Arm tremor in cervical dystonia differs from essential tremor and can be classified by onset age and spread of symptoms

The pathophysiology of arm tremor in patients with cervical dystonia (CD) and its relationship to other types of tremor is unclear. In the present study, we have compared the tremor in these patients with that seen in patients with essential tremor (ET) using two neurophysiological techniques: the triphasic EMG pattern accompanying ballistic wrist flexion movements; and reciprocal inhibition between forearm muscles. During ballistic wrist flexion movements, the latency of the second agonist EMG burst was later in ET than CD patients. This suggests that the mechanism of the arm tremor in CD may differ from that in ET. There was no group difference between reciprocal inhibition in patients with ET or CD. However, there was much more variability in the data from patients with CD. Because of this, we subdivided the CD patients into two groups, group A with normal levels of presynaptic inhibition and group B with reduced or absent presynaptic inhibition. A posteriori, it turned out that the patients in these two subgroups had similar clinical symptoms, but different clinical histories. The arm tremor of patients in group A started simultaneously with torticollis (mean onset age of arm tremor 40 years ± 20.7 SD, interval between onset of arm tremor and torticollis 0 ± 2.9 years) whereas it began much earlier (mean onset age 14 years ± 6 SD) and preceded onset of torticollis by a longer interval (21.6 ± 17.5 years) in patients of group B. Patients in group A also had less co-contraction in their ballistic wrist movements between the first agonist and the antagonist burst than those patients in group B. We conclude that arm tremor in patients with CD may have a mechanism different from that seen in patients with ET. Moreover, the data imply that there are two subgroups of CD patients with arm tremor, one with a late and simultaneous onset of arm tremor and torticollis (group A), and another with an early onset of arm tremor and later development of torticollis (group B). These groups do not correspond to the currently proposed clinical subdivision of 'dystonic tremor' and 'tremor associated with dystonia'.