Guest Editorial
From Counting to Understanding: The Evolving Epidemiologic Approach to Dementia
Ian McDowell, PhD1
The aging of populations has raised concerns over the impact of an aging population on the prevalence and incidence of age-associated diseases, such as the dementias. This demographic shift began earlier in Europe than elsewhere; during the 1980s, several epidemiologic studies recorded the prevalence and incidence of dementia in Europe (1–4). Other studies investigated US populations (5–7). By the early 1990s, comparative summaries had been written (8), and general consensus arose on several key findings. From age 65 to age 90 years, incidence and prevalence rates double with roughly every 5 years of age and then slow. The balance between Alzheimer’s disease (AD) and vascular dementias may vary from place to place, but prevalence and incidence rates are comparable among countries when all forms of dementia are combined. Some studies showed different rates for men and women; however, this is far from consistent, and there seems to be little overall difference for all forms of dementia combined. Because of their greater number, there are more women with dementia.
In Canada, epidemiologic studies of dementia began relatively late. Several regional studies were undertaken during the 1980s (9). Only late in the decade, however, did the federal government begin to consider the need for a large, national study of dementia (10) that would parallel a similar study of osteoporosis (11). This study, intended to provide statistical evidence to inform policies, led to the development of the longitudinal Canadian Study of Health and Aging (CSHA), described in this issue by Lindsay and colleagues (12). By then, it was clear that merely replicating existing studies had little virtue; further, policy development requires more than estimates of prevalence and incidence. By the time the CSHA was being planned, a second generation of epidemiologic studies was turning the focus toward identifying risk factors for Alzheimer’s disease and other forms of dementia and toward studying patterns of providing care for people with dementia. Both the CSHA and the Indianapolis–Ibadan dementia project (13) were designed in the context of the wave of new studies undertaken during the 1990s and built on the experience of earlier studies to expand the epidemiologic perspective on dementia. Both studies formed part of a set of 6 international investigations of dementia coordinated by the World Health Organization (13,14).
Early studies of risk factors for AD had shown that no single factor accounted for the condition. Further, the associations that were established (such as associations with age or educational attainment) represented a range of etiologic channels requiring careful interpretation. These findings led to revisions in the diagnostic classifications of the dementias and included a closer investigation of the relation between Alzheimer’s disease and vascular dementia (15,16). They also led to finer conceptual distinctions, such as that between dementia as an age-associated vs an aging-related condition (17). Similarly, epidemiologic studies, such as the Religious Orders Study, showed that the association between AD and education occurs primarily with the clinical manifestation of disease, rather than with the underlying pathology (18). This finding prompted the interpretation of education as a marker for cognitive reserve. “Cognitive reserve” is a general term that refers to some aspect of brain structure or function that enables a person possessing greater reserve to function longer in the presence of brain pathology than a person with less reserve (18–21). The subsequent finding that people who engage in mentally stimulating activities during their adult lives (which may in part be predicted by their educational level) are at reduced risk of dementia (22) coincided felicitously with growing evidence from the basic sciences that the human brain remains plastic and capable of continued growth into adulthood (23,24). Hence, the epidemiologic approach to the dementias evolved from identifying associations to studying the aging process in a more dynamic manner—one that recognized the interactions among biological, personal, and environmental factors. This issue’s paper by Hendrie and colleagues describes ways of thinking about gene–environment interactions and illustrates work of this type (25).
Traditionally, etiologic studies in epidemiology focused on explaining why people get sick; only recently has attention turned to the corollary—why many people in very similar circumstances nonetheless remain healthy. For example, the objectives for the 10-year follow-up of the CSHA were broadened to include studies of healthy and successful aging. Studies of susceptibility are familiar to psychologists and psychiatrists and, with growing interactions between disciplines, have begun to influence epidemiologic approaches. This “new epidemiology” has also begun to consider the social and other contextual factors that may constitute independent influences on health. The CSHA, for example, has linked individual-level data on dementia to contextual information available through geographic information systems databases and also to compositional information drawn from the Canadian census. This linkage will permit multilevel analyses to identify the influence of separate factors (such as socioeconomic status) measured at the individual, family, and community levels.
It is hoped that this fuller conceptualization of etiologic factors and attention to explaining both cases and noncases, combined with the incorporation of factors (such as genetic and environmental factors) that operate at different levels in the causal web, may furnish greater insights into the causes of complex conditions such as AD. No study can address all the issues involved in understanding a complex disease. Although the CSHA was not able to collect pathological samples or to undertake brain imaging of all participants, it does have important strengths. With an initial sample size of more than 10 000 elderly people, and using a representative sample that included subjects from both the community and institutions, it is one of the largest studies of dementia yet undertaken. The diverse data set spans genetics; quantitative, and some qualitative, self-reported data; neuropsychological testing and clinical examinations; health services utilization; death certificate information from provincial sources; and contextual data describing the environment in which people live. This breadth characterizes the approach of the new epidemiology; time will tell what additional insights it is able to offer.
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Author(s)
1Professor, Department of Epidemiology and Community Medicine, Faculty of Medicine, University of Ottawa, Ottawa, Ontario
Address for correspondence: Dr I McDowell, Department of Epidemiology and Community Medicine, Faculty of Medicine, University of Ottawa, 451 Smyth Road, Ottawa, ON K1H 8M5
e-mail: mcdowell@uottawa.ca
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