Prevalence of dementia in mainland China, Hong Kong and Taiwan: an updated systematic review and meta-analysis

Abstract Background There are several existing systematic reviews of prevalence of dementia for mainland China, Hong Kong and Taiwan, but several studies have been newly reported. The aim of this study is to update prevalence data in this region and test for variation across geographical areas and time periods using the new dataset. Methods Twenty prevalence studies identified from World Alzheimer Report 2015 (January 2011–March 2015) and an updated search (March 2015–February 2017) were added to the original dataset (N = 76). Meta-regression was used to investigate geographical variation and time trends, taking methodological factors and characteristics of study population into account, and to estimate prevalence and number of people with dementia by geographical area. Results Compared with northern China, the prevalence of dementia was lower in the central China [-1.0; 95% confidence interval (CI):−2.2, 0.3], south China (−1.7; 95% CI: −3.1, −0.3), Hong Kong and Taiwan (−3.0; 95% CI: −5.0, −1.0) but appeared to be higher in western China (2.8; 95% CI: 0.1, 5.5) after adjusting for methodological variation. The increasing trend from pre-1990 to post-2010 periods was considerably attenuated when taking into account methodological factors and geographical areas. The updated estimated number of people with dementia in all these areas is 9.5 million (5.3%; 95% CI: 4.3, 6.3) in the population aged 60 or above. Conclusions Geographical variation in dementia prevalence is confirmed in this update, whereas evidence on increasing trends is still insufficient. Differing societal development across areas provides an opportunity to investigate risk factors at the population level operating across diverse life course experiences. Such research could advance global primary prevention of dementia.

Appendix S1. Protocol, search strategy, included and excluded studies 1

. Systematic review protocol
Background Dementia has become a major public health concern across the globe. 1 The worldwide epidemiology of dementia has been an important topic as it provides fundamental information for research and policy planning. Compared to other low and middle income countries, China has a comparatively rich body of literature on prevalence studies of dementia. 2 Although there are several existing systematic reviews of prevalence of dementia for mainland China, Hong Kong and Taiwan (latest search until April 2012), 2-4 the recent reporting of contemporary provides a need for these to be updated.

Aim
The aim of this study is to update the prevalence estimates of dementia in mainland China, Hong Kong and The same search strategy and inclusion/exclusion criteria reported in the earlier reviews will be used to select included studies. 4 An electronic search of English and Chinese databases: PubMed, Web of Science, Chinese Inclusion criteria (1) Cases were collected by field survey, not based on hospital data (2) The study involved population sampling rather than recruiting volunteer participants (3) The study reported prevalence in the people aged 50 and over (4) Dementia case identification was not solely decided by a screening test and that specific instruments and criteria were reported The following information will be extracted from the included studies: (1) Study design: methods of screening, diagnosis and confirmation, interviewers and sampling method; (2) Participants: sample size and response rate, characteristics of participants, such as age group, study location, urban or rural area; (3) Dementia identification: screening tools, diagnostic criteria and instruments; (4) Results: overall prevalence of all type dementia and stratified prevalence by age, gender and educational level.

Data analysis
The same analytical methods reported in the previous analysis 4 will be used to analyse the up to date prevalence data. To adjust for the effect of age, prevalence estimates from individual studies will be standardised to the Census Population of China 2010. 8 A random-effect meta-analysis will be used to calculate the overall and regional pooled estimate of dementia prevalence. Meta-regression will be conducted to test whether the variation in prevalence estimates can be related to methodological factors or characteristics of study populations and to investigate difference across geographical areas and time periods taking into account study design and methodological factors. The results of meta-regression modelling will be applied to population structures by different areas and used to estimate the numbers of people with dementia.    Taiwan focused on the population aged 65 or above, the estimation was only generated from age 65. Since studies from western areas did not report prevalence estimate in age 90 or above, the estimate was calculated for age 85 or above. Using the same method, Figure S3-2 shows age-stratified prevalence across the five areas.  Eligibility criteria 6 Specify study characteristics (e.g., PICOS, length of follow-up) and report characteristics (e.g., years considered, language, publication status) used as criteria for eligibility, giving rationale.

RESULTS
Study selection 17 Give numbers of studies screened, assessed for eligibility, and included in the review, with reasons for exclusions at each stage, ideally with a flow diagram. Study characteristics 18 For each study, present characteristics for which data were extracted (e.g., study size, PICOS, follow-up period) and provide the citations.
p.10-13, S2 Risk of bias within studies 19 Present data on risk of bias of each study and, if available, any outcome level assessment (see item 12). S2 Results of individual studies 20 For all outcomes considered (benefits or harms), present, for each study: (a) simple summary data for each intervention group (b) effect estimates and confidence intervals, ideally with a forest plot.