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 direction on the excitation of the hand-associated motor cortex and the median nerve by magnetic stimulation. Monophasic and biphasic current pulses with the same peak rise time of 80 μs and a maximum rate of current change (dI/dt) were discharged through an eight-shaped coil of the stimulator used (Dantec MagPro). Two current directions with opposite orientation in the coil axis were studied. Findings: (1) for both, cortex and nerve stimulation, biphasic stimuli were more effective and elicited compound muscle action potentials (CMAPs) with lower thresholds and larger amplitudes. (2) Using biphasic pulses the direction of the currents in the first phase of the pulse did not influence the CMAP amplitude. (3) Using monophasic pulses induced currents oriented postero-anteriorly in the motor cortex or orthodromically along the nerve axis elicited larger CMAPs than currents in the opposite orientation. (4) Pulse configuration did not influence the CMAP-latencies and by this the stimulation site (cortex, nerve). Conclusion: Monophasic stimuli are useful to investigate excitation effects which are dependent on the current direction. The application of biphasic stimuli with their stronger excitation effects might be advantageous when patients with high cortical thresholds or deep lying nerves shall be investigated.