Temporal trend in dementia incidence since 2002 and projections for prevalence in England and Wales to 2040: modelling study

Objective To forecast dementia prevalence with a dynamic modelling approach that integrates calendar trends in dementia incidence with those for mortality and cardiovascular disease. Design Modelling study. Setting General adult population of England and Wales. Participants The English Longitudinal Study of Ageing (ELSA) is a representative panel study with six waves of data across 2002-13. Men and women aged 50 or more years, selected randomly, and their cohabiting partners were recruited to the first wave of ELSA (2002-03). 11392 adults participated (response rate 67%). To maintain representativeness, refreshment participants were recruited to the study at subsequent waves. The total analytical sample constituted 17 906 people. Constant objective criteria based on cognitive and functional impairment were used to ascertain dementia cases at each wave. Main outcome measures To estimate calendar trends in dementia incidence, correcting for bias due to loss to follow-up of study participants, a joint model of longitudinal and time-to-event data was fitted to ELSA data. To forecast future dementia prevalence, the probabilistic Markov model IMPACT-BAM (IMPACT-Better Ageing Model) was developed. IMPACT-BAM models transitions of the population aged 35 or more years through states of cardiovascular disease, cognitive and functional impairment, and dementia, to death. It enables prediction of dementia prevalence while accounting for the growing pool of susceptible people as a result of increased life expectancy and the competing effects due to changes in mortality, and incidence of cardiovascular disease. Results In ELSA, dementia incidence was estimated at 14.3 per 1000 person years in men and 17.0/1000 person years in women aged 50 or more in 2010. Dementia incidence declined at a relative rate of 2.7% (95% confidence interval 2.4% to 2.9%) for each year during 2002-13. Using IMPACT-BAM, we estimated there were approximately 767 000 (95% uncertainty interval 735 000 to 797 000) people with dementia in England and Wales in 2016. Despite the decrease in incidence and age specific prevalence, the number of people with dementia is projected to increase to 872 000, 1 092 000, and 1 205 000 in 2020, 2030, and 2040, respectively. A sensitivity analysis without the incidence decline gave a much larger projected growth, of more than 1.9 million people with dementia in 2040. Conclusions Age specific dementia incidence is declining. The number of people with dementia in England and Wales is likely to increase by 57% from 2016 to 2040. This increase is mainly driven by improved life expectancy.


Assessment of Covariates in risk factor analysis
Trained interviewers asked participants about any doctor diagnosis of cardiovascular disease, stroke, diabetes, and depression as well as frequency of alcohol intake and smoking habits. 2 3 To assess participants' levels of physical activity, participants were asked how often they participated in vigorous, moderate, and mild physical activities during their leisure time.
Examples for the types of activities and their associated intensities were shown to the participants to assist their response. The questions were extracted from a validated physical activity questionnaire and previously used in the Health Survey for England. A summary measure for physical activity status was compiled for each participant and categorized into sedentary, mild, moderate, and vigorous levels. 4 5 Social class status was based on the National Statistics Socio-Economic Classification (NS-SEC) indicator and classified in 5 categories of managerial and professional; intermediate; small employers and own-account workers; lower supervisory and technical; semi-routine. 6 Total income was calculated from all sources of income including employment, pensions, benefits, assets (interests from savings, shares, and bonds, rental income) and other sources adjusted (equivalised) for family size. Net wealth was calculated from net housing and nonhousing wealth, including financial and physical wealth, equivalised for family size. 7 Height was measured to the nearest millimetre using a free standing stadiometer and weight to the nearest 0.1 kg using portable electronic scales. Body mass index was calculated by dividing weight in kilograms over height in square meters. Systolic and diastolic blood pressure was measured using the Omron HEM-907 blood pressure monitor after the participant was seated for at least 5 minutes. Three readings were taken and the mean of the second and third reading was used for analysis.
Blood samples were taken using standard protocols and cholesterol levels were determined using the DAX oxidase assay. Detailed information on the protocols, analyses, internal and external quality control protocols are provided elsewhere.        Error bars represent 95 % uncertainty intervals for estimates from IMPACT-BAM, and 95% confidence intervals for estimates from CFAS II. Figure 11: IMPACT-BAM predicted prevalence of dementia in men and women compared to estimates from wave 7 of the English Longitudinal Study of Ageing (2014-2015)* *In assessment of cognitive and functional impairment at wave 7, we were unable to exclude cases of transient cognitive or functional impairment due to absence of data at later (post wave 7) dates. We calculated the numbers of persons who would have been marked as dementia based on transient impairment in cognition and function at previous waves of ELSA and applied the age-specific proportion to the dementia prevalence at wave 7 to correct for the false positives.

Supplement Figure 12: IMPACT-BAM predicted cardiovascular disease prevalence compared with observed estimates from the Health Survey for England in 2011.
Error bars represent 95% uncertainty intervals. Figure 13: IMPACT-BAM predicted mortality compared with observed estimates from the UK Office for National Statistics.

Supplement
Supplement Figure 15: Sensitivity analysis for prevalence of dementia assuming no calendar trend in incidence of dementia, standardized to the population of England and Wales in 2015.

Supplement Figure 16: Sensitivity analysis for numbers of cases of dementia assuming cardiovascular incidence does not decline after 2014
Dashed lines represent 95% uncertainty intervals.