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.