Effects of Olfactory Training on Olfactory and Cognitive Function: A Systematic Review

Abstract

Background: Olfactory training (OT) interventions exhibit dual therapeutic potential, enhancing both chemosensory function and multiple cognitive domains through neuroplastic mechanisms. Objective: To systematically review the efficacy of OT on olfaction, cognition, and neuroimaging changes across a range of populations, from healthy adults to patients with olfactory and cognitive impairments. Methods: A systematic review was conducted based on the Preferred Reporting Items for Systematic Reviews and Meta-Analyses guidelines. A comprehensive search of PubMed, Embase, Web of Science, and Cochrane databases performed to identify studies that evaluate the effect of OT on olfaction and cognition in different populations, focusing on its potential to enhance cognition and mitigate the progression of cognitive decline. Results: Of the initial 412 abstracts reviewed, 24 full-text articles were included. Risk-of-bias assessment revealed 7/12 randomized controlled studies (RCTs) had high risk and 11/12 non-RCTs had serious bias. The duration of OT interventions mostly ranged from 3 to 6 months. In healthy adults (6 studies) and adults with olfactory dysfunction (13 studies), OT improved olfactory function in a population-dependent manner and exerted domain-specific cognitive benefits. Meta-analysis revealed that the pooled proportion of individuals achieving a clinically-significant olfactory improvement was 27.01% (95% confidence interval, 0.16-0.40). Improvements were observed in global cognition, verbal fluency, and working memory. In adults with cognitive impairment (5 studies), classical OT showed limited olfactory and cognitive improvements; however, intensive OT with enriched odors, longer sessions, and greater adherence demonstrated potential for enhanced cognitive and emotional outcomes in patients with dementia. Furthermore, the reviewed neuroimaging studies consistently demonstrate that OT can increase volumes in several brain regions and promote neuronal reorganization, providing a potential mechanism of OT in olfactory recovery and cognitive improvement. Conclusions: OT effectively improves olfaction, cognition, and brain health, particularly in cognitively-normal populations.