Do age-standardised dementia incidence rates really increase in England and Wales? – Authors' reply
Authors: Yuntao Chen, Eric J Brunner
Journal: The Lancet Public Health (2024)

We thank Sara Ahmadi-Abhari and Mika Kivimäki for their comments on our Article.1Chen Y Bandosz P Stoye G et al.Dementia incidence trend in England and Wales, 2002–19, and projection for dementia burden to 2040: analysis of data from the English Longitudinal Study of Ageing.Lancet Public Health. 2023; 8: e859-e867Summary Full Text Full Text PDF PubMed Scopus (6) Google Scholar We agree that there are methodological complexities in estimating the dementia incidence trend and projecting future dementia cases.2Nichols E Lee J Updating estimated trends in dementia incidence: evidence of increases in England.Lancet Public Health. 2023; 8: e830-e831Summary Full Text Full Text PDF PubMed Scopus (2) Google Scholar One aspect is the different sources of bias involved in estimating dementia incidence trends. The multistate model used in our Article is useful to account for death as a competing risk. It allows for unobserved dementia cases to occur in those who died between survey waves, and for the increased death rate in patients with dementia compared with those without dementia. However, the multistate model does not address potential bias due to informative drop-out from the survey, in contrast to the joint model method.3Ahmadi-Abhari S Guzman-Castillo M Bandosz P et al.Temporal trend in dementia incidence since 2002 and projections for prevalence in England and Wales to 2040: modelling study.BMJ. 2017; 358j2856PubMed Google Scholar We used consistent algorithmic dementia case definition across nine waves. Verbal fluency was not assessed at wave 6 and we imputed it using information at waves 5 and 7. Although we recognise that this approach might not entirely mitigate bias, our sensitivity analysis, in which survey weights were applied, showed a similar incidence time trend. 1Chen Y Bandosz P Stoye G et al.Dementia incidence trend in England and Wales, 2002–19, and projection for dementia burden to 2040: analysis of data from the English Longitudinal Study of Ageing.Lancet Public Health. 2023; 8: e859-e867Summary Full Text Full Text PDF PubMed Scopus (6) Google Scholar We used the 4-year epoch method for point estimates, as it is relatively robust to incidence fluctuation at individual waves (table).4Satizabal CL Beiser AS Chouraki V Chêne G Dufouil C Seshadri S Incidence of Dementia over Three Decades in the Framingham Heart Study.N Engl J Med. 2016; 374: 523-532Crossref PubMed Scopus (707) Google Scholar, 5Power MC Bennett EE Turner RW et al.Trends in relative incidence and prevalence of dementia across non-Hispanic Black and White individuals in the United States, 2000–2016.JAMA Neurol. 2021; 78: 275-284Crossref PubMed Scopus (64) Google Scholar Epoch estimates are not adjusted for missing dementia cases because of death bias. The magnitude of this bias is likely to differ across epochs, owing, for example, to the slowdown in the mortality rate decline in the UK after 2010. The multistate model uses the epoch estimates as input data to generate the non-linear trend that is the main novel finding in our study.TableDementia incidence rates (per 1000 person-years) based on 4-year and 2-year epochs2002–062004–082006–102008–122010–142012–162014–182016–184-year subcohortsEvents371305284291269324341..Person-years42 52833 43233 89539 07136 36137 30533 227..Crude8·79·18·47·47·48·710·3..Standardised*Age-standardised and sex-standardised rates based on England and Wales 2011 Census population estimates.10·711·110·610·38·610·211·3..2-year subcohortsEvents185161139171125142175153Person-years21 84417 13617 33619 92818 53819 07117 02414 820Crude8·59·48·08·66·77·410·310·3Standardised*Age-standardised and sex-standardised rates based on England and Wales 2011 Census population estimates.10·311·110·111·87·58·710·810·5* Age-standardised and sex-standardised rates based on England and Wales 2011 Census population estimates. Open table in a new tab Another important aspect of uncertainty arises in future dementia projection. Increasing public awareness has contributed to a diagnostic trend and an artefactually increasing dementia incidence rate since 2010.6Donegan K Fox N Black N Livingston G Banerjee S Burns A Trends in diagnosis and treatment for people with dementia in the UK from 2005 to 2015: a longitudinal retrospective cohort study.Lancet Public Health. 2017; 2: e149-e156Summary Full Text Full Text PDF PubMed Scopus (104) Google Scholar Our sensitivity analysis based only on the algorithmic case definition1Chen Y Bandosz P Stoye G et al.Dementia incidence trend in England and Wales, 2002–19, and projection for dementia burden to 2040: analysis of data from the English Longitudinal Study of Ageing.Lancet Public Health. 2023; 8: e859-e867Summary Full Text Full Text PDF PubMed Scopus (6) Google Scholar estimated a slower upward trend between 2010 and 2018. Recognising the uncertainties, we generated three contrasting predictions of dementia burden to 2040. We echo Nichols and Lee that it is always important to update our findings when new waves of data are available. We declare no competing interests.