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

Abstract

BACKGROUND/OBJECTIVES 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. DESIGN 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. MEASUREMENTS Age‐specific dementia/AD incidence rates and their standard errors reported in each study. RESULTS 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. CONCLUSION 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.

The Importance of Birth Year for the Incidence of Dementia

“…In addition, median birth year in different age groups was calculated, thus decreasing the precision of measurements. However, analyses using multiple cohorts for examination of time trends have come to similar conclusions as those of Gao et al…”

“…In this issue of the Journal of the American Geriatrics Society , Gao et al report a meta‐analysis on the influence of birth year on the incidence of dementia and Alzheimer disease (AD). They included 38 articles with 53 cohorts (comprising a total of 123 335 individuals) examining dementia incidence and 31 articles with 35 cohorts (comprising a total 89 376 individuals) on AD incidence.…”

“…Thus, knowledge of the epidemiology of dementia, its prevalence, incidence, and risk factors, is based mainly on cohorts from the Western world born in the late 19th century or the early 20th century, when living conditions were poor in large parts of the Western world . In the meta‐analysis of Gao et al, most participants were born in the 1920s and early 1930s.…”

“…This was, to a large extent, explained by the higher educational level in the later‐born cohort, but also by a decreased association between stroke and dementia, leading to a larger decrease in vascular dementia. This illustrates two factors, which are also highlighted by Gao et al: the importance of education and the better treatment of vascular disorders, such as stroke. It needs to be emphasized that treatment of elevated diastolic blood pressure was not initiated until the 1960s, and treatment of elevated systolic blood pressure and of hypercholesterolemia was not initiated until the 1990s.…”

“…It is noteworthy that changes between birth cohorts seem to be more accentuated among women, including changes in personality, depression, and blood pressure . The meta‐analyses by Gao et al did not report any sex differences in relation to birth cohort changes in the incidence of dementia. Considering changes in sex roles worldwide, this needs to be examined in more detail in the future.…”