Could cohort composition be driving this? Re: Temporal trend in dementia incidence since 2002 and projections for prevalence in England and Wales to 2040: modelling study
Ahmadi-Abhari and coauthors are to be congratulated for a timely and methodologically sophisticated work on past trend and future projection of dementia in England. In such an accomplished work results are bound to be nuanced. Although the future numbers of people with dementia are likely to increase in concert with life expectancy, the age specific prevalence is likely to decrease. This trend entails cost implications, and we will be better prepared to make the necessary allocation were we to have some idea as to the drivers of reduced prevalence.
Using the same English Longitudinal Study of Ageing 2002–2013 with joint model, I identified a comparable trend and found a new driver: delayed onset of cognitive impairment and cohort effect (1). The projected reduced prevalence in these authors’ new work is likely driven by delayed onset of dementia among the younger (post-War) cohort alive today and will increase in preponderance in future. Some evidence of precisely this delayed onset has recently arrived from across the pond thanks to a sister study to the English one, the US Health and Retirement Study. Over the last decade, dementia onset in older Americans has been delayed from an average age of 80.7 to one of 82.4 years (2, 3). This delayed onset has been posited on this side of the Atlantic (1, Figure 4).
But can we say more? Recent results comparing cohort effects on both sides of the Atlantic suggest a positive answer. We estimated that the three cohorts composing older people today (pre-War, War, and post-War) followed markedly different trajectories in each of the two countries which can be consequential for trends in health outcomes (4). Importantly, we found that the War cohort in America have better mental health trajectories than the English War cohort. (The other two cohorts are broadly similar: the pre-War cohorts in both countries are relatively similar to each other, likewise the post-War cohorts). Moreover, mental health of the English study members, as well as their gait speed and cognitive function have been shown to be significantly influenced by their childhood or cohort conditions (5). In sum, cohort effect shapes trends in dementia prevalence and onset in both countries. Since dementia onset in America has been found to be markedly delayed, and cohort composition in America is advantageous, it is likely that dementia onset in England will be delayed by at most the same amount. Future cost models of dementia burden may benefit from such a reasoned estimate.
1. Tampubolon. 2015. Cognitive ageing in Great Britain in the new century: Cohort differences in episodic memory. PLoS One. 10(12): e0144907 DOI: 10.1371/journal.pone.0144907
2. Crimmins et al. 2016. Change in cognitively healthy and cognitively impaired life expectancy in the United States: 2000–2010. SSM – Population Health. DOI: 10.1016/j.ssmph.2016.10.007
3. Kolata. 2016. U.S. dementia rates are dropping even as population ages. The New York Times. 21 November 2016. https://www.nytimes.com/2016/11/21/health/dementia-rates-united-states.html. Accessed: 19 July 2017.
4. Tampubolon and Maharani. 2017. When did old age stop being depressing? Depression trajectories of older Americans and Britons 2002-2012. American Journal of Geriatric Psychiatry. DOI: 10.1016/j.jagp.2017.06.006
5. Tampubolon. 2015. Growing up in poverty, growing old in infirmity: The long arm of childhood conditions in Great Britain. PLoS One. 10 (12): e0144722 DOI: 10.1371/journal.pone.0144722
Competing interests: No competing interests