Olfactory function in mild cognitive impairment and Alzheimer's disease: A meta-analysis.
OBJECTIVE
Olfactory function is altered in mild cognitive impairment (MCI) and Alzheimer's disease (AD); therefore, it may serve as a useful tool for the early detection of MCI before its advancement to AD. The aim of this meta-analysis was to investigate olfactory deficits in patients with MCI and AD.


STUDY DESIGN
Literature search.


METHODS
A search was conducted of the electronic databases PubMed, Embase, and Web of Science from their inception until 2017. We included original articles with adequate data on the identification, threshold, and/or discrimination of olfactory function in MCI or AD. The standard mean difference and 95% confidence interval (CI) were calculated. The studies were weighted according to inverse variance estimates. The effect sizes were categorized as small [Cohen's d = 0.2], medium (d = 0.5), or large (d ≥ 0.8) based on these methods. Subgroup analyses were performed based on mean age and sex differences between the groups.


RESULTS
Twelve articles (reporting 21 effects) examining 563 patients with MCI and 788 patients with AD, were included in the meta-analysis. Compared to MCI, AD had moderate to large heterogeneous effects on olfactory function (Cohen's d = 0.64, 95% CI: 0.50, 0.78). Olfactory identification tests demonstrated larger effects (d = 0.71, 95% CI: 0.51, 0.91) than did tests of other olfactory domains.


CONCLUSIONS
Meta-analysis results revealed that olfactory identification was more profoundly impaired in patients with AD than in those with MCI. These findings suggest that a simple test of odor identification is valuable in differentiating individuals at a risk of AD.


LEVEL OF EVIDENCE
NA Laryngoscope, 129:362-369, 2019.
['Jung|Hahn Jin|HJ|0000-0002-6015-7048', 'Shin|In-Soo|IS|', 'Lee|Ji-Eun|JE|0000-0001-9043-140X']
D000368:Aged D000544:Alzheimer Disease D060825:Cognitive Dysfunction D005260:Female D006801:Humans D008297:Male D008875:Middle Aged D000857:Olfaction Disorders D012903:Smell
Mild cognitive impairment Alzheimer's disease meta-analysis dementia olfaction