Risk of cognitive impairment or dementia in relatives of patients with Parkinson disease.

BACKGROUND The evidence for increased risk of dementia in relatives of patients with Parkinson disease (PD) remains conflicting. OBJECTIVE To study the risk of cognitive impairment or dementia in first-degree relatives of patients with PD. DESIGN, SETTING, AND PARTICIPANTS We conducted a historical cohort study of 1019 first-degree relatives of 162 patients with PD and of 858 relatives of 147 matched controls representative of the population of Olmsted County, Minnesota. In addition, we studied 2716 first-degree relatives of 411 patients with PD referred to Mayo Clinic. MAIN OUTCOME MEASURES We administered via telephone a cognitive test directly to relatives or a dementia questionnaire to proxies. For relatives reported by proxies to have dementia, we obtained copies of their medical records to confirm the diagnosis. We also obtained dementia information from a medical records-linkage system. RESULTS In the overall population-based sample, the risk of cognitive impairment or dementia was increased in relatives of patients with PD compared with relatives of controls (hazard ratio, 1.37; 95% confidence interval, 1.03-1.81; P = .03) and was particularly increased in relatives of patients with onset of PD at age 66 years or younger (youngest tertile; hazard ratio, 1.73; 95% confidence interval, 1.21-2.46; P = .003). The findings were consistent in several sensitivity analyses. In the referral-based sample, the risk of cognitive impairment or dementia in relatives increased with younger age at onset of PD but did not vary by other clinical characteristics. CONCLUSION Cognitive impairment or dementia may share familial susceptibility factors with PD (genetic or nongenetic).