Are Fewer People Developing Dementia?

Megan Brooks

May 11, 2012

May 11, 2012 — New data from the Rotterdam Study hint that the incidence of dementia is declining, perhaps because of preventive measures and better control and treatment of vascular risk factors.

"Although the differences in dementia incidence were nonsignificant, our study suggests that dementia incidence has decreased between 1990 and 2005," the study team reported in Neurology May 8.

"Our study suggests that the dramatic rise in absolute numbers of people living with dementia in the coming years may be slightly less enormous than has been previously reported," note Monique M. Breteler, MD, PhD, from Erasmus MC University Medical Center, Rotterdam, the Netherlands, and colleagues.

Optimistic Evidence

In a linked commentary, Eric B. Larson, MD, MPH, from the Group Health Research Institute in Seattle, Washington, and Kenneth M. Langa, MD, PhD, from the University of Michigan, Ann Arbor, say this report from the "highly regarded Rotterdam Study provides some optimistic evidence that recent trends in a number of social (e.g., increased years of education) and medical (e.g., increased treatment of vascular risk) factors may have led to a decline in the age-specific incidence of dementia in older adults."

Dr. Breteler and colleagues compared dementia incidence in 2 independent subcohorts of persons aged 60 to 90 years from the Rotterdam Study. The first subcohort started in 1990 and consisted of 5727 individuals; the second started in 2000 and comprised 1769 adults. All participants were free of dementia at baseline and were followed for a maximum of 5 years.

A total of 286 participants developed dementia in the 1990 subcohort, compared with 49 in the 2000 cohort. The researchers report that the age-adjusted incidence of dementia was 25% lower in the 2000 subcohort in all strata, reaching borderline significance in the overall analysis (incidence rate ratio [IRR], 0.75; 95% confidence interval [CI], 0.56 - 1.02).

For men, the difference was largest in the 70- to 79-year age stratum (IRR, 0.48; 95% CI, 0.21 - 1.11), while there was no difference in dementia incidence in the 80- to 89-year age stratum (IRR, 0.98; 95% CI, 0.51 - 1.90).

In their commentary, Dr. Larson and Dr. Langa say the findings of a relatively greater decline in incidence for the 70- to 79-year age strata compared with the 80- to 89-year strata might be "at least a hint of a compression of cognitive morbidity, i.e., the younger-old are postponing dementia incidence to older ages and thereby presumably spending less total time with dementia."

For women, the estimated reduction in dementia incidence increased from 9% in the lowest stage stratum (60 to 69 years; IRR, 0.91; 95% CI, 0.17 - 4.94) to 23% in the highest age stratum (80 to 89 years; IRR, 0.77; 95% CI, 0.45 - 1.29), the researchers say.

Healthier Brains on Imaging

Mortality rates were also lower in the 2000 subcohort (rate ratio, 0.63; 95% CI, 0.52 - 0.77) relative to the 1990 subcohort.

Evidence for a declining trend in dementia incidence, say the researchers, is further supported by the observation that participants from the 2000 subcohort had on average less brain atrophy (larger total brain volumes, P < .001) and less cerebral small-vessel disease on magnetic resonance imaging (albeit nonsignificant in men) than the earlier cohort.

They note that their results are in line with a recent report on trends in the incidence and prevalence of dementia in the United States, where a decline in dementia incidence was observed in Rochester, Minnesota, between 1985 and 1994.

However, the current findings contrast with earlier reports from Rochester that reported no change in incidence between 1960 and 1974 and a possible increase in dementia among adults aged 85 years and older between 1965 and 1984. "The reported increase, however, might very well be the result of an increasing awareness of dementia and an increased case identification over the years," Dr. Breteler and colleagues point out.

Profound Implications

The researchers think their estimates reflect a true decline in incidence rates for several reasons: The same methods were used to diagnose dementia in both subcohorts, there was a risk reduction across all strata of sex and age (except for men older than 80 years), and there was a clear reduction in mortality.

"A person who dies," they explain, "is no longer at risk of developing dementia, while underlying risk factors, especially vascular factors, are associated with both dementia and mortality risk. The mortality rate, and thereby the competing risk effect, was higher in the 1990 cohort. Should mortality rates have been the same in both subcohorts, the difference in incidence rates of dementia would probably have been larger."

The brain imaging data showing larger brain volumes and less cerebral small-vessel disease in the 2000 subcohort also support the finding of declining dementia incidence, they say.

Higher education levels and greater attention to vascular risk factors at a population level may be factors in the declining incidence of dementia. Although the prevalence of hypertension and obesity significantly increased between 1990 and 2000, this was paralleled by a significant increase in use of antithrombotics (mainly platelet aggregation inhibitors) and lipid-lowering drugs, the researchers point out.

Dr. Larson and Dr. Langa say, "Ongoing research undoubtedly will inform further our answer to the question: Is dementia incidence declining? The clinical and public policy implications of that research are profound, given the vast and growing number of people reaching advanced old age."

Dr. Breteler and the editorial writers have disclosed no relevant financial relationships. A complete list of disclosures for the study team is provided with the article.

Neurology. 2012;78:1452-1453,1456-1463. Abstract Editorial