Background The connection between Alzheimer's disease (AD) and olfactory deficits is well documented and further, alterations in olfactory functioning may signal declines in functions associated with dementia. The aim of the present comprehensive meta-analysis was to investigate the nature of olfactory deficits in mild cognitive impairment (MCI).
Methods Articles were identified through computerised literature search from inception to 30 June 2016 using PubMed, MEDLINE and PsychInfo databases. In order to control for differences in sample size during effect size computation, studies were weighted according to their inverse variance estimates.
Results 31 articles (62 effects) were identified, which included 1993 MCI patients and 2861 healthy older adults (HOA). Included studies contrasted odour identification, discrimination, detection threshold and/or memory between cases and controls. Moderate to large and heterogeneous effects were seen for olfactory deficits in MCI relative to HOA (d=−0.76, 95% CI −0.87<δ<−0.64). Moderator analysis revealed that tests of odour identification yielded larger effect sizes than those of odour detection threshold or memory. In addition, a potential interaction between age and sex was observed, with male patients carrying a larger burden of olfactory deficit and older female patients performing better on olfactory tests.
Conclusions and relevance Olfactory deficits are present and robust in MCI. Odour identification is most impaired in MCI, which parallels the most prominent sensory deficit seen in AD. As such, a simple-to-administer test of odour identification warrants inclusion in the screening of individuals at risk for developing AD.
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Contributors DRR and PJM contributed equally to the study. They had full access to all data and take responsibility of the integrity of the data and the accuracy of the data analysis. MJM and DRR conceptualised and designed the study. Acquisition, analysis or interpretation of data was carried out by PJM, DRR, LB, MJM and SK. DRR, MJM and PJM drafted the manuscript. DRR, MJM, BIT, LB, SK, DAW and PJM critically revised the manuscript for intellectual content. Statistical analysis was performed by MJM and DRR. DRR, MJM and DAW obtained the funding. Administrative, technical or material support was provided by MJM and DRR. DRR and MJM supervised the study.
Funding This work was supported by National Institute of Mental Health (K01 MH102609) and National Institute on Aging (P30 AG10124).
Competing interests None declared.
Provenance and peer review Not commissioned; externally peer reviewed.
Data sharing statement This is a meta-analysis of published work. All data are in the public domain in the form of manuscripts. Any and all data that we report are presented in either the body of the manuscript or the online supplemental material. We are happy to share the final collated data set.