In a large nationally representative survey of older Americans we found that, between 1993 and 2002, the prevalence of cognitive impairment consistent with dementia decreased from 12.2% to 8.7%, representing an absolute decrease of 3.5 percentage points, and a relative decrease of nearly 30%. In addition, we found an increased risk of death among those with moderate or severe CI, and this increased mortality was most evident among those with CI who had higher levels of education.
The decline in the prevalence of CI suggests that, overall, the combined impact of recent trends in medical, lifestyle, demographic, and social factors has been positive for the cognitive health of older Americans. Although the prevalence of some cardiovascular risks that are also associated with a higher risk of dementia15
increased significantly, other factors showed trends that favored a reduced prevalence of CI. Most importantly, we think, individuals who were 70 or older in 2002 had significantly higher levels of education, on average, than those who were 70 or older in 1993. Our trend analyses suggest that increasing levels of education and net worth among older Americans explained about 40% of the observed relative decrease in CI prevalence between 1993 and 2002.
Higher levels of education are likely associated with greater “cognitive reserve,” in that brains of the more educated are able to sustain greater damage (e.g., AD pathology or ischemia) before reaching the threshold of clinically significant CI.21,26
However, at the time this threshold is finally crossed, brain pathology is more advanced in those with more education, resulting in a more rapid cognitive decline22,26
and greater risk of mortality.22,27
Our findings support the cognitive reserve hypothesis in that we found a significant protective effect of education on CI risk in both the 1993 and 2002 cohorts, and increased risk of 2-year mortality among those with CI who had higher levels of education in both the 1993 and 2002 cohorts. Our findings of a declining prevalence of CI between 1993 and 2002, and the strong association of education with decreased risk for CI, are consistent with similar trends found between 1982 and 1999 in a recent study using data from the National Long Term Care Survey,3
and our findings extend those of Freedman and colleagues who also found a decline between 1993 and 1998 in “severe cognitive impairment” using HRS data.23,41
Potential mechanisms leading from more education to better cognitive function and reserve include a direct positive effect of schooling on brain development,3,24,26
greater mental stimulation throughout the life course due to more cognitively demanding occupations42,43
and leisure time activities,20,24,44
and more “brain healthy” lifestyles such as better control of cardiovascular and cerebrovascular risk factors, as well as better access to health care interventions that may help preserve cognitive function.30,31
Our finding that the increasing prevalence of cardiovascular risks was not accompanied by an increasing prevalence of CI suggests that these risks were treated more successfully in 2002 compared to 1993. For instance, a recent analysis of Medicare Current Beneficiary Survey (MCBS) data showed that the use of statin medications to treat high cholesterol increased from 4% to 22% of older individuals with heart disease between 1993 and 2002, while use of any antihypertensive medication increased from 46% to 62%.45
This more intensive treatment was accompanied by significantly better blood pressure control and improved cholesterol profiles among those 65 to 84 years old, as measured in the National Health and Nutrition Examination Survey (NHANES). In sum, both more intensive and successful treatment of cardiovascular risks in 2002 compared to 1993 may have had a “spill-over” benefit for population cognitive health. The association of self-reported hypertension with lower risk of dementia in our study is consistent with a possible protective effect of antihypertensive medications,46,47
however we were unable to test this hypothesis directly.
There have been significant changes in the treatment of Alzheimer’s disease, the most common cause for dementia, during the time period of our study. Since 1993, cholinesterase inhibitor medications have been approved for treatment of mild to moderate AD. The use of these medications has increased rapidly since their introduction; about 25% of patients with AD were using a ChI in the late 1990’s in one population-based study,48
and prescriptions have increased steadily since then.49
Since these medications are used mainly only after diagnosis of dementia, and since their impact on cognitive function is modest, it is highly unlikely that they are an important explanation for the decreased prevalence of CI that we found in our study between 1993 and 2002.
One prior study using HRS data to study trends in cognitive function did not show the same results as our study. Rodgers and colleagues found no significant decline in the proportion of those with CI, after adjusting for a number of survey design issues, including whether respondents had taken the HRS cognitive test at a prior wave.50
The exclusion from that study of proxy respondents, a significant proportion of whom have CI, may be one source for the difference in findings. However, Freedman and colleagues found a significant decline in “severe cognitive impairment” in the community-dwelling sample (both self respondents and proxy respondents) between 1993 and 1998,23
and these findings were robust to various assumptions regarding loss to follow-up, trends in the size and composition of the nursing home population, and the handling of item non-response on the HRS cognitive scale.41
Our study adds to this prior work by tracking important changes in the mortality associated with CI during this time period, and by using more recent HRS data.
This study has at least two potential limitations. The HRS cognitive measures provide an assessment of cognitive function, but they do not allow the determination of a clinical diagnosis of dementia. We used cognitive categories and cut-off scores that have shown good correlation with dementia in prior studies; specifically, limitations in ADLs and IADLs,37
extent of informal caregiving,37
and the likelihood of nursing home admission.51
Another limitation is that we did not have data on the use of cardiovascular and dementia medications, so we could not directly assess how the increasing use of these agents during the time period of our study may have affected overall brain health.
The strengths of this analysis include its large nationally representative samples of U.S. adults using the same cognitive tests in both years. In addition, the HRS measured cognitive function directly and in a consistent way, and also utilized a proxy informant to provide an assessment of memory and judgment for those respondents unable to participate. These features overcome the shortcomings of using dementia diagnoses obtained from administrative data and excluding those who are significantly impaired because data are not gathered from a proxy. The representative community sample of the HRS included a wide range of educational attainment, allowing a better assessment of the relationship of education to CI and mortality than most clinical samples where individuals with low levels of education are often under-represented. We also had nearly complete 2-year mortality follow-up of the more than 7,000 individuals in each cohort. Hence, our mortality analyses are unlikely biased by non-random attrition from the cohorts.
In summary, our findings engender optimism regarding trends in the overall cognitive health and quality of life of older Americans. There appears to have been a “compression of cognitive morbidity” between 1993 and 2004, with fewer older Americans reaching a threshold of significant cognitive impairment, and a more rapid decline to death among those who did. Societal investment in building and maintaining cognitive reserve through formal education in childhood, as well as continued cognitive stimulation during work and leisure in adulthood, may help limit the burden of dementia among the growing number of older Americans, especially the oldest-old.