Familial risks of hospitalization for Parkinson's disease in first-degree relatives: a nationwide follow-up study from Sweden.

The aim of this study was to estimate familial risks of Parkinson's disease (PD) in first-degree relatives of probands with PD compared with first-degree relatives of control probands. Standardized incidence ratios (SIRs) of PD were estimated in the total Swedish population for the period January 1, 1987 to December 31, 2001. SIRs were calculated by age, sex, occupation, geographic region, family size, and type of related proband on the basis of first hospital diagnoses of PD during the study period. Results showed that during the study period, there were 65 cases of first-degree relatives who were hospitalized for PD out of the total 13,276 events (first hospital diagnoses of PD) between 1987 and 2001. Age-specific analyses of familial PD revealed that there was no apparent difference in SIRs by age category. Overall, significant SIRs for PD in first-degree relatives were 3.1 (95% CI 2.1-4.3) for men and 4.0 (95% CI 2.8-5.7) for women. When the related PD proband was a sibling, the SIR was significantly higher (8.7) than when the related proband was a parent (SIR=2.9, p=0.01) or a child (SIR=3.6, p=0.04). For spouses, no increased risks were found. In conclusion, the findings of the present study suggest that genetic factors are important in early- (age < or =50 years) and later (age >50 years) onset PD, and that shared environmental factors during childhood or recessive effects may partly be important for familial aggregation of the disease.