Mild tremor in relatives of patients with essential tremor - What does this tell us about the penetrance of the disease?

Background: Mild tremor may occur in relatives of patients with essential tremor (ET). However, this phenomenon has not been studied quantitatively or with a comparison group. Such a study may provide information on the penetrance of ET. Objective: To obtain data on the magnitude of tremor in case and control relatives who did not meet diagnostic criteria for ET. Methods: Cases with ET and control subjects from the Washington Heights-Inwood community in northern Manhattan, NY, were enrolled in a family study. Their first- and second-degree relatives underwent a videotaped tremor examination. Two neurologists rated the severity of tremor, assigning a total tremor score (0-36 [maximum]). Data were analyzed on 201 case relatives and 212 control relatives who did not meet diagnostic criteria for ET. Results: The mean total tremor score of first-degree case relatives was higher than that of first-degree control relatives (4.9 vs 3.9; P < .003). Total tremor scores for second-degree relatives did not differ (4.1 vs 4.2; P=.68). A larger percentage (55.2% vs 36.6%; P=.01) of first-degree case relatives had total tremor scores of 4 or more. Among first-degree relatives who were older than 60 years, 13 case relatives (59.1%) and 18 control relatives (45.0%) had total tremor scores of 4 or more. Conclusions: A considerable number of seemingly normal case relatives may have a genetic predisposition or tremor. Even among older case relatives (greater than or equal to 60 years of age), there was an increased prevalence of higher tremor scores, suggesting that in that age group, subclinical ET may be present and penetrance still may not be complete.