Familial coaggregation of Alzheimer's disease and Parkinson's disease: systematic review and meta-analysis.

BACKGROUND Familial aggregation has been shown for Alzheimer's disease (AD) and Parkinson's disease (PD) separately, and it has been hypothesized that these diseases also coaggregate in families. METHODS The authors investigated familial coaggregation of AD and PD by conducting a systematic review and meta-analysis. PubMed was searched for relevant studies published through the end of October 2012. Three independent investigators screened publications and extracted data. Relative risk estimates of AD risk associated with family history of PD or parkinsonism, or PD risk associated with family history of AD or dementia, were summarized into metaestimates using random effects models. Heterogeneity and publication bias were tested using Higgins' and Egger's tests, respectively. RESULTS We included 16 studies in the review, with 14 included in any meta-analysis. AD risk associated with family history of PD yielded a summary hazard ratio of 1.18 (95% CI: 1.00-1.39) based on 5 reconstructed cohort studies and a summary odds ratio (OR) of 1.40 (95% CI: 0.92-2.12) based on 7 case-control studies. PD risk associated with family history of AD yielded a summary OR of 0.75 (95% CI: 0.49-1.16) based on 3 studies. There was no significant heterogeneity among studies, nor significant publication bias. CONCLUSIONS There may be familial coaggregation of AD and PD, although the association was modest and only apparent when studying AD risk associated with family history of PD.