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Is Dementia in Decline? Historical Trends and Future Trajectories

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Perspective

HISTORY OF MEDICINE

Is Dementia in Decline? Historical Trends and Future Trajectories

David S. Jones, M.D., Ph.D., and Jeremy A. Greene, M.D., Ph.D.

N Engl J Med 2016; 374:507-509February 11, 2016DOI: 10.1056/NEJMp1514434


In 2005, researchers from the Duke Center for Demographic Studies reported a “surprising trend”: data from the National Long-Term Care Surveys showed that the prevalence of severe cognitive impairment in the Medicare population had decreased significantly between 1982 and 1999.1 At a time when baby-boomer demographics led to predictions of a looming dementia crisis, this finding offered hope. Since that time, other reports have similarly shown that the incidence or prevalence of dementia is decreasing in various populations. Researchers have offered many possible explanations, including increased wealth, better education, control of vascular risk factors, and use of statins, antihypertensive agents, and nonsteroidal antiinflammatory drugs.1,2 However, even as researchers describe their “cautious optimism” about specific populations, they still project a quadrupling of global prevalence over the coming decades.3


In this issue of the Journal, Satizabal and colleagues report more “robust evidence” of dementia’s decline (pages 523–32). Using surveillance data collected from the Framingham Heart Study from 1975 to the present, they found a 20% decrease in dementia incidence each decade, even as average body-mass index, diabetes prevalence, and population age have increased. Can we now conclude that the tide has turned in the dementia epidemic? The potential decline of dementia, seen in light of the rise and fall of other major diseases, raises an even more tantalizing prospect: Can we control our burden of disease?


This is not the first time that the medical profession and the public health community have struggled to interpret reports of an unexpected reversal of a chronic-disease epidemic.4 In 1964, California health officials reported that rates of coronary artery disease (CAD) had begun to decrease. This finding, which defied the widespread belief that the CAD epidemic would only worsen as life expectancy grew, garnered scant attention. Even a decade later, most health officials assumed that CAD was still on the rise. It was only in 1974 that researchers began taking the prospect of decline seriously. By 1978, they had accepted that CAD’s national decline had begun in the mid-1960s. Similar decreases were soon reported in many other high-income countries, from Australia to Finland. This recognition triggered debate over the contribution of medical and public health interventions, in hopes that knowledge of the causes of decline would guide policies and resource allocation and ensure continuation of these health benefits.


The history of the debate on CAD decline carries important lessons for emerging reports of dementia’s decline. First, it can be extremely difficult to produce timely and convincing data about the trajectories of chronic diseases.4 When physicians began to debate CAD trends in 1974, they had to rely on government data that were 5 years out of date. It took 4 years of concerted effort to reach consensus about an inflection that had occurred more than a decade earlier. Even though better and timelier data are now available, dementia researchers must still be resourceful in seeking convincing data. As Satizabal et al. indicate, each existing report has limitations. Their new data, which overcome many of these limitations, demonstrate the value of investments in long-term, longitudinal epidemiologic research such as the Framingham Heart Study. But the data still reflect only one population sample. Whether they are accepted as conclusive evidence of a broad-based reduction in dementia incidence will become clear only over time.


Second, since trajectories of chronic-disease incidence reflect complex interactions of many causal factors, it will almost always be uncertain whether decreases will continue or reverse. Even as consensus about international CAD reduction consolidated between the 1970s and the 1990s, worrisome evidence about countervailing trends also appeared.4 Enthusiasm for anti-CAD public health campaigns has been fragile, even in countries like Finland that demonstrated their promise so well. The widespread increases in obesity and diabetes could fuel CAD resurgences. Many researchers have warned that CAD’s decline could stall or even reverse — something that has happened among young adults and other subpopulations in Europe, Australia, and the United States. Other countries, such as China, continue to see increases in CAD with no evidence of plateau or reversal.


All these countervailing trends could affect dementia as well. Rocca and colleagues have warned that increases in obesity, diabetes, and hypertension could undermine the gains achieved through improved education, wealth, and control of vascular risk factors. Even if a dementia decline has begun, it might not last: the outcome depends on the balance of diverging trends.2,3


Third, these ambiguities open up a battleground for conflicting interpretations by interested parties. Policymakers can use the same data to tell vastly different stories about public health. Forecasts of CAD’s future continue to swing between narratives of triumph and catastrophe.4 The good news is that more and more countries are reporting evidence of decline. The bad news is the evidence of the fragility of these gains.


Narratives of dementia remain similarly malleable. In the early 1980s, even after CAD’s decline had been accepted and despite knowledge that dementia shares many risk factors with CAD, physicians began to warn about an exploding dementia epidemic.5 The decrease in prevalence that surprised Manton and colleagues in 2005 could have been predicted decades earlier. But dementia will remain a problem despite these decreases. The prevalence of dementia can increase, even if the incidence falls, if global populations live longer. The absolute number of people with dementia can increase, even if both incidence and prevalence fall, if the size of the elderly population grows. That explains why, 10 years into the era of reports of decreasing dementia in selected populations, Satizabal and colleagues still write that the “prevalence of dementia is expected to soar as our societies age.” Even researchers rigorously examining the evidence of decreases continue to worry about what the future will bring.


History offers reasons for hope. Evidence of dementia’s decline shows once again that our burden of disease is malleable. This lesson has been hard won. Mid-19th-century physicians saw cholera and tuberculosis as inevitable scourges of urban environments. But those epidemics yielded to sanitary reform, improved standards of living, and eventually medical care. As control of infectious disease led to dramatic gains in life expectancy, physicians in the early 20th century came to see CAD and cancer as the inevitable scourges of long lives. Over recent decades, that pessimism has largely given way as well: CAD and many forms of cancer are increasingly preventable and curable. The burden of disease of the 20th century, like that of the 19th, was not an inevitable fact of life, but a product of lives lived amid specific — and malleable — conditions.


What should we expect as cancer and heart disease come under control? Many people think that we can live even longer lives — but lives compromised by dementia, vision loss, and hearing loss. Whether that fate is inevitable or whether these, too, are malleable scourges remains to be seen. Such questions are better left to futurists and geriatricians than to historians. Yet Satizabal et al. believe there’s cause for “cautious hope.” Primary and secondary prevention might diminish the magnitude of the long-feared dementia epidemic. Something else might save our vision and hearing.


Faced with choices between equally defensible epidemiologic projections, physicians and researchers must think carefully about what stories they emphasize to patients and policymakers. The implications, especially for investment in long-term care facilities, are enormous. Our explanations of decline are equally important, since they guide investments in behavior change, medications, and other treatments.


With this latest contribution, optimism about dementia is more justified than ever before. Even if death and taxes remain inevitable, cancer, CAD, and dementia may not. But cautious optimism should not become complacency. If we can elucidate the changes that have contributed to these improvements, perhaps we can extend them. Today, the dramatic reductions in CAD-related mortality are under threat. The incipient improvements in dementia are presumably even more fragile. The burden of disease, ever malleable, can easily relapse.


 


Incidence of Dementia over Three Decades in the Framingham Heart Study

Claudia L. Satizabal, Ph.D., Alexa S. Beiser, Ph.D., Vincent Chouraki, M.D., Ph.D., Geneviève Chêne, M.D., Ph.D., Carole Dufouil, Ph.D., and Sudha Seshadri, M.D.

N Engl J Med 2016; 374:523-532February 11, 2016DOI: 10.1056/NEJMoa1504327

 

Journal pre-amble: In the Framingham Heart Study, the incidence of dementia among participants 60 years of age or older has declined over three decades; the 5-year cumulative hazard rate declined from 3.6 per 100 persons in the 1970s and 1980s to 2.0 per 100 persons in the 2000s and 2010s.

 

Abstract

 

Dementia is the leading cause of dependence and disability in the elderly population worldwide.1-3 As the average life expectancy increases, the prevalence of dementia4 and associated monetary costs are expected to increase exponentially.5 A few studies have suggested that the age-specific incidence of dementia (i.e., the risk of dementia at any specific age) might be decreasing, but these studies either have shown a trend that failed to reach significance6,7 or have relied on comparisons of prevalence data that were ascertained at multiple time points.8-10 One study showed no decline in incidence.11 Temporal trends are best derived through continuous monitoring for new cases in a representative community-based sample over an extended observation period, with the use of consistent diagnostic criteria; however, such data from published studies are limited. We estimated temporal trends in the incidence of dementia over three decades among participants in the Framingham Heart Study.

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