Cognitive function and prediction of dementia in old age.

Longitudinal changes in cognitive functioning were examined for a sample of aging twins, some of whom developed dementia while others did not. Individuals who were judged to be demented at a mean age of eighty-five years had achieved lower scores on most tests twenty years prior to diagnosis, and experienced greater declines in vocabulary and forward digit span over time, than those surviving to a comparable age without dementia. These trends were observed for individuals with mild, as well as moderate-to-severe, dementia and were unrelated to physical health status or premorbid activity patterns. It is suggested that dementing illness may develop very slowly, and that the likelihood of exhibiting clinically significant dementia may vary with premorbid intellectual level.