A 16-year-old girl was admitted for breathlessness. Her only medical history was asthma treated with a bronchodilator and steroids. Examination revealed hepatomegaly, lower limb oedema and a soft systolic murmur with loud second heart sound. Chest radiography revealed cardiomegaly with pulmonary congestion, and the electrocardiogram was suggestive of right ventricular hypertrophy ( Fig. 1 A) . Echocardiography showed an abnormal dilated right atrium and right ventricle and an estimated pulmonary artery systolic pressure, on tricuspid regurgitation, of 124 mmHg (supra-systemic pulmonary hypertension) ( Fig. 1 B). In addition, a thick membrane was noted in the left atrium, separating it into two chambers ( Fig. 1 B, white arrow). This had an eccentrically placed orifice nearer to the septal margin of the membrane on transesophageal echography ( Fig. 1 C, white arrow), with a significant diastolic gradient (38 mmHg) ( Fig. 1 B), causing postcapillary pulmonary hypertension ( Supplementary data ). A diagnosis of cor triatriatum was made (without atrial septal defect or anomalous pulmonary venous connection) and the findings of pulmonary hypertension were attributed to the restrictive membrane, mimicking mitral stenosis. A decision was made to proceed to surgical resection of the obstructing atrial membrane. The left atrium was divided by a thick, fibromuscular membrane, with a 3 mm eccentrically placed orifice. The membrane was excised completely; the anatomy of the left atrium was inspected and showed no other abnormalities. The patient made an uneventful recovery following surgery; nevertheless, pulmonary hypertension persisted (80 mmHg), and was treated by an endothelin receptor antagonist (bosentan) and oral anticoagulation. Two years later, pulmonary pressures were normalized and treatment interrupted definitively.
