Abstract Eosinophilic pleural effusion (EPE), characterized by ≥ 10% eosinophils in pleural fluid, is a clinical syndrome often secondary to known causes like trauma, infection, autoimmune diseases or malignancy. When exhaustive evaluation fails to identify an underlying etiology, it is classified as idiopathic EPE (IEPE). This condition is poorly characterized in literature, and poses a significant diagnostic challenge. Its relationship with other eosinophilic disorders, such as hypereosinophilic syndrome (HES), remains unclear. We present a case series of four patients diagnosed with IEPE, of whom three are male, aged 20-58 years old (see Table 1). Duration of the illness ranges from 2 weeks to 4 months. All present with pleural effusion and mild peripheral blood eosinophilia (peak blood eosinophil are 780-2460/μL). Pleural fluid analysis confirms EPE in all cases (eosinophils account for 14%-61% in pleural effusion). Three patients have polyserositis, including pericardial and peritoneal effusions. A comprehensive workup, including etiological examination, autoimmune panels (including IgG4) and tumor screening, is negative. To rule out clonal disorders and occult malignancies, bone marrow biopsy, thoracoscopy or PET-CT are carried out when necessary. Two patients with rapid progression and significant pericardial involvement receive oral prednisone (equivalent to 0.5 mg/kg per day, then gradually tapering) with complete resolution of effusions. The other two, with milder symptoms, are managed conservatively with drainage and surveillance, and remain stable. The cases illustrate the critical role of a systematic exclusion process. This series also addresses a significant knowledge gap regarding IEPE, a condition with scarce reports and often overshadowed by cases of HES with pleural involvement. Our findings align with small cohorts suggesting IEPE predominantly affects young adults with only mild or even normal blood eosinophil counts. In clinical practice, due to the above reasons and limitations of testing methods, it is hard to early detect elevated eosinophils in pleural fluid. We hypothesize that IEPE may represent a localized or precursor form of systemic eosinophilic disorders, where eosinophilic infiltration is confined to the pleura before systemic blood elevation. Management for IEPE was poorly defined before. Here, two patients with rapid progression and significant pericardial effusions require urgent corticosteroid therapy and have excellent response, while the stability of untreated cases suggests a spectrum of disease severity, advocating for an individualized treatment strategy and long-term follow-up. This highlights the clinical relevance of recognizing IEPE as a distinct entity and emphasizes the need for larger studies to define its natural history and optimal management. This abstract is funded by: None
Cheng et al. (Fri,) studied this question.