Transudative Pleural Effusion Associated with Extramedullary Hematopoiesis

The authors report a case of transudative pleural effusion associated with extramedullary hematopoiesis due to the presence of a myeloproliferative neoplasm, which was unclassified. A 71-year-old man presented with right pleural effusion during an exacerbation of thrombocytosis. The pleural effusion was transudative, although there was no history of cardiac failure or hypoalbuminemia, and treatment with diuretics failed. Extramedullary hematopoiesis was diagnosed in bilateral paravertebral soft tissue and the liver on (111)In bone marrow scintigraphy. The administration of hydroxyurea simultaneously reduced peripheral blood platelet count and pleural effusion within 2 weeks. The possible cause of transudative pleural effusion in association with extramedullary hematopoiesis is discussed.