Intermittent bilateral coherence in physiological and essential hand tremor

•The two hand oscillations are frequently mutually coherent in physiological and essential tremor.•Epochs of strong coherence alternate with intervals of insignificant coherence.•Transient entrainment of physiological tremor by ballistocardiac forcing is inferred.

ObjectiveTo investigate the prevalence and the temporal structure of bilateral coherence in physiological (PT) and essential (ET) hand tremor.MethodsTriaxial accelerometric recordings from both hands in 30 healthy subjects and 34 ET patients were analyzed using spectral coherence and wavelet coherence methods. In 12 additional healthy subjects, the relation between the hand tremor and the chest wall acceleration was evaluated using partial coherence analysis.ResultsThe majority of both PT and ET subjects displayed significant bilateral coherence. While in PT, bilateral coherence was most frequently found in resting hand position (97% of subjects), in ET the prevalence was comparable for resting (54%) and postural (49%-57%) positions. In both PT and ET, epochs of strong coherence lasting several to a dozen seconds were separated by intervals of insignificant coherence. In PT, bilateral coherence at the main tremor frequency (8-12 Hz) was coupled with the ballistocardiac rhythm.ConclusionThe oscillations of the two hands are intermittently synchronized in both PT and ET. We propose that in postural PT, bilateral coherence at the main tremor frequency arises from transient simultaneous entrainment of the left and right hand oscillations to ballistocardiac forcing.SignificanceBilateral coherence of hand kinematics provides a sensitive measure of synchronizing influences on the left and right tremor oscillators.