Despite the concern of an explosion of dementia cases in an ageing population over the next few decades, a new study based on data from the Framingham Heart Study (FHS) suggests that the rate of new cases of dementia actually may be decreasing.
These findings provide hope that some cases of dementia might be preventable or delayed and encourage funding agencies and the scientific community to further explore demographic, lifestyle and environmental factors underlying this positive trend.
“Currently there are no effective treatments to prevent or cure dementia; however, our study offers hope that some of the dementia cases might be preventable – or at least delayed – a through primary (keep the disease process from starting) or secondary (keep it from progressing to clinically obvious dementia) prevention,” explained corresponding author Dr Sudha Seshadri, professor of neurology at Boston University School of Medicine and FHS senior investigator. “Effective prevention could diminish in some measure the projected explosion in the number of persons affected with the disease in the next few decades,” she added.
It is believed that the number of Americans with Alzheimer’s disease and other dementias will grow each year as the size and proportion of the US population age 65 and older continues to increase. By 2025 the number of people age 65 and older with Alzheimer’s disease is estimated to reach 7.1m – a 40% increase from the 5.1m of the same age affected in 2015. By 2050, the number of people in this population with Alzheimer’s disease may nearly triple, from 5.1m to a projected 13.8m, barring the development of medical breakthroughs to prevent or cure the disease.
The World Health Organisation estimates that 47.5m people have dementia in the world and the total number of people with dementia is projected to reach 75.6m in 2030 and almost triple by 2050 to 135.5m.
FHS participants have been monitored continuously for the occurrence of cognitive decline and dementia since 1975. Thanks to a rigorous collection of information, FHS researchers have been able to diagnose Alzheimer’s disease and other dementias using a consistent set of criteria over the last three decades. These sources of information include FHS exams, outside clinical records, interviews with family members, and the examination of participants suspected of having a neurological problem by neurologists and neuropsychologists.
Researchers looked at the rate of dementia at any given age and attempted to explain the reason for the decreasing risk of dementia over a period of almost 40 years by considering risk factors such as education, smoking, blood pressure and medical conditions including diabetes, high blood pressure or high cholesterol among many others.
Looking at four distinct periods (the late 1970s, late 80s, 90s and 2000s), the researchers found that there was a progressive decline in incidence of dementia at a given age, with an average reduction of 20 percent per decade since the 1970s, when data was first collected. The decline was more pronounced with a subtype of dementia caused by vascular diseases, such as stroke. There also was a decreasing impact of heart diseases, which suggests the importance of effective stroke treatment and prevention of heart disease. Interestingly, the decline in dementia incidence was observed only in persons with high school education and above.
The FHS consistently has been shown to be a reliable source of data. However the authors concede that the sample population is overwhelmingly of European ancestry and that further studies are needed to extend the findings to other populations. In addition, the authors also did not look at the effects of key variables such as changes in diet and exercise.
Despite these limitations, “it is very likely that primary and secondary prevention and better management of cardiovascular diseases and stroke, and their risk factors, might offer new opportunities to slow down the currently projected burden of dementia for the coming years” add Carole Dufouil, Inserm research director in Bordeaux, France. Yet, the authors warn that this does not mean that the total number of persons with dementia will decrease anytime soon. Since baby boomers are ageing and people are living longer, the burden of dementia will continue to grow.
This particular type of study requires an enormous amount of data collected over many years on the same persons, and the study was only possible thanks to the generosity and dedication of FHS participants, who contribute their time and data with a missionary zeal. There is a plaque in the centre of Framingham town that states, Framingham, the town that changed America’s heart! released at the 50th anniversary of the FHS in 1998. Now the town can claim some credit for changing America’s brain health, too.
The study also was possible thanks to the work of earlier generations of researchers, as well as colleagues at the FHS who contribute to ongoing data collection. This study was carried out in collaboration between Inserm researchers at the Bordeaux School of Public Health, France, and BUSM.
The prevalence of dementia is expected to soar as the average life expectancy increases, but recent estimates suggest that the age-specific incidence of dementia is declining in high-income countries. Temporal trends are best derived through continuous monitoring of a population over a long period with the use of consistent diagnostic criteria. We describe temporal trends in the incidence of dementia over three decades among participants in the Framingham Heart Study.
Participants in the Framingham Heart Study have been under surveillance for incident dementia since 1975. In this analysis, which included 5205 persons 60 years of age or older, we used Cox proportional-hazards models adjusted for age and sex to determine the 5-year incidence of dementia during each of four epochs. We also explored the interactions between epoch and age, sex, apolipoprotein E ε4 status, and educational level, and we examined the effects of these interactions, as well as the effects of vascular risk factors and cardiovascular disease, on temporal trends.
The 5-year age- and sex-adjusted cumulative hazard rates for dementia were 3.6 per 100 persons during the first epoch (late 1970s and early 1980s), 2.8 per 100 persons during the second epoch (late 1980s and early 1990s), 2.2 per 100 persons during the third epoch (late 1990s and early 2000s), and 2.0 per 100 persons during the fourth epoch (late 2000s and early 2010s). Relative to the incidence during the first epoch, the incidence declined by 22%, 38%, and 44% during the second, third, and fourth epochs, respectively. This risk reduction was observed only among persons who had at least a high school diploma (hazard ratio, 0.77; 95% confidence interval, 0.67 to 0.88). The prevalence of most vascular risk factors (except obesity and diabetes) and the risk of dementia associated with stroke, atrial fibrillation, or heart failure have decreased over time, but none of these trends completely explain the decrease in the incidence of dementia.
Among participants in the Framingham Heart Study, the incidence of dementia has declined over the course of three decades. The factors contributing to this decline have not been completely identified.