Usefulness of intermuscular coherence and cumulant analysis in the diagnosis of postural tremor

This article investigate the potential value of two advanced EMG measures as additional diagnostic measures in the polymyographic assessment of postural upper-limb tremor.  We investigated coherence as a measure of dependency between two EMG signals, and cumulant analysis to reveal patterns of synchronicity in EMG activity in muscle pairs. Eighty datasets were analyzed retrospectively, obtained from four groups: essential tremor (ET), Parkinson’s disease (PD), enhanced physiological tremor (EPT), and functional tremor (FT).  Intermuscular coherence was highest in the PD group (0.58), intermediate in FT (0.43) and ET (0.40), and weakest in EPT (0.16) (p=0.002). EPT patients could be distinguished by low coherence: coherence<0.18 in the wrist + elbow extensors differentiates EPT in this sample with a sensitivity of 86% and specificity of 84%.  Cumulant analysis showed predominantly alternating activity between wrist and elbow extensor in ET patients, while a more synchronous pattern was predominant in PD, EPT and FT (p=0.008). EMG activity in wrist and elbow flexors tended to be more synchronous in PD (p=0.059).Our results suggest that coherence and cumulant analysis may be of additional value in the diagnostic work-up of postural tremor.  These additional measures may be helpful in diagnosing difficult tremor cases.