REPETITIVE MAGNETIC STIMULATION AND MOTOR EVOKED-POTENTIALS

The aim of the study was to evaluate the effect of varying stimulus ra tes of repetitive transcranial magnetic stimulation (RTMS) on the moto r evoked potentials (MEPs) recorded from the right abductor pollicis b revis muscle (APE). Thirteen normals were included. Stimuli were appli ed to the cortex and to the median nerve at the wrist. The cortical st imuli were applied without and with facilitation. Stimulus intensity w as 1.2 times the motor threshold (Tm-APB) 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 ME P with increased amplitude, resembling clonic contraction. At 20 Hz th e inhibition of the MEPs decreased (P < 0.05) compared to 10 Hz stimul ation. Facilitation decreased the inhibition at 5 and 10 Hz stimulatio n (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 com plex influence on the MEPs depending upon stimulus rates. The pronounc ed inhibition and excitation of the MEPs at 5 and 10 Hz stimulation po ssibly reflects inhibition and excitation in the cortico-spinal neuron s.