<scp>IgG4</scp> Related Disease Presenting With Eosinophilic Pleural Effusion

A 56-year-old man presented with right-sided pleuritic chest pain and intermittent cough for 4 months, with a background history of well-controlled asthma and allergic rhinitis. Chest radiographs revealed right-sided pleural effusion, the fluid chemistry showed an exudate effusion with negative cytology and abundant eosinophils. Thoracoscopic pleural biopsy showed moderate lymphoplasmacytic chronic inflammation. Immunohistochemistry showed IgG4 staining up to 90 plasma cells/hpf (> 10% diagnostic), and the IgG4: IgG ratio was 40% (> 40% diagnostic), confirming the diagnosis of IgG4-related disease (IgG4-RD). Elevated serum IgG4 level (300 mg/dL; normal range: 3-201 mg/dL), further supported the diagnosis. While eosinophilic pleural effusion (EPE) accounts for 5%-16% of all exudative pleural effusions, IgG4-related pleural disease is an extremely rare cause of EPE.