. Global prevalence of dementia: a Delphi consensus study. Lancet. 2005 Dec 17;366(9503):2112-7. PubMed.


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  1. This report of a Delphi consensus conference predicting the number of dementia cases endeavors to replace qualitative awareness of rapidly growing prevalence with specific figures to guide care planning in the coming decades, both in the U.S. and other developed countries, and in the much larger population of the developing world. The Delphi method of evidence-based consensus through repeated estimation by experts, rather than a simple meta-analysis, was employed to supplement evidence with expert opinions where data were sparse. The experts' estimates—of 40 million dementia cases worldwide by 2020, and 80 million by 2040—are sobering, indeed. Still more sobering are the specific figures for countries such as India, China, and their neighbors, which are experiencing more rapid demographic shifts.

    The overall figures are thoughtfully derived, taking into account regional and national demographic differences, and are likely to represent a fair estimate of prevalence in the coming decades. Even so, several areas of uncertainty should be borne in mind. One is the apparently lower age-specific rates of dementia in developing countries, particularly in Africa and the poorer parts of Southeast Asia. Although there are plausible reasons that such differences might be real, they could also result from the limitations of applying assessment tools developed in Western countries in a very different context. If the actual rates in these developing regions are equivalent to those in the developed world, then the estimated numbers of cases would be that much greater.

    Another source of uncertainty is dementia rates in the "85 and over" category. This is the fastest-growing segment of the population in the developed world, and the mean age of this segment is rising as increasing numbers live into their nineties and beyond. Although only limited data are available, there is some evidence that the risk for dementia may actually fall after age 90. Whether this would affect the overall number of cases depends on a number of factors, particularly life expectancy in those with dementia.

    Last, of course, these numbers presume no change in demographic patterns around the world, and no significant impact of treatment or prevention. Alas, the former are largely beyond our control, and some changes (such as an effective AIDS vaccine or reduction of early mortality in Eastern Europe) would only increase the number of dementia cases. The latter, on the other hand, may be within our reach, and numbers like these should inspire ongoing efforts for treatment and prevention.

    View all comments by Deborah Blacker
  2. In this article, the authors present worldwide prevalence figures of dementia that are very close to the figures we published in 2003 (Wimo et al., 2003) (24.3 million in 2001 worldwide in the Lancet paper; 25.5 million in our ADAD paper). The authors contrast their figures to ours, particularly regarding the prevalence in Africa (0.49 million in their paper, 1.5 million in ours). However, table 2 in our paper contains continent-specific values where we present alternative figures, that is, 0.41 million for Africa, which is rather close to their figure for Africa. The worldwide prevalence figure on our alternative calculation was 24.2 million in 2000 (to be compared to 24.3 million in 2001 in their paper). So one may ask, what's new in the Lancet paper?

    Furthermore, we have updated the prevalence to 2003 in a forthcoming paper (Dementia and Geriatric Cognitive Disorders), in which we also present an estimate of the worldwide direct costs of dementia.


    . The magnitude of dementia occurrence in the world. Alzheimer Dis Assoc Disord. 2003 Apr-Jun;17(2):63-7. PubMed.

    View all comments by Bengt Winblad

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