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
five times higher than that of the general population. Even patients who reported a negative family history of PD actually had a prevalence rate in relatives more than three times higher than that of the general population. A study of the Icelandic population (16) revealed the presence of genetic as well as environmental components in the pathogenesis of late-onset PD (onset at >50 years of age). The risk ratio for PD in relatives of patients with late-onset PD was 6.7 (95% CI: 4.3-9.6) for siblings, 3.2 (95% CI: 1.2-7.8) for offspring, and 2.7 (95% CI: 1.6-3.9) for nephews and nieces. The findings of Maher et al. (17) for 203 sibling pairs with PD also supported a genetic contribution to the pathogenesis of PD. Sibling pairs with PD were found to be more similar in age at symptomatic disease onset than in year of symptomatic disease onset. The frequency of PD in parents (7%) and siblings (5.1%) was greater than that in spouses (2%).
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