The study evaluated 134 older men and women who didn't have cognitive impairment at the time of their death. Participants came from the Religious Orders Study and the Memory and Aging Project. Both are longitudinal, clinical-pathologic studies of older persons without dementia who underwent annual clinical evaluations and several cognitive performance tests. After they died, their brains were examined at autopsy for evidence of pathology.
More than a third of the participants (50) met criteria for a pathologic diagnosis of Alzheimer's disease. Criteria included lesions of brain tissue on the autopsy. This group also scored significantly lower than the other participants on tests for episodic memory, such as recalling stories and word lists.
"The results provide evidence in support of the idea that some type of neural reserve can allow a large number of older persons to tolerate a significant amount of Alzheimer's pathology without manifesting obvious dementia," said study author David A. Bennett, MD, of the Rush Alzheimer's Disease Center in Chicago.
Scores on the Mini Mental State Examination, a mental status screening test of cognitive functions, were nearly identical for participants with and without a pathologic diagnosis of Alzheimer's disease.
"This study questions the acceptability of minor episodic memory loss in older adults as 'normal'," said Carol F. Lippa, MD, who wrote an editorial in the same issue of Neurology. "Maybe this early decline in episodic memory precedes mild cognitive impairment and should be the target of research efforts in the early detection of Alzheimer's disease."