A new method to measure caudal motor conduction time using magnetic stimulation

Although central motor conduction time (CMCT) has been used for the diagnosis of lumbosacral spinal stenosis (LSSS), its diagnostic value is limited due to the short length of the involved segment compared to the long length of the total conduction distance. To overcome this, we introduce a new method to measure the caudal motor conduction time (caudal MCT) using magnetic stimulation. Magnetic stimulation was applied to the vertex and the T12 and S1 spinous processes for transcortical, thoracic, and sacral stimulation, respectively, and compound muscle action potentials were recorded simultaneously from the rectus abdominis (RA) and the right and left abductor hallucis (AH) muscles using three channels. CMCT was calculated by the latency difference in the AH response between transcortical and sacral stimulation, and between transcortical and thoracic stimulation for RA. Caudal MCT was calculated by subtracting CMCT for RA from that for AH. Caudal MCT was delayed in patients with LSSS compared to normal persons. We suggest that measuring caudal MCT may be useful for the diagnosis of LSSS, but its diagnostic sensitivity and specificity requires prospective study.