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) a nd its relationship to other types of tremor is unclear. In the present stu dy, we have compared the tremor in these patients with that seen in patient s with essential tremor (ET) using two neurophysiological techniques: the t riphasic EMG pattern accompanying ballistic wrist flexion movements; and re ciprocal inhibition between forearm muscles. During ballistic wrist flexion movements, the latency of the second agonist EMG burst was later in ET tha n 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 i nhibition in patients with ET or CD. However, there was much more variabili ty 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 inhibi tion and group B with reduced or absent presynaptic inhibition. A posterior i, it turned out that the patients in these two subgroups had similar clini cal symptoms, but different clinical histories. The arm tremor of patients in group A started simultaneously with torticollis (mean onset age of arm t remor 40 years +/- 20.7 SD, interval between onset of arm tremor and tortic ollis 0 +/- 2.9 years) whereas it began much earlier (mean onset age 14 yea rs +/- 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 a nd the antagonist burst than those patients in group B. We conclude that ar m 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 a rm 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 tre mor' and 'tremor associated with dystonia'.