Brain, Vol. 126, No. 2, 326-342,
February 2003
© 2003 Guarantors of Brain
doi: 10.1093/brain/awg043
Abnormal corticomuscular and intermuscular coupling in high-frequency rhythmic myoclonus
1 Sobell Department of Clinical Neurophysiology, Institute of Neurology and 2 Neurosciences Unit, Institute of Child Health and Great Ormond Street Hospital for Children, London, UK, 3 Neurologische Klinik und Poliklinik, Charité, Campus Virchow-Klinikum, Berlin, Germany, and 4 Institute of Child Neurology and Psychiatry, University of Pisa and IRCCS Fondazione Stella Mars, Pisa, Italy
*Present address: Institute of Child Neurology and Psychiatry, University of Pisa and IRCCS Fondazione Stella Mars, Pisa, ItalyCorrespondence to: Dr Pascal Grosse, Sobell Department of Neurophysiology, Clinical Motor Physiology Group, Institute of Neurology, 8–11 Queens Square, London WC1N 3BG, UK E-mail: p.grosse{at}ion.ucl.ac.uk
Frequency analysis may have some advantages over back-averaging in the neurophysiological assessment of patients with suspected cortical myoclonus in whom myoclonic EMG bursts repeat rhythmically at high frequency. However, the clinical utility of EEG–EMG coherence and related EMG–EMG coherence is not established. Equally, there is an incomplete understanding of the physiology of the systems contributing to the coherence evident between signals in cortical myoclonus. Here we address these issues in an investigation of EEG–EMG and EMG–EMG coupling in proximal and distal muscles of the upper extremities in nine patients with multifocal high frequency rhythmic myoclonus due to non-progressive conditions. We found exaggerated coherence between EEG and contralateral EMG and between pairs of ipsilateral EMG signals. The results of frequency analysis of EMG–EMG mirrored those for EEG–EMG, but the former technique was superior in distinguishing a pathologically exaggerated common drive in distal upper limb muscles. Both techniques were more sensitive than back-averaging. Frequency analysis also revealed important disparities between proximal and distal upper limb muscles. In the latter case, the functional coupling between cortex and muscle was dominated by efferent processes. In contrast, there was considerable inter-individual variation in the extent to which EEG–EMG and EMG–EMG coupling in proximal muscles reflected afferent and efferent loops. Thus, the processes sustaining myoclonic discharges may differ for proximal and distal muscles and between patients.
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