Mohammed T. Abou-Saleh

Principles and Practice of Geriatric Psychiatry.Editors: Professor John R. M. Copeland, Dr Mohammed T. Abou-Saleh and Professor Dan G. BlazerCopyright & 2002 John Wiley & Sons LtdPrint ISBN 0-471-98197-4 Online ISBN 0-470-84641-038Dementia Epidemiology:Prevalence and IncidenceA. F. JormCentre for Mental Health Research, The Australian National University, Canberra, AustraliaPrevalence is the proportion of cases of a disease present in apopulation at any one time, while incidence is the rate ofoccurrence of new cases over a given period of time, usually 1year. Prevalence is a function of both the incidence of disease andits duration: the prevalence of a disease will rise if the rate of newcases increases or if the average case survives longer. Prevalence isuseful for assessing the likely need for service provision. However,for purely scientific purposes, such as assessing risk factors,incidence is preferred over prevalence. The reason is that anydifferences between groups in prevalence may be due todifferences in either incidence or disease duration.The notions of prevalence and incidence are based on theassumption that a population can be neatly divided into cases andnon-cases. However, for dementia this division is not straightforward.There is a gradation from normal cognitive ageing throughto severe dementia, without clear breaks to define where normalityends and dementia begins. The threshold for dementia is usuallydefined in terms of interference with daily living, but even this is afuzzy boundary. Furthermore, prevalence and incidence studiestypically examine different levels of severity, described as ‘‘mild’’,‘‘moderate’’ or ‘‘severe’’, but these descriptors are not always usedconsistently to divide up the continuum of severity. Variousdiagnostic criteria for dementia are known to divide thecontinuum in different ways, which can result in very differentprevalence and incidence rates. For example, Erkinjuntti et al. 1examined the rates of dementia in the same sample using sixdifferent sets of diagnostic criteria. They found that the percentagewith dementia varied from 3.1% using the ICD-10 criteria to29.1% using DSM-III. Thus, there are no ‘‘true’’ prevalence orincidence rates for dementia, but rather various rates dependenton the definition of dementia used.PREVALENCE OF DEMENTIAThe number of prevalence studies is now very large and severalmeta-analyses have been carried out to pool the data for thosethat give rates for specific age groups (e.g. 65–69 years). The firstmeta-analysis, by Jorm et al. 2 , involved fitting a statistical modelto data from 22 studies published between 1945 and 1985. Theyfound that methodological differences between studies contributedto variation in prevalence rates. For example, studies thatused a broad definition of dementia (to include all cognitiveimpairment) had rates 64% higher than those using a morenarrow definition. They fitted an exponential statistical model tothe data 2 . The essence of this model is that prevalence risesexponentially with age, doubling every 5.1 years, but the actualrates differ from study to study. Although there were differencesbetween studies, it is possible to derive average rates acrossstudies. These are shown in column 1 of Table 38.1. Theexponential model is an adequate description up to age 90 butshould not be applied above that age. If prevalence continued todouble every 5.1 years above age 90, it would soon be greater than100%, which is impossible. This limitation of the exponentialmodel has led some researchers to use the logistic model, in whichprevalence at first rises steeply, but then levels out to a maximumof 100% 3 . For prevalence rates of 0–50% the exponential andlogistic curves are difficult to distinguish, and the exponentialcurve may be preferred because of its simplicity.The second meta-analysis involved a pooling of data from 12European studies dating 1980–1990, which used DSM-III orequivalent criteria 4 . This meta-analysis did not involve fitting astatistical model to the data or testing for the effects ofmethodological differences. Rather, the researchers simply dividedthe data from each study into 5 year age groups and pooled them.The results are shown in column 2 of Table 38.1. Despite thedifferences in approach, the results are remarkably close to thoseof Jorm et al. 2 .A third meta-analysis was carried out by Ritchie and Kildea 5(this superseded an earlier meta-analysis by Ritchie et al. 6 , whichwill not be described here). They were particularly interested inwhat happens to prevalence in extreme old age, in particularwhether dementia is inevitable if a person lives long enough.Ritchie and Kildea 5 pooled data from nine studies that includedsamples of elderly people aged over 80. These data are shown incolumn 3 of Table 38.1 and are very similar to the earlier metaanalysesup to age 90. Ritchie and Kildea 5 fitted various curves toTable 38.1Prevalence rates (%) for dementia from three meta-analysesMeta-analysisAge group Jorm et al. 2 Hofman et al. 4 Ritchie and Kildea 560–64 0.7 1.0 —65–69 1.4 1.4 1.570–74 2.8 4.1 3.575–79 5.6 5.7 6.880–84 11.1 13.0 13.685–89 23.6* 24.5* 22.390–94 — — 33.095–99 44.8* Rates for ages 85+.Principles and Practice of Geriatric Psychiatry, 2nd edn. Edited by J. R. M. Copeland, M. T. Abou-Saleh and D. G. Blazer&2002 John Wiley & Sons, Ltd