Effect of discharge desynchronization on the size of motor evoked potentials: an analysis Kai M. Ro ¨sler a, * , Elisabeth Petrow a , Johannes Mathis a , Zsuzsanna Ara ´nyi a,1 , Christian W. Hess a , Michel R. Magistris b a Department of Neurology, University of Berne, Berne, Switzerland b Department of Neurology, University of Geneva, Geneva, Switzerland Accepted 31 July 2002 Abstract Objective: Motor evoked potentials (MEPs) after transcranial magnetic brain stimulation (TMS) are smaller than CMAPs after peripheral nerve stimulation, because desynchronization of the TMS-induced motor neurone discharges occurs (i.e. MEP desynchronization). This desynchronization effect can be eliminated by use of the triple stimulation technique (TST; Brain 121 (1998) 437). The objective of this paper is to study the effect of discharge desynchronization on MEPs by comparing the size of MEP and TST responses. Methods: MEP and TST responses were obtained in 10 healthy subjects during isometric contractions of the abductor digiti minimi, during voluntary background contractions between 0% and 20% of maximal force, and using 3 different stimulus intensities. Additional data from other normals and from multiple sclerosis (MS) patients were obtained from previous studies. Results: MEPs were smaller than TST responses in all subjects and under all stimulating conditions, confirming the marked influence of desynchronization on MEPs. There was a linear relation between the amplitudes of MEPs vs. TST responses, independent of the degree of voluntary contraction and stimulus intensity. The slope of the regression equation was 0.66 on average, indicating that desynchronization reduced the MEP amplitude on average by one third, with marked inter-individual variations. A similar average proportion was found in MS patients. Conclusions: The MEP size reduction induced by desynchronization is not influenced by the intensity of TMS and by the level of facilitatory voluntary background contractions. It is similar in healthy subjects and in MS patients, in whom increased desynchronization of central conduction was previously suggested to occur. Thus, the MEP size reduction observed may not parallel the actual amount of desynchronization. q 2002 Elsevier Science Ireland Ltd. All rights reserved. Keywords: Collision technique; Corticospinal tract; Motor evoked potentials; Multiple sclerosis; Transcranial magnetic stimulation; Phase cancellation 1. Introduction The size of motor evoked potentials (MEPs) obtained by transcranial magnetic stimulation (TMS) is influenced by a variety of factors, the most important being the number of activated motor neurones (MNs). If the number of dischar- ging MNs is reduced, the MEP size will decrease, as observed in healthy subjects when low stimulus intensities or insufficient facilitation manoeuvres are applied (Ro ¨ sler et al., 1999a), and in patients with lesions of the central motor pathways (Magistris et al., 1999). The size of a MEP is additionally influenced by desyn- chronization of the stimulus-induced MN discharges. Desynchronization induces phase cancellation and reduces the MEP size (phase cancellation: the negative phases of individual motor unit potentials are cancelled by positive phases of others; Kimura et al., 1986). Therefore, MEPs are smaller than compound muscle action potentials (CMAPs) after maximal peripheral nerve stimulation, even if 100% of the spinal MNs supplying a target muscle have been excited by the transcranial stimulus (i.e. ‘maxi- mal TMS’) (Magistris et al., 1998). Recently we introduced a triple stimulation technique (TST), which suppresses the effects of the TMS-induced MN discharge desynchroniza- tion (Magistris et al., 1998). As a consequence, this collision technique provides a quantitative measure of the percentage of spinal MNs driven to discharge by TMS. Using maximal TMS intensities and sufficient facilitation manoeuvres, the TST always demonstrates excitation of nearly 100% of the Clinical Neurophysiology 113 (2002) 1680–1687 1388-2457/02/$ - see front matter q 2002 Elsevier Science Ireland Ltd. All rights reserved. PII: S1388-2457(02)00263-8 www.elsevier.com/locate/clinph CLINPH 2001717 * Corresponding author. Department of Neurology, Inselspital, CH 3010 Bern, Switzerland. Tel.: 141-31-632-30-98; fax: 141-31-632-30-11. E-mail address: kai.roesler@insel.ch (K.M. Ro ¨sler). 1 Present address: Semmelweis University Faculty of Medicine, Buda- pest, Hungary.