A Rare Case of Concurrent Urinothorax and Myelomatous Pleural Effusion.

Introduction: Myelomatous pleural effusion (MPE) is a rare complication of multiple myeloma, occurring in < 1% of cases and associated with poor prognosis. Urinothorax, another rare cause of pleural effusion, results from urine leakage into the pleural cavity, typically due to obstructive uropathy or trauma. Although both conditions are individually uncommon, their concurrent occurrence has not been previously reported. Case Presentation: An 81-year-old man with relapsed multiple myeloma presented with dyspnea, hematuria, and acute renal failure. Imaging revealed bilateral hydronephrosis, obstructive uropathy from a pelvic mass, and bilateral pleural effusions. Thoracentesis showed an exudative, hemorrhagic effusion with a pleural fluid-to-serum creatinine ratio > 1, initially suggesting urinothorax. Despite bilateral nephrostomy, the effusion recurred. Cytology and immunohistochemistry confirmed MPE. Biopsy of the pelvic mass revealed extramedullary myeloma (plasmacytoma), which explained both the ureteral obstruction and pleural involvement. Conclusion: This is the first reported case of simultaneous MPE and urinothorax. It underscores the importance of a comprehensive, multidisciplinary diagnostic approach when evaluating complex pleural effusions with overlapping features.