Teaching Case

49-year-old woman with palpitations and fatigue.

Echocardiography: Left atrial enlargement divided into two compartments by a membrane. The proximal (upper) portion receives venous blood, whereas the distal (lower) portion is in contact with the atrioventricular valve.

Cardiac MR: shows moderate left atrial dilation divided into two distinct chambers by a membrane of 2mm thickness; the proximal chamber received the pulmonary veins and the distal chamber contained the atrial appendage and mitral valve. The fibromuscular membrane has an eccentric fenestration which communicates both chambers without signs of obstruction. It is associated with small interatrial communication with left-to-right shunt.

Cor triatriatum (or triatrial heart) is a very rare congenital abnormality, first described by Church in 1868. It is usually diagnosed in childhood but in some cases remain silent into adulthood. There are three main embryological theories explaining the development of this condition: malseptation (overgrowth of septum primum), malincorporation (entrapment of the common pulmonary vein by the left horn of the sinus venosus), and entrapment (incomplete incorporation of the common pulmonary vein into the left atrium) [1].
It involves the left atrium (cor triatriatum sinistrum) and rarely the right atrium (cor triatriatum dextrum). Cor triatriatum sinistrum is more frequent, is characterized by a proximal left atrial chamber (where the pulmonary veins drain) that is separated from the distal true left atrial chamber (containing the mitral valve and left atrial appendage) by a fibromuscular fenestrated membrane with transverse or horizontal orientation, band-like or funnel-shaped [2]. The size, number of septations and transmembrane flow determine the severity of the lesion and the clinical presentation. If the orifice is small it is usually diagnosed during infancy by obstructive symptoms mimicking mitral stenosis, but if the orifice is large it can also be asymptomatic. Symptoms include dyspnoea, orthopnoea, cyanosis, haemoptysis, chest discomfort and palpitations [3]. The transthoracic echocardiography is the imaging modality of choice and has 98% accuracy in the diagnosis of cor triatriatum [4]. It shows a differential filling of the two distinct atrial chambers, demonstrates the membrane, evaluates turbulent flow on colour Doppler and the associated cardiac lesions (mitral or aortic regurgitation, ostium secundum atrial septal defect, and the presence of left superior vena cava with unroofed coronary sinus) but may miss small openings in the fibromuscular membrane [1]. Transoesophageal echocardiography and 3D-Transoesophageal echocardiography provides better imaging of the left atrium, morphology of the membrane and the degree of obstruction allowing diagnosis of cor triatriatum with small orifices. CT and MR allows diagnosis of cor triatriatum in asymptomatic patients (incidental diagnosis), or when echocardiographic assessment is unavailable or inconclusive. CT gives additional information including vascular anomalies, cardiac anatomy, wall motion and myocardial perfusion [5]. MR and Cine-MR is also effective for visualization of anatomic abnormalities, evaluates dynamic cardiac function and haemodynamics but with limited spatial resolution, partial volume artefacts and poor data acquisition due to cardiac arrhythmias [6]. Surgical resection of the intraatrial membrane is indicated with severe obstruction as for severe mitral stenosis.