Introduction
The olfactory system is important for our response to the environment and olfactory dysfunction (OD) represents a critical loss of information. The causes are diverse, including upper airway respiratory tract infections, head traumas, idiopathic reasons, and impairments secondary to sinonasal or neurodegenerative diseases.1OD can be categorized into qualitative and quantitative impairments. Qualitative OD can be further subdivided into parosmia, defined as distorted odour perception in the presence of an odour and phantosmia, defined as odour perception in the absence of an apparent odour source.2 Both parosmia and phantosmia can occur alone but are most commonly present along with quantitative OD.1 Parosmia has been associated with better clinical outcome in terms of spontaneous olfactory recovery.3–5 However, literature on the significance of parosmia as a predictor of olfactory rehabilitation in patients with OD receiving therapy remains sparse. Therefore, further elucidating its role as prognostic factor in olfactory recovery is needed for clinical counselling, especially when considering its prevalence of up to 60 percent among patients with certain etiologies of OD.6
While quantitative impairments of the sense of smell are common and may affect up to one quarter of the general population, the prevalence of qualitative impairments appears significantly lower.7,8 Notably, presence of parosmia varies among patients with quantitative OD, depending on the underlying cause of smell loss. While parosmia is most commonly found in patients with postinfectious OD, distorted odour perceptions are also reported in posttraumatic, idiopathic, and sinonasal causes.9Previous studies on parosmia as prognostic factor in olfactory recovery provided first evidence, that the presence of parosmia at the initial visit might be associated with a higher number of clinically relevant improvements compared to the parosmia-free group.3–5
Treatment for smell loss relates to its underlying cause and pathophysiology. While treatment strategies for OD secondary to (chronic) sinonasal diseases aim to resolve the underlying conditions, olfactory training (OT) aims to enhance olfactory recovery based on the neuronal plasticity of the olfactory system.10 OT is recommended as conscious sniffing of at least four different odours at least twice daily for several months and has emerged as a simple and side-effect free treatment option for various causes of smell loss. Previous studies and meta-analysis provided evidence that OT is effective in patients with OD, but also healthy subjects of different age groups to improve olfactory function.10 It has been suggested, that aetiology of smell loss (i.e. postinfectious) and longer duration of OT might serve as prognostic factor for better outcomes in terms of olfactory recovery.10 However, the literature on symptoms of qualitative OD as predictor of olfactory recovery after OT remains sparse. Understanding its impact would be of great clinical significance in counselling patients who may otherwise be confused by distorted odour perceptions in quantitative smell loss. Hence, the aim of this study was to elucidate the prognostic value of parosmia and phantosmia in terms of olfactory rehabilitation in a cohort of patients with various causes of OD receiving OT.