Clinical Characteristics and Outcomes of Pediatric Idiopathic Hypereosinophilic Syndrome: A 22-Year Single-Center Case Series.

OBJECTIVE: Idiopathic hypereosinophilic syndrome (HES) is a rare and heterogeneous disorder in childhood, and data regarding its clinical course and treatment outcomes are limited. MATERIALS AND METHODS: We aimed to evaluate the clinical characteristics, organ involvement patterns, treatment approaches, and follow-up outcomes of pediatric patients diagnosed with idiopathic HES. This retrospective study included pediatric patients diagnosed with idiopathic HES between 2003 and 2025 at a tertiary pediatric hematology center. Diagnosis was based on persistent peripheral eosinophilia (≥1,500 /mm³), evidence of organ involvement attributable to eosinophilia, and exclusion of secondary and clonal causes. Demographic features, laboratory findings, organ involvement, treatment responses, relapse frequency, and outcomes were analyzed descriptively. RESULTS: Seven patients (2 females, 5 males) were included. The median age at diagnosis was 5 years (range, 3 months-9 years). The median absolute eosinophil count at diagnosis was 16,000 /mm³ (range: 5,600-112,000). Organ involvement was heterogeneous, with hepatic involvement being the most frequent (6/7, 85.7%), followed by cardiac (3/7, 42.9%), cutaneous (2/7, 28.6%), and pulmonary and central nervous system involvement (1/7 each, 14.3%). All patients received corticosteroids as first-line therapy, with generally rapid initial hematologic and clinical responses. However, recurrent relapses occurred in several patients. One patient with frequent relapses and progressive pulmonary involvement died despite multiple additional therapies. CONCLUSION: Pediatric idiopathic HES demonstrates variable clinical behavior. Although corticosteroids are generally effective as initial therapy, relapse risk and potential for severe organ involvement necessitate close monitoring and individualized management strategies.