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Alzheimer's disease (AD) is the most common cause of dementia. Of the 5 to 10 per cent of the population aged over 65 years who have some kind of cognitive decline, over 50 per cent of cases will be due to AD and, although accounting for a smaller percentage of presenile cases, AD is still the single largest cause. The initial disease description by Alzheimer in 1907 was of a woman in her fifties with a progressive dementia and behavioral disturbance, who was found to have neurofibrillary tangles and amyloid plaques throughout her cerebral cortex. The term 'Alzheimer's disease' was then applied to similar cases with a presenile dementia, before it was realized that identical pathological changes were seen in the majority of elderly demented patients.

Since plaques and tangles are found in a very high proportion of non-demented elderly subjects, debate continues about whether AD represents a continuum or a distinct disease process that increases in
frequency with age. With recognition of a number of causative gene mutations (see below) AD is now generally believed to be a multifactorial disease with familial and sporadic forms.

Alzheimer's disease Video

The definition of dementia has evolved from one of progressive global intellectual deterioration to a syndrome consisting of progressive impairment in memory and at
least one other cognitive deficit (aphasia, apraxia, agnosia, or disturbance in executive function) in the absence of another explanatory central nervous system
disorder, depression, or delirium (according to the Diagnostic and Statistical Manual of Mental Disorders, 4th edn (DSM-IV) ). Even this recent syndrome concept is
becoming inadequate, as researchers and clinicians become more aware of the specific early cognitive profiles associated with different dementia syndromes. For
instance, in early Alzheimer's disease there may be isolated memory impairment many years before more widespread deficits develop.
Since dementia is predominantly a disorder of later life, it represents an increasing problem for individuals and society with the projected increase in the elderly
population. It is estimated that the 18 million people with dementia worldwide will increase to 34 million by the year 2025. This increase is most marked in the
developing countries, where the 11 million people with dementia in the year 2000 will reach 24 million by 2025. In the developed world, the equivalent figures are 7
million in 2000 and 11 million in 2025. In Europe alone, 4 million people will be affected by the year 2004.
Although the incidence of dementia is difficult to establish, community prevalence studies suggest that about 8 per cent of all people over 65 years of age suffer from
dementia, this shows a marked increase with advancing age. The prevalence below 65 years is about 1:1000, this rises to 1:50 to the age of 70 and 1:20 from 70 to
80. Over 80 years of age the prevalence is 1:5.