An elusive diagnosis: pericardial effusion and hypereosinophilia revealing seronegative eosinophilic granulomatosis with polyangitis: a case report.

Eosinophilic granulomatosis with polyangiitis (EGPA) is a small- and medium-vessel necrotizing vasculitis characterized by asthma, eosinophilia, and multiorgan involvement. Cardiac manifestations, though uncommon, are more prevalent in antineutrophil cytoplasmic antibody (ANCA)-negative patients and carry significant morbidity. Diagnosis can be challenging due to variable presentations and the lack of a definitive test. We present a 60-year-old woman with asthma and chronic rhinosinusitis who developed progressive dyspnea and was found to have a large pericardial effusion with tamponade physiology, bilateral pulmonary infiltrates, and marked eosinophilia. She underwent an urgent pericardial window and a lung wedge resection. Lung biopsy revealed eosinophilic vasculitis and eosinophilic pneumonia. Extensive infectious and autoimmune workups, including ANCA testing, was negative. She demonstrated rapid clinical improvement following the initiation of corticosteroids. The combination of asthma, sinus disease, eosinophilia, pulmonary infiltrates, and biopsy findings met the 2022 ACR-EULAR classification criteria for EGPA. She was discharged on high-dose steroids and was later started on mepolizumab for maintenance. This case highlights the diagnostic complexity of EGPA, particularly in ANCA-negative patients with life-threatening features such as cardiac tamponade. It underscores the importance of maintaining a broad differential in patients with systemic eosinophilia. Early recognition, timely immunosuppressive therapy, and multidisciplinary care are critical for improving outcomes and preventing severe complications.