A rare case of partial cor triatriatum diagnosed at an adult age: a case report.

Background: Cor triatriatum sinistra (CTS) is a rare condition where the left atrium (LA) is divided by a thin membrane into an upper and lower chamber. Incidentally, the diagnosis is made in late adulthood, usually because of a favourable variant such as in our patient who presented with partial CTS. Case summary: We present the case of a 62-year-old female who presented with COVID-19. She was known for longstanding symptoms of dyspnoea on exertion as well as a minor stroke several years ago. Computed tomography on admission suggested there was a mass in the LA but transthoracic echocardiography and cardiac magnetic resonance imaging revealed the diagnosis of partial CTS in which the superior compartment received pulmonary venous drainage from the right lung and the left-sided pulmonary veins drained into the inferior chamber. Since there were signs of chronic pulmonary oedema she successfully underwent balloon dilatation of the membrane resulting in remission of symptoms and normalization of the pressure in the accessory chamber. Discussion: Partial CTS is a rare variant of CTS. Since part of the pulmonary veins drains in the lower chamber of the LA (and thereby unload the right ventricle), it is a favourable variant and patients may present later in life when membrane orifices calcify or it may be discovered as an incidental finding. In some patients requiring intervention, balloon dilatation of the membrane may be considered as an alternative to surgical removal of the membrane by thoracotomy.