Hypereosinophilic Syndrome in a Patient With Cystic Fibrosis: A Rare Case of Cardiac Involvement and Response to Mepolizumab.

Hypereosinophilic syndrome (HES) is a rare condition characterized by persistent eosinophilia (eosinophil count ≥1.5 × 10/L) and end-organ damage in the absence of an identifiable cause. Cardiac involvement is common and may lead to life-threatening complications. Cystic fibrosis (CF) is a chronic multisystem disease predominantly associated with neutrophilic inflammation, and eosinophilic disorders are less often reported in this population. A 32-year-old woman with CF, complicated by CF-related diabetes and pancreatic insufficiency, presented with chest pain and peripheral eosinophilia (3.2 × 10⁹/L); infectious, autoimmune, and allergic evaluations were negative. Imaging revealed perimyocarditis, and systemic corticosteroids were initially effective but discontinued due to cushingoid side effects and anasarca. She subsequently experienced a recurrence of chest pain accompanied by eosinophilia (1.7 × 10⁹/L), and a diagnosis of idiopathic HES was made based on persistent eosinophilia, cardiac involvement, and exclusion of secondary causes. She responded favorably to monthly subcutaneous mepolizumab, a monoclonal antibody that prevents interleukin-5 (IL-5) from binding to its receptor, thereby inhibiting the recruitment and activation of eosinophils, with resolution of eosinophilia and improvement in symptoms. This case underscores the importance of considering HES in CF patients presenting with unexplained eosinophilia and extrapulmonary symptoms. It also illustrates the efficacy of targeted biologic therapy in managing idiopathic HES when corticosteroids are poorly tolerated.