Organ Involvement and Constitutional Symptoms in Idiopathic Hypereosinophilic Syndrome: A 72-Patient Single-Centre Cohort

: Background: Idiopathic hypereosinophilic syndrome (iHES) is a rare disorder of persistent hypereosinophilia with secondary organ damage, but the full spectrum of organ involvement and the constitutional-symptom burden under systematic ascertainment remain incompletely characterised. Objectives: To describe the clinical phenotype of iHES in a large single-centre cohort, with attention to symptoms typically under-reported in observational series. Methods: We followed 72 consecutive patients with iHES at a tertiary referral centre between January 2017 and May 2025. Defined HES variants were excluded by FIP1L1-PDGFRA RT-PCR and aberrant T-cell phenotyping on flow cytometry; secondary causes were excluded clinically. Each potential organ manifestation was systematically assessed at every visit using a structured clinician-administered checklist. Wilson 95% confidence intervals (CIs) are reported for all proportions. Results: Median age at diagnosis was 45 years (interquartile range [IQR] 31-58); 83% were female. The median number of involved organ systems was 3 (IQR 2-4, range 1-6). Pulmonary (91.7%, 95% CI 83.0-96.1), sinonasal (75.0%, 95% CI 63.9-83.6), musculoskeletal (72.2%, 95% CI 61.0-81.2) and gastrointestinal (66.7%, 95% CI 55.2-76.5) involvement predominated. Constitutional symptoms-principally fatigue-were recorded in 97% of patients (95% CI 90.4-99.2) and tracked clinically with disease activity. Cardiac involvement was infrequent (8.3%, 95% CI 3.9-17.0) but uniformly severe. Conclusions: Under systematic symptom probing, constitutional features and pulmonary/sinonasal involvement were recorded more frequently in this single-centre idiopathic HES cohort than in earlier series that pooled HES variants. However, selection bias from a pulmonology and severe-asthma referral pattern, together with differences in symptom ascertainment, likely contributes to these estimates and limits their generalisability to the broader iHES population. Routine echocardiographic screening and structured assessment of fatigue and cognitive symptoms should be incorporated into iHES monitoring.