Awareness of olfactory deficits in healthy aging, amnestic mild cognitive impairment and Alzheimer's disease

Background: Olfactory dysfunction is present in early Alzheimer's disease (AD), and has now been reported in people with amnestic mild cognitive impairment (aMCI). Recent evidence suggests that unawareness of an olfactory deficit may predict which MCI patients will subsequently meet AD criteria. However, important methodological limitations challenge this suggestion. While addressing some of the limitations of previous research, this preliminary study explores unawareness of olfactory deficits as a predictive factor of future AD among people with aMCI. Methods: Twenty-five participants with aMCI, 25 AD patients, and 22 healthy elderly participants underwent testing of olfactory identification. Subjective reports regarding perceived decline in olfactory detection and olfactory identification were also obtained. A subset of participants was reassessed 12 months later. Results: Control participants performed better than both aMCI and AD patients on olfactory identification. Almost uniformly, participants did not report decline in either olfactory detection or identification. Prediction of olfactory identification scores from subjective reports of olfactory function was poor, and awareness of olfactory decline bore no relationship to the likelihood of aMCI patients progressing to AD by the 12-month review. Conclusions: Treating awareness of olfactory function as a unitary construct can be misleading, and there is a poor relationship between subjective and objective measures of olfactory ability. Our preliminary data suggest that unawareness of olfactory decline does not improve the identification of patients with MCI who are more likely to be in the prodromal phase of AD. Replication in a larger cohort is needed to support these findings.