The Efficacy of Olfactory Training as a Treatment for Olfactory Disorders Caused by Coronavirus Disease-2019: A Systematic Review and Meta-Analysis

Background: There have been reports investigating the use of olfactory training in olfactory dysfunction after COVID-19. Objective: We evaluated the effect of olfactory training on the olfactory dysfunction of patients infected with COVID-19. Methods: We searched PubMed, EMBASE, the Web of Science, the Cochrane database, SCOPUS, and Google Scholar up to May 2022. We retrieved studies that compared the extents of olfactory dysfunction before and after olfactory training. We performed a subgroup analysis by the duration of olfactory dysfunction. Results: The olfactory score after olfactory training (standard mean difference [SMD] = 1.0830, 95% confidence interval [CI] [0.6416; 1.5245], P <.0001, I² = 90.4%) was higher than that before training. The olfactory dysfunction rate differed significantly (OR = 0.0232, 95% CI [0.0052; 0.1044], P <.0001, I² = 63.1%) before and after olfactory training. On subgroup analysis, although patients with both acute (onset < 30 days prior) and chronic (onset > 30 days prior) olfactory dysfunction evidenced clinically significant improvements, training during acute dysfunction (compared to acute dysfunction) increased the olfactory score to a greater extent (SMD = 1.7779, 95% CI [1.0077; 2.5481] vs 0.6928 [0.2143; 1.1712], P = 0.0190). Moreover, as a result of subgroup analysis by dividing the included studies into2 using 2-month training period as standard, there was no statistically significant difference in the effect of the training period in the included study. Conclusion: Olfactory training improved olfactory disorders caused by COVID-19. Such training was effective in both the acute and chronic phases.