[Mediastinal pseudotumor due to atrial hernia caused by a left pericardial defect].

Radiological examination in a young adult revealed the presence of an opacity facing the second left arch of the heart shadow. This finding associated with data from computed tomography suggested a diagnosis of a thymic tumor. Surgical exploration demonstrated a defect in the left pericardium through which there was a rhythmic protrusion of the auricle and fatty tissue. Aplasia of the pericardium is rarely observed, and usually involves its left side. It results from premature atrophy of the left Cuvier's canal, and is associated with cardiac or pulmonary anomalies in half of the cases. Diagnosis should be suggested by the abnormal appearance of the second left arch, very often clinically asymptomatic, and is confirmed when the creation of a pneumothorax produces a simultaneous pneumopericardium. A thoracic scan can visualize the left auricular hernia beyond the mediastinal limits. However, pericardial aplasia must remain a differential diagnosis of pathological opacities in the middle mediastinum.