Epidemiology of aging and associated cognitive disorders: Prevalence and incidence of Alzheimer's disease and other dementias.

The study of the incidence and prevalence of dementia is important to understand the distribution of dementing illness among age and sex groups, and for the detection of possible causes of these disorders. A variation in the incidence or prevalence of dementia, especially Alzheimer's disease (AD) by region or specific populations can be because of greater or lesser exposure to the causal agents of dementia. For example, in the past the striking differences in the incidence of coronary heart disease (CHD) led to the understanding of the relationship between dietary factors, such as saturated fat and dietary cholesterol, and the incidence of CHD. However, there is a high prevalence of dementia in elderly individuals around the world, and multiple studies conducted in industrialized and nonindustrialized countries have shown an age-standardized prevalence of dementia ranging from 5% to 7% in most countries. Dementia is not a specific disease but rather a constellation of cognitive changes and disability due to several "causes," i.e., AD, Lewy bodies, vascular disease, drugs, and alcohol. Whether there is a trend for reduced incidence of dementia has to be further evaluated. It is possible that the improvement in the treatments of risk factors, especially vascular disease, has resulted in decreased incidence. However, this could result in an increase in prevalence, since the improved therapies for risk factors will lead to increased longevity in patients with dementia.