How normal is 'normal'? Mild tremor in a multiethnic cohort of normal subjects

Background: While many 'normal' subjects exhibit mild clinically detectable tremor, the extent to which this tremor is present has received little attention. Objective: To characterize the prevalence and clinical characteristics of mild, clinically detectable tremor in a multiethnic cohort of normal subjects. Methods: Normal control subjects (n=36) and their relatives (n=67) were enrolled in a community-based case-control study of the familial aggregation of essential tremor. Subjects underwent a tremor interview and videotaped tremor examination. Two neurologists independently rated the severity of tremor during different postures and tasks, and a total tremor score (maximum score, 36) was calculated for each subject. Eight subjects were randomly selected to undergo quantitative computerized tremor analysis. Results: In 103 normal subjects (36 control subjects and 67 relatives of control subjects) the mean total tremor score was 4.8 (range, 0- 12.5). Ninety-nine (96%) of 103 subjects had tremor, as defined by a total tremor score of 0.5 or higher. For most tasks, the mean tremor score was greater in the nondominant than in the dominant arm. The total tremor score correlated highly with age (r=0.28; P=.004). There were no sex or ethnic differences in the mean total tremor scores. On tremor analysis, the amplitude and frequency of the tremor differed from that in a group of similarly studied subjects with essential tremor. Conclusions: Normal subjects almost uniformly have a clinically detectable tremor that is mild and age dependent. Characterization of this tremor helps to establish standards for normal tremor. These standards are crucial for accurate diagnostic classification in population-based studies of essential tremor.