Cor Triatriatum Sinister 

In cor triatriatum, the left atrium is subdivided by a thin membrane.  The pulmonary veins enter an accessory chamber that joins the left atrium through a narrow oriface.  There is a proximal chamber that receives the pulmonary veins and a distal (true left atrium) chamber located more anteriorly where it empties into the mitral valve. The membrane that divides the atrium into two parts may vary significantly in size and shape. It may appear similar to a diaphragm or be funnel-shaped, bandlike or entirely intact (imperforate) or contain one or more openings (fenestrations) ranging from small, restrictive-type to large and unrestrictive. In addition, the accessory chamber may communicate with the right atrium through an anomalous channel.  The most common form of cor triatriatum in one in which all four pulmonary veins drain to an accessory atrial chamber, but other more rare variants do exist in which there is partial or total anomalous pulmonary venous connection.  The most widely accepted theory for the cause of cor triatriatum is secondary to an incomplete incorporation of the common pulmonary vein into the posterior aspect of the left atrium.  Cor triatriatum is frequently associated with orther cardiac abnormalities such as a PFO, secundum ASD and left SVC to coronary sinus and less commonly with anomalous pulmonary venous connection. 

References:
Chen Q, Guhathakurta S et al. Cor triatriatum in adults: three new cases and a brief review. Tex Heart Inst J. 1999: 26;206-210.
Moss and Adams. Heart disease in infants, children and adolescents including the fetus and young adult. 7th edition. 761-790.
Lai W, Mertens L, Cohen M, Geva T. Echocardiography in pediatric and congential heart disease from fetus to adult. 2009