Periodic limb movement disorder is associated with normal motor conduction latencies when studied by central magnetic stimulation--successful use of a new technique.

This study prospectively tested the hypothesis that patients with periodic limb movement disorder (PLMD) have longer motor conduction latencies than normals. Six healthy adults, 13 patients with PLMD, and 8 patients with long-term multiple sclerosis (MS) had recordings of motor conduction latencies during wake and sleep. MS subjects were included only to show that we could detect prolongation of central conduction; nonMS subjects were used to test the hypothesis. Subjects had no other medical or sleep problems. A novel magnetic stimulator, the Cadwell MES-10, was discharged over the vertex and the C7 cervical spine. It triggered compound muscle action potentials that were recorded in the abductor digiti minimi in the hand. The conduction latencies were the total conduction time (TCT), measured vertex to hand, and the peripheral conduction time (PCT), measured C7 to hand. The difference was the central conduction time (CCT). Only TCT could be obtained during sleep. Supporting the use of TCT as an indirect measure of central conduction was that, in all waking subjects, TCT correlated with CCT (r = 0.91, p = 0.001) but not with PCT. Reliabilities during wake and sleep were 0.95 or higher for TCT and PCT measurements. Waking CCT was greater in MS subjects (13.77 milliseconds) than those without MS (9.21 milliseconds), p = 0.001. Sleeping TCT was much less impressive in distinguishing MS subjects [27.08 milliseconds in nonrapid eye movement (NREM) sleep; 28.64 milliseconds in rapid eye movement (REM) sleep] from nonMS subjects (24.45 milliseconds in NREM; 24.84 milliseconds for REM), p = 0.07 for NREM and p = 0.04 for REM.(ABSTRACT TRUNCATED AT 250 WORDS)