Cystic fibrosis (CF) is the most common genetic disorder in Belgium, affecting 1 in 2850 children. A defect in the CFTR channel results in increased viscosity of extracellular secretions and decreased mucociliary clearance in the airways. As a result of this mechanism, chronic rhinosinusitis (CRS) occurs in nearly 100% of CF patients. CRS can lead to pronounced sinonasal complaints and can have a negative impact on the quality of life. In addition, several studies have shown that poor upper airway control has a negative impact on the lower airways. Unfortunately, treatment options are limited. Our previous study has shown that only 21% of patients have well-controlled CRS.

The aim of this single-center, randomized cross-over study is to compare the effect of nasal rinses with (XNI) and without (SNI) Xylitol in the treatment of CF-CRS. Xylitol is a sugar alcohol that is already used as an antibiofilm agent,eg in the prevention of caries. Previous pilot studies have already shown a beneficial effect of XNI in the treatment of non-CF CRS.

Cystic fibrosis (CF) is the most common genetic disorder in Belgium, affecting 1 in 2850 children. A defect in the CFTR channel results in increased viscosity of extracellular secretions and decreased mucociliary clearance in the airways. As a result of this mechanism, chronic rhinosinusitis (CRS) occurs in nearly 100% of CF patients. CRS can lead to pronounced sinonasal complaints and can have a negative impact on the quality of life of CF patients. In addition, several studies have shown that poor upper airway control has a negative impact on the lower airways. Unfortunately, treatment options are limited. Our previous study has shown that only 21% of patients have well-controlled CRS.

The aim of this study is to compare the effect of nasal rinses with (XNI) and without (SNI) Xylitol in the treatment of CF-CRS. Xylitol is a sugar alcohol that is already used as an antibiofilm agent, eg. in the prevention of caries.

Patients with cystic fibrosis, >18 years of age, who are known to have CRS with/without nasal polyps and who experience sinonasal symptoms in daily life, will be recruited (n=38). Patients with a visual analogue scale (VAS) score of >30/100 mm are included. Patients with a VAS <30 mm or patients who started <6 months ago with a CFTR modulator (eg Trikafta, Symkevi, Kalydeco) are excluded. After signing the ICF, patients are randomly allocated to a treatment arm "AB" or "BA". Patients in the study group 'AB' start, after a washout period of one week, with nasal rinses with Xylitol for six weeks. Afterwards, after another one week washout period, patients are asked to rinse the nose with standard saline nasal rinses without Xylitol for six weeks. Patients in the 'BA' group first start with standard nasal rinses, followed by Xylitol.

In total there are three planned study visits: a screening visit, a follow-up visit after the first treatment period and a follow-up visit after the second treatment period. During these visits, a nasal endoscopy (Lund-Kennedy and Modified Davos score) and smell tests (Sniffin' Sticks) are performed. In addition, the patients are asked to complete a questionnaire with VAS scores, the SNOT-22 score and the TRE score. Data is collected in a REDCAP database. Afterwards, the outcome parameters are statistically compared between the SNI and the XNI group.

Inclusion Criteria:

• Patients with established diagnosis of cystic fibrosis > 18 years old
• Routine follow-up at UZ Leuven
• Patients with established diagnosis of chronic rhinosinusitis, with or without nasal polyps, and with a VAS-score of >30 out of 100 for at least one individual sinonasal symptom (nasal obstruction, rhinorrhea, postnasal drip, facial pain/pressure, reduced smell)
• Willing to participate in this study and compliant to the treatment

Exclusion Criteria:

• Patients <18 years old
• Start of CFTR gene-modulator less than 6 months prior to inclusion
• Not willing to participate in this trial or incompliant to the trial
• History of severe epistaxis (<6 months), requiring intervention by an ENT specialist
• History of hypersensitivity/allergic reaction/anaphylactic shock to one of the compounds of the sachets (NaCl, bicarbonate, mint, Xylitol)

The study will be designed as a blinded trial, in which nor the investigator, nor the patient will know the sequence of the treatments (AB or BA). The patients will receive a box with sachets for treatment period 1 at the screening visit. The box will have a number from 1 to 76 and the sachets from both treatment groups will not contain any information regarding the content.