Alzheimers disease and other dementias Prevalence and incidence of dementia

An important distinction to be drawn in epidemiology is that between prevalence and incidence. Put simply, prevalence refers to the proportion of the population which is affected by a disease at a particular point or period in time, whereas incidence is the rate at which new cases arise over a period (typically 1 year). Prevalence is determined by both the incidence and how long people continue to have the disease. Prevalence is relevant for administrative purposes since it tells how many people there are to care for, whereas incidence is of more scientific interest in evaluating risk factors because it is unaffected by survival differences.

The prevalence of dementia has been researched extensively and several meta-analyses have pooled data from a large number of studies. Figure.3 shows the prevalence of dementia by age group from three of these meta-analyses/,7,,8 and 9) The prevalence of dementia rises steeply with age, following approximately an exponential form up to the age of 90 years. There is dispute about what happens at older ages: some authorities argue that the prevalence of dementia would eventually reach 100 per cent, whereas others believe that the rise in prevalence will eventually level off. However, the issue of what happens in extreme old age is best settled by incidence data, rather than prevalence, because any levelling off could be due to a shorter survival time after developing dementia.

Fig. 3 Prevalence rates for dementia across age groups: data from three meta-analyses.

Of course, dementia is simply a syndrome caused by many different diseases. The prevalence of the underlying diseases is of more interest. Alzheimer's disease and cerebrovascular disease are the two major causes of dementia, but their relative importance differs from country to country. (1..0) In Europe, North America, and Australasia, Alzheimer's disease is more prevalent than vascular dementia. However, in Oriental countries, vascular dementia is sometimes found to be more prevalent, although more recent studies indicate a trend towards a predominance of Alzheimer's disease. We have much less information about the prevalence of dementing diseases in other parts of the world. It is unwise to generalize findings from the developed countries to the developing ones, because there could be major differences. For example, one cross-national study found that the age-adjusted prevalence of dementia was only 2 per cent in Nigerians compared with 8 per cent in African-Americans, and the prevalence of Alzheimer's disease was also lower (1 per cent versus 6 per cent). (H) Recent evidence from this continuing study suggests that incidence may also be lower.

Incidence has been much less researched than prevalence because it involves following up, over a period of time, a large group of people who are initially disease free. However, the results from a meta-analysis of the incidence of dementia and Alzheimer's disease show that incidence rises approximately exponentially with age, but that it is lower in Oriental countries than in Europe or North America. (!2) The cause of these regional differences is unknown; they could be real or the result of differences in diagnostic practices. While there is no gender difference in the incidence of dementia, females may have a higher incidence of Alzheimer's disease in very old age.

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