Diagnostic significance of rhythmicity in postural hand tremor

Rhythmicity is an important feature of tremor that is widely viewed as having diagnostic significance. We hypothesized that rhythmicity might be a generic function of tremor severity, reflecting greater oscillatory entrainment of motor pathways. Postural hand tremor and forearm electromyograms were recorded from 49 controls, 78 Parkinson patients, and 133 essential tremor patients. Rhythmicity was quantified in terms of approximate entropy and three measures of cycle-to-cycle frequency variability: tremor stability index, cycle-to-cycle variability, and power spectral bandwidth. Physiological tremor was less rhythmic than Parkinson tremor and essential tremor, but the two pathological tremors did not differ significantly. Hand tremor amplitude and forearm electromyogram-hand tremor coherence were moderate statistical predictors of rhythmicity in both pathological tremors. Adding a 1-kg weight to the hand had little effect on the rhythmicity metrics, except for a moderate reduction in the approximate entropy of physiological tremor. We conclude that these four rhythmicity metrics are not helpful in distinguishing postural tremors in Parkinson disease and essential tremor. The moderate correlations of rhythmicity with tremor amplitude and electromyogram coherence suggest that rhythmicity of pathological tremors is largely a generic reflection of oscillatory neuronal entrainment. Comparisons of tremor rhythmicity in pathological conditions must control for tremor amplitude and electromyogram coherence.