Magnetic stimulation over different brain regions: no differential effects on the elicited sympathetic skin responses

Peak latencies and amplitudes of sympathetic skin responses (SSRs) of the hand following magnetic stimulation at different sites with two or five consecutive 10-Hz stimuli were investigated with regard to safety aspects of repetitive transcranial magnetic cortex stimulation (rTMS). The amount of sympathetic activation as assessed by the amplitudes of the SSRs depended on the stimulation site and decreased in the following order: brachial plexus stimulation>nerve root stimulation>stimulation over the brain>activation by acoustic coil artefact. When stimulating over six different regions of the cortex (frontal, central, parieto-occipital, and both hemispheres), the elicited SSRs had similar amplitudes and peak latencies. The SSRs elicited by rTMS over the motor cortex were not related to the sum of the amplitudes of excitatory muscle compound responses. Currents with opposite directions over the motor cortex markedly influenced the size of the motor responses but not of the SSRs. The number of consecutive 10-Hz stimuli did not influence the latencies or amplitudes of the SSRs. It can be concluded that SSRs after magnetic stimulation over peripheral nerves or the brain are a correlate of an unspecific arousal reaction. A therapeutic application of short series of rTMS should not be limited by the amount of sympathetic activation.