The Framingham Heart Study has been ongoing since 1948 and it continues to provide answers, or at least insights, into all kinds of important medical questions. The latest news from Framingham is that the incidence of dementia, the rate at which new cases develop, has been declining over a period of 30 years, and that’s good news indeed.
It’s not exactly news, as several other epidemiologicstudies have pointed in the same direction, but many of those studies were either methodologically flawed or, though suggestive, did not yield statistically significant results The Framingham Study has the great virtue of using consistent, standard diagnostic criteria for dementia; it has monitored the cognitive status of the people it’s been following since 1975 (with even more extensive monitoring beginning in 1981); and it makes use of a special “dementia review panel,” which includes a neurologist and neuropsychologist to evaluate the evidence in every case of possible dementia. Because the Framingham Study collects all sorts of other data as well, including blood pressure readings, cholesterol levels, body-mass index, and information about diabetes, heart disease, cigarette smoking, and education, the investigators are able to control for all these factors.
The conclusion: since 1977, there has been a decline in the incidence of dementia averaging 20 percent per decade. But there are a few caveats. This study identified a total of 371 cases of dementia. That’s it. The 371 cases were scrupulously identified from among just over 5000 study participants, but it’s nonetheless a fairly small number of people. And if you look at the rate at which new cases of dementia were identified by “epoch,” you will see that the rate went from 3.6/100 in the first epoch to 2.8/100 in the second period, to 2.2/100 in the third period, to 2.0/100 in the final epoch. That is, the rate has fallen steadily but the most dramatic decline took place years ago. Things seem to be leveling off.
And there’s another issue that’s worth dwelling on: if you separate out the cases of Alzheimer’s disease from the cases of vascular dementia, you find that the rate of new cases of vascular dementia has fallen significantly, but there has been no statistically significant fall in the rate at which Alzheimer’s disease develops. This is not entirely surprising as we have made enormous inroads in cutting the rate of heart disease, in large measure by controlling blood pressure, treating high cholesterol, and persuading people to stop smoking—or better yet, never start. Exactly the same risk factors affect the development of heart disease and vascular dementia: prevent one, and you’ll prevent the other. But we have made no progress at all in preventing Alzheimer’s disease, which is thought to be responsible for the majority of cases of dementia.
The good news—and it really is good news—is that the progress in preventing heart disease has spilled over into the prevention of vascular dementia. The not-so-good news is that most dementia isn’t due to vascular disease. We should still vigorously combat vascular dementia, which after all causes some cases of dementia, and which also interacts with Alzheimer’s disease, in those unlucky people who are afflicted with both, to produce symptoms that are worse than would be expected from the combination of the two conditions. But there’s still a long way to go.
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