Central motor conduction differs between acute relapsing –remitting and chronic progressive multiple sclerosis A.M. Humm a , M.R. Magistris b , A. Truffert b , C.W. Hess a , K.M. Ro ¨sler a, * a Department of Neurology, University of Berne, Inselspital, CH-3010 Bern, Switzerland b Department of Neurology, University of Geneva, Geneva, Switzerland Accepted 21 June 2003 Abstract Objective: To characterize central motor conduction in relation to the clinical deficits and to the disease duration in 90 patients with acute relapsing – remitting MS (RR-MS) and in 51 patients with chronic primary or secondary progressive MS (P-MS). Methods: The triple stimulation technique (TST) was used to quantify the central motor conduction failure (expressed by the TST amplitude ratio) and conventional motor evoked potentials (MEPs) were used to measure the central motor conduction time (CMCT). Results: The TST amplitude ratio was reduced in presence of a clinical motor deficit (p ¼ 0:02 for RR-MS, p , 0:01 for P-MS), but did not significantly differ in RR-MS and P-MS (p . 0:05) when patients with similar clinical motor deficit were compared. The CMCT was not related to the clinical motor deficit in both RR-MS and P-MS. However, the CMCT was markedly prolonged in P-MS, when patients with similar clinical motor deficit and with similar disease duration were compared (p , 0:01). The differences were not attributable to differential involvement of the spinal cord, which was similar in RR-MS and P-MS. Conclusions: Our results disclose differences between the central motor conduction in RR-MS and P-MS that are not related to disease severity, spinal cord involvement or disease duration. q 2003 International Federation of Clinical Neurophysiology. Published by Elsevier Ireland Ltd. All rights reserved. Keywords: Central motor conduction time; Central motor conduction failure; Transcranial magnetic stimulation; Motor evoked potentials 1. Introduction Multiple sclerosis (MS) is characterized by multiple white matter lesions, which are disseminated in time and space. MS typically presents with a relapsing –remitting course (80% of patients), followed after 5–10 years by a secondary progressive phase with or without superimposed relapses (70% of patients). In 10–20% of patients, the disease course is primary progressive. The pyramidal tract is frequently affected, and impaired motor performance is a major cause of disability in MS. Pyramidal tract function can be assessed using transcranial magnetic stimulation (TMS), yielding motor evoked potentials (MEPs) in leg, arm or cranial and neck muscles (Barker et al., 1985; Benecke and Meyer, 1991; Berardelli et al., 1991). TMS participates in the diagnosis of MS (Hess et al., 1987; Mayr et al., 1991; Beer et al., 1995; Ravnborg, 1996) and helps to follow its course with or without treatment (Kandler et al., 1991; Salle et al., 1992; Fuhr et al., 2001; Fuhr and Kappos, 2001). Theoretically, central motor conduction in MS may be altered in different manners, namely (i) by increased conduction time, or (ii) by conduction deficit (e.g., by central conduction block (Smith and McDonald, 1999), or by loss of conducting axons), or (iii) by both. Both, increased central motor conduction time (CMCT) and central conduction deficits are found in many, yet not in all patients, for reasons that are not always obvious. A number of factors could cause this interindividual varia- bility, including pathophysiological mechanisms (such as ongoing myelin degradation, differential axonal involve- ment, localization of the lesions, or pathophysiological heterogeneity of the disease) and methodological factors (e.g., sampling errors due to a limited number of recording muscles, lack of methodological sensitivity to detect conduction abnormalities). The goal of the present study was to sort out pathophysiological mechanisms affecting Clinical Neurophysiology 114 (2003) 2196–2203 www.elsevier.com/locate/clinph 1388-2457/$30.00 q 2003 International Federation of Clinical Neurophysiology. Published by Elsevier Ireland Ltd. All rights reserved. doi:10.1016/S1388-2457(03)00231-1 * Corresponding author. Tel.: þ41-31-632-30-98; fax: þ 41-31-632-30- 11. E-mail address: kroesler@insel.ch (K.M. Ro ¨sler).