Rates of New Dementia Cases May Be Falling

Despite concerns about an explosion of dementia cases in an aging population over the next few decades, a new study, based on data from Boston University-administered Framingham Heart Study (FHS), suggests that the rate of new cases of dementia actually may be decreasing over time.

The study in the New England Journal of Medicine, co-authored by researchers from BU’s schools of Medicine (MED) and Public Health (SPH), offers hope that some cases of dementia might be prevented or delayed and encourages funding agencies and the scientific community to further explore demographic, lifestyle, and environmental factors underlying the positive trend. Co-authors include Alexa Beiser, professor of biostatistics at SPH and of neurology at MED.

Experts believe 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 aged 65 and older continues to increase. By 2025, the number of people 65 and older with Alzheimer’s disease is projected to reach 7.1 million—a 40 percent increase from the 5.1 million people affected in 2015. By 2050, the number of people in this age group with Alzheimer’s disease may nearly triple, to a projected 13.8 million, barring the development of medical breakthroughs to prevent or cure the disease.

Worldwide, the World Health Organization (WHO) estimates that 47.5 million people have dementia, and that the total number of people with dementia could reach 75.6 million in 2030.

FHS participants have been continuously monitored for the occurrence of cognitive decline and dementia since 1975. Sources of information on their cognitive health include FHS exams, outside clinical records, interviews with family members, and the examination of participants suspected of having neurological problems by neurologists and neuropsychologists.

For the new study, researchers looked at the rate of dementia at various ages and attempted to explain the reason for a decreasing risk of dementia over a period of almost 40 years. They considered risk factors such as education and smoking, and medical conditions including diabetes, high blood pressure, and high cholesterol.

Looking at four distinct periods in the late 1970s, late 1980s, 1990s, and 2000s, the researchers found that there was a progressive decline in the incidence of dementia at a given age, with an average reduction of 20 percent per decade since the 1970s. The decline was more pronounced with a subtype of dementia caused by vascular diseases, such as stroke.

Interestingly, the decline in dementia incidence was observed only in people with a high school education and above.

The five-year, age- and sex-adjusted cumulative hazard rates for dementia were 3.6 per 100 persons during the first period (late 1970s and early 1980s), 2.8 per 100 persons during the second period (late 1980s and early 1990s), 2.2 per 100 persons during the third period (late 1990s and early 2000s), and 2.0 per 100 persons during the fourth period (late 2000s and early 2010s).

The authors say that while 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, none of those trends completely explains the decrease in the incidence of dementia.

“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—through primary (keep the disease process from starting) or secondary (keep it from progressing to clinically obvious dementia) prevention,” explains corresponding author Sudha Seshadri, professor of neurology at MED 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.”

The FHS consistently has been considered a reliable source of data. However, the study 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 did not look at the effects of key variables, such as changes in diet and exercise.

Despite the limitations, the authors say that better management of cardiovascular diseases and strokes and their risk factors could offer new opportunities to slow the projected burden of dementia.

Researchers from Inserm in France and the University of Bordeaux contributed to the study. Funding came from FHS and from grants from the National Institute on Aging and National Institute on Neurological Disorders and Stroke.