Recent studies (Pascual-Leone et al 1994, Berardelli et al. 1998) have demonstrated that repetitive transcranial magnetic stimulation (rTMS), at a 5-Hz frequency with a suprathreshold intensity, can induce a progressive increase in the amplitude of motor-evoked potentials (MEP) and a prolongation of the cortical silent period duration (Berardelli et al. 1999). In particular Pascual-Leone et al. (1994) described a spread of the rTMS-induced excitation. We studied the spread of excitation induced by focal rTMS in 8 normal subjects. Trains of 10 stimuli were delivered at 5, 10 and 20 Hz with a Cadwell Magnetic High Speed Stimulator when the subjects were at rest and during contraction. The EMG activity was simultaneously recorded from distal muscles (first dorsal interosseous or forearm flexor) and in the biceps muscle. For each subject, the intensity of stimulation was chosen to obtain responses (MEP or SP) in distal muscles and not in the biceps muscle. At rest rTMS produced a facilitation of the MEP in FDI muscles at all the frequencies studied. Higher frequencies were only able to induce (after the fifth stimulus of the train) a MEP in the biceps muscle, which increased in amplitude during subsequent stimuli (P0.05) in all the frequencies studied. Our data suggest that during rTMS excitatory and inhibitory network behaved differently. The spread of the rTMS-induced excitation seems to engage only the excitatory intracortical interneurons as no SP in the biceps muscle has been recorded. The imbalance between facilitatory and inhibitory network could lead to an increased risk of developing seizures at high-frequency stimulation.
|Issue number||4 SUPPL.|
|Publication status||Published - 2000|
ASJC Scopus subject areas
- Clinical Neurology