Contrasting cognitive trajectories of impending death and preclinical dementia in the very old.

BACKGROUND: Both impending death and preclinical dementia are associated with cognitive impairment in old age, although their effects on cognitive functioning have rarely been examined within the same study.
METHODS: Participants (n = 1,200, aged 75+ years) from a community-based study completed a measure of global cognitive performance (the Mini-Mental State Examination [MMSE]) at 3-year intervals over an 11-year period. Level and change of MMSE performance were compared for three groups: persons in close proximity to death, persons in a preclinical phase of dementia, and persons who remained alive and nondemented throughout the study.
RESULTS: There were significant group differences in MMSE performance 3 years before each outcome (death, dementia, or end of study). Those with preclinical dementia performed the poorest and declined twice as fast on the MMSE relative to the other groups. Although persons in close proximity to death declined faster in general, no accelerated decline was observed for the impending death group after persons with dementia or preclinical dementia had been excluded. Group differences were attenuated for the oldest-old (81+ years) compared to the old-old (75 to 80 years).
CONCLUSIONS: The lack of accelerated decline in proximity to death after excluding persons with dementia or preclinical dementia suggests that part of the terminal decline effect demonstrated in previous investigations may reflect preclinical dementia deficits. Further, accelerated cognitive decline might be a more reliable indicator of preclinical dementia than a low cognitive score due to confounds associated with cross-sectional cognitive performance.