Effects of Olfactory Training on Olfactory Bulb Volume Among Patients With Non-Sinonasal-Related Olfactory Dysfunction: A Systematic Review and Meta-Analysis

Objective Olfactory training (OT) is being used increasingly among patients with olfactory dysfunction (OD). Evidence suggests that OT could significantly improve olfactory function among patients with non-sinonasal-related OD. Neuroplasticity of olfactory bulb (OB) is associated with the input of olfactory receptor neurons, and exposure to a wide variety of odors facilitates neuroplasticity of OB. This study aimed to perform a meta-analysis to evaluate the effect of OT on olfactory bulb volume (OBV) among patients with non-sinonasal-related OD. Methods We performed a systematic review and meta-analysis of studies sourced from PubMed, Google Scholar, Web of Science, and Embase, covering the period from their inception to May 2025. The included studies provided magnetic resonance imaging measurements of OBV in individuals with non-sinonasal-related OD or normosmia individuals. These participants either underwent OT or were part of a non-intervention control group. Results Seven studies with 310 participants were analyzed to assess changes in OBV before and after OT. The training significantly improved overall unilateral OBV by 2.14 mm 3 [ Z = 5.27; 95% confidence interval (CI) = 1.34-2.93; P < .001], with the OBV of patients with non-sinonasal OD increasing by 1.91 mm 3 ( Z = 4.47; 95%CI, 1.07-2.75; P < .001) and OBV of healthy individuals by 4.22 mm 3 ( Z = 3.26; 95%CI, 1.68-6.75; P = .001). Among patients with non-sinonasal-related OD, OT resulted in an increase of 1.52 mm 3 in the left OBV ( Z = 2.66; 95%CI, 0.40-2.64; P = .008) and an increase of 2.40 mm 3 in the right OBV ( Z = 3.74; 95%CI, 1.14-3.66; P < .001). There were no significant differences in OBV changes between the left and right sides ( P = .30). In contrast, patients with non-sinonasal-related OD who did not receive OT did not show significant changes in OBV after OT ( Z = 0.74, P = .46). Conclusions This meta-analysis provided evidence that OT was associated with a small but statistically significant increase in OBV among patients with non-sinonasal-related OD. The increase in OBV was comparable between the left and right OB of patients with non-sinonasal-related OD.