Cor triatriatum sinister resembling Lucas type C1a in a cat

A six-year-old neutered male cat was presented for acute respiratory distress. Transthoracic echocardiography identified a left atrial membrane consistent with cor triatriatum sinister (CTS) and turbulent flow across a single fenestration. Additionally, it was suspected that the distal chamber may receive at least one pulmonary vein. Because respiratory signs improved with corticosteroids but not with a short diuretic trial, and because echocardiography suggested that the distal chamber might receive at least one pulmonary vein, cardiac computed tomography angiography was performed to further characterize the pulmonary venous anatomy and to differentiate CTS from pulmonary vein stenosis and other congenital vascular malformations. Cardiac computed tomography angiography confirmed left atrial subdivision with one interchamber orifice (5.33 mm) and demonstrated a pulmonary venous connection pattern in which three pulmonary veins connected to the proximal chamber, whereas the left cranial pulmonary vein connected to the distal chamber, which was continuous with the mitral valve. Adapting the human Lucas classification scheme, the anatomic configuration most closely resembled Lucas type C1a, and no congestive heart failure attributable to CTS was identified.