When I was a young doctor working in poor countries, I made a casual observation that old people in them did not seem to suffer very often from dementia. It occurred to me that properly conducted surveys, of the kind that I was not suited to performing, might be valuable, because a difference in the incidence and prevalence of dementia in various countries might offer a clue to the cause of the disease. As populations throughout the world age, this is a matter of increasing importance.
But was my initial casual observation true? And, if true, might the explanation not be that people who survived to old age in poor countries were simply the kind of people who, in richer countries, would not have got the disease anyway?
A survey has been published recently in the Lancet, in which researchers interviewed and tested 12,887 people over the age of 65 in various locations in the Dominican Republic, Cuba, Venezuela, Peru, Mexico, and China. Rather strangely, they write that “11,718 were dementia-free,” rather than that 1,169 showed signs of dementia. This means that 9.1 percent of their initial sample showed such signs, and oddly enough the authors do not attempt to answer the rather obvious question of whether this percentage is higher or lower than you would have expected of a similar sample taken in richer countries.
They did, however, note that the death rate among those who initially showed signs of dementia during a 3-5 year period of follow-up was 1.56 to 5.69 times as high as among those with no such signs. The relative risk of death for people with dementia in poor countries was higher than that in richer countries; not altogether surprisingly, since it is likely that the medical treatment for the intercurrent but curable illnesses to which the demented are prone would be less assiduously treated in poor countries.
The researchers followed up 69 percent of their initial cohort who showed no signs of dementia for between 3 and 5 years (31 percent were lost to follow-up), to establish a rate at which dementia developed, and also to try to identify what factors, if any, protected against it.