Transcranial magnetic brain stimulation was used to study central motor conduction (CMCT) to small hand muscles in patients with peroneal muscular atrophy and hereditary spastic paraplegia at the National Hospital and Institute of Neurology, Queen Square and the Department of Neurological Science, Royal Free Hospital, London, UK. Proximal motor roots were excited at the intervertebral foramina, the stimulating cathode placed at C7-T1 and the anode 6 centimeters laterally on the ipsilateral side. Central motor conduction time was estimated by subtracting the latency of this potential from that of the response to brain stimuli. CMCT was normal in HSMN I, HSMN II, and HSP. In patients with HSMN I with pyramidal signs, central motor conduction time was greatly prolonged bilaterally. The results reflected an involvement of the central motor pathways. 
COMMENT: Dyck PJ and Lambert EH  subdivided patients with HSMN into two main groups: HSMN I with demyelination in peripheral nerves and HSMN II without demyelination. Patients with HSMN who had pyramidal signs were designated type V. Pyramidal signs may occur as a regular feature in some families but do not reflect disease severity. The authors of the above study found no correlation between the degree of general disability and the occurrence of abnormal CMCT.