The incidence of dementia may be declining, new research suggests.
Investigators at the Albert Einstein College of Medicine, Bronx, New York, found dementia incidence rates decreased significantly among individuals born after 1929.
However, adjustment for age, sex, education level, and prevalence of cardiovascular comorbidities did not explain this trend.
While the study results won’t have a major impact on clinical practice, they’re important for public health planning, lead study author, Carol A. Derby, PhD, Saul R. Korey Department of Neurology, told Medscape Medical News.
“The paper highlights the need for continued efforts toward prevention of risk factors we know how to handle, such as cardiovascular disease risk factors.”
She added that the research may also be useful in terms of projecting the future burden of dementia.
The study was published online September 5 in JAMA Neurology.
Dramatic Rise in Prevalence
The incidence of dementia may be declining, but because of an aging population, the prevalence is set to rise dramatically in coming years. The current worldwide dementia prevalence is estimated to be more than 47 million, but by 2050, it’s projected to reach 115 million.
The new analysis included 1348 participants, mean baseline age 78.5 years and 61.6% female, who were enrolled in the Einstein Aging Study (EAS) from October 20, 1993, to November 17, 2015. Since 1993, the EAS has recruited and followed noninstitutional individuals aged 70 years and older from Bronx County, New York.
Annual assessments for study participants included a clinical neurologic examination, comprehensive neuropsychological assessments, medical history, blood pressure, and psychosocial assessment.
Researchers determined the prevalence of myocardial infarction (MI), stroke, and diabetes though self-report of physician diagnosis. They assessed global cognition using the Blessed Information Memory Concentration test and depression using the Geriatric Depression Scale.
A dementia diagnosis was based on standardized clinical criteria from the Diagnostic and Statistical Manual of Mental Disorders, fourth edition. It required impairment in memory (a score of at least 1.5 standard deviations below the age-adjusted mean for the Logical Memory test or a score of 24 or less on the Free and Cued Selective Reminding Test-Free Recall) and in at least one additional cognitive domain, plus evidence of functional decline.
Researchers examined dementia incidence according to year of birth. “That allowed us to disentangle the effects of age from birth cohort effects; people are born in different years and have different experiences,” said Dr Derby.
The birth cohorts included before 1920, 1920-1924, 1925-1929, and after 1929.
There were 150 incident cases of dementia during 5932 person-years of follow-up.
As expected, dementia incidence overall increased with increasing age. However, there was a consistent pattern of decreasing incidence in sequential birth years, with an accelerated decrease in incidence rates for those born in the middle to late 1920s.
The incidence per 100 person-years was 5.09 in birth cohorts before 1920, 3.11 in the 1920-1924 birth cohorts, 1.73 in the 1925-1929 birth cohorts, and 0.23 in cohorts born after 1929.
The researchers carried out further analyses to explore the trends. Poisson regression models adjusted for age, sex, race, and education level indicated a significant change point for those born after July 1929 (95% confidence interval [CI], June 1929 to January 1930).
The relative rate for birth cohorts before July 1929 vs after was 0.13 (95% CI, 0.04 – 0.41).
Need for Continued Vigilance
Decreased dementia prevalence is likely due to lower risk for dementia onset and not to increased rates of cure or remission, given the lack of disease-modifying dementia treatments, said Dr. Derby.
The researchers investigated the role of cardiovascular disease (CVD) and cardiovascular risk factors in the dementia incidence rates. Vascular risk factors increase the risk for dementia.
Consistent with national trends, the study showed the age-specific prevalence of MI and stroke decreased in the cohort across sequential birth cohorts while diabetes prevalence increased in later birth cohorts.
After adjusting for MI, stroke, or diabetes, the significant decrease in dementia incidence among individuals born after mid-1929 remained.
Dr Derby said that while declining prevalence of MI and stroke did not explain the trends observed in the study, it could be that the researchers were not able to control for all the factors related to better control of CVD. The study did not, for example, control for how well hypertension or diabetes was controlled.
Despite a shift toward higher education in the cohort, this trend did not explain the findings. However, Dr Derby noted that the shift was not huge and the cohort did not have a large number of participants with very low education levels.
Improved nutrition over successive birth cohorts might have affected dementia risk. However, the researchers were unable to assess this possibility because the EAS has not routinely collected dietary data.
Dr Derby noted the “huge increase” in the global prevalence of diabetes in recent decades.
“Whether reduced dementia incidence rates will translate into a lower burden of dementia in the population remains to be seen, given the aging population and also given that this other major risk factor is an epidemic worldwide,” she said.
With the rise in diabetes and still no effective treatments for AD, the new research “highlights the need for continuing vigilance in terms of prevention,” said Dr Derby.
A limitation of the study was the small number of dementia cases for later birth years. Also, the researchers could not analyze dementia subtypes, although a decrease in all-cause dementia likely also reflects changes in Alzheimer’s disease rates.
Commenting on the study for Medscape Medical News, Dean Hartley, PhD, director of science initiatives, medical and scientific relations, Alzheimer’s Association, said it adds to the body of literature identifying a similar trend.
“There are now a number of studies that reinforce this concept that in developed countries, the dementia incidence is actually going down, which is good,” said Dr Hartley. “We are excited to see this sort of news.”
But even though this trend “is probably real,” it’s still not exactly clear what is driving it, he said.
Dr Hartley pointed out that the information on cardiovascular risks was self-reported by study participants and that the study lacked information on diet. He noted that research released at this year’s Alzheimer’s Association International Conference showed that adherence to the Mediterranean diet or the low-sodium DASH diet can improve cognition by 30%.
Nutritional counseling and modification along with other lifestyle interventions will be tested for their impact on cognitive decline or dementia in a new 2-year $20 million clinical trial (US POINTER [US PrOtect through a lifestyle INTErvention to Reduce risk]) announced recently by the Alzheimer’s Association.
The study will include 2500 older adults with no cognitive symptoms but at increased risk for later cognitive decline. Recruiting for the study will begin next year.