A 43-year-old female, a known case of dilated cardiomyopathy with chronic atrial fibrillation presented with symptoms of congestive heart failure. She had a long standing history of hypertension and palpitation. There was no history of stroke or embolic event in the past. Transthoracic echocardiography revealed a large freely mobile round mass in the left atrium (LA), which was partially prolapsing in to the left ventricle (LV). The mass was moving freely in the LA and was bouncing back after striking the mitral valve leaflets like a ball in a pinball machine. Severe LV dysfunction (ejection fraction- 15-20%) and mitral regurgitation were also present. Patient was scheduled for surgical removal of the mass under cardiopulmonary bypass. Intra-operative transesophageal echocardiography (TEE) confirmed the pre-operative findings [Video 1]. In addition, it revealed two small masses near the left atrial appendage (LAA) [Figure 1]. At surgery, a large spherical pink colored mass with a smooth surface was removed. Evidence of peduncle or wall attachment was not demonstrated during surgery. Two small masses, one from the mouth of LAA and other from the wall of LA were also found and removed during the surgery. Histopathology examination of the mass revealed features consistent with thrombus. Post-operative course was uneventful and patient was discharged on 13 th post-operative day.
Figure 1: Midesophageal two-chamber view showing a round mass in the left atrium) and two small masses (arrow) near the left atrial appendage, left ventricle
The probable predisposing factors for thrombus formation are mitral valve disease, non-valvular atrial fibrillation, severe LV dysfunction, and other causes of atrial contractile failure.  Among these factors, mitral valve disease or atrial fibrillation are the most common. Thrombus formation in the presence of sinus rhythm is rare. Any condition predisposing to low flow state can lead to thrombus formation. Thus, in the present patient the risk factors for thrombus formation included atrial fibrillation and LV dysfunction. Patients with LA thrombus are at a high risk for thromboembolic events and sudden death as a freely mobile LA thrombus can occlude the mitral valve by a ball-valve mechanism. The differential diagnosis for an intra-cavitary cardiac mass includes thrombus, myxoma, lipoma and non-myxomatous neoplasm.  Among them, cardiac myxoma is the most common benign primary tumor of the heart and generally found in the LA. On echocardiogram cardiac myxomas typically appear as a mobile mass attached to the endocardial surface by a stalk, usually arising from the fossa ovalis. Cardiac thrombi, which appear more frequently than cardiac myxomas are typically located more often in the LA or LAA and generally occur in patients with organic heart disease. In the present patient, features were not consistent with thrombus as it was spherical and mobile, surface was smooth and echogenicity was more consistent with myxoma. The present thrombus may have been attached earlier, but got dislodged due to lysis at some stage as patient was on warfarin therapy. Other intra-cardiac masses such as lipoma or non-myxomatous neoplasm are rare. TEE is the extensive diagnostic tool to investigate suspected intra-cardiac mass involving the LA. 
Agmon Y, Khandheria BK, Gentile F, Seward JB. Clinical and echocardiographic characteristics of patients with left atrial thrombus and sinus rhythm: Experience in 20 643 consecutive transesophageal echocardiographic examinations. Circulation 2002;105:27-31. [PUBMED]