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.