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Age of onset was bimodally distributed in clinic cases. There were differences in the anatomic distribution of the tremor (arm tremor only vs head and arm tremor vs isolated head tremor). Rate of progression was distributed exponentially; there was a large cluster of subjects with slower rates of progression, and a smaller number who had faster rates. There was an association between age of onset and rate of progression (r=0.46-0.50, P<.002); cases with older age of onset (>60 years) progressed more rapidly (P<.001). In addition, upper limb tremor progressed more slowly among those with concomitant head tremor (P=.03). Essential tremor is not a homogeneous condition. There are differences in age of onset, anatomic distribution of tremor, and rate of progression. The ET in several groups of patients in this study 9those with age of onset >60 years and those without head tremor) progressed more rapidly, suggesting that these ET cases may define distinct clinical subtypes. These subtypes should be further assessed for etiologic and genetic heterogeneity as well as differences in responsiveness to therapeutic agents. |
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