Transcranial magnetic stimulation of the motor cortex was performed in 10 normal subjects and 10 patients with radiographical abnormalities of the corpus callosum. Seven patients had a complete or partial agenesis or hypoplasia of the corpus callosum, two had a thin corpus callosum due to hydrocephalus or white matter degeneration and one had a circumscript contusion lesion of the corpus callosum. The patients served as a clinical model to investigate transcallosal influences on excitatory and inhibitory effects of motor cortex stimulation and to assess the potential diagnostic use of interhemispheric conduction studies and the contribution of interhemispheric interaction on transcranially elicited contralateral excitatory and inhibitory motor responses. Stimulation over one motor cortex suppressed tonic voluntary electromyographic activity in ipsilateral hand muscles in all subjects with preserved anterior half of the trunk of the corpus callosum. Since this suppression was lacking or had a delayed onset latency in patients with absence or abnormalities of the anterior half of the trunk of the corpus callosum it can be concluded that it is due to a transcallosal inhibition (Ti) of the opposite motor cortex mediated by fibres passing through this part of the corpus callosum. In normal subjects Ti had an mean onset latency of 36.1 +/- 3.5 ms (SD) and a duration of 24.5 +/- 3.9 ms. The calculated mean transcallosal conduction time was 13 ms. The threshold of Ti recorded in muscles ipsilateral to stimulation tended to be higher than the one for eliciting excitatory contralateral motor responses (56 +/- 6% versus 46 +/- 10% maximum stimulator output). Cortical thresholds (at rest) for contralateral excitatory hand motor responses were higher in patients with developmental abnormalities of the corpus callosum than in normals (66 +/- 17% versus 46 +/- 10% maximum stimulator output), which probably reflects also a facilitatory transcallosal interaction of both motor cortices in normals. In contrast, facilitation of cortically elicited motor responses in one hand by strong contraction of the other hand was the same in the patients with agenesis of the corpus callosum and normals, which suggests that this facilitatory spread takes place on a spinal rather than on a cortical level. Central motor latencies and amplitudes of contralateral hand motor responses were the same in patients with developmental abnormalities of the corpus callosum and normals (6.1 +/- 0.7 ms versus 6.3 +/- 0.7 ms and 6.7 +/- 2.4 mV versus 6.6 +/- 2.9 mV) so that callosal transfers do not seem to influence corticospinal conduction properties.(ABSTRACT TRUNCATED AT 400 WORDS)

In 1 split-brain patient and 13 patients with circumscript surgical lesions within different parts of the corpus callosum (CC), the topography of fibers within the CC mediating an interhemispheric inhibition between the motor cortices was investigated in the phase of a stable disconnection syndrome. The aim of the study was to characterize the system of callosal fibers activated by transcranial magnetic brain stimulation in more detail as a basis for a diagnostic application of this technique. The locations and areas of the lesions were measured on sagittal magnetic resonance (MR) slices and attributed to five segments of the CC, because the borders of the anatomic subdivisions of the CC are not clearly defined. Transcallosal inhibition (TI) of tonic electromyographical hand muscle activity was elicited by focal transcranial magnetic stimulation over the primary motor cortex. In a reference group of normal subjects, TI started after 35.8+/-3.6 msec had a duration of 24.8+/-2.7 msec and an estimated transcallosal conduction time of 15.5+/-3.0 msec (range, 10.2-20.0 msec, n = 24 hands). No significant differences were found for the TI originating from the right and left motor cortices. From the absence and presence of TI in patients with lesions in different parts of the CC it can be concluded that most of the fibers mediating TI pass through the posterior third and anterior fourth segment of the CC, that is, through the posterior half of the trunk of the CC. The clinical findings suggest a role for TI in the performance of fast and complex hand motor tasks.

The combination of central motor latencies and transcallosal inhibition evoked by transcranial magnetic stimulation yields objective data to estimate disease progression in MS as assessed by the EDSS.

Reorganization of the motor system can be present more than 20 years after amputation. Furthermore, differential patterns of reorganized corticospinal output were found for different stump muscles, which might be due to varying amounts of ipsilateral corticospinal projections.