Back in February, I wrote that “some news is good news.” The news in question came from the Framingham study and it showed that the incidence of dementia had been falling in the US by 20 percent each decade since the 1980s. However, I was concerned about the generalizability of the finding since the total number of people identified as having dementia was 371—and they were all from Framingham, MA. Now another, larger study points to a similar decrease. That’s something to be grateful for this Thanksgiving season.
Before we get too excited—or complacent—let’s consider a few caveats. The new study was based on data from the Health and Retirement Study, a large, nationally representative cohort study that’s been going on since 1992. It looked at the prevalence of dementia in all those in their survey who were over age 65 and lived either in the community or a nursing home in one or both of two sample years, 2000 and 2012. That turned out to be 21,057 people, of whom a total of about 2000 developed dementia. So far, so good: a bigger sample, drawn from the entire country, with a respectable sized group of individuals with dementia.
But now for the problems. Dementia was diagnosed using a modified version of the Telephone Interview on Cognitive Status, a measure involving a 27-point scale. Sounds good, except that the full Telephone Interview on Cognitive Status is not an established way to diagnose dementia, and the abridged version used by the investigators is even less well established. And indeed, when compared to a different test which the authors regard as the gold standard, the telephone interview correctly classified dementia in 78 percent of respondents. The other 22 percent of people were either falsely diagnosed as having dementia or incorrectly assessed as normal. That’s worrisome. The point of the study was to compare the prevalence of dementia in 2000 and 2012, which is awfully difficult to do if you can’t accurately determine prevalence.
Still, the findings of a decline in prevalence from 11.6 percent in 2000 to 8.6 percent in 2012 (corrected for the change in age and sex distribution of the population) are consistent with those of the Framingham Heart Study and of a British study. They show a 24 percent decline in prevalence of the disease, despite an increase in obesity and diabetes during the same period. And, as with the earlier studies, increases in education and improvement in control of cardiovascular risk factors (high blood pressure, smoking, and diabetes) are associated with the fall in dementia.
So maybe it’s really true. Maybe the risk for each us individually is not quite as bleak as I have been suggesting in this blog. On the other hand, the projection is that by 2050, there will be 83.7 million people age 65 or older. If even 8.6 percent of them have dementia, as suggested in the current study, that’s over 7 million people. Unless we find a cure soon, which doesn’t seem terribly likely, we’re still going to be faced with an enormous public health problem.