A study published Monday in JAMA Neurology found that almost a third of Americans aged 65 and older had either dementia or mild cognitive impairment in 2016—but prevalence of the conditions varied by age, education, and race and ethnicity.

Study details and key findings

For the study, researchers gathered data from 3,496 participants in the Harmonized Cognitive Assessment Protocol (HCAP) project, which is part of the ongoing, longitudinal Health and Retirement Study (HRS). The HCAP included a cross-sectional random sample of HRS participants aged 65 and older in 2016.

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Between June 2016 and October 2017, HCAP participants were required to complete a comprehensive neuropsychological test and an in-person interview.

On average, participants were 76.4 years old. Overall, 60% were female, 71% self-identified as white, 16% as Black, 11% as Hispanic, and 2% as another race.

Dementia and mild cognitive impairment were classified through an algorithm that was based on standard diagnostic criteria. Researchers then compared participants' test performance to a normative sample. To estimate the prevalence of dementia and mild cognitive impairment in the United States, the researchers used population weights.

According to study author Jennifer Manly of Columbia University Irving Medical Center, and colleagues, 10% of participants had dementia in 2016 and 22% had mild cognitive impairment.

While dementia prevalence was similar among men and women, it varied by age, education, and race and ethnicity.

For instance, when compared with white participants, dementia was more common among Black participants, and mild cognitive impairment was more common among Hispanic participants.

With every five-year increase in age, participants experienced a higher risk of dementia and mild cognitive impairment. For example, 35% of people ages 90 and older had dementia—a significant jump from just 3% of participants between ages 65-69.

The study also found that "each additional year of education was associated with a decrease in risk of dementia." 

Commentary

While the findings mark the first representative study of cognitive impairment conducted in over 20 years, the study authors noted that the findings resemble other recent estimates of dementia prevalence in the United States.

Because the findings represent a snapshot in time, they cannot accurately evaluate cognitive impairment rates or rates of progression among individuals who have mild cognitive impairment, the authors said.

In addition, the authors noted that other longitudinal studies have determined that mild cognitive impairment classifications can be inconsistent, with many people falling into a normal cognition classification at follow-up appointments.

Ultimately, the HCAP study's cross-sectional design "does not allow for examination of survival bias, which could inflate prevalence if some groups are living longer with dementia or decrease estimates in groups with higher mortality," the authors added.

Still, "[b]ecause the HCAP study is part of the nationally representative and long-running Health and Retirement Study, these data not only show the burden of dementia now, but will be used in the future to track the trends in dementia burden in the decades ahead," said co-author Kenneth Langa of the University of Michigan.

"With increasing longevity and the aging of the Baby Boom generation, cognitive impairment is projected to increase significantly over the next few decades, affecting individuals, families, and programs that provide care and services for people with dementia," Manly said.

"Following those trends will be especially important given the likely impact of COVID and other recent population health changes on the risk for dementia in the coming decades," Langa added. (George, MedPage Today, 10/24; Mueller, The Hill, 10/24; Manly et al., JAMA Neurology, 10/24 [subscription required])