The oracle at Delphi had legendary powers of foresight. But would the prophetess have predicted a staggering 81 million cases of dementia worldwide by 2040? Probably not. That onus falls on Cleusa Ferri, from King’s College, London, and colleagues at Alzheimer’s Disease International, an umbrella organization for national AD groups worldwide. In the December 17 Lancet, Ferri and coworkers report a reanalysis of published epidemiology data. Using the predictive power of a technique called the Delphi consensus, they divined some sobering predictions, most notably that a new case of dementia would arise every seven seconds worldwide and that dementia in the developing world will increase at an alarming rate over the next 40 years.

Researchers use the Delphi consensus method to make quantitative predictions from qualitative analysis—and yes, it is named after the ancient Greek oracle. The consensus emerges when members of a panel of experts independently review data and then anonymously share their findings. Each member of the panel then gets an opportunity to revise his or her own estimates while considering their colleagues’ predictions. Human nature being what it is, the panel tends to converge on a reasonably solid consensus without the added stress of peer pressure. These consensus methods have become widely accepted in recent years. They offer an advantage over the more quantitative meta-analyses because, being based on qualitative assessment, they can include studies that have sparse or poor data. In fact, when numbers are missing, the method allows the experts to fill in the blanks by extrapolating from similar data obtained elsewhere. One hopes that Sybil, the guardian priestess, would approve.

Ferri and colleagues trained the consensus technique on epidemiology data published between 1980 and 2004 and came up with predictions for the prevalence of dementia in 14 World Health Organization (WHO) regions. WHO defines these based on both geography and patterns of child and adult mortality. The authors stuck to dementia rather than AD or other specific dementias because most of the published data does not document the ratio of different subtypes.

The consensus panel of 12 experts estimated that the total number of people with dementia worldwide today is just over 24 million, which fits some previous predictions (see, for example, Wimo et al., 2003). But they paint a dire picture of how dementia numbers are set to increase over the next 35 years or so, especially in developing countries.

The authors predict that the number of people with dementia in developed countries, which already have a high prevalence, will double between 2001 and 2040. In developing countries, however, the numbers are set to increase by up to fourfold. China and the developing western Pacific, for example, will have more than 26 million dementia patients, aged 60 and older, in 2040. That is up from an estimated six million in 2001. Worldwide, cases are set to double every 20 years until 2040.

The authors caution that their predictions have limitations. “Although the expert panel achieved high levels of consensus, this was often on the basis of scant epidemiological evidence,” they write. Nonetheless, they also suggest that their estimates for developing countries are, if anything, conservative. As patterns of morbidity and mortality in developing countries converge with those in the developed West, dementia prevalence will do likewise, they suggest. “The implication is that our projections for dementia in developing regions might be conservative,” they write.

The bigger question here is how this data will be used. In ancient Greece, the government consulted the oracle at Delphi before making major policy decisions. One can only hope that current lawmakers will heed these latest predictions, too.—Tom Fagan


  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.

  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.

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Paper Citations

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

External Citations

  1. Alzheimer’s Disease International

Further Reading

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Primary Papers

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