University of California researchers found that the incidence rate for all causes of dementia in people age 90 and older is 18.2% annually and significantly increases with age in both men and women. This research, called "The 90+ Study," is one of only a few to examine dementia in this age group, and the first to have sufficient participation of centenarians.
Findings of the study appear in the February issue of Annals of Neurology.
Dementia (senility) is a progressive, degenerative disorder that affects memory, language, attention, emotions, and problem solving capabilities. A variety of diseases cause dementia including Alzheimer's disease, stroke, and other neurodegenerative disorders. According to a 2000 report from the World Health Organization (WHO), approximately 6%-10% of the population 65 years and older in North America have dementia, with Alzheimer's disease accounting for two-thirds of those cases.
For their population-based, longitudinal study of aging and dementia, Maria Corrada, Sc.D., and colleagues invited members who were originally part of The Leisure World Cohort Study and 90 years of age or older as of January 1, 2003. As of December 31, 2007 there were 950 participants in The 90+ Study and 539 who had completed a full evaluation that included neurological testing, functional ability assessments and a questionnaire covering demographics, past medical history, and medication use. Evaluations were repeated every 6-12 months with a final dementia questionnaire completed shortly after death.
Analysis was completed on 330 participants who were primarily women (69.7%) between the ages of 90 to 102, and who showed no signs of dementia at baseline. Researchers identified 140 new cases of dementia during follow-up with 60% of those cases attributed to Alzheimer's disease (AD), 22% vascular dementia, 9% mixed AD and vascular dementia and 9% with other or unknown cause.
Dr. Corrada explained, "Our findings show dementia incidence rates almost double every five years in those 90 and older." Researchers found the overall incidence rate based on 770 person-years of follow-up was 18.2% per year. Rates increased with age from 12.7% per year in the 90-94 age group, to 21.2% per year in the 95-99 age group, to 40.7% per year in the 100+ age group. Incidence rates were very similar for men and women. Previous results from The 90+ Study found higher estimates of dementia prevalence in women (45%) compared to men (28%), a result also seen in other similar studies.
Prior reports estimate there were 2 million Americans aged 90 and older in 2007 and the number is expected to reach 8.7 million by 2050, making the oldest-old the fastest growing segment of the U.S. population. "In contrast to other studies, we found that the incidence of dementia increases exponentially with age in both men and women past age 90," said Dr. Corrada. "Given the population projections for this age group along with our findings, dementia in the oldest-old threatens to become an epidemic with enormous public health impact."
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