Assessment of motor conduction times using magnetic stimulation of brain, spinal cord and peripheral nerves

Central conduction times (CCTs) in the motor pathways were assessed in normal subjects using a magnetic stimulus for the recording of cortical and spinal (Cv7 and L4-L5) Motor Evoked Potentials (MEPs) as well as F-waves in upper and lower limbs. M-responses of maximal amplitude could be reliably obtained by using a 4.5 Tesla coil and a stimulus intensity of 85% of the maximal output. It was found that the CCTs derived from the latency difference between cortical and spinal MEPs were, on average, 0.90 ms and 3.15 ms longer, respectively for upper and lower limbs, than those obtained by combining cortical MEPs and F-wave latency measurements. This difference is similar to that reported when electric stimulation is used to elicit F-waves and/or spinal MEPs. It suggests that motor root fibers are depolarized at a distance of 7.8 and 17 cm from the cord by a spinal magnetic stimulus applied respectively at Cv7 and L4-L5 levels. This study shows that magnetic stimulation alone permits to assess CCT in motor pathways with the same reliability as any of the other stimulation modalities hitherto proposed in literature.