Most patients with cystic fibrosis (CF) develop multisystemic clinical manifestations, the minority having mild or atypical symptoms. We describe an adolescent with chronic cough and purulent rhinorrhoea since the first year of life, with diagnoses of asthma, allergic rhinitis and chronic rhinosinusitis. Under therapy with long-acting bronchodilators, antihistamines, inhaled corticosteroids, antileukotrienes and several courses of empirical oral antibiotic therapy, there was no clinical improvement. There was no reference to gastrointestinal symptoms. Due to clinical worsening, extended investigations were initiated, which revealed Pseudomonas aeruginosa in sputum culture, sweat test with a positive result and heterozygosity for F508del and R334W mutations in genetic study which allowed to confirm the diagnosis of CF. In this case, heterozygosity with a class IV mutation can explain the atypical clinical presentation. It is very important to consider this diagnosis when chronic symptoms persist, despite optimised therapy for other respiratory pathologies and in case of isolation of atypical bacterial agents.
