Published in the November 2007 issue of Neuroepidemiology, the study is based on data from 856 men and women who participated in the Aging, Demographics and Memory Study conducted in 2002 by researchers at the University of Michigan Institute for Social Research (ISR) and Duke University Medical Center and funded by the National Institute on Aging. The study was part of the larger ISR Health and Retirement Study, a nationally representative survey of Americans age 51 and older.
About 3.4 million people, or 13.9 percent of the population age 71 and older, have some form of dementia, the study found. As expected, the prevalence of dementia increased dramatically with age, from five percent of those aged 71 to 79 to 37.4 percent of those age 90 and older.
About 2.4 million of those with dementia, or 9.7 percent of the population age 71 and older, were found to have Alzheimer's disease, the most common cause of dementia, according to the study.
Overall, the researchers found that Alzheimer's disease accounted for approximately 69.9 percent of all dementia, while vascular dementia -- often caused by stroke -- accounted for 17.4 percent. With increasing age, Alzheimer's disease accounted for progressively more of the dementia cases, so that in the 90+ age group, it comprised 79.5 percent of dementia cases, compared to 46.7 percent among those in their 70s.
The study provides the first prevalence estimates based on a nationally representative sample of older adults from all regions of the United States. Four previous national estimates of the prevalence of Alzheimer's disease were all obtained by extrapolation from regional samples. These estimates ranged from 2.1 million to 4.5 million.