Large, ball-shaped left atrial hematoma encapsulated by a thin membrane-like organized thrombus requiring urgent surgical resection.

A 57-year-old female patient presented with dyspnea. Chest radiography revealed pulmonary congestion and bilateral pleural effusion. Electrocardiogram revealed sinus tachycardia. Transthoracic echocardiography revealed a large mass measuring 32 × 25 mm in the left atrium that caused mitral valve obstruction during diastole. Emergency surgery was performed, and the mass was found to be a hematoma encapsulated by a thin membrane-like organized thrombus. No atrial fibrillation or coagulation abnormalities were observed during follow-up. Learning objective: A ball-shaped left atrial thrombus without atrial fibrillation or mitral stenosis is a rare lesion. Previous studies have not reported a large left atrial hematoma encapsulated by a thin membrane-like organized thrombus. Blood flow into the left atrial mural thrombus may have caused the rapid enlargement of the large, ball-shaped left atrial thrombus.