The rate of decline in function in Alzheimer's disease and other dementias

Background. Functional impairment over time is a necessary condition for th e diagnosis of dementia. Increasingly, it is recognized that rates of decli ne may not follow a linear progression. This variability may indicate that dementia in Alzheimer's disease represents disease rather than inevitable a ging. In order to investigate decline in function in dementia, we developed a model of the rate of decline in functions in Alzheimer's disease and in other dementias in comparison with normal aging. Methods. Secondary analysis of a cross-sectional, representative sample of Canadians aged 65 and older (N = 2,914) was performed. We calculated a meas ure identified as an impairment index, defined as the probability of the oc currences of an impairment or disability in a structured clinical examinati on. Results. The rate of functional decline varies for different diagnostic gro ups and increases with severity of the disease. The distribution for the ra te of decline in dementia is distinct from that in aging without cognitive impairment. In those without cognitive impairment, the distribution is expo nential. Elderly persons with dementia of any type showed a log-normal dist ribution. Conclusions. The difference in the distributions between aging with and wit hout dementia likely reflects fundamental differences in the processes of d ecline in functions in the two groups. This suggests that the declines seen in persons with dementia are distinct from normal aging. It also has impli cations for the testing of antidementia medications, in that modeling treat ment effects based on an assumption of linear decline is likely to be flawe d.