27 July 2013. In line with recent analyses in the U.S. and Europe, a July 16 paper in the journal Lancet reported a substantial drop in dementia prevalence among older adults in England over the last two decades. According to the new research, dementia rates fell 24 percent—from 8.3 to 6.5 percent of seniors aged 65 years and older—between 1991 and 2011. “This study was designed to test for differences across sites and across time by using methods that allow comparisons with the power to detect such differences. It is the only study that does this,” noted senior investigator Carol Brayne, Cambridge Institute of Public Health, UK, in an email to Alzforum. The data received wide attention at the Alzheimer’s Association International Conference held July 13-18 in Boston, Massachusetts, and in lay media outlets (e.g. see New York Times and National Public Radio). Alzforum recently covered this trend in depth (see ARF related news story).
Estimates of 1991 dementia rates in England came from the country’s first Cognitive Function and Ageing Study (CFAS I)—a two-stage process with screening followed by diagnostic assessment. The first phase of CFAS I analyzed data from 7,635 seniors at residential home and care facilities in Cambridgeshire, Newcastle, and Nottingham. They were interviewed between 1990 and 1993 about their lifestyle, health, cognition, and sociodemographic characteristics. The investigators selected 20 percent of those people, representing a broad spectrum of cognition, to take the geriatric mental state (GMS) examination. From this they estimated dementia prevalence using a computerized algorithm. CFAS II similarly analyzed data collected between 2008 and 2011 from a near identically sized cohort from the same geographic areas; the only difference was that screening and assessment occurred in one phase.
Age- and sex-specific prevalence estimates from CFAS I put the number of demented elderly in England at 664,000. Population aging effects would increase that figure to 884,000, the researchers report. However, CFAS II found that only 670,000 seniors had dementia in 2011—214,000 (24 percent) fewer than predicted by population aging.
To account for possible skewing due to difficulty enrolling participants, the researchers tested various scenarios that could have nudged the 2011 prevalence figures upward. For instance, they re-calculated the figures assuming that non-responders’ dementia risk was 50 percent, or even double, that of study participants. These scenarios did increase prevalence estimates, but not to 1991 levels, lending credence to the drop in dementia rates in the past two decades.
Other research has also hinted at declining dementia burden in high-income countries. Recent research from Sweden suggests that dementia incidence, or the number of new cases per year, has plunged as public health improved (Qiu et al., 2013). In this study, dementia prevalence hardly budged, though the study was smaller and had a shorter time period than the UK analysis, Brayne noted. Dementia incidence seems to be on the wane in the Netherlands (Schrijvers et al., 2012) and the U.S. (see Rocca et al., 2011), and analysis of a southern European cohort found a drop in dementia prevalence, as well (Lobo et al., 2007).
Trends seem different in Asia. One study documents a rising number of dementia and Alzheimer’s disease cases over the past two decades in Japan (Sekita et al., 2010), and a large meta-analysis found that dementia burden in China may be far larger than previously estimated (ARF related news story). Meanwhile, Yu Tzu Wu, a graduate student in Brayne’s lab, analyzed epidemiological studies of dementia in China, Hong Kong, and Taiwan (Wu et al., 2013). Wu found that apparent increases in prevalence “melted away when she took into account methodological changes over time,” Brayne said. “This highlights the need to hold the methodology steady when making inferences about change (in dementia estimates).”—Esther Landhuis
Matthews FE, Arthur A, Barnes LE, Bond J, Jagger C, Robinson L, Brayne C on behalf of the Medical Research Council Cognitive Function and Ageing Collaboration. Lancet. 16 July 2013. Abstract