Incident cognitive impairment occurred, over an average interval of four years, among 8.1% of U.S. adults 45 and older in this study. The odds of incident cognitive impairment were 18% higher among residents of the Stroke Belt than among non-Belt residents after adjusting for strong independent predictors of cognitive decline including age, sex, and education level. When impairment at the two most recent consecutive assessments was required for incident case definition, the adjusted odds increased to 40% higher risk in the Stroke Belt region, suggesting an even greater regional disparity in persisting impairment. This study is the first known documentation of higher incident cognitive impairment in the Stroke Belt region of the United States than in the rest of the nation.
The higher adjusted incidence of impairment among Stroke Belt residents also remained after controlling for the significant association with race, thereby obviating the potential confounding of race with region. Historically, there has been a greater concentration of African American adults living in the Stroke Belt than in the remaining U.S.26
, and living in the Stroke Belt and being African American both increase risk for stroke mortality. The excess stroke mortality borne by African Americans appears to be due to higher stroke incidence rates among African Americans, which is particularly apparent at younger ages.26–29
The same factors that increase risk for stroke--hypertension, diabetes, kidney disease, and metabolic syndrome—also increase risk for cognitive impairment.10–16
With the exception of kidney disease16
, these conditions disproportionately affect African American adults.
Incident impairment based on SIS performance was robustly associated with well-established risk factors for cognitive decline, including older age and fewer years of education. Using the SIS, we found approximately 2% incident impairment annually—somewhat lower than annual incidence rates reported by studies that used clinical diagnostic assessments for dementia (3.2%)30
and mild cognitive impairment (5.1%)31
, likely due in part to the younger mean age of our cohort. In addition, practice effects among those with opportunity for multiple annual exposures to the Six-item Screener might have served to lower detection of subtle impairment.32
Our relatively low annualized percentage of incident impairment is consistent with a report demonstrating in two independent population-based cohorts that estimated declines in cognitive function are smaller, irrespective of statistical approach, when using long (e.g., 5-year) compared to short (e.g., 1-year) follow-up periods, due to health and survival effects that bias longer follow-up periods (e.g., selective attrition of participants with lower levels of cognitive function).33
The net impact of such processes would be underestimation of incident impairment.
Inspection of the geographic patterns of cognitive impairment reveals a preponderance of incident cases among participants throughout the south, extending from the east coast to Texas and Oklahoma in a pattern that is generally concordant with shifting stroke mortality patterns which have spread toward the Mississippi River Valley and westward within the past 50 years.34
Casper and colleagues have suggested that the changing geographic pattern of stroke mortality may reflect geographic changes in economic resources which are associated with patterns of medical and behavioral risk factors, migration patterns, and health care standards.34
It is plausible that these same factors influence trajectories of cognitive function.
The findings of this study are subject to limitations. First, the Six-item Screener used to assess cognitive status likely lacks sensitivity to subtle cognitive changes. Even so, our documentation of clear regional differences in the probability of cognitive decline provides evidence that the screener is sufficiently sensitive for epidemiological research. We confirmed our SIS findings using two statistical methods and two definitions of incident impairment. Our incident impairment classifications coincided with lower performance on two more sensitive measures of cognitive function, and adjusted scores on these additional measures were lower in the Stroke Belt than in the rest of the nation, providing corroboration of geographic variations in incident cognitive impairment. Previous findings from REGARDS attest to the utility of the SIS in detecting broad patterns of association with conditions affecting cognition, such as traditional cardiovascular risk factors2,35,36
, chronic kidney disease16
, and congestive heart failure.37
A second limitation is that our cooperation and response rates are moderate, though comparable to those achieved by similar population-based studies. These rates leave open the possibility that nonresponse bias could affect interpretation of our results. In addition, a large block of the western United States was not included in the state-level analyses. The small number of REGARDS participants in those states—a product of simple random sampling, which selects fewer participants from states that are relatively sparsely populated—was deemed insufficient to provide stable estimates of incident decline. Participants in these states were, however, included in the primary analysis of regional differences. Additionally, although we employed a random sampling strategy, cohorts of participants in the state level analyses may not be representative of the states as a whole.
Future work should examine the influence of migration patterns, urban/rural residence, life course socioeconomic factors, and educational quality in relation to cognitive decline, as well as proximate causes of incident cognitive impairment. Earlier work by our group suggests several likely cardiovascular and stroke risk factors to pursue.2,35,36
Pinpointing regional patterns in the contribution of modifiable risk factors to incident cognitive impairment will allow for geographically concentrated prevention and intervention efforts. Such efforts will be particularly important for those segments of the population who are most vulnerable to incident impairment. The information gained by tracking the physical and cognitive health of the REGARDS cohort of over 12,000 AA and 18,000 EA adults may be used to design and implement appropriate programs and services for older Americans at both state and national levels.