Relationship of age of onset and family history in Parkinson disease

Background: The aim of this study was to determine whether age of onset of Parkinson disease (PD) is associated with differences in PD risk and PD age of onset in parents and siblings. Methods: Clinical and detailed family history data were available for 1,114 PD probands. Results: Proband age of onset was not associated with differences in PD prevalence or PD age of onset in parents. Proband age of PD onset <50, compared with ≥50 years, was associated with significantly greater risk of PD in siblings (hazard ratio: 2.4; P=0.002; 95% confidence interval: 1.4, 4.1), and proband age of onset was significantly correlated with sibling age of onset (Somer's D=0.20; P=0.018). Conclusions: Proband age of PD onset is not associated with differences in parental PD risk. Siblings of PD patients with onset before age 50 are at increased risk of PD and are more likely to have early-onset disease.