Introduction Cardiac tumours are rare and can present themselves as primary or metastatic tumours. Up to 90% of cardiac tumours are benign in nature, and they may present either as an embolic phenomena or with atypical chest symptoms.

Case Presentation In this study, we report a case of a 50-year-old male with no known medical illness who presented to the emergency department with acute onset of palpitations associated with mild dyspnoea. He did not have angina, failure, or constitutional symptoms. General examination was unremarkable; however, his pulse was irregularly irregular. There was mild pansystolic murmur.

Results The blood tests were within the normal range. Electrocardiogram revealed atrial fibrillation with a ventricular rate of 75 bpm. Echocardiography revealed a huge heteroechoic mass in the left atrium (LA) measuring 32.7 cm², causing LA dilatation associated with moderate mitral regurgitation, and the mass appeared to partially obscure mitral inflow. Subsequent cardiac computed tomography showed a mass measuring 7 cm × 7 cm, arising from the interatrial septum (IAS), with specks of calcification within and minimal contrast enhancement. There was mild coronary artery disease. He underwent urgent cardiac surgery. Intraoperatively, the mass had a wide base stalk at the IAS that did not involve the mitral annulus. Rooftop right atrial incision made to remove the mass with manipulations of the Triangle of Koch. Intraoperatively, mitral valves appeared degenerated, causing severe mitral regurgitation (MR) upon excision of the mass, which required mitral valve replacement. Postoperatively, the patient remained in complete atrioventricular block, which required permanent pacemaker insertion. Histopathological examination of the mass in keeping with the myxoma and valve, which were noted to have degenerative changes.

Discussion Atrial fibrillation rarely presents as feature of LA mass. Meanwhile, mitral valve degeneration can be attributed to inflammatory cytokines in myxoma. Postoperative complete atrioventricular block is rare in LA myxoma arising from fossa ovalis that requires a permanent pacemaker.

Conclusion Atrial myxoma with atypical features presented solely as symptomatic atrial fibrillation. The severity of MR can be underestimated in the presence of myxoma. Echocardiography should be performed during index hospitalization to prevent delayed diagnosis and other sequelae of myxoma.