4 Ethnic Differences in Dementia and Alzheimer’s Disease

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The proportion of ethnic minorities among the elderly in the United States is increasing. The U.S. Census Bureau estimates that the proportion of elders who are white and non-Hispanic will decline from 87 percent in 1990 to 67 percent in 2050. As compared to the 1990 Census, the population of Hispanic elders is expected to double in 2010, and will be 11 times greater by 2050. Of the 80.1 million elderly projected for 2050, 8.4 million (10.4 percent) will be black, as compared to 8 percent of elders in 1990. With these changes, ethnic minority populations will bear an increased share of the economic and social burden associated with diseases that predominantly affect the elderly, such as Alzheimer’s disease (AD). This presents the potential for a major public health issue because ethnic minorities may be at higher risk for AD and dementia than non-Hispanic whites.

Investigations of ethnic populations that have migrated across several cultures offer the opportunity to study groups for which genetic factors essentially remain the same but environmental and cultural forces undergo dramatic change. At the same time, comparison of different racial groups residing in the same environment with similar socioeconomic status and equal exposure to risk factors may help to uncover genetic factors responsible for AD (Osuntokun et al., 1992).

The studies reviewed in this chapter examine ethnic differences in rates of broad categories such as “cognitive impairment” or “dementia” as well as specific neurodegenerative diseases such as Alzheimer’s disease and vascular dementia. Cognitive impairment, a necessary prerequisite for diagnosis of any dementia, is determined using either screening tests, such as the Mini-Mental State Exam (MMSE), or more sensitive and extensive neuropsychological test batteries incorporating individual measures such as Logical Memory from the Weschler Memory Scale. To meet clinical criteria for dementia, cognitive impairment must be of sufficient severity to interfere with activities of daily living. Cross-cultural research on dementia must contend with the fact that assessments of both cognitive impairment and daily functioning are susceptible to culturally dependent definitions and are quantified by measures that are sensitive to cultural and educational background.

There are many possible etiologies of progressive dementia, but the most frequent causes are Alzheimer’s pathology and cerebrovascular disease. Although the exact etiology cannot be definitively determined before an autopsy, there are research criteria for AD and vascular dementia that have been shown to predict the specific pathological determination upon autopsy with up to 90 percent accuracy. However, the supporting research has involved almost exclusively white subjects. Few autopsy studies have been performed to confirm the accuracy of these diagnoses among ethnically diverse groups.

This chapter will first review the findings of epidemiological studies of dementia and AD among different ethnic groups within the United States and other countries. This review is not intended to be a comprehensive survey of AD epidemiology, which is available elsewhere (Chang, Miller, and Lin, 1993Hendrie, 1998Jorm, 1990Larson and Imai, 1996Yeo, Gallagher-Thompson, and Lieberman, 1996); rather, it is intended to highlight specific studies that emphasize the issues in research of ethnicity and AD. We will then explore some potential explanations for ethnic differences in rates of AD and dementia: (1) statistical limitations, (2) bias in measurement of cognitive functioning, (3) genetic factors, (4) nongenetic medical risk factors, and (5) social factors.


Ethnic Comparisons Within the United States

A number of studies have compared the rates of dementia and AD between ethnic groups residing in the United States. Despite differences in sampling methods and definitions of dementia as well as in definitions of race/ethnicity, the most frequent findings in reviewing this literature are that African Americans and Hispanics have higher prevalence and incidence of dementia and AD than whites. Native Americans appeared to have lower rates of AD in comparison to whites. Asian Americans had rates of dementia comparable to whites; however, whether there is the same proportion of AD compared to vascular dementia among Asian Americans and Asian immigrants remains uncertain. Opinion differs on whether correction for education accounted for the different rates of dementia and AD found among these cultural groups.

Most U.S.-based studies have focused on comparing rates of dementia or AD among African Americans and Hispanics to rates among whites. These studies found higher rates of cognitive impairment, dementia, and AD among ethnic minorities than among whites (Folstein, Bassett, Anthony, Romanoski, and Nestadt, 1991George, Landerman, Blazer, and Anthony, 1991Gurland et al., 1998Haerer, Anderson, and Schoenberg, 1987Perkins et al., 1997Prineas et al., 1995Schoenberg, Anderson, and Haerer, 1985Still, Jackson, Brandes, Abramson, and Macera, 1990Teresi, Albert, Holmes, and Mayeux, 1999). One of the largest projects, a population-based, longitudinal study of 2,126 elderly residents of New York City, examined the incidence of AD among three ethnic/racial groups, self-defined according to U.S. Census criteria: Non-Hispanic whites, non-Hispanic blacks, and Hispanics (mostly Caribbean) (Tang et al., 2001). These individuals were identified as Medicare recipients residing in selected Census tracts of the neighborhoods of Washington Heights and Inwood. Using National Institute of Neurological and Communicative Disorders and Stroke and the Alzheimer’s Disease and Related Disorders Associations (NINCDS-ADRDA) criteria, neurological examination, and results from an extensive neuropsychological test, the standardized incidence rate for non-Hispanic black elders (4.2 percent per person-year) and Caribbean Hispanics (3.8 percent per person-year) was significantly higher than that of the referent group, non-Hispanic whites, even after correcting for differences in years of education.

Another large study, the Duke Established Populations for Epidemiological Studies of the Elderly project, found no differences in frequency of dementia between African Americans and whites. This study described a sample of 4,136 participants (Fillenbaum et al., 1998), 55 percent of whom were African American. The sample was defined using multistage probability sampling with unequal probabilities of selection to sample community-dwelling residents age 65 and older within five adjacent counties, one urban and four rural. However, the way in which the racial groups were defined is unclear. The authors used the Consortium to Establish a Registry for Alzheimer’s Disease (CERAD) Neuropsychological Test battery to assess cognitive functioning, and norms correcting for years of education (Unverzagt, Hall, Torke, and Rediger, 1996) were used for the determination of significant cognitive deficit and dementia. The prevalence of dementia among elders above age 67, as determined by clinical consensus, was 7 percent for African Americans and 7.2 percent for whites. There were also no differences in the 3-year incidence of dementia for African Americans (5.8 percent) versus whites (6.2 percent). The authors did not report incidence of dementia subtypes; therefore, it is possible that although the overall rates of dementia were similar among African Americans and whites, the frequencies of AD and vascular dementia may differ within the groups.

The rate of dementia on admission to nursing homes is higher among black residents than among white residents (Weintraub et al., 2000); however, findings from studies of long-term outcomes for African-American elders with dementia are not consistent. Mortality associated with dementia was found to be higher among blacks than non-Hispanic whites, especially among black males (Lanska, 1998). However, there were no statistically significant differences in survival from time of entry into the CERAD study of whites and African Americans after accounting for the effects of age, gender, and severity of dementia (Heyman, Peterson, Fillenbaum, and Pieper, 1996). However, for each of these studies, the exact way in which racial groups were defined was not stated.

The role of immigration and changes in environmental risk factors was examined in several epidemiological studies of elders with Japanese ancestry. The age-standardized prevalence of dementia (using Diagnostic and Statistical Manual of Mental Disorders—Third Edition [DSM-III] criteria) among Japanese-American men aged 71 to 93 living in Hawaii (White et al., 1996) was 7.6 percent. This rate was higher than Japanese men living in Japan (4 percent to 6 percent), and similar to prevalence rates in European populations. The age-standardized prevalence of AD (using NINCDS-ADRDA criteria) in this Japanese-American population was 4.7 percent. The authors suggested that environmental or cultural exposures associated with migration from Japan to Hawaii influenced the development of AD in these Japanese Americans. Similar results were reported in a study of 1,985 Japanese-American participants in the Kame project in King County, Washington (Graves et al., 1996). A cross-sectional study of dementia prevalence using the California Alzheimer’s Disease and Diagnostic Treatment Centers found that, as compared to whites, Asian Americans had a greater proportion of vascular dementia and lower proportion of AD (Still et al., 1990), similar to studies of Asians in Asia (to be discussed).

Native Americans appear to have a lower rate of AD than whites, but equivalent rates of overall cognitive impairment or dementia. Hendrie et al. (1993) examined 192 Cree, aged 65 and older, living on two reserves in Manitoba, Canada, and an age-stratified sample of 241 English-speaking whites living in Winnipeg. Using the Community Screening Interview for Dementia (CSID) to screen for cognitive impairment, the authors found a significant difference between the age-adjusted prevalence of AD among the Cree Indians (0.5 percent) as compared to whites (3.5 percent), despite the two groups having an equivalent age-adjusted prevalence of dementia (4.2 percent in each population).

A study of Cherokee Indians living in northeastern Oklahoma (Rosenberg et al., 1996) used NINCDS-ADRDA criteria to identify 26 people aged 65 and older with AD, and then assessed an equal number of normal controls. The investigators found that as the genetic degree of Cherokee ancestry increased, the frequency of AD decreased. That is, after taking into account whether the ε4 allele of the apolipoprotein E (APOE) gene is present, elders with more than 50 percent genetic Cherokee ancestry were less likely to be in the AD group than the control group. Genetic degree of ancestry for each participant was calculated using genealogical records provided by the Cherokee Nation Tribal Registration Department. A limitation of this study is its case-control design; however, this study represents a unique method of examining the relationship of race/ethnicity to disease because the degree of ethnic ancestry was assessed (albeit not through formal genetic analysis), as opposed to classifying individuals into racial groups based on self-report or investigator observation.

**National Library of Medicine

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