Declining prevalence of dementia in the U.S. elderly population.

A decline in chronic disability prevalence occurred 1982 to 1999 in the U.S. elderly population parallel to declines in severe cognitive impairment. Comparative analysis of factors contributing to the incidence of dementia led us to suggest explanations for this decline. 42,000 disabled and non-disabled individuals aged 65+ participating in National Long Term Care Surveys (NLTCS) were drawn from Medicare enrollment lists to ensure the US population aged 65+ is represented. Severe cognitive impairment (SCI) was defined by the subject not being able to successfully answer any cognitive screen questions in survey interviews. This definition thus covered cases of dementia of different origin and clinical manifestation: Alzheimer's, non-Alzheimer's, stroke-related, vascular etc. Age-specific prevalence of SCI was calculated for 1982, 1984, 1989, 1994 and 1999, and Medicare record physician determined diagnoses of vascular, mixed and Alzheimer's dementia in 1994 and 1999 was determined by gender and age. We found 310,000 fewer severely cognitively impaired elderly in 1999 than in 1982. The average decline in prevalence was from 5.7% to 2.9% for this period. This was associated with a significant decline in mixed but not Alzheimer's dementias. On a gender basis, the male proportional decline was larger than that of female. Several possible explanations of such a surprising trend in elderly age dementias are discussed, including (i) increased proportion of better educated people among the oldest old; (ii) recent declines in stroke rates (these may contribute to decreasing risks of post-stroke dementias); (ii) expanding use of neuro-protective medications working prophylactically for selected dementias. A significant component of disability decline in the U.S. elderly population is the decline in vascular and mixed dementias, but not in Alzheimer's disease alone. Improved medical therapies and better education among the old appear to play important roles in this decline.