Repetitive magnetic stimulation and motor evoked potentials.

The aim of the study was to evaluate the effect of varying stimulus rates of repetitive transcranial magnetic stimulation (RTMS) on the motor evoked potentials (MEPs) recorded from the right abductor pollicis brevis muscle (APB). Thirteen normals were included. Stimuli were applied to the cortex and to the median nerve at the wrist. The cortical stimuli were applied without and with facilitation. Stimulus intensity was 1.2 times the motor threshold (TmAPB) for cortical stimulation and supramaximal for peripheral stimulation. Stimulus rates were 1, 2, 3, 5, 10 and 20 Hz. Nine pulses were applied in each stimulus series. At stimulus rates between 1 and 3 Hz no amplitude changes were observed throughout the stimulation. At 5 Hz stimulation inhibition of some of the MEPs was observed followed by MEPs with increased amplitude. At 10 Hz stimulation some MEPs were totally inhibited, interrupted by an MEP with increased amplitude, resembling clonic contraction. At 20 Hz the inhibition of the MEPs decreased (P < 0.05) compared to 10 Hz stimulation. Facilitation decreased the inhibition at 5 and 10 Hz stimulation (P < 0.01). On peripheral stimulation a decrement was observed with stimulus rates of 10 and 20 Hz. The study shows that RTMS exerts a complex influence on the MEPs depending upon stimulus rates. The pronounced inhibition and excitation of the MEPs at 5 and 10 Hz stimulation possibly reflects inhibition and excitation in the cortico-spinal neurons.