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What Does This Tell Us About the Penetrance of the Disease?

Elan D. Louis, MD, MS; Blair Ford, MD; Steven Frucht, MD; Ruth Ottman, PhD

Arch Neurol. 2001;58:1584-1589.

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 (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.

From the Gertrude H. Sergievsky Center (Drs Louis and Ottman) and the Department of Neurology (Drs Louis, Ford, and Frucht), College of Physicians and Surgeons, and Mailman School of Public Health (Epidemiology Division) (Dr Ottman), Columbia University, New York, NY; and the Epidemiology of Brain Disorders Research Department, New York State Psychiatric Institute, New York (Dr Ottman).

Corresponding author and reprints: Elan D. Louis, MD, MS, Unit 198, Neurological Institute, 710 W 168th St, New York, NY 10032 (e-mail: EDL2{at}Columbia.edu).

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