Influence of pulse configuration and direction of coil current on excitatory effects of magnetic motor cortex and nerve stimulation

This paper describes the influence of pulse configuration and current direc tion on the excitation of the hand-associated motor cortex and the median n erve by magnetic stimulation. Monophasic and biphasic current pulses with t he same peak rise time of 80 mu s and a maximum rate of current change (dI/ dt) were discharged through an eight-shaped coil of the stimulator used (Da ntec MagPro). Two current directions with opposite orientation in the coil axis were studied. Findings: (1) for both, cortex and nerve stimulation, bi phasic stimuli were more effective and elicited compound muscle action pote ntials (CMAPs) with lower thresholds and larger amplitudes. (2) Using bipha sic pulses the direction of the currents in the first phase of the pulse di d not influence the CMAP amplitude. (3) Using monophasic pulses induced cur rents oriented postero-anteriorly in the motor cortex or orthodromically al ong the: nerve axis elicited larger CMAPs than currents in the opposite ori entation. (4) Pulse configuration did not influence the CMAP-latencies and by this the stimulation site (cortex, nerve). Conclusion: Monophasic stimul i are useful to investigate excitation effects which are dependent on the c urrent direction. The application of biphasic stimuli with their stronger e xcitation effects might be advantageous when patients with high cortical th resholds or deep lying nerves shall be investigated. (C) 2000 Elsevier Scie nce Ireland Ltd. All rights reserved.