Cor triatriatum (or triatrial heart)^[1] is a congenital heart defect where the left atrium (cor triatriatum sinistrum) or right atrium (cor triatriatum dextrum) is subdivided by a thin membrane, resulting in three atrial chambers (hence the name).

Cor triatriatum represents 0.1% of all congenital cardiac malformations and may be associated with other cardiac defects in as many as 50% of cases. The membrane may be complete or may contain one or more fenestrations of varying size.

Cor triatriatum sinistrum is more common.^[2] In this defect, there is typically a proximal chamber that receives the pulmonic veins and a distal (true) chamber located more anteriorly where it empties into the mitral valve. The membrane that separates the atrium into two parts varies significantly in size and shape. It may appear similar to a diaphragm or be funnel-shaped, band-like, entirely intact (imperforate) or contain one or more openings (fenestrations) ranging from small, restrictive-type to large and widely open.

In the pediatric population, this anomaly may be associated with major congenital cardiac lesions such as tetralogy of Fallot, double outlet right ventricle, coarctation of the aorta, partial anomalous pulmonary venous connection, persistent left superior vena cava with unroofed coronary sinus, ventricular septal defect, atrioventricular septal (endocardial cushion) defect, and common atrioventricular canal. Rarely, asplenia or polysplenia has been reported in these patients. In the adult, cor triatriatum is frequently an isolated finding.

Cor triatriatum dextrum is extremely rare and results from the complete persistence of the right sinus valve of the embryonic heart. The membrane divides the right atrium into a proximal (upper) and a distal (lower) chamber. The upper chamber receives the venous blood from both vena cavae and the lower chamber is in contact with the tricuspid valve and the right atrial appendage.

The natural history of this defect depends on the size of the communicating orifice between the upper and lower atrial chambers. If the communicating orifice is small, the patient is critically ill and may succumb at a young age (usually during infancy) to congestive heart failure and pulmonary edema.^[3] If the connection is larger, patients may present in childhood or young adulthood with a clinical picture similar to that of mitral stenosis. As the malformed membrane calcifies with age, thus further narrowing such opening, decreased cardiac output produces features of pulmonary venous hypertension and right heart failure—including symptoms of dyspnea and orthopnea, easy fatigability, palpitations and shortness of breath, among others.^[3] Cor triatriatum may also be an incidental finding when it is nonobstructive.

Primarily diagnosed with imaging, such as echocardiogram (ultrasound of the heart), CT, and/or MRI.^[3]

Treatment of Cor triatriatum varies among cases and is dependent upon presentation of symptoms—incidental finding of the condition in asymptomatic patients does not typically require immediate medical management, but for those exhibiting dyspnea and pulmonary congestion, surgical intervention is required. The disorder can be treated surgically by removing the membrane dividing the atrium. The surgery, which usually occurs by first excising the diaphragm and then closing the atrial septum, has a reported survival of 90% at five years, with almost all patients becoming asymptomatic post-surgery.

• Cor triatriatum at eMedicine