Borst first coined the term cor triatriatum in 1905,  whereas the diagnostic entity of cor triatriatum sinister (CTS) was first described in 1868 by Church.  It represents a membranous diaphragm dividing the left atrium into 2 chambers; the proximal chamber, located in a posterior-superior position, accepting the pulmonary veins and the distal chamber, inferior-anterior in position, communicating with the mitral valve and the left atrial appendage.  This results in 3 atrial chambers, hence the name triatriatum. A rare similar membranous malformation in the right atrium is referred to as cor triatriatum dexter. This article discusses the surgical nuances of the more common, left-sided CTS.
History of the Procedure
The first successful repair was achieved by Lewis et al in 1956,  which was performed using inflow occlusion with hypothermia. Several small cohorts have been reported in the literature, with an operative mortality rate of up to 25%. A recently published report from Boston Children’s Hospital included 65 patients with cor triatriatum sinister (CTS). The 30-day survival for this cohort was 97%, with only one noncardiac death reported on long-term follow-up. 
Cor triatriatum sinister (CTS) is often observed in infants with a small opening between the common pulmonary venous chamber and the left atrium. These infants usually present with evidence of low cardiac output syndrome and pulmonary venous obstruction, including poor peripheral pulses, pallor, pulmonary edema, pulmonary hypertension, tachypnea, and growth failure. If an opening is present between the proximal left atrial chamber and the right atrium, associated left-to-right shunting and evidence of pulmonary overcirculation are observed.
When obstruction is mild, the patient is usually asymptomatic, and if this is an isolated lesion, disease is not usually suspected. Patients with mild obstruction can present with progressive pulmonary congestion over months to years. Symptoms are determined by the size of the connection between the pulmonary venous chamber and the left atrium.
Cor triatriatum is a rare congenital anomaly with an incidence of up to 0.4% on autopsy.  Fyler reported cor triatriatum in only 25 of 14,000 patients with cardiac disease and in 5 of 2251 infants with heart disease.  This calculates to 1 case per 700 children, 1 case per 450 infants with heart disease, or 0.0045 cases per 1000 live births.
When considering frequency, one must account for the fact that nonobstructive CTS may be more frequent at autopsy than is clinically observed. Pathological demonstration of incomplete fusion of the pulmonary venous contribution to the left atrium causing a membrane between the 2 chambers is somewhat common in a very mild form on autopsy, and is not clinically significant. This may explain the higher autopsy incidence of cor triatriatum compared with the much lower clinical incidence, with just over 250 having been reported in the literature.
The smooth part of the left atrium is formed when the primordial common pulmonary veins incorporate with the developing left atrial chamber. It is thought that the common wall, which is produced as a result of incorporation of the primordial pulmonary veins, fails to regress. This arrest in regression can occur at any point in this process, resulting in a wide spectrum in the size of the connection between the pulmonary vein chamber and the left atrial chamber. The range in the size of this connection accounts for the variety of presentations. The larger the size of the connection, the milder the symptoms, while the smaller the size of this connection, the more severe the symptoms. Cor triatriatum has many different degrees, ranging from a normal left atrium to almost complete obstruction between the chambers. Cases that present clinically are usually at the moderate or severe end of the spectrum; thus, the mild form is rarely clinically seen.
Normally during cardiogenesis, the common pulmonary vein is absorbed into and becomes part of the left atrium. Incomplete absorption, leaving varying degrees of obstructing tissue at the line of connection between the left atrium and the common pulmonary vein, results in the development of CTS. Typically, the foramen ovale and the left atrial appendage communicate with the distal left atrium and this helps to distinguish between CTS and an obstructing supravalvar ring. However, atrial septal defects can communicate between the right atrium and either the proximal or the distal left atrial chamber. Lam and colleague described anatomical classification of cor triatriatum in 1962. 
The physiologic consequences of CTS are a direct function of the size of the orifice between the pulmonary venous chamber and the left atrium. When obstruction is sufficient to raise the pulmonary venous pressure, pulmonary hypertension may reach or exceed systemic levels. In this regard, the clinical picture and the course of patients with CTS are comparable to those for infants with obstructed total anomalous pulmonary venous return. However, other forms of CTS have been noted in which the proximal chamber is connected to the right atrium and to the left atrial chamber.
If the proximal left atrial chamber only connects to the right atrium via a defect in the septum primum, then this is total anomalous pulmonary venous return to the right atrium. However, if the proximal left atrium connects to the distal left atrial chamber and the right atrium, then this is CTS with a proximal connection to the right atrium. However, these patients can present with partial anomalous pulmonary veins. For example, a patient may have CTS with a connection to the right atrium and a separate connection to the left atrium, no atrial septal defect, 3 pulmonary veins to the proximal chamber, and a right upper pulmonary vein to the superior vena cava (SVC) orifice. This is thought to be a partial anomalous pulmonary venous return with CTS.
When obstruction is mild, no symptoms are present, and if this is an isolated lesion, disease is not suspected. With severe obstruction, the patient may have pulmonary edema, pulmonary hypertension, and respiratory symptoms (eg, tachypnea, dyspnea) early in infancy. Cor triatriatum should be suspected whenever a patient has unexplained pulmonary edema.
Infants with classic CTS with a small opening between the common pulmonary venous chamber and the left atrium usually present with evidence of low cardiac output, including pallor, cyanosis, tachypnea, poor peripheral pulses, and growth failure. 
In children and young adults, the classic presentation is signs and symptoms of pulmonary vascular hypertension.  A recent case describes a rare presentation of CTS associated with extensive mediastinal lymphadenopathy as a result of pulmonary venous hypertension causing inflammation. 
There are reports that describe rare presentations of CTS, including heart failure, [12, 13] hemolytic anemia,  and asthma, [15, 16, 17] all consistent with pulmonary congestion. Coexistence of CTS with other congenital heart defects such as tetralogy of Fallot,  partial or total anomalous pulmonary venous return, left superior vena cava, atrial septal or atrioventricular defect,  and transposition of the great arteries  have also been reported.
As discussed previously, the natural history of cor triatriatum is closely linked to the aperture, or opening, between the common pulmonary venous chamber and the portion of the left atrium containing the mitral valve. Classic cor triatriatum, with a restrictive opening in the partition between the proximal common pulmonary venous sinus and the distal left atrium, is an urgent indication for operation.
In most infant patients with symptoms, this opening is critically narrowed, and approximately 75% of patients born with classic cor triatriatum die in infancy if not recognized. As such, symptoms usually develop early, and operation is usually necessary in the first year of life.
Similarly, operation is indicated on an urgent basis in more complex forms of cor triatriatum, when the common pulmonary venous chamber empties into the right atrium and a restrictive opening is present between the common pulmonary venous chamber and the distal left atrium associated with a restrictive patent foramen ovale.
Finally, cor triatriatum can appear later in life, often in the teenage years. These patients present with chronic symptoms and evidence of restricted inflow into the distal left atrium. Over time, this may have caused pulmonary vascular disease and changes similar to chronic mitral stenosis. An operation is usually indicated to halt the chronic changes of elevated left atrial pressure. Published reports have described the presentation of CTS in much older patients with pregnancy,  pulmonary hypertension,  pulmonary artery thrombosis,  rheumatic mitral stenosis,  severe mitral regurgitation,  atrial fibrillation,  and lipomatous hypertrophy of the interatrial septum. 
In classic CTS, the common pulmonary venous chamber is usually somewhat larger than the distal left atrium. The wall partitioning the 2 chambers often appears rather thick and fibromuscular.
The proximal chamber is typically thick-walled, while the distal chamber is usually thin-walled, indicating the difference in pressures seen by the chambers. The anatomic differentiation between CTS and a supramitral ring is based on the position of the wall relative to the left atrial appendage. In CTS, the left atrial appendage is located in the distal chamber, whereas in a supramitral ring, it is located in the proximal chamber.
The pulmonary veins are not typically dilated in CTS, despite the high pressure present in them. The anatomic relationship of the pulmonary veins to the right atrium and superior/inferior vena cava is usually normal. However, as stated above, they do not have to be, and a myriad of combinations of CTS and some form of anomalous pulmonary venous return have been noted.
The fossa ovalis may be located on either side of the partition, joining the right atrium to the proximal chamber or the distal chamber, depending upon its location.
No contraindications to surgical correction of isolated CTS exist, except the general contraindications that exist for all cardiac surgeries on cardiopulmonary bypass.
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