The classic definition of cor triatriatum is based on the recognition that either the left atrium (cor triatriatum sinister) or the right atrium (cor triatriatum dexter) is divided into two chambers by an abnormal fibromuscular membrane. The terms subdivided left or right atrium^1–4 will be used throughout this chapter, highlighting in a more comprehensible fashion the basic anomaly of this uncommon cardiac malformation.

In the commonest left-sided variant of this lesion, the pulmonary veins enter a proximal common pulmonary venous chamber, which is separated by a fibromuscular membrane from a distal chamber containing the mitral valve and left atrial appendage.

Subdivided right atrium is a much rarer anomaly in which a membrane, originating from the crista terminalis, divides the trabecular portion of the right atrium from the venae cavae. This entity should be distinguished⁵ from the more common occurrence of prominent eustachian or thebesian valves.

The typical findings of subdivided left atrium were first described in 1868 by Church,⁶ although the term cor triatriatum was initially applied to this anomaly by Borst in 1905.⁷ Subdivided left atrium was first diagnosed with angiography by Miller in 1964⁸ and with echocardiography by Ostman-Smith in 1984.⁹ Surgical repair was first accomplished in 1956.^10,11 Cor triatriatum dexter was described by Rokitanski in 1875,¹² yet noninvasive diagnosis by means of echocardiography has been increasingly reported only since 1987.¹³ Successful surgical repair of subdivided right atrium was first reported in 1972.^14,15

The morphogenesis of subdivided left atrium remains unclear, and the various hypotheses reported thus far⁵ fail to explain all the anatomic variants of this anomaly. The entrapment theory proposed by Van Praagh and Corsini¹⁶ is consistent with the findings observed in most cases of subdivided left atrium as well as with the hourglass and the tubular types described by Marin-Garcia and colleagues,¹⁷ where no membrane is seen to divide the common pulmonary venous chamber from the mitral valve and the left atrial appendage. The entrapment theory postulates that subdivided left atrium results from the entrapment of the left atrial ostium of the common pulmonary vein by tissue of the right horn of the sinus venosus, leading to failure in the incorporation of the common pulmonary veins into the left atrium during the fifth week of embryonic development. Consequently, these authors use the term common pulmonary vein chamber to describe the proximal chamber, which receives the pulmonary veins. The entrapment theory fails, however, to explain the coexistence of subdivided left atrium with either total or partial anomalous pulmonary venous drainage.⁴

The morphogenesis of subdivided right atrium is less controversial.^3–5 During early cardiac development, the opening of the sinus venosus in the right portion of the common atrium is delineated by two valves, which join superiorly to form the septum spurium. The left valve becomes incorporated in the septum secundum, whereas the superior portion of right valve (which regresses at around 12 weeks of gestation) forms the crista terminalis; the remnants of the inferior portion form the eustachian and thebesian valves. According to Gharagozloo,⁵ the obstructing membrane in subdivided right atrium originates from the crista terminalis and should be distinguished from the more commonly observed prominent eustachian or thebesian valves. In essence, subdivided right atrium is caused by the persistence of an unusually prominent right valve of the sinus venosus.

The most comprehensive classification of the anatomic variants of subdivided left atrium was reported by Krabill and Lucas.¹⁸ In its classic form (Fig. 80-1), the right and left pulmonary veins enter a common pulmonary venous chamber (proximal chamber), which is, in most instances, separated by a fibromuscular diaphragm from a distal chamber where the mitral valve and the left atrial appendage are found. The dividing membrane may show a single orifice ranging in diameter between less than 3 mm to about 1 cm; occasionally, several small defects might be seen. In two unusual variants of subdivided left atrium (the so-called hourglass and tubular types), no membrane is found to divide the left atrium (Fig. 80-2).¹⁶ In these variants, the thicker-walled common pulmonary vein chamber is usually posterior and somewhat superomedial to the thin-walled proximal chamber. Associated anomalies may be present in more than 50 percent of patients with this lesion. In addition to the more frequent finding of partial and total anomalous pulmonary venous connection, associated anomalies include ventricular septal defect (VSD), atrioventricular septal defect (AVSD or AV canal), tetralogy of Fallot, and coarctation of the aorta. A left superior vena cava¹⁹ is found more frequently than in other types of congenital heart defects and may be associated with a partially or totally unroofed coronary sinus.²⁰

The typical finding of subdivided right atrium is the presence of a membrane separating the right atrial appendage and the tricuspid orifice from the venae cavae and the coronary sinus. The term subdivided right atrium should be reserved for those hearts where the dividing membrane originates from the crista terminalis (Fig. 80-3) and, as mentioned above, this entity should be distinguished from those cases of unusual prominence of either the eustachian or thebesian valves, which do not originate from the crista terminalis.^3,5 Yet the former lesion has often been described at necropsy, whereas the latter anatomic variant has been increasingly diagnosed clinically by echocardiography. The most commonly observed cardiac malformations associated with subdivided right atrium include pulmonary valve stenosis or atresia, hypoplastic right ventricle, and tricuspid valve stenosis or atresia.^21,22

Obstruction of pulmonary venous return to the left atrium is the key hemodynamic feature of subdivided left atrium. The severity of obstruction is multifactorial and depends not only on the size of the fenestration in the dividing membrane but also on both the presence and size of an ASD between the chamber of the common pulmonary veins and the right atrium, as well as on associated cardiac lesions. Severe pulmonary venous hypertension, indistinguishable from that observed in obstructed total anomalous pulmonary venous connection, is present when the size of the fenestration is severely restrictive (3 mm or less) and the atrial septum is intact. Conversely, when a large ASD is present between the common pulmonary venous chamber and the right atrium or in presence of either a partial or a total unobstructed pulmonary venous return, left-to-right shunting will be present and pulmonary venous obstruction may not be obvious.

The hemodynamic features of subdivided right atrium are equally based on the severity of obstruction to systemic venous return and on the presence of an ASD. Cyanosis may in fact be frequent due to shunting of inferior caval blood into the left atrium through an ASD. Right-sided congestive heart failure with elevated central venous pressure may be present in cases with intact atrial septum and right ventricular inflow or outflow obstruction.