The goal of this clinical trial is to understand the mechanisms leading to olfactory dysfunction (OD) in chronic rhinosinusitis (CRS). The main questions it aims to answer are:

• Is the type 2 inflammation impacting the olfactory epithelium regeneration, leading to OD?
• Does a treatment interfering with one of the major T2 inflammatory cytokines, have an impact on the mechanisms of OD in CRS?

Participants will undergo olfactory testing and nasal brushing before and after 3 months of treatment.

Inclusion Criteria:

• At least two of the following symptoms: nasal obstruction, rhinorrhoea, smell loss, facial pain for 12 weeks.
• Endoscopic visualisation of bilateral sinonasal polyps
• SSI (TDI) < 30.75 and decreased sense of smell subjectively
• Eosinophilia > 300/μl of blood Ku/l
• On a stable nasal steroid treatment for at least 4 weeks

Exclusion Criteria:

• Active smoking
• Primary and secondary immune deficiencies
• Ciliary diseases (cystic fibrosis, primary ciliary diskinesia)
• History of or current nasal malignancies
• < 18 years and > 70 years
• Patients unable to express consent
• Use of systemic corticosteroids for < 4 weeks before visit 1 • Use of systemic or local antibiotics for < 4 weeks before visit 1