14 November, 2016
Failure to identify odors may be an early sign of cognitive impairment. A recent study included persons aged 65 years or older without dementia (males and females, n = 1037) . They were asked to identify 40 different odors, and their success rate was scored. Also, a brain MRI and a battery of cognitive tests were performed. Follow-up at 2 and 4 years in 757 participants showed that low baseline scores correlated with cognitive decline and the appearance of Alzheimer's disease. MRI hippocampal volume did not show predictive utility in this cohort. The investigators suggested that the inexpensive smell test could be useful as a predictor of future cognitive impairment.
The above study by Devanand and colleagues is one of several similar ones showing that low performance in smell testing correlated with a higher risk of cognitive impairment . In a recent study among 1430 cognitively normal participants (mean age 79.5 ± 5.3 years, 49.4% men, mean 3.5 years of follow-up), there were 250 incident cases of minimal cognitive impairment (MCI). An association between decreasing olfactory identification, as measured by a decrease in the number of correct responses in Brief Smell Identification Test score, and an increased risk of MCI was established . The same was demonstrated in patients with Parkinson's disease: worse baseline olfaction was associated with long-term cognitive decline . Interestingly, even a simple test, using a container of 14 g of peanut butter, which was opened and moved up 1 cm at a time during the participant's exhale-until-odor detection, while measuring the distance between the subject's nostril and container, appeared to be a sensitive and specific test for probable Alzheimer's disease . A nice overview on the influence of age on the olfactory system and pathways mentioned that the magnitude of olfactory deficits, which occur in neurodegenerative and neurodevelopmental diseases, appears to be associated with the relative damage to the basal cholinergic system . Perhaps the link between cognition and olfactory function involves the apolipoprotein E É›4 allele (ApoE) that has been associated with increased cognitive and olfactory deficits .