Incidence of Dementia in England and Wales: The MRC Cognitive Function and Ageing Study

BACKGROUND The greatest challenge to the study of dementia for epidemiologists is the definition of this syndrome and its subtypes. Ideally there would be a fixed definition which could be easily operationalized with consistency and validity in any location, language, culture, and time period. The definitions of the condition surrounding dementia—whether they be subtypes, clinical, neuropsychologic, neuropathologic, neuroimaging, boundary syndromes or what dementia is not—‘‘normal’’ and ‘‘successful ageing’’—are all themselves subject to intense research activity including their very existence. Thus although there have been enormous advances in knowledge in all these fields we are still anchored to a combination of decisions based on a combination of assumptions in different domains about the absence or presence of particular impairments in an individual. The brief period in the 1980s when it seemed that clinical diagnosis of dementia, subtypes, and borderline conditions might be superseded by algorithmic and decisionbased diagnosis has been passed by with, instead, a movement away from the earlier gold standard of single physician diagnosis to multidisciplinary review consensus diagnosis with a range of investigations to hand. The field of descriptive epidemiology has benefited greatly from these developments—making it possible to adopt more uniform, if hotly contested and imperfect, diagnostic criteria for dementia using the internationally used sets of criteria, most often DSM, originally III, then IIIR and IV. As governments recognized the importance of disorders associated with ageing in the late 1970s and 1980s they started investing in dedicated population studies and any review of descriptive epidemiology today is drawing on that era’s decisions for national investment including many of the wealthier countries of the world. There were many issues for those seeking to organize high quality population studies to grapple with. What population base? Which age groups, stratified by age and sex or not? Nationally representative of locally based populations? Institutions in or out? How high does the response rate have to be? What is the drop out, or is it better to have a volunteer sample with minimal drop out? One stage or 2 stages? How frequent the follow-up? How long should the study go on for? What should the methods of screening be, how intense the diagnostic instrument or should it be single interview? What is the impact of drop out within and between the phases of the study? Are medical records adequate for estimating impact of loss to follow up? How best to analyze to address all the different elements of the study, design and drop out, informative or missing at random? What is the exact definition of incidence and at what point does this occur and how should this be estimated? How to achieve sufficient investment at baseline to be able to examine risk at a later stage? Funding agencies watched all this and worried about the likely open-ended cheques being requested. The research was funded, although often with compromise. But, in reality, most of the studies launched in the 1980s and early 1990s have continued with fairly extensive follow up and have contributed enormously to the current understanding of dementia with descriptive and risk outputs. Each has been subject to the particular funding arrangements of their context, our own not least. Comprehensive reviews of incidence studies have already been published; this paper will not replicate this exercise but draw on these for illustration. There have been 2 main approaches: one to conduct a systematic review followed by meta-analysis and formal analysis. The other a systematic review followed by a Delphi approach to create estimates of prevalence, linked to global burden mortality to create incidence rates. These show that there is general agreement about rates in the developed world and that the estimates for less developed regions tend to be in the middle. The intermediate countries are those with relatively unstable estimates— many of these have rapidly changing mortality patterns and the survey data quality provide some difficulties for comparison. The rest of this contribution will concentrate on a single study. The Medical Research Council Cognitive Copyright r 2006 by Lippincott Williams & Wilkins From the Department of Public Health and Primary Care, University of Cambridge, Cambridge, UK. Supported by the Medical Research Council and the Department of Health, UK. Reprints: Carol Brayne, MSc, MD, MRCP, MFPHM, Department of Public Health and Primary Care, University of Cambridge, Cambridge CB2 2SR, Cambridge, UK (e-mail: cb105@medschl. cam.ac.uk). REVIEW ARTICLE