Dementia incidence continues to increase with age in the oldest old: the 90+ study

Abstract

Objective: The oldest old are the fastest growing segment of the US population, and accurate estimates of dementia incidence in this group are crucial for healthcare planning. Although dementia incidence doubles every 5 years from ages 65 to 90 years, it is unknown if this exponential increase continues past age 90 years. Here, we estimate age- and sex-specific incidence rates of all-cause dementia in people aged 90 years and older, including estimates for centenarians.

Methods: Participants are from The 90+ Study, a population-based longitudinal study of aging and dementia. Three hundred thirty nondemented participants aged 90 years and older at baseline were followed between January 2003 and December 2007. Age- and sex-specific incidence rates of all-cause dementia were estimated by person-years analysis.

Results: The overall incidence rate of all-cause dementia was 18.2% (95% confidence interval [CI], 15.3-21.5) per year and was similar for men and women (risk ratio, 0.94; 95% CI, 0.65-1.37). Rates increased exponentially with age from 12.7% per year in the 90-94-year age group, to 21.2% per year in the 95-99-year age group, to 40.7% per year in the 100+-year age group. The doubling time based on a Poisson regression was 5.5 years.

Interpretation: Incidence of all-cause dementia is very high in people aged 90 years and older and continues to increase exponentially with age in both men and women. Projections of the number of people with dementia should incorporate this continuing increase of dementia incidence after age 90 years. Our results foretell the growing public health burden of dementia in an increasingly aging population.

Figure 1

Flow chart for participant inclusion in incidence estimates. ^aOf the 411 participants with no in-person evaluation at baseline, using other sources of information available we determined that 201 were demented at baseline, 107 were not demented at baseline and had a follow-up evaluation, 88 were not demented at baseline but did not have a follow-up evaluation, and 15 did not have enough information for a cognitive status determination.

Figure 2

Age-specific incidence rates and 95% confidence intervals of all-cause dementia in the 90+ Study: January 1, 2003 to December 31, 2007. Incidence rates were computed for 3 age categories using a person-years analysis and are plotted at the average age for each age category: 92.7 years for the 90–94-year category, 96.4 years for the 95–99-year category, and 101.3 years for the 100+-year category. The incidence curve is from a Poisson regression with age as a continuous variable. The time for the incidence rates to double was estimated at 5.5 years.