Epidemiologic studies indicate a genetic contribution to the pathogenesis of PD. Laz-zarini et al. (11) found that the likelihood of persons in New Jersey having PD at age 80 years was about 2% for the general population and about 5% to 6% if a parent or sibling was affected. However, if both a parent and a sibling were affected, the probability of having PD increased to 20% to 40%. Marder et al. (12) assessed the risk of PD among first-degree relatives living in the same geographic region (northern Manhattan, New York, U.S.A.). They found a 2% cumulative incidence of PD to age 75 years among first-degree relatives of patients with PD compared with a 1% incidence among first-degree relatives of control subjects. The risk of PD was greater in male than in female first-degree relatives [relative risk, 2.0; 95% confidence interval (CI): 1.1-3.4]. The risk of PD in any first-degree relative was also higher for whites than for African Americans or Hispanics (relative risk, 2.4; 95% CI: 1.4-4.1). Rocca et al. (13) reported that relatives of probands who were younger (<66 years) at onset of PD had a significantly increased risk (relative risk, 2.62; 95% CI: 1.66-4.15; P = 0.02), whereas relatives of probands with later onset had no increased risk.
In an Italian case-control study (14), history of familial PD was the most relevant risk factor (odds ratio, 14.6; 95% CI: 7.2-29.6). In a Canadian study of PD patients (15), the prevalence rate in first- and second-degree relatives was more than
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