Left atrial division may present in adults, often with an atrial septal defect (ASD), causing pulmonary hypertension and breathlessness. A 28 year old man presented with a six month history of breathlessness and palpitations. Atrial fibrillation, pansystolic murmur, and bi-basal crepitations were noted. Transoesophageal echocardiogram (TOE) (panel A) showed a left ventricular ejection fraction of 32%, severe central mitral regurgitation with normal leaflets, chordae and papillary muscles, and a left atrial dividing membrane with a small posteromedial communication. There was no ASD. Pulmonary artery pressure on Doppler ultrasound was 90 mm Hg.

A direct operative approach entered the superior chamber (with forceps through the communication) (panel B). Membrane excision and ring mitral valvoplasty were performed (TOE confirmed competence). The patient regained sinus rhythm and recovered well. TOE four months post-surgery showed an ejection fraction of 41% and a pulmonary artery pressure of 40 mm Hg.

Cor triatriatum sinistrum consists of a fibromuscular membrane dividing the left atrium into a proximal chamber receiving pulmonary veins and distal chamber leading to the left ventricle. There may be communications, and an ASD may vent the pulmonary circulation. Here there was a 0.5 cm transmembrane defect (panel B). Left ventricular failure and atrial fibrillation probably induced deterioration. Surgical intervention is the principal treatment with excellent postoperative results.