BOSTON—Findings from a new population-based study indicate that dementia incidence may have decreased over the past two decades among residents of Stockholm ages 75 and older. In addition, researchers have found a declining dementia prevalence in the United Kingdom and evidence that people in Denmark are living to older ages with better overall functioning, according to two recent studies published in Lancet. The question that remains is: How applicable are these results to comparable populations in other developed countries and throughout the world?

In the Swedish study, investigators at the Karolinska Institutet compared a cohort from a 1987–2000 study called the Kungsholmen Project (KP) (n = 1,810; ages 75 and older) with a cohort from an ongoing study begun in 2001, the Swedish National Study on Aging and Care (SNAC-K) (n = 3,363; ages 60 and older). After identifying a subgroup in the latter cohort of 1,575 participants 75 and older, the researchers found that standardized prevalence rates of dementia for the subgroup and the earlier cohort were virtually identical—a finding that remained when the investigators examined age-specific and sex-specific prevalence rates between the two study groups.

However, when the researchers compared the two main cohorts by subgroups based on age—75 to 79, 80 to 84, 85 to 90, and older than 90—as well as between demented and nondemented patients, they found, with the exception of the age 75 to 79 subgroups, a clear increase in survival for the SNAC-K cohort versus the KP cohort, according to coinvestigator Laura Fratiglioni, MD, PhD, Director of the Aging Research Center at the Karolinska Institutet in Stockholm.

“For this period of about 15 to 20 years, there appears to be a change in the natural history of the disease,” she said at the 2013 Alzheimer’s Association International Conference. “We see that the prevalence is the same, but the mortality has changed…. The data, taken together, suggest that dementia incidence may have decreased in two decades—at least in this Swedish, urban, 75-plus-old population. And I have to stress that, even with this difference … it’s important to think that, with survival, many factors can [play a role].”

A Global Trend in Decreasing Dementia Incidence
The findings by Dr. Fratiglioni and colleagues echo several recent studies that analyzed trends of dementia occurrence and demonstrated evidence of a declining incidence, with important implications for improved health and quality of life among the very elderly. For example, in a comparison of physical and cognitive functioning of two Danish cohorts of individuals older than 90 born 10 years apart, Christensen and colleagues found that the cohort born in 1915 scored significantly better than the cohort born in 1905 on both the cognitive tests and the Activities of Daily Living score.

Dr. Fratiglioni offered several possible interpretations for these findings. “Vascular risk factors and related diseases are implicated in dementia,” she said. “And we know also that the incidence and mortality of major cardiovascular diseases have decreased in high-income countries since the 1980s.”

In terms of the wider implications, she cited as welcome news to all older people the decreased risk seen for individuals but questioned the “generalizability” of the findings beyond more highly educated subjects who may have healthier lifestyles and beyond people older than 75. “I don’t know if this is the same for people with early-onset dementia,” Dr. Fratiglioni cautioned.

She also wondered about the effect of a declining dementia incidence on public health. Would the impact really decrease given the absolute number of dementia cases and despite the presence of aging populations throughout the developed world?

Clues to answering that question may be found in a recent study that examined whether declining incidence rates for myocardial infarction, stroke, and cancer could counterbalance the future demographic challenges inherent among the aging population of Sweden. Modig and colleagues found that the required changes in incidence rates are, in Dr. Fratiglioni’s words, “well in line” with historical figures for stroke.

“This is almost [true] for myocardial infarction but not true for cancer,” she added. “If we have a disease that has a strong association with age, a reduction of incidence rates on the order of 1% to 2% is sufficient to offset the challenges of the aging population.”

Reason for Optimism
In Dr. Fratiglioni’s estimation, there is some cause for optimism, but caution should be exercised. “My major conclusion is … that we have shown that dementia risk can be decreased, and this is due to the fact that we can act especially against vascular risk factors,” she said.

—Fred Balzac
Contributing Writer

Is Dementia Incidence Declining?