Magnetic stimulation of the corticospinal tracts in Pelizaeus-Merzbacher disease.

To evaluate conduction abnormalities of the corticospinal tracts (CSTs) in Pelizaeus-Merzbacher disease (PMD), magnetic stimulation at three levels was carried out in 3 boys with PMD aged between 9 and 12 years. They were all diagnosed as having a duplicated proteolipid protein gene. The motor cortex and cervical spinal roots were stimulated with a round coil, whereas a double cone coil was used for brain-stem stimulation. Surface electromyographic (EMG) recording was performed on the first dorsal interosseous muscles. Despite a normal EMG response to cervical stimulation, magnetic shock of the motor cortex elicited no EMG activity, even in the case with less motor symptoms. This discrepancy between the electrophysiological and clinical findings is likely due to slowing conduction, which reduces the temporal summation of multiple descending volleys magnetically elicited. A partial conduction block may also occur because of the lack of an EMG response to brain-stem stimulation. Thus, we speculated that the spastic paresis in PMD is associated with both slowing conduction and a partial conduction block in the CSTs.