Increased family risk of PD limited to early-onset disease

Mayo Clinic researchers report in the Annals of Neurology, that their study, "provides further evidence that the familial aggregation of PD is modest overall, and it is restricted to the families of probands with younger onset of PD."Dr Rocca and colleagues studied 3,714 relatives of Parkinson's disease (PD) probands and 851 relatives of control probands. Probands had either been referred to the Mayo Clinic over a four-year period or part of an historical cohort of the population of Olmsted County, Minnesota. An additional 625 spouses of PD probands were included in the study as controls.The researchers used the "family study method" to determine which relatives showed signs of parkinsonism. They explain, "Whenever possible, relatives were interviewed and screened for parkinsonism either directly or through a proxy, and those who screened positive were examined or a copy of their medical record was obtained to confirm the diagnosis." The authors argue that this is a more precise approach to the family history method, where only the proband or a proxy for each family provides information on the occurrence of parkinsonism. The study found that relatives of probands with younger onset PD (at age 66 years or younger) had a significantly increased risk (relative risk, 2.62; 95% confidence interval, 1.66-4.15) of developing the disease, whereas relatives of probands with later onset PD had no increased risk. When considering all types of parkinsonism, the authors found no significantly increased risk. They say, "This finding suggests that the familial aggregation observed is related to PD, not to other types of parkinsonism."The authors note, "Our study confirms three previous reports suggesting an increased risk of PD among first-degree relatives of probands with younger onset of PD, but not among relatives of probands with later onset of PD."Reference...