While a new study shows the dementia incident rate has dropped 20% each decade on average since the 1970s, the actual reasons for the decline aren't fully understood. Preventive factors, such as improvements in cardiovascular health and early detection may have played a role.
The study found 371 cases of dementia in a group of more than 5,000 people from Massachusetts. The rates dropped from 3.6 per 100 people between 1977 and 1983, to 2.0 per 100 people between 2004 and 2008, according to findings published Wednesday in the New England Journal of Medicine.
Attempts to make drugs to treat Alzheimer's disease and other forms of dementia have failed. Meanwhile, projections anticipate millions of baby boomers will suffer severe cognitive decline over the next few decades. Still researchers say the new findings offer “cautious hope.”
While the magnitude of projected increases in dementia may be buffered by preventive factors, it's not completely clear why the incidence of the disease has declined, concluded the NEJM study authors, writing from the Boston University School of Medicine and the University of Bordeaux in Talence, France.
In an accompanying editorial, Dr. David Jones from Harvard Medical School and Dr. Jeremy Greene from the Johns Hopkins University School of Medicine, added that unexpected reversals of chronic-disease epidemics have historically been challenging to interpret.
Ambiguities, they said, can “open up a battle ground for conflicting interpretations by interested parties,” such as policy makers determining where to allocate funding and resources. And, any perceived improvements could be reversed if efforts to curb other health problems linked to increased rates of dementia, such as diabetes and heart disease, are diverted.
"Even if a dementia decline has begun, it might not last,” they wrote. “The burden of disease, ever malleable, can easily relapse."
The NEJM Study is part of the Framingham Heart Study, a longitudinal cohort study started in 1948 looking at more than 5,200 residents of Framingham, Mass. An additional 5,200 individuals, offspring of the original patients, were added in the 1970s. The current analysis consisted of 5,200 people between the ages of 60 and 101. The researchers looked at four different windows of time between 1971 and 2008.
Overall, there was a 44% relative decline in the incident rate of dementia. Patients also developed the condition later in life over time.
Researchers observed a trend toward higher educational levels, and a parallel trend toward a lower prevalence of most heart disease factors, except for obesity and diabetes.
The risk of developing dementia was nine times higher for patients who had had a stroke between 1977 and 1983, compared to those who had not. Between 2004 and 2008, the risk of dementia for patients who had had a stroke versus those who had not was two times higher.
The authors pointed out several limitations, however. The study did not examine factors such as the patients' diet, physical activity, or the burden of vascular brain injury. Participants in the Framingham Study are also “overwhelmingly of European ancestry,” and therefore the findings would need to be replicated for other racial groups.
A separate study published online Wednesday in Alzheimer's & Dementia, the Journal of the Alzheimer's Association, found the incidence of dementia to be highest in African-Americans and lowest in Asian-Americans. Researchers from the University of California in San Francisco used electronic health records of more than 274,000 Kaiser Permanente patients followed from January 2000 through December 2013.
In the UCSF study, the dementia incidence ranged from an average annual rate of 26.6 cases per 1,000 people for African-Americans and 22.2 cases per 1,000 people among Native Americans, to 15.2 cases per 1,000 people for Asian-Americans. In between were Latinos and Pacific Islanders, with an average annual rate of 19.6 cases per 1,000 people, and whites with 19.3 cases per 1,000 people.