitude and latency. We observed a significant influence of current direction on latencies in MT adapted recording of IO-curves but not on MEP amplitudes.

Discusion: Our results consistently demonstrate for the first time a significant influence of current direction on MT, MEP amplitude and latency at fixed stimulus intensities as well as in IO curves due to the hypotheses generated from the specific I-wave pattern. The high consistency of the results when changing the current direction without altering coil position offers the opportunity of I wave specific evaluation and stimulation in TMS.