Declining incidence of dementia seen in Framingham Heart Study

1. In the Framingham Heart Study, there has been a declining incidence of dementia over three decades, spanning from the late 1970s to the present.

2. The declining incidence of dementia has been largely limited to those with at least a high school diploma and was only partially explained by improvements in cardiovascular risk factors.

Evidence Rating Level: 1 (Excellent)

Study Rundown: As the world’s population ages, the prevalence of dementia—a driver of dependence and disability in the elderly—is expected to increase exponentially. Recent studies have suggested that the age-adjusted incidence of dementia might be decreasing. However, these studies have not consistently supported a statistically significant decreasing trend in this figure and have been limited by differing methodologies in ascertaining dementia over time.

Drawing on rich, longitudinal data collected in the Framingham Heart Study, this prospective cohort study provides additional evidence for a declining incidence of dementia over three decades, spanning from the late 1970s to the present. There was a monotonically decreasing 5-year age- and sex-adjusted cumulative incidence of dementia across 4 epochs from this time period, with a 44% reduction in incidence of dementia in the most recent epoch compared to the first epoch in the late 1970s. Importantly, these improvements were seen only in subjects who had achieved at least a high school diploma.

During this time period, statistically significant improvements in blood pressure and cholesterol control as well as smoking rates were seen, though there was an increase in the rates of type 2 diabetes and obesity; however, the combined effect of these improvements in many cardiovascular risk factors did not entirely explain the decline in incidence of dementia, suggesting that there may be as-of-yet unmeasured factors at play.

These results derive strength from the effort to harmonize standards for diagnosing dementia across several decades of the cohort study but would be further strengthened by data such as the diet and exercise habits of the participants, and neuropathology results.

Click to read the study in NEJM

Relevant Reading: The global prevalence of dementia

In-Depth [prospective cohort]: This longitudinal cohort study involved 5,205 individual patients contributing 40,192 person-years of data across 4 five-year epochs starting between 1977 and 2004; subjects were at least 60 years of age and did not have a diagnosis of dementia at the start of each epoch if they were included. Patients in the Framingham Heart Study underwent screening exams for incident dementia; if they screened positive, further neuropsychological testing was pursued, and the diagnosis of dementia was ultimately confirmed in each case by an expert panel. The 5-year age- and sex-adjusted cumulative hazard rates of dementia declined over time, from 3.6 per 100 persons (95% [CI], 2.9 to 4.4) during the 1^st epoch to 2.0 per 100 persons (95% [CI], 1.5 to 2.6) during the 4^th epoch. Importantly, the decline in the incidence of dementia was limited to those subjects with at least a high school diploma, who saw an average decline in risk of 23% per decade (HR, 0.77; 95% [CI], 0.67 to 0.88). During this time period, there was also an improvement in systolic and diastolic blood pressure control, cholesterol control, and smoking rates, though there was an increasing prevalence of type 2 diabetes and obesity. When the observed decline in the incidence of dementia was adjusted for vascular risk factors (present at baseline evaluation and at midlife) or preexisting or incident stroke, it did not change significantly, suggesting that these factors do not entirely explain the central findings of the study.

Image: PD

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