Mild cognitive impairment, dementia, and their subtypes are common in the "oldest old" women, which includes those 85 years of age and older, according to a report in the May issue of Archives of Neurology, one of the JAMA/Archives journals.

The oldest old is "the fastest growing segment of the U.S. population and is expected to increase in number by 40 percent during the next decade alone," the authors write as background information in the article. "Initial evidence suggests that the incidence of all-cause dementia almost doubles with every 5 years of age and that the prevalence of dementia rises from approximately 2 percent to 3 percent in those 65 to 75 years to 35 percent in those 85 years and older."

To characterize the prevalence of mild cognitive impairment and its subtypes in oldest old women, Kristine Yaffe, M.D., of the University of California at San Francisco, and colleagues, evaluated data from 1,299 women enrolled in the Women Cognitive Impairment Study of Exceptional Aging, an ancillary of the Study of Osteoporotic Fractures. The women had a mean (average) age of 88.2 years and 27.0 percent were older than 90 years. Of the women in the study, 231 (17.8 percent) were diagnosed as having dementia and 301 (23.2 percent) as having mild cognitive impairment, for a combined total of 41.0 percent with clinical cognitive impairment.

The prevalence of mild cognitive impairment was higher in women 90 years or older than in women 85 to 89 years (24.5 percent vs. 22.7 percent). Of the subtypes of mild cognitive impairment, amnestic multiple domain (affecting multiple cognitive functions, including memory difficulty) was most common, followed by non-amnestic single domain (affecting one type of cognitive function, not affecting memory) accounting for 33.9 percent and 28.9 percent respectively, with amnestic single domain (affecting one type of cognitive function, including memory difficulty) affecting 21.9 percent of women.

The prevalence of dementia in women 90 years and older was approximately double that of women ages 85 to 89 years (28.2 percent vs. 13.9 percent), however the distribution of dementia subtypes (Alzheimer's disease, vascular dementia, mixed or other) was similar across all age groups. Compared with women with normal cognition, those with dementia were on average older, less likely to have completed high school and more likely to live in a nursing home. Women with dementia were also more likely to have reported depression, a history of stroke, and to have an apolipoprotein E ε4 (APOE ε4) allele (an alternative form of the gene).

As treatment and course of dementia differ by subtype, the authors note that, "the distribution of dementia subtypes is vital for public health planning." In the present sample of oldest old women, Alzheimer's disease and mixed dementia accounted for nearly 80 percent of dementia cases combined, and vascular dementia accounted for 12.1 percent of cases. Therefore, "screening for cognitive disorders in the oldest old is of the utmost importance, especially in high-risk groups," the authors conclude.

Arch Neurol. 2011;68[5]:631-636.