Since transcranial magnetic stimulation with series of stimuli is increasingly used as a physiological research instrument and recently has been introduced as a promising therapeutic tool, safety aspects and the identification of the underlying activation processes become an important issue. Our aim is to investigate which mechanisms mediate the influence of rTMS on cerebral perfusion. Therefore, the changes of cerebral haemodynamics following hemisphere selective rTMS are determined by using transcranial Doppler sonography. Furthermore, we study the effects of rTMS on the autonomous nervous system by measuring sympathetic skin responses and cardiovascular parameters. The following two abstracts illustrate our approach and give some preliminary results of our studies.
The aim of the study was to investigate the influence of repetitive transcranial magnetic stimulation (rTMS) with different pulse configurations on cerebral blood flow velocitiy.
rTMS was performed with an eight-shaped coil (Dantec Mag Pro) over the motor cortex in 5 healthy volunteers. Ten series of 5 stimuli (inter-stimulus-interval 100msec) with monophasic and biphasic pulses were administered every 40-60 seconds. Stimulus intensity was set 1.2times the threshold of responses in the first interosseus dorsalis muscle. Mean blood flow velocities (MBFV) in both middle cerebral arteries (MCA) were recorded simultaneously by transcranial Doppler sonography. Changes of MBFV in each MCA were calculated in relation to a five seconds prestimulus-period as baseline.
rTMS with biphasic pulses was followed by larger amplitudes (4.8%) and shorter latencies (3sec) of MBFV changes in the ipsilateral MCA than the monophasic stimulation (peak amplitude 3.2%, peak latency 7sec). In comparison to the contralateral side the MBFV increase ipsilateral to stimulation was more pronounced in both stimulation conditions. Maximal side-to-side-differences were reached two seconds after stimulation.
The rapid ipsilateral MBFV change following rTMS with biphasic pulses indicates an increase of cerebral blood flow consecutive to the activation of cortical neuronal structures. The late MBFV increase following monophasic stimuli reflects an unspecific sympathetic activation.
The aim of the study was to investigate whether the SSR following magnetic cortex stimulation is different in comparison to nerve or acoustic stimulation and whether it depends on the cortical stimulation site.
In 21 healthy subjects the SSR was recorded bilaterally from the palm of the hand. There were 9 different conditions of magnetic stimulation (circular coil, Dantec Mag Pro, biphasic pulses): 3 positions over each hemisphere (frontal, central, parieto-occipital), cervical nerve roots, brachial plexus, acoustic coil artefact. At each stimulation site 5 stimulation series were given. Paired stimuli (PS) and series of 5 stimuli (SS) were given (ISI 100ms, 1.3times of cortical motor threshold).
No significant latency differences of the SSR (mean peak latencies: 2.1s) elicited at different stimulation sites were observed. Irrespective of whether the stimulation site was ipsi- or contralateral to the recording site, the largest amplitudes were elicited by plexus (PS:0.99mV, RS:0.84mV) and root stimulation (PS:1.0mV, RS:0.71mV). Cortex stimulation produced SSR with the second largest amplitudes (PS:0.51mV, RS:0.33mV) without differential effects of the stimulation site. The acoustic artefact produced significantly smaller SSRs than cortex and nerve stimulation (PS:0.14mV, RS:0.07mV).
In conclusion, stimulation over different parts of the cortex of both hemispheres showed no differential effects on sympathetic autonomic function. No safety risk regarding sympathetic activation was found for repetitive transcranial stimulation.