The association between diabetes and olfactory impairment in adults: A systematic review and meta-analysis

Background: Evidence suggests that olfactory impairment (OI) may be a degenerative neurologic complication of diabetes; however, the association is not yet well established. The objective of this work was to systematically review existing literature on the association between diabetes and OI in adults, with meta-analysis of evaluable studies. Methods: A literature search encompassing 358 abstracts from the last 75 years in PubMed, EMBASE, and Cochrane was performed. English-language articles investigating adults with diabetes and OI in comparison to control groups with original data and ≥7 subjects were included. The Newcastle-Ottawa scale was applied for quality assessment. Two investigators independently reviewed all articles. For meta-analysis, the odds ratio of OI in diabetes compared with control groups was calculated using the fixed effects model. Results: The initial search yielded 358 abstracts, from which 21 articles were reviewed and 11 articles (n = 6,747) were included. Studies included were case-control (64%) or cross-sectional (36%) with evidence level 3b. On the Newcastle-Ottawa scale, the mean quality assessment score for case-control and cross-sectional studies was 7.4 (maximum of 9) and 7.0 (maximum of 10), respectively. A statistically significant association between diabetes and olfaction compared with controls was found in 6 (55%) of the 11 articles. Four studies were eligible for meta-analysis, which yielded an overall odds of having OI with diabetes as 1.58 times more likely than in control groups (95% CI [1.16, 2.16]; I² = 10.3%). Conclusions: The reviewed studies support a significant association between diabetes and OI. Further studies are warranted to characterize this association. Level of Evidence: 3a.