Overall, reported rates for dementia among older adults have ranged from 4% to 7% (Pfeffer, Afifi, & Chance, 1987), although more recent studies have estimated rates as high as 10% (Evans et al., 1989). The rates increase with age, with estimates of 3-5% among adults aged 65 to 74,18-20% of those aged 7584, and as many as 50% of those aged 85 and older. It has been estimated that as much as 90% of dementia is of the Alzheimer's type, with another 10% accounted for by vascular dementia (Evans et al., 1989; Tatemichi, Sacktor, & Mayeau, 1994). In comparison to non-Hispanic whites, however, the rates of vascular dementia among African Americans, Chinese Americans, and Japanese Americans, appear to be 2-4 times higher, as a result of higher rates of conditions such as hypertension, whereas the rates among Hispanics are lower, a result of lower rates of cardiovascular disease (Hasegawa, Homma, & Imai, 1985).
Based on the results of screening instruments such as the Mini-Mental State Examination (MMSE; Folstein, Folstein, & McHugh, 1975), the rates of cognitive impairment have been reported to be twice as high among African Americans and Hispanic Americans as among non-Hispanic whites (Black et al., 1999; Escobar et al., 1986). The rates among Native Americans may be considerably lower, although data are available from only a very limited number of studies (Hendrie et al., 1993). Using data from the five Epidemiologic Catchment Area (ECA) study sites, for example, George, Landerman, Blazer, and Anthony (1991) reported unadjusted rates of cognitive impairment to be 42% for African Americans and 29% of Hispanic Americans, in comparison to 12% for non-Hispanic whites. The rate differences found in the ECA data were much less dramatic for severe impairment (8% for African Americans and 3% for Hispanics, compared to 2% for non-Hispanic whites), than the rate differences for mild impairment (33% for African Americans, 26% for Hispanics, and 10% for non-Hispanic whites).
The issue then arises as to whether these are true differences, or inaccurate reflections of impairment resulting from sociodemographic and cultural bias, particularly confounded by level of education and adaptation into the mainstream society. On the basis of education alone, the validity of assessments among older minorities becomes questionable. A substantial amount of convincing research has demonstrated that older individuals who are poorly educated or illiterate perform poorly on screening instruments such as the MMSE
(Taussig, Henderson, & Mack, 1996), and will score like brain-damaged individuals on neuropsychological tests (Ardila, Roselli, & Puente, 1994).
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