**1. Introduction**

### **1.1. Global aging**

The aging of populations has become a worldwide phenomenon [1]. In 1990, 26 nations had more than two million elderly citizens aged 65 years and older, and the projections indicate that an additional 34 countries will join the list by 2030. In 2000, the number of old people (65+ years) in the world was estimated to be 420 million and it was projected to be nearly one billion by 2030, with the proportion of old people increasing from 7 to 12%. The largest increase in absolute numbers of old people will occur in developing countries; it will almost triple from 249 million in 2000 to an estimated 690 million in 2030. The developing regions' sharing the worldwide aging population will increase from 59 to 71% [2]. Developed coun‐ tries, which have already shown a dramatic increase in people over 65 years of age will ex‐ perience a progressive aging of the elderly population. Underlying global population aging is a process known as the "demographic transition" in which mortality and then fertility de‐ cline [3]. Decreasing fertility and lengthening life expectancy have together reshaped the age structure of the population in most regions of the planet by shifting relative weight from younger to older groups.

Both developed and developing countries will face the challenge of coping with a high fre‐ quency of chronic conditions, such as dementia, which is a characteristic of aging societies. These conditions impair the ability of older persons to function optimally in the community and reduce well-being among affected individuals and their families. Further, these condi‐ tions are associated with significant health care costs that must be sustained by the society at large. Thus, the global trend in the phenomenon of population aging has a dramatic impact on public health, healthcare financing and delivery systems throughout the world [4]. Due to the aging of the population, dementia has become a major challenge to elderly care and public health.

© 2013 Xu et al.; licensee InTech. This is an open access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2013 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

#### **1.2. Dementia and Alzheimer's disease**

Dementia is defined as a clinical syndrome, and characterized by the development of multi‐ ple cognitive deficits that are severe enough to interfere with daily functioning, including social and professional functioning. The cognitive deficits include memory impairment and at least one of the other cognitive domains, such as aphasia, apraxia, agnosia or disturbances in executive functioning [5, 6]. Alzheimer's disease is the most common cause of dementia in the elderly, accounting for 60-70% of all demented cases [7]. Alzheimer's disease is strictly a neuropathological diagnosis determined by the presence of neurofibrillary tangles and se‐ nile plaques in the brain of patients with dementia. The disease frequently starts with mem‐ ory impairment, but is invariably followed by a progressive global cognitive impairment [8]. Vascular dementia is the second most common cause of dementia in the elderly after Alz‐ heimer's disease. Vascular dementia is defined as loss of cognitive function resulting from ischemic, hypoperfusive, or haemorrhagic brain lesions due to cerebrovascular disease or cardiovascular pathology. Diagnosis of vascular dementia requires cognitive impairment; vascular brain lesions, often predominantly subcortical, as demonstrated by brain imaging; a temporal link between stroke and dementia; and exclusion of other causes of dementia [9]. The combination of Alzheimer's disease and vascular dementia pathological changes in the brains of older people are extremely common, making mixed dementia probably the most common type of dementia [10].

**2. Occurrence of Alzheimer's disease**

estimated as incidence × average disease duration.

**2.1. Prevalence**

of 9.7 % [15].

tively, of all dementia cases.

The occurrence of a disease can be measured as proportion of people affected by the disease in a defined population at a specific time point (prevalence), or as number of new cases that occur during a specific time period in a population at risk for developing that disease (inci‐ dence). The prevalence reflects the public health burden of the disease, whereas the inci‐ dence indicates the risk of developing that disease. The prevalence is determined by both incidence and duration of the disease, and in certain circumstances, the prevalence may be

Epidemiology of Alzheimer's Disease http://dx.doi.org/10.5772/54398 331

Based on the available epidemiological data, a group of experts estimated that 24.3 million people have dementia today, with 4.6 million new cases of dementia every year (one new case every 7 seconds). The number of people affected will double every 20 years to 81.1 mil‐ lion by 2040 [11]. Similar estimates have been reported previously [12]. Most people with dementia live in developing countries. China and its western Pacific neighbours have the highest number of people with dementia (6 million), followed by the European Union (5.0 million), USA (2.9 million), and India (1.5 million). The rates of increase in the number of dementia cases are not uniform across the world; numbers in developed countries are fore‐ casted to increase by 100% between 2001 and 2040, but to increase by more than 300% in In‐ dia, China, and other south Asian and western Pacific countries [11]. About 70% of these cases were attributed to Alzheimer's disease [11, 13]. The pooled data of population-based studies in Europe suggests that the age-standardized prevalence in people 65+ years old was 6.4 % for dementia and 4.4 % for Alzheimer's disease [14]. In the US, a study of a national representative sample of people aged >70 years yielded a prevalence for Alzheimer's disease

Worldwide, the global prevalence of dementia was estimated to be 3.9 % in people aged 60+ years, with the regional prevalence being 1.6 % in Africa, 4.0 % in China and Western Pacific regions, 4.6 % in Latin America, 5.4 % in Western Europe, and 6.4 % in North America [11]. A meta-analysis including 18 studies from China during 1990-2010 showed prevalence of Alzheimer's disease of 1.9% [16]. More than 25 million people in the world are currently af‐ fected by dementia, most suffering from Alzheimer's disease, with around 5 million new cases occurring every year [11]. The number of people with dementia is anticipated to dou‐ ble every 20 years. Despite different inclusion criteria, several meta-analyses and nationwide surveys have yielded roughly similar age-specific prevalence of AD across regions (Figure 1) [17]. The age-specific prevalence of Alzheimer's disease almost doubles every 5 years af‐ ter aged 65. Among developed nations, approximately 1 in 10 older people aged ≥ 65 is af‐ fected by some degree of dementia, whereas more than one third of very old people aged ≥85 years may have dementia-related symptoms and signs [18, 19]. There is a similar pattern of dementia subtypes across the world, with Alzheimer's disease and vascular dementia, the two most common forms of dementia, accounting for 50 % to 70 % and 15 % to 25 %, respec‐

Alzheimer's disease was first identified more than 100 years ago, but research into its symp‐ toms, causes, risk factors and treatment has gained momentum only in the last 30 years. Al‐ though research has revealed a great deal about Alzheimer's, the precise physiologic changes that trigger the development of Alzheimer's disease largely remain unknown. The only exceptions are certain rare, inherited forms of the disease caused by known genetic mu‐ tations. Alzheimer's disease affects people in different ways, but the most common symp‐ tom pattern begins with gradually worsening ability to remember new information. This occurs because disruption of brain cell function usually begins in brain regions involved in forming new memories. As damage spreads, individuals experience other difficulties. The following are warning signs of Alzheimer's disease: memory loss that disrupts daily life; challenges in planning or solving problems; difficulty completing familiar tasks at home, at work or at leisure; confusion with time or place; trouble understanding visual images and spatial relationships; new problems with words in speaking or writing; misplacing things and losing the ability to retrace steps; decreased or poor judgment; withdrawal from work or social activities; and changes in mood and personality. As the disease progresses, the in‐ dividual's cognitive and functional abilities decline. In advanced Alzheimer's disease, peo‐ ple need help with basic activities of daily living, such as bathing, dressing, eating and using the bathroom. Those in the final stages of the disease lose their ability to communicate, fail to recognize loved ones and become bed-bound and reliant on around-the-clock care. When an individual has difficulty moving because of Alzheimer's disease, they are more vulnera‐ ble to infections, including pneumonia (infection of the lungs).
