ementia is becoming a major public health problem as D the life expectancy of the population continues to increase. In 1997, 2.32 million Americans were affected by Alzheimer’s disease, which accounts for almost 50% of all the cases of dementia. The prevalence of persons living with Alzheimer’s disease is 4.3%, 8.5%, 16.0%, and 28.5% among 75-$0-, 8 5 , and 90-year-olds, respectively. Projections from large epidemiological studies indicate that, in the next 50 years, the annual number of new cases of Alzheimer’s disease is expected to increase from 360,000 in 1997 to 1.14 million in 2047, a threefold increase.’ The trends for nonAlzheimer’s dementia will likely show similar or only slightly lower figures. The longitudinal study by Bryane et al., published in this issue: suggests that these epidemiological data, although dramatic, probably still underestimate the true burden of dementia in older populations. The authors examined the change in cognitive performance of an initial cohort of 2130 unselected 75-year-old persons in Cambridge, UK, who were screened using Folstein’s Mini-Mental State Examination (MMSE). Those who were judged not demented were followed up 2.4, 6, and 9 years after the baseline assessment. From the available data, only a modest change in MMSE score, from 24.3 to 22.9, was observed after 9 years. However, 35%, 47%, and 54% of the participants who were alive at each follow-up had cumulatively dropped-out at the 2.4-, 6-, and 9-year follow-up visits, respectively. Therefore, only a fraction of those potentially eligible were actually reexamined. To correct for attrition, the authors imputed the mean MMSE score in the participants who had dropped-out at each wave under two different assumptions: that drop-out is a random phenomenon (“missing-at-random”) or, conversely, that it is associated with identifiable characteristics (“informative drop-out”). In the first scenario, the hypothesized rate of change in MMSE score was similar to the one observed, although the absolute values were lower, and the mean MMSE score at year 9 was 21.7. An alternative and probably more realistic scenario was depicted by taking into account that the risk of attrition was inversely related to baseline MMSE. This implies that the reported data were likely biased and, therefore, underestimated the true change in MMSE score because persistence in the study was itself associated with a more preserved cognitive performance. Indeed, under the assumption of an “informative drop-out,” the rate of decline in MMSE score was much more rapid, and the mean scores at year 9 ranged from 15.8 to 19.2, according to different hypothesized rates of change. As the authors suggest, this analysis implies that age-associated cognitive decline might be more severe, and/or might affect a larger proportion of the aging population, than previously thought. The consequences for both the clinician and the public health administrator are evident.
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