Relationship between jerky and sinusoidal oscillations in cervical dystonia

Introduction: Dystonia is often associated with repetitive jerky oscillations (i.e. dystonic tremor), while tremor is characterized by sinusoidal oscillations. We propose two competing predictions for dystonic tremor and sinusoidal tremor relationship. In any given patient, (1) the oscillation could be characterized as either sinusoidal or jerky based on the degree of distortion in the waveforms, (2) the oscillation consists of both sinusoidal and jerky waveforms mixed in a certain proportion that varies among individuals. We objectively test these predictions in patients with cervical dystonia. Methods: We recorded head oscillations in 14 subjects with cervical dystonia using a high-resolution magnetic field search coil system. Distortion in the signal was used as a measure of jerkiness. A hierarchical clustering classified the oscillations based on distortion characteristics. Results: Signal analysis in the frequency domain allowed identification of the components of the waveforms at frequencies other than the fundamental frequency. The distortion from the component at fundamental frequency was present in both low and high frequency range. Based on varying levels of distortions, i.e. jerkiness, the head oscillations were grouped into 4 clusters: one cluster with lowest distortion (sinusoidal waveforms), one cluster with highest distortion (jerky waveforms), and two intermediate clusters – one with distortion at low frequency and another with distortion at high frequency. The distribution of 4 clusters varied across subjects suggesting co-existence of sinusoidal and jerky waveforms. Conclusion: These results support the prediction that jerky and sinusoidal waveforms concur in cervical dystonia. Amount of concurrence varies amongst patients.