Is essential tremor symmetric? Observational data from a community-based study of essential tremor

Background: Essential tremor (ET) has been variably portrayed in the litera ture both as a symmetric arm tremor and as an asymmetric arm tremor. Few qu antitative clinical or neurophysiological data specifically address the iss ue of tremor asymmetry in ET. Objectives: To examine a community-dwelling cohort of subjects with ET to ( 1) estimate the prevalence of tremor asymmetry and (2) quantify the magnitu de of tremor asymmetry. Methods: Fifty-four subjects with ET, identified in a community-based study of ET in New York City, underwent a Tremor Interview and a videotaped Trem or Examination. The examination included 6 tasks: sustained arm extension, pouring water, drinking water, using a spoon, finger-to-nose movements, and drawing spirals with each arm. Two neurologists rated the severity of trem or using a 0 to 3 clinical rating scale and a total tremor score was calcul ated (range, 0-36). Fourteen (25%) of 54 subjects also underwent quantitati ve computerized tremor analysis. Results: The prevalence of asymmetry depended on the definition of asymmetr y; small to moderate differences between sides were common. The mean side-t o-side difference in clinical ratings for each of the 6 tasks was 0.54 of 3 points, which represented a 1.32-fold difference between sides. Clinical r ating scores were higher in the nondominant arm in 39 subjects (72%), highe r in the dominant arm in 9 (17%), and equal in 6 (11%). The 2 left-handed s ubjects had higher clinical ratings on the right. During quantitative compu terized tremor analysis, there was a 1.71-fold mean difference between trem or amplitudes in the dominant and nondominant sides, and in 12 subjects (86 %), the maximum tremor amplitude was in the nondominant arm. Conclusions: Small to moderate differences between sides were common in ET. In most community-dwelling subjects, tremor amplitude was greatest in the nondominant arm. In contrast, clinic-based studies have reported greater tr emor in the dominant arm; those with ET who seek medical attention are more likely to exhibit severe tremor in their dominant arms. This study documen ts that mild asymmetry is a fundamental property of ET and that tremor is m ore severe in the nondominant arm.