Dementia rates falling

As more people live into old age, the prevalence of some diseases, like dementia, has been predicted to rocket. But those suggestions may paint a gloomier picture than the reality. Public health researcher Carol Brayne has looked at similar groups of people in two large studies 20 years apart and found that fewer people - and particularly men - are developing the disorder than they were two decades ago, Chris Smith went to find out more..

Carol - In England, we have found that the incidence of dementia has dropped by around 20% in the population if we look at people aged 65 and over compared with people of 65 and over 20 years ago.

Chris - Now why is that a revelation?  You're saying that fewer people are developing dementia in that age range now than they did historically but, aren't we all getting healthier?

Carol - We didn't have any reason to believe that dementia would be one of the disorders that changed as population health changes. The fact is that we didn't know whether dementia is changing in the population, so whilst we can look at heart attack mortality or stroke mortality from routine statistics in many countries, we can't do that with dementia. You have to go out into the population and measure it and see how many people have dementia in a given population and how many people develop dementia over time in a given population.  So you have to that one time and then in order to test whether it's changing, you have to then repeat that at a sufficiently long interval to see whether it's changed.

Chris - Where did the initial idea that is was going to be more common or, indeed, it was more common 20 years ago come from?

Carol - Because of health improvements and public health improvements in general, we've had a remarkable extension of life across the globe and that's continuing now, and it's continuing within the older age groups as well.  Governments around the world and in Europe began to be aware that this longevity was going to create perhaps a problem for society and we ought to be studying it and working out what were the consequences of having a much older population, and those consultations led to and investment into dementia studies. Studies looking at dementia in the population, and risk factors, and what might mitigate dementia in the population, and the size of the problem in order to try to plan services and model into the future.

Chris - How did you do this study?

Carol - So 20 years ago we out to the populations in multiple sites and recruited them through General Practice lists which are whole populations lists.  So it was people aged 65 and over...

Chris - How many of them?

Carol - Over 18,000 then. And we interviewed folk with comprehensive interviews from which we can extract a study diagnosis of dementia and we followed people over time. From those sites we selected three (Nottingham, Newcastle and Cambridgeshire), and we repeated the same methods, as far as we possibly could, 20 years later.

Chris - Critically what do you see when you compare those two groups you've seen this drop but, if you drilled down into the data, what are the trends in there?

Carol - For the prevalence in both men and women, the proportion of both sexes who meet the criteria of dementia has dropped.

Chris - By how much has it gone down?

Carol - 23%. For the incidence...

Chris - These are new cases.  Incidence means new cases cropping up in the two years you follow people up afterwards?

Carol - Yes exactly. The observed over expected has dropped by 20% and it's almost entirely driven by the drop in men, which is effectively a 40% drop with about a 1% drop in women.

Chris - That's a huge difference and for such a big bias in the sexes. How do you account for that?

Carol - Probably because the men don't survive for as long and even once they develop the condition, don't survive as long. So that if you have heart disease or if you have particular conditions, often it's seen that men die at an earlier stage.

Chris - But politicians, David Cameron included, have put dementia at the top of the agenda saying this a huge problem and, within the next 50 years, we might be facing a third of the population of western countries who are afflicted. So this flies in the face of these predictions.  Could it be a blip? Could it be that the people you're looking at are those people who came through the Second World War, they have actually had a very healthy diet a lot of them, they haven't had all these other risk factors like putting on lots of weight and actually my generation coming along next, we're actually going to be the ones that will have a much higher rate of dementia?

Carol - That may be the case. We're seeing similar changes in other European countries and the U.S. and we are seeing such effects as the flin effect, which is that I.Q. is going up across time, and there are factors such as increased education which is a protective factor. So it is a question of how does each generation experience risks and protective factors across their life course.  Do you have a healthy birth, do you have an adequate early nutrition to nourish your brain, and so on. Do you then have sufficient physical activity and a good diet, as you said. And then are you exposed to an environment in which smoking is very common. So all these things probably influence how we age. The fact is that we are still seeing increases in life expectancy. We can only know what's going to happen across time by continuing to conduct the kinds of studies that we have just done. In ten years time it would be sensible - we've only got two time points here and you really only begin to understand trends when you get three plus.