In a Policy View published in The Lancet Neurology journal, a group of leading experts on the epidemiology of dementia state that the number of people with dementia - both new cases and total numbers with the disease - in some Western European countries is stabilising despite population ageing, in direct contrast to the "dementia epidemic" reported in some recent studies.
The Policy View discusses data from five large epidemiological studies done in Sweden, the Netherlands, the UK, and Spain that compare dementia occurrence in old people across two periods of time using the same methods of diagnosing dementia in the same geographical regions (figure 1 ). The findings suggest that prevalence (ie, the percentage of the population with dementia) and incidence (the number of new dementia cases over a given time) of dementia in specific age groups are falling across time and generations (figure 3 and panel 3).
Estimates of the proportion of dementia cases within countries are needed to plan for the provision of care, yet much of the evidence used at both national and local levels (eg, the UK's NHS primary care targets) is based on research started in the 1980s. "These old studies support the idea of a continuing 'dementia epidemic', but are now out of date because of changes in life expectancy, living conditions, and improvements in health care and lifestyle,"  says Carol Brayne, lead author and Professor of Public Health Medicine at the Cambridge Institute of Public Health (CIPH), University of Cambridge in the UK.
Findings from four of the five studies analysed in the Policy View showed non-significant changes in overall dementia occurrence over the past 20 to 30 years. The UK study showed a significant reduction (about 22%) in overall prevalence in people aged 65 years in 2011 than the predicted estimates in 1990, resulting in stabilisation of estimated numbers of people with dementia. Results from the study done in Zaragoza (Spain) showed a significant decline in dementia prevalence in men aged 65 and older (about 43%) between 1987 and 1996. The studies done in Stockholm (Sweden) and Rotterdam (the Netherlands) showed that the age-specific incidence of dementia is falling in these regions.
"The suggested decrease in dementia occurrence coincides with improvements in protective factors (such as education and living conditions) for dementia and a general reduction in risk factors (such as vascular diseases) over recent decades," explains Brayne. "Incidence and deaths from major cardiovascular diseases have decreased in high-income countries since the 1980s. We are now potentially seeing the results of improvements in prevention and treatment of key cardiovascular risk factors such as high blood pressure and cholesterol reflected in the risk of developing dementia." 
According to the researchers, although the decrease in dementia occurrence is a positive sign, dementia care will remain a crucial challenge for many years because of population ageing. "It is important to remember that the number of people over age 85 is the fastest growing age demographic, with about 40% currently estimated to be affected by dementia,"  says co-author Yu-Tzu Wu from CIPH, University of Cambridge in the UK.
Professor Brayne concludes, "Our up-to-date evidence suggests a relatively optimistic picture of possible future trends in dementia occurrence and strengthens the need to shift more of our societal and research focus to primary prevention across the lifecourse, with a rebalancing from what could be seen as the current overemphasis on diagnostics and drug interventions for dementia (which detect early or later assumed pathology). Policies which address determinants of health in earlier life stages and enhance cognitive reserve for populations may have the greatest long term impact on reduction of dementia risk at given ages in later life as well as on population health more generally." 
NOTES TO EDITORS:
 The studies allow meaningful comparisons of prevalence and incidence over time because they use the same methods of diagnosing dementia between two time-points.
 Quotes direct from authors and cannot be found in text of Policy View.
The Lancet Neurology