Decreasing trend in the incidence and prevalence of dementia: a systematic review

The progressive ageing of the population is one of the main socio-demographic phenomena, taking place at a global level. Several recent population-based studies conducted worldwide suggest that the age-specific risk of dementia may be changing in some countries and areas.

This systematic review was performed using the methodology proposed by the Cochrane handbook for systematic reviews and reported following the PRISMA statement. A structured bibliographic search was performed on the databases PubMed, ISI Web of Science and the Cochrane Database of Systematic Reviews. All included studies were qualitatively assessed using the Methodological Evaluation of Observational REsearch (MORE).

The bibliographic search yielded 2394 records. Three more articles were retrieved from other sources. A total of ten studies were included, five reported data on a possible reduction in the prevalence of dementia, and five reported data on a possible reduction in its incidence.

The present systematic review focused on the recent observations of a possible decrease in the frequency of dementia and cognitive impairment in some Western countries (USA, UK, Sweden, the Netherlands, France, Iceland) between 1977 and 2014. The included studies have a high heterogeneity in terms of the clinical criteria used to diagnose dementia, and of the criteria used to define the clinical condition preceding dementia, such as isolated cognitive impairment. Moreover, the methodological quality with which they were conducted was also heterogeneous, with scores ranging from 1 to 7 using the MORE tool.