Abstract Body (Do not enter title and authors here): A 28-year-old male with no chronic illness presented with pleuritic chest pain, progressive dyspnea, low-grade fever, and vomiting. Over days, he developed hemodynamic instability and worsening dyspnea. Initial ECG showed diffuse ST elevation and PR depression. Transthoracic echocardiography revealed a large pericardial effusion with signs of tamponade. Urgent pericardiocentesis yielded 400 mL of inflammatory exudate. Despite empirical antimicrobial therapy, the patient remained febrile with elevated markers.

Further evaluation showed bilateral pleural effusions, mediastinal lymphadenopathy, and pulmonary nodules. A subxiphoid pericardial window and thoracotomy with biopsy were performed. Histology revealed, necrosis, Masson bodies, and abscess formation without granulomas. c-ANCA/PR3 antibodies were strongly positive. A diagnosis of granulomatosis with polyangiitis (GPA) was made. High-dose IV corticosteroids followed by methotrexate induced clinical improvement. The patient was discharged on immunosuppression and heart failure therapy.

Cardiac involvement occurs in only 3.3% of GPA cases, with pericarditis being the most common manifestation, yet presentation with tamponade is exceedingly rare. GPA typically presents with ENT, renal, or pulmonary involvement and has a mean diagnostic age of 53.6; our patient’s age and isolated cardiac onset significantly delayed diagnosis. Inflammatory pericardial effusions unresponsive to antibiotics should prompt consideration of vasculitis. Fewer than 5% of pericarditis cases are autoimmune, yet they are often severe, refractory, and associated with systemic inflammation. Lack of granulomas on biopsy should not exclude GPA if clinical and serologic features support the diagnosis.

Corticosteroids were initiated only after ruling out infection, aligning with current recommendations to avoid early immunosuppression in uncertain cases. Although rituximab or cyclophosphamide is preferred for organ-threatening disease, methotrexate was selected due to clinical stability and limited access, highlighting real-world barriers to guideline adherence.

This case underscores the importance of expanding diagnostic considerations for treatment-unresponsive pericardial disease and recognizing GPA as a rare but life-threatening cause of tamponade, even in young patients. Hemodynamic compromise, diagnostic uncertainty, and timely immunosuppressive decision-making required close multidisciplinary coordination and critical care management.