The impact on prevalence of dementia in the oldest age groups of differential mortality patterns: a deterministic approach.

Background: Until recently relatively little data have been available on the prevalence of dementia in the oldest age groups, and yet it is these age groups which are expanding fastest. It is therefore important to understand whether the prevalence of dementia rises inexorably with age ('age-dependent'), or, as some suggest, levels off or even declines in the very oldest age groups ('age-related'). Combined analysis of the many prevalence studies now available has led to modelled curves which do suggest a slowing of rise in prevalence at these great ages, and has been interpreted as meaning that dementia is age-related. This interpretation does not take into account the differential survival of individuals with cognitive impairment compared with normals of the same age.

Method: Flexible prevalence-incidence-duration models were generated using a deterministic approach applied to published combined analyses of prevalence rates, population death rates and mortality odds ratios.

Results: The variation in observed prevalence patterns with age is explained to a great extent by the mortality observed in the cognitively impaired. Simple examination of age patterns in prevalence does not answer the fundamental question surrounding the age-dependence of cognitive impairment.

Conclusion: Inferring biological meaning from these observed curves is not valid without examining the mathematical phenomena of the relationship of incidence, mortality and prevalence. This approach allows an examination of the impact of varying mortality and incidence on the prevalence of dementia and cognitive impairment and will be useful in determining the potential impact of preventive strategies on the population.