Introduction: Patients with cervical dystonia (CD) often have limb tremor that is clinically indistinguishable from essential tremor (ET). Whether a common central mechanism underlies the tremor in these conditions is unknown. We addressed this issue by quantifying limb tremor in 19 patients with CD and 35 patients with ET.
Method: Postural, resting and kinetic tremors were quantified (amplitude, mean frequency and regularity) using a three-axis accelerometer.
Results: The amplitude of limb tremor in ET was significantly higher than in CD, but the mean frequency was not significantly different between the groups. The cycle-to-cycle variability of the frequency (ie the tremor irregularity), however, was significantly greater (∼50%) in CD. Analysis of covariance excluded the possibility that the increased irregularity was related to the smaller amplitude of tremor in CD (ANCOVA: p = 0.007, F = 5.31).
Discussion: We propose that tremor in CD arises from oscillators with different dynamic characteristics, producing a more irregular output, whereas the tremor in ET arises from oscillators with similar dynamic characteristics, producing a more regular output. We suggest that variability of tremor is an important parameter for distinguishing tremor mechanisms. It is possible that changes in membrane kinetics based on the pattern of ion channel expression underlie the differences in tremor in some diseases.
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↵* Defining individual cycle: First, we normalised the de-trended data with the mean amplitude (ie normalized amplitude = actual amplitude − mean amplitude). This allows the amplitude to realign along the abscissa and the peaks of the cycles always remain positive and the troughs always negative. The ‘x’ co-ordinate of the intersection of the data trace (moving from the negative value to the positive value) with the abscissa was recorded. The value of the first data point marks the beginning and the subsequent data points mark the end of the given cycle.
Funding: Funding was provided by the Gustavus and Louise Pfeiffer Foundation, Ataxia-telangiectasia Children's Project.
Competing interests: None.