Is the Incidence of Dementia Declining?

TABLE. 5-Year age- and sex-adjusted cumulative hazard rates for dementia

Dementia is the leading cause of dependence and disability among older adults worldwide, with a total cost of US $604 billion- equivalent to almost 1% of the world’s gross domestic product.^1-3 Currently, 35.6 million people worldwide have dementia, and it is estimated that the number of individuals with dementia in the world will double every 20 years to reach 115.4 million by 2050.⁴

Although it is likely that the incidence of dementia will increase significantly worldwide in the coming years, a recent study that evaluated temporal trends from the Framingham Heart Study found that the incidence of dementia had declined over the past 4 decades (Table).⁵

The declining trend was more rapid for the incidence of vascular dementia than for that of Alzheimer disease (P = .004 vs P = .052), although the analyses of the dementia subtypes were based on smaller numbers than those for overall dementia. Note, however, that the risk reduction was seen only in persons who had at least a high school diploma.

When adjusted for vascular risk factors present at baseline or during midlife or for preexisting and incident stroke and other cardiovascular diseases, there was no significant difference in the results. The researchers also found that most indicators of cardiovascular health (excluding diabetes and obesity) had improved over the 4 spans of time studied. In addition, the investigators found a decrease in cardiovascular events and benefit for the use of antihypertensive medications on the risk of dementia during successive spans. This benefit was also more pronounced among individuals who had a high school diploma.

Wu and colleagues⁶evaluated data from studies done in Western Europe (Sweden, the Netherlands, the UK, and Spain). The investigators found a 22% reduction (P = .003) in the overall incidence of dementia in the UK study. In the study done in Spain, a 43% reduction was seen in dementia incidence in men (P = .0002). The studies done in Sweden and the Netherlands indicated a non-significant reduction.

Grasset and colleagues⁷ found that the incidence of dementia in France had declined significantly in women (hazard ratio [HR], 0.62) when they compared data between 1988 and 1989 and between 1999 and 2000. Differences in education, vascular factors, and depression accounted only to some extent for this reduction (HR, 0.73).

In contrast to the data from these studies, a study from Sweden found that the incidence of dementia among people aged 85 years or older increased from 26.5% in 2000 to 2002 to 37.2% in 2005 to 2007 (P = .001).⁸ Dodge and colleagues⁹ evaluated data from 8 large population-based studies in Japan. They found that there was a higher incidence of all-cause dementia in 2008 compared with 1992 after controlling for age groups and sex. Similarly, a review by Chan and colleagues¹⁰ showed that the incidence of dementia in China in 1990 was 1.8% for 65- to 69-year-olds and 42.1% for 95- to 99-year-olds. By 2010 the incidence of dementia had increased to 2.6% for 65- to 69-year-olds and 60.5% for 95- to 99-year-olds.

One concern with a majority of epidemiological studies in dementia is that they often do not take into account the effects of prevention strategies, lifestyle factors, or risk modification strategies. Norton and colleagues¹¹ estimated the population-attributable risk of Alzheimer disease worldwide for 7 risk factors: diabetes, depression, smoking, physical inactivity, midlife hypertension, midlife obesity, and low educational attainment. They found that the combined worldwide risk was approximately 30%. This suggests that a little under one-third of Alzheimer disease cases might be attributable to these potentially modifiable risk factors. It is postulated that a 10% to 25% reduction in these risk factors could prevent almost 3 million cases of Alzheimer disease worldwide.¹² Lifestyle (ie, social, physical, and mental leisure activities) has been found to be protective against dementia.¹³

Based on available evidence, it appears that the incidence of dementia is declining in the US and some countries of Western Europe. Improvements in the level of education and a reduction in vascular risk factors seem to be partly responsible for this decline. Education and lifestyle modification may also play a role in reducing the occurrence of dementia. Given these emerging trends, it is appropriate to be cautiously optimistic regarding a decline in the occurrence of dementia in certain parts of the world. However, with complex diseases such as dementia, only time will tell whether these positive trends are transient or the benefits obtained from improved educational status, risk factor modification, and lifestyle changes will be maintained.

Disclosures:

Dr Tampi is Professor of Psychiatry at Case Western Reserve University School of Medicine and the Vice Chairman for Education and Faculty Development in the department of psychiatry at MetroHealth in Cleveland, OH. Ms Tampi is the Executive Director of Behavioral Health Services at Saint Francis Hospital and Medical Center in Hartford, CT. The authors report no conflicts of interest concerning the subject matter of this article.

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