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 for tremor. Even among older case relatives (> or =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.