Incidence of Dementia and Alzheimer Disease Over Time: A Meta-Analysis.

Abstract

Population-based incidence estimates of dementia and Alzheimer disease (AD) provide important information for public health policy and resource allocation. We conducted a meta-analysis of published studies that reported age-specific incidence rates of dementia and AD to determine whether dementia and AD incidence rates are changing over time. PubMed and MEDLINE were searched for publications through June 30, 2017, using key words "dementia", "Alzheimer", and "incidence." Inclusion criteria for the meta-analysis are: (1) population-based studies using personal interviews and direct examinations of the study subjects, (2) standardized clinical diagnosis criteria, (3) reporting age-specific incidence rates, (4) published in English, and (5) sample size of 500 or greater and length of follow-up of 2 years or greater. Mixed-effects models were used to determine the association between birth year and incidence rates. Age-specific dementia/AD incidence rates and their standard errors reported in each study. Thirty-eight articles with 53 cohorts on dementia incidence and 31 articles with 35 cohorts on AD incidence met the inclusion criteria. There were significant associations between later birth years and decreased dementia incidence rates in all three age groups (65-74, 75-84, and 85 years and older). There were no significant associations between birth year and AD incident rates in any of the three age groups. In particular, AD incidence rates reported from Western countries stayed steady in all age groups, while studies in non-Western countries showed significantly increased AD incidence rates for the 65 to 74 years age group (odds ratio = 2.78; P = .04), but a nonsignificant association for the 75 to 84 or 85 years and older groups. Dementia incidence declined over the past four decades, but AD incidence did not decline. Further research, especially from non-Western countries, is needed to elucidate the mechanism underlying the trends in dementia and AD incidence over time.