Mohammed T. Abou-Saleh

196 PRINCIPLES AND PRACTICE OF GERIATRIC PSYCHIATRYthe data, including exponential and logistic models. The best fitwas by a modified logistic curve in which prevalence levelled off ataround 40% at age 95. The authors concluded that dementia isnot inevitable in extreme old age. However, this conclusion hasbeen criticized by McGee and Brayne 7 because it was based onprevalence rather than incidence data. A decrease in survival withdementia in very old age could explain the flattening of the agecurvethat was observed.PREVALENCE OF ALZHEIMER’S DISEASE ANDVASCULAR DEMENTIAThe clinical diagnosis of Alzheimer’s disease and vasculardementia in community surveys involves additional problemsbeyond those in diagnosing global dementia, so such studies havebeen fewer. Nevertheless, several meta-analyses have attempted tointegrate data on the issue.The original meta-analysis of Jorm et al. 2 also examined sevenstudies that gave age-specific data on Alzheimer’s disease andvascular dementia. One study could not be fitted well by theTable 38.2 Prevalence rates (%) for Alzheimer’s disease according to ameta-analysis by the US General Accounting Office 10All severity levels Moderate–severe casesAge Men Women Men Women65 0.6 0.8 0.3 0.670 1.3 1.7 0.6 1.175 2.7 3.5 1.1 2.380 5.6 7.1 2.3 4.485 11.1 13.8 4.4 8.690 20.8 25.2 8.5 15.895 35.6 41.5 15.8 27.4exponential model, but the remaining six could. The prevalence ofAlzheimer’s was found to double every 4.5 years of age, whilevascular dementia doubled every 5.3 years. In other words, therise with age was steeper for Alzheimer’s disease.The pooling of data from European studies also examinedspecific dementing diseases 8 . The pooled prevalence rates forAlzheimer’s disease from six studies were: 30–59 years, 0.02%; 60–69 years, 0.3%; 70–79 years, 3.2%; and 80–89 years, 10.8%. It wasnot possible to arrive at pooled rates for vascular dementiabecause of the variation across studies.Later, Corrada et al. 9 fitted a logistic model to data from 15studies giving age-specific data on prevalence of Alzheimer’sdisease. They found considerable variation in rates betweenstudies depending on the methodology used. However, theunderlying trend was for an 18% increase in the odds forAlzheimer’s disease with every year of age.The most recent meta-analysis was carried out by the USGeneral Accounting Office 10 in response to controversy in thatcountry about the number of people with Alzheimer’s disease. Alogistic curve was fitted to data from 18 studies with predominantlyWhite populations. The data were grouped by sex andseverity level of the dementia and are shown in Table 38.2. It canbe seen that the results vary, depending on severity, and thatfemales have a higher prevalence than males.INCIDENCE STUDIESIncidence studies are much rarer than prevalence studies becausethey require longitudinal data and large sample sizes to arrive atage-specific rates. It is only fairly recently that sufficient studieshave become available to permit meta-analysis.Two meta-analyses have been published at around the sametime. The first of these, by Jorm and Jolley 11 , used data from 23published studies. The incidence of both dementia and Alzheimer’sdisease was found to increase exponentially with age up toTable 38.3. Incidence rates (%) for dementia from meta-analyses by Jorm and Jolley 11 and Gao et al. 12Jorm and Jolley 11 Gao et al. 12Age group Europe (mild+) Europe (moderate+) USA (moderate+) East Asia (mild+) 12 studies60–64 — — — — 0.1165–69 0.91 0.36 0.24 0.35 0.3370–74 1.76 0.64 0.50 0.71 0.8475–79 3.33 1.17 1.05 1.47 1.8280–84 5.99 2.15 1.77 3.26 3.3685–89 10.41 3.77 2.75 7.21 5.3390–94 17.98 6.61 — — 7.2995+ — — — — 8.68Mild+ results from USA and Moderate+ results from East Asia are missing from the table because insufficient data were found in the literature.Table 38.4 Incidence rates (%) for Alzheimer’s disease from meta-analyses by Jorm and Jolley 11 and Gao et al. 12Jorm and Jolley 11Gao et al.Age group Europe (mild+) Europe (moderate+) USA (mild+) USA (moderate+) East Asia (mild+) 8 studies60–64 — — — — — 0.0665–69 0.25 0.10 0.61 0.16 0.07 0.1970–74 0.52 0.22 1.11 0.35 0.21 0.5175–79 1.07 0.48 2.01 0.78 0.58 1.1780–84 2.21 1.06 3.84 1.48 1.49 2.3185–89 4.61 2.26 7.45 2.60 3.97 3.8690–94 9.66 4.77 - - - 5.4995+ - - - - - 6.68Moderate+ results from East Asia are missing from the table because insufficient data were found in the literature.