Introduction
Olfactory dysfunction (OD) can be classified into conductive and sensorineural types, although these are not mutually exclusive. In conductive losses, such as nasal polyps and chronic rhinosinusitis (CRS), inspired odorants are unable to enter the olfactory cleft in the nasal cavity. In sensorineural loss, the damage of olfactory receptor neurons or their central projection contribute to OD.1Attempted treatments have included medical (topical and systemic steroids, zinc, etc.) and surgical treatment. Hummel et al.2 studied the effectiveness of olfactory training(OT) in a group of patients with olfactory loss due to post-infectious, post-traumatic, or idiopathic etiologies. OT has shown promise as an alternative treatment modality for several causes of OD, with the exception of sinonasal disease.
Previous research found that exposing various odors in patients with post-infectious and post-traumatic OD for 16 weeks increased their olfactory function.3 According to a recent meta-analysis, OT is a promising clinical therapy for patients with OD, and many other trials have shown that OT has good olfactory outcomes with no serious side effects. 4 Recently, we reported that OT resulted in olfactory improvement reflected in the total threshold, discrimination and identification (TDI) score, threshold score, and identification score in patients with post-infectious olfactory dysfunction (PIOD) as compared to a control group using five odorants familiar to Koreans.5 Moreover, OT can be considered for patients with persistent COVID-19-related OD because this therapy is inexpensive and has negligible adverse effects.6
In patients with chronic rhinosinusitis (CRS), OD is a common complaint.7-9 Although the main mechanism of CRS-induced OD is unclear, it is thought to be a combination of mechanical obstruction from edematous mucosa or polyposis, as well as sensorineural damage from chronic inflammatory injury to the olfactory neuroepithelium.10 Endoscopic sinonasal surgery (ESS) is used to improve sinus function and access to topical medical treatment in patients with medically refractory CRS. However, olfactory function after ESS can be unpredictable.11-12 An early study endorsed the effect of ESS in reducing CRS-related OD. However, several recent studies on the outcomes of OD after ESS have reported conflicting results.13 A study found that OT improved the olfactory activity of sinonasal patients, but did not evaluate the effects of OT on post-sinonasal-operative patients. A separate study reported that no significant changes in individual component values of TDI scores were observed in sinonasal patients. However, that research did not include a control group.14 Therefore, the effectiveness of OT in CRS patients who experienced mixed (conductive and sensorineural) olfactory dysfunction after resolving the conductive cause by sinonasal surgery has not been investigated.