Irregularity distinguishes limb tremor in cervical dystonia from essential tremor.

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 (approximately 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.