Abstract

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 CD and 35 ET patients.

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 mean frequency was not significantly different between the groups. The cycle-to-cycle variability of the frequency (i.e., 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 upon the pattern of ion channel expression underlie the differences in tremor in some diseases.