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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