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Dementia rates may be falling, not rising

22 September 2017

World Alzheimer's Day is a timely occasion to examine dementia statistics commonly quoted by advocacy groups and researchers.

Take, for example, the number of people forecast to develop dementia by Alzheimer's Disease International: a doubling every 20 years to reach 75 million by 2030 and 131 million by 2050.

In the US, the cost of dementia by 2050 is predicted to reach a staggering $USD1.1 trillion while Alzheimer’s Australia’s latest report predicts that 1.1 million Australians will be affected by 2056.

Given the importance of dementia to all concerned it is vital to understand exactly how these numbers are generated.

In Australia, Australian Bureau of Statistics (ABS) population age profile data together with models for the evolution of age-structure over the next 40 years is used to predict future dementia numbers. It should come as no surprise that this method predicts dynamic changes because the nation is ageing.

For example, over time, more people will move from lower age brackets to higher ones. Today, 1.3 million people are in the 60-64 age band, a number this is projected to reach 2.2 million by 2056. No quibbles there because the ABS is very good at these kinds of forecasts.

These numbers are foundational because advanced age is the number one risk factor for dementia. In general, the prevalence of dementia (the number of people affected at any given time) doubles with every 5-year age increment after the age of 60.

This is where the second kind of information comes in and age-specific prevalence data is now marshalled for the first time in the latest Alzheimer’s Australia report.

The report notes that the prevalence of dementia among 65-69 year olds is currently four percent, rising to 22 percent in 85-89 year olds.

At the heart of predictions by Alzheimer’s Australia, Alzheimer’s Disease International UK and the Alzheimer’s Association in USA, is a multiplication of the projected dynamic changes in age-structure (we are getting older) together with static estimates of increasing dementia prevalence with age.

This then provides dementia numbers that can be extrapolated for 10, 20, 40 or even 100 years. And it is these forecasts that tend to drive the dementia conversation because they make the headlines and feed economic models. Advocacy groups in turn use these numbers for policy and political purposes and researchers adopt them to argue relative primacy. 

What’s the problem?

Dementia rates are not fixed or immutable, and if anything, they are shrinking. Falling age-specific dementia rates have now been observed in the UK, Denmark, and most recently in the US.

For example, the most accurate nationally representative dementia data using time-invariant methodology comes from the UK Cognitive Function and Ageing Study (CFAS) published in the prestigious Lancet journal. That study reports a 20-30 percent decline in the age-specific dementia rates over a ten-year period. To put this in concrete terms, in 1991, an 80-year old man in the UK had a 15 percent chance of having dementia, a figure that has fallen to 11 percent in 2001.

The most recent Framingham study result published in the equally prestigious New England Journal of Medicine has similarly reported a 30-40 percent decline in dementia rates over the past three decades.

To be clear, neither of these studies challenges the idea that dementia becomes more common as we get older. Rather, they suggest that over the years the risk for having dementia at a particular age has dropped.

Could similar improvements in ‘cohort risk’ be happening in Australia? In truth, we don’t know because we are yet to carry out a nationally-representative dementia survey, let alone analyse trends over time. This is woeful and must become a national research priority.

But there is a fascinating hint that this trend might be happening here too. Buried in the source study used by Alzheimer’s Australia were data from the ABS National Survey of Mental Health, carried out in 1997 and 2007. This study used a sophisticated sampling technique to generate nationally representative data from more than 1700 individuals aged 65 or more on each occasion.

It reveals that older Australians’ general cognitive abilities increased over that decade. For example, among people aged 70-74 years, the average test score increased from 27.2 to 28.4 (out of a maximum of 30). While this may seem modest, these differences were statistically robust, and if applied to the whole population could be telling.

Assuming that such test scores are reflective of dementia burden – questionable, but this was the explicit assumption by Alzheimer’s Australia in their report – then it is not inconceivable that Australians’ risk for dementia may have declined over this decade.

In the UK, this has meant that while the population did get older between 1991-2001, the expected bulge in dementia numbers did not materialise. Instead of a projected 33 percent increase in dementia numbers they only saw a one percent increase.

What could be behind this welcome development? This is still far from clear, but the most likely candidates are long term improvements in education, literacy and building up of ‘cognitive reserve’, as well as better quality and access to healthcare.

The ‘problem’ is therefore actually good news.

Yet mainstream media, advocacy groups and researchers have been conspicuously quiet about this, perhaps because it is not in their interest to announce that the predicted tsunami was a fizzer.

In any case, on this World Alzheimer’s Day let’s celebrate this rare win for the community in the long-term fight against dementia.

Dr Michael Valenzuela is Professor of Regenerative Neuroscience, Brain and Mind Centre and Sydney Medical School, University of Sydney.

Dan Gaffney

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