Transcranial magnetic brain stimulation is now a well established non-invasive tool to investigate the following functions of the motor system which are investigated in patients with different lesions within the central and peripheral nervous system:
The used stimulation technique of transcranial magnetic brain stimulation bases on the principle of electromagnetic induction. Currents flow through the stimulation coil and induce a magnetic field which in turn induces currents in conductive neuronal elements and lead by this to an excitation of these structures. Brain stimulation is performed with focal stimulation coils to ensure a circumscript activation of corticospinal efferent, cortical inhibitory, and callosal neurons. Stimulation effects are assessed by surface compound emg-responses of different muscles. Parameters of the activation of excitatory and inhibitory corticospinal neurons are the thresholds, sizes, central motor latency times of evoked responses and the durations of postexcitatory inhibition. Parameters of callosally mediated effects are the onset latencies and durations of transcallosal inhibition.
Magnetic resonance imaging is used to localize the lesions and to determine their sizes. Clinical and computerized examinations are performed to characterize motor disability.
In 1996 we investigated 35 patients with strokes, 5 patients with spinal lesions, 20 patients after arm amputation and 10 patients after loss of one hand and replantation of this hand. To improve our normative data for leg motor responses, we investigated another normal 59 subjects to establish a new index of central motor latency times which might allow a more sensitve evaluation of spinal conduction deficits.
In addition to the aims of our project we investigated 15 patients with different types of hydrocephalus before and after decompression by shunt operation, because we had discovered that these patients offered an exellent model for a potentially reversible compression of the corpus callosum. To further characterize the callosal motor system also 15 patients with circumscript lesions of different parts of the corpus callosum were investigated.
Exciting results were found in a group of patients with hemisphere lesions after stroke. In these patients we were able to establish transcallosal activation of the motor cortex as a new way to assess the function of the motor cortex in patients with white matter lesions at subcortical level below the course of the callosal fibres (e.g. capsular infarcts). This approach was found to be useful even when clinical testing gave no further information than hemiplegia. Other interesting results were obtained in patients after arm amputation. Beside changes of the motor cortical representation of proximal stump muscles we could relate the occurrence of phantom limb pain with plastic processes within the cortex. Astonishingly, transcranial magnetic brain stimulation sometimes abolished phantom limb pain for a period of days to months. This vaguely hints at a probable therapeutic use of single pulse brain stimulation in such patients. In patients with compression of the corpus callosum due to hydrocephalus we could for the first time exclude that the remarkable thinning of the corpus callosum and of the subcortical white matter produced an axonal fibre degeneration, functional block of conduction processes or fibre demyelination. Furthermore we could show that shunting led to a good recovery of callosal and corticospinal motor functions in the investigated patient group.