Directionality of corticomuscular coupling in essential tremor and cortical myoclonic tremor

S Sharifi,F Luft,S Potgieter,T Heida,W Mugge,A.C Schouten,L.J Bour,A.F Van Rootselaar

doi:10.1016/j.clinph.2021.04.011

Abstract

ObjectiveA role of the motor cortex in tremor generation in essential tremor (ET) is assumed, yet the directionality of corticomuscular coupling is unknown. Our aim is to clarify the role of the motor cortex. To this end we also study ‘familial cortical myoclonic tremor with epilepsy’ (FCMTE) and slow repetitive voluntary movements with a known cortical drive. MethodsDirectionality of corticomuscular coupling (EEG-EMG) was studied with renormalized partial directed coherence (rPDC) during tremor in 25 ET patients, 25 healthy controls (mimicked) and in seven FCMTE patients; and during a self-paced 2 Hz task in eight ET patients and seven healthy controls. ResultsEfferent coupling around tremor frequency was seen in 33% of ET patients, 45.5% of healthy controls, all FCMTE patients, and, around 2 Hz, in all ET patients and all healthy controls. Ascending coupling, seen in the majority of all participants, was weaker in ET than in healthy controls around 5–6 Hz. ConclusionsPossible explanations are that tremor in ET results from faulty subcortical output bypassing the motor cortex; rate-dependent transmission similar to generation of rhythmic movements; and/or faulty feedforward mechanism resulting from decreased afferent (sensory) coupling. SignificanceA linear cortical drive is lacking in the majority of ET patients.

Highlights

Essential tremor (ET), characterized by a postural and intentional tremor predominantly of the hands, is a highly prevalent and disabling movement disorder of unknown cause (Hopfner et al, 2016)
Efferent coupling around tremor frequency was seen in 33% of essential tremor (ET) patients, 45.5% of healthy controls, all familial cortical myoclonic tremor with epilepsy’ (FCMTE) patients, and, around 2 Hz, in all ET patients and all healthy controls
Possible explanations are that tremor in ET results from faulty subcortical output bypassing the motor cortex; rate-dependent transmission similar to generation of rhythmic movements; and/or faulty feedforward mechanism resulting from decreased afferent coupling

Summary

Introduction

Essential tremor (ET), characterized by a postural and intentional tremor predominantly of the hands, is a highly prevalent and disabling movement disorder of unknown cause (Hopfner et al, 2016). A prominent role of the central sensorimotor cortex was suggested following several EEG-EMG and MEG-EMG studies that demonstrated corticomuscular coupling at tremor frequency (Hellwig et al, 2001; Raethjen et al, 2007; Schnitzler et al, 2009). Previous studies that investigated corticomuscular coherence over time have shown intermittent coherence despite a constant tremor power (Raethjen et al, 2007; Sharifi et al, 2017). Over time despite a strong tremor intensity there was a loss of significant corticomuscular coherence at tremor frequency.

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Conclusion