Pelizaeus-Merzbacher disease: electrophysiological study of two sibs with the classic form and of their relatives.

We examined two sibs with the classic form of Pelizaeus-Merzbacher disease (PMD) and their relatives. Electromyographic-electroneurographic studies and magnetic stimulation of motor pathways were performed. In both patients we found an absence of compound motor action potential (cMAP) after stimulation of the motor cortex and a normal conduction time by stimulating the cervical roots. Despite reported sparing of the peripheral nervous system in PMD, our conduction study of the tibial nerve revealed a slightly decreased motor nerve conduction velocity in one patient. In both patients the EMG study showed neurogenic findings. The elder sister showed a prolonged central motor conduction time. This study demonstrates abnormalities of motor corticospinal pathways also in PMD relatives suggesting that magnetic stimulation could be useful in detecting "subclinical" abnormalities in this dysmyelinating condition. Furthermore, in accordance with previous studies, we suggest that a slight involvement of the peripheral nervous system could be observed in PMD.