At a given level of medical technology, as the average age of the population rises, the number of people suffering age-related diseases at any given time also tends to rise. The only way to offset that is improved prevention or improved therapies, with the former being more important in the past, and the latter becoming increasingly dominant as technological progress accelerates. The rates of incidence and mortality have fallen over the past few decades for many common age-related conditions, a trend that reflects some combination of prevention and better treatment, especially for cardiovascular disease. Given that there has been a large reduction in cardiovascular disease impact and mortality, and the aging of the cardiovascular system can drive the development of dementia, it isn't terribly surprising to see evidence for similar reductions in dementia incidence:
The UK has seen a 20% fall in the incidence of dementia over the past two decades, according to new research, leading to an estimated 40,000 fewer cases of dementia than previously predicted. Reports in both the media and from governments have suggested that the world is facing a dementia 'tsunami' of ever-increasing numbers, particularly as populations age. However, several recent studies have begun to suggest that the picture is far more complex. Although changing diagnostic methods and criteria are identifying more people as having dementia, societal measures which improve health such as education, early- and mid-life health promotion including smoking reduction and attention to diet and exercise may be driving a reduction in risk in some countries.
As part of the Medical Research Council Cognitive Function and Ageing Study (CFAS), researchers interviewed a baseline of 7,500 people in three regions of the UK (Cambridgeshire, Newcastle and Nottingham) between 1991 and 1994 with repeat interviews at two years to estimate incidence. Then 20 years later a new sample of over 7,500 people from the same localities aged 65 and over was interviewed with a two year repeat interview again. This is the first time that a direct comparison of incidence across time in multiple areas, using identical methodological approaches, has been conducted in the world.
The researchers found that dementia incidence across the two decades has dropped by 20% and that this fall is driven by a reduction in incidence among men at all ages. These findings suggest that in the UK there are just under 210,000 new cases per year: 74,000 men and 135,000 women - this is compared to an anticipated 250,000 new cases based on previous levels. Incidence rates are higher in more deprived areas. Even in the presence of an ageing population, this means that the number of people estimated to develop dementia in any year has remained relatively stable, providing evidence that dementia in whole populations can change. It is not clear why rates among men have declined faster than those among women, though it is possible that it is related to the drop in smoking and vascular health improving in men. The researchers argue that while influential reports continue to promote future scenarios of huge increases of people with dementia across the globe, their study shows that global attention and investment in reducing the risk of dementia can help prevent such increases.