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.