Discussion

Congenital left atrial appendage aneurysm is rare, with approximately 50 congenital left atrial appendage aneurysm cases (including several cases diagnosed during the fetal period) previously reported. Although asymptomatic patients are sporadically diagnosed during adulthood, ^{Reference Aryal, Hakim and Ghimire1} there are no reports of rapid left atrial appendage aneurysm enlargement and symptomatic features in the neonatal period, or of cardiac failure symptoms associated with left atrial appendage aneurysm expansion. Previous surgical cases in neonates have reported left atrial appendage aneurysm sizes of approximately 30 mm ^{Reference Fountain-Dommer, Wiles and Shuler2–Reference Strecker, Koch and Schild4} , while the aneurysm in the present case was nearly twice as large. With rapid left atrial appendage aneurysm enlargement, the circulatory blood volume decreased and the main pulmonary artery was compressed. Thus, the heart failure symptoms (i.e., respiratory distress and feeding difficulties) were likely a result of the left atrial appendage aneurysm enlargement, which is supported by the improvement in heart failure symptoms after left atrial appendage aneurysm resection.

Transthoracic echocardiography is considered an adequate and primary method for left atrial appendage aneurysm detection. However, in our case, the mass was found by fetal echocardiography and was misidentified as a cyst. By contrast, MRI is important for assessing giant left atrial appendage aneurysms, overall aneurysm spread (i.e., by accurate size measurement), the presence of thrombi, and in ruling out a differential diagnosis. ^{Reference Oz, Oguz, Karcaaltincaba, Yilmaz and Haliloglu5} In the present case, dynamic contrast-enhanced MRI showed enhancement in the arterial phase and continuity with the left atrium, which contributed to the definitive diagnosis. Interestingly, stasis maps provided by 4D flow Cardiovascular MRI were reported to provide objective information in therapeutic decision-making. ^{Reference Pradella6}

Congenital left atrial appendage aneurysms can be intrapericardial (associated with an intact pericardium) or extrapericardial (associated with pericardial defects). The intrapericardial type is thought to be caused by congenital abnormalities in the left atrial wall and/or appendage. ^{Reference Cho, Park, Lee, Choo and Sung7} The congenital form arises from dysplasia of the musculi pectinati and related atrial muscle bands. Acquired forms often occur in association with conditions leading to elevated left atrial pressure, including organic mitral valve diseases. In the present case, pathological examination revealed no epicardial defect, an intrapericardial type left atrial appendage aneurysm, and marked thinning in part of the atrial muscle, accompanied by myxoid degeneration. There was no evidence of inflammation, abnormal muscle bundles, or degeneration. Increased blood flow after birth may have contributed to thinning of the left atrial appendage. Further enlargement of the left auricular mass may have also caused aneurysm wall thinning, creating a vicious cycle that facilitated further aneurysm enlargement.

In rare cases, mitral regurgitation or cardiac tamponade in the neonatal period due to left ventricular compression can occur. ^{Reference Mansour, Aldousany, Arce, Subramanian and Ashraf8} Thus, careful follow-up with consideration for early surgery is required. In the present case, surgery was chosen because of the rapid aneurysm enlargement and symptoms of heart failure. During the operative period (including anaesthesia), we had concerns over potential circulation failure following left atrial appendage aneurysm excision. However, his vital signs were stable and cardiac failure symptoms resolved post-operatively. Resection through a median sternotomy during cardiopulmonary bypass is a safe method. ^{Reference Sankhyan9}