Cor triatriatum, pulmonary vein stenosis, and atresia of the common pulmonary vein are very rare but related congenital lesions. The original Latin terminology (which we are trying to update) referred to cor triatriatum sinister and cor triatriatum dexter. Cor triatriatum refers to a “heart with three atriums,” sinister referring to the left atrium and dexter to the right atrium. Of the two lesions, cor triatriatum sinister is so much more common than cor triatriatum dexter that common use of the phrase “cor triatriatum” refers to the left (sinister) version. Moreover, cor triatriatum dexter is unrelated developmentally and clinically to cor triatriatum sinister. A more appropriate (English) name for cor triatriatum may be “divided atrium” [1]. Cor triatriatum sinister (divided left atrium) occurs when the left atrium is subdivided by a fibromuscular membrane into two separate chambers (Figure 36.1). The pulmonary veins enter a posterosuperior chamber. The fibromuscular membrane separates this chamber from an anteroinferior chamber, which communicates with the mitral valve and left atrial appendage. There is often a communication between either the proximal or distal chamber with the right atrium. There is nearly always a fenestration in the membrane that allows communication between the two atrial chambers (Figure 36.2). Surgical excision of the membrane is curative in most cases. The embryologic origin of these malformations is related to abnormal incorporation of the common pulmonary vein into the left atrium during cardiac development. In this regard, these lesions have similar embryologic development to total anomalous pulmonary venous connection (TAPVC). Their pathophysiology is also similar to TAPVC in that they can present with varying degrees of pulmonary venous obstruction or diversion of pulmonary venous return to the left atrium. Because of these similarities, the clinical presentations of these lesions may mimic each other and may mimic TAPVC. Because cor triatriatum dexter (divided right atrium) is unrelated developmentally and clinically, it will be considered separately at the end of this chapter.

Embryology of the Pulmonary Venous System

The origins of cor triatriatum sinister (divided left atrium), atresia of the common pulmonary vein, and pulmonary venous stenosis can best be understood within the context of the morphogenesis of the pulmonary venous system. During the sixth to eighth weeks of development, between the stages known as Carnegie 12 and 23, the primordial lung buds are formed subsequent to branching of the tracheal rudiment from the esophagus. As they form, they share the developing splanchnic plexus with other foregut derivatives, with drainage into the systemic circulation through the paired cardinal and umbilico‐vitelline veins. During these initial stages, there is no direct connection between the developing plexuses within the lung buds and the cavity of the atrial component of the developing heart tube. The atrial component of the developing heart retains its connections with the pharyngeal mesenchyme through the so‐called dorsal mesocardium. At this early stage, the reflections of the walls of the atrial component in the region of the mesocardium surround an area known as the pulmonary pit (Figure 36.3).

During the seventh week of gestation, at Carnegie stage 16, there is canalization of a midline strand within the pharyngeal mesenchyme of the dorsal mesocardium. This produces the common pulmonary vein, which establishes the necessary connections between the right and left pulmonary veins and the cavity of the left atrium. The initial communication with the left atrium is located inferiorly, adjacent to the developing atrioventricular canal (Figure 36.4).

With ongoing development, the right and left pulmonary veins are “cannibalized” by the developing left atrium, at the same time being incorporated into the roof of the chamber. It is only with incorporation of the veins to form the chamber that it becomes possible to recognize the superior interatrial fold that is often incorrectly described as the “septum secundum.” This can be seen at the end of the eighth week of development during cardiac development.

The definitive left atrium is then made up of the body of the initial atrial component of the heart tube, the vestibular myocardium of the atrioventricular canal, and the pulmonary venous sinus, with the pulmonary component forming the dome of the atrial chamber (Figure 36.5). It then requires further “cannibalization” of the pulmonary veins to produce the situation in which four veins open to the corners of the atrial roof, which is not complete until the end of the twelfth week of human development.

Pathologic Anatomy

The morphology of the pulmonary venous abnormalities is readily related to abnormalities in development and incorporation of the common pulmonary vein. Timing is important, as connections to the pathways for systemic venous drainage are still present if the defect occurs early. If the common pulmonary vein becomes atretic, or fails to canalize in the early stages, a connection is retained to the splanchnic plexus and the cardinal or umbilico‐vitelline venous systems. This results in various patterns of TAPVC. Atresia of only the left or right pulmonary veins leads to partial anomalous pulmonary venous return. If atresia of the common pulmonary vein occurs late, however, subsequent to the involution of the connections to the systemic venous system, then the vein itself remains atretic, with the right and pulmonary veins draining into a blind cul‐de‐sac that has no connection to the left atrium or systemic venous drainage.

It was originally suggested that failure of incorporation of the right and left pulmonary veins could result in division of the left atrium. Careful analysis of the shelf dividing the atrium into its pulmonary and vestibular components, however, suggested this to be a simplistic approach to the lesion. Instead, it was proposed that the lesion reflected “entrapment” of the pulmonary venous component. Others suggested that the lesion was the consequence of persistence of the left superior caval vein, but this can be discounted simply because the majority of cases do not show this venous malformation. In truth, the origin of the dividing shelf remains unknown. The very fact that the left atrium can rarely be divided in association with TAPVC gives lie to the notion that the lesion can be explained on the basis of inappropriate incorporation of the developing pulmonary veins. Pulmonary vein atresia or stenosis, in contrast, is readily explained on the basis of our current knowledge of development.

Cor Triatriatum (Divided Left Atrium)

The initial description of division of the left atrium is usually ascribed to Church, in 1868, but Andral had described a heart with “3 auricles” in 1829 [2, 3]. The term “cor triatriatum” was subsequently used to describe such lesions. In the current classification and anatomic nomenclature, hearts cannot, properly speaking, have three atria because the cardinal feature of an atrial chamber is its appendage. Such appendages can be of only two types (right and left), and even hearts with the misnomer “cor triatriatum” have only two atria. The defect is, rather, the consequence of division of the chamber into its pulmonary and vestibular components by a fibrous shelf. Divided left atrium is rare, comprising less than 0.1% of all congenital cardiac malformations. In a 50‐year period at the Mayo Clinic only 25 patients were operated on for cor triatriatum [4]. Males and females are equally affected. Initial surgical repair was accomplished separately in 1956 by two pioneers of early cardiac surgery, Drs. Arthur Vineberg and John Lewis [5, 6]. Additional intracardiac malformations are found in up to four‐fifths of patients, notably patency of the oval foramen [7–10]. The incidence of coexisting lesions, however, tends to be higher in series encountered in the autopsy as opposed to the operating room [11].

Anatomy

The hallmark of cor triatriatum (divided left atrium) is the presence of an oblique fibromuscular shelf that divides the left atrium into pulmonary and vestibular components. Histologically, this shelf is composed of a rim of smooth and cardiac muscle cells in communication with the atrium and a central component that is predominantly composed of dense fibrous tissue [12]. The oval fossa can then communicate with either component of the divided atrium (Figure 36.1).

It is the size of the fenestration in the shelf that determines whether there is obstruction to pulmonary venous flow. In some cases the shelf can be imperforate, and the pulmonary chamber then drains directly, or indirectly, into the right atrium. Several complex schemes of classification have been proposed over time, but they are all unduly complicated. The key features are the site of drainage of the oval fossa, and the size of the hole between the pulmonary and vestibular components [13]. Examples of variants of cor triatriatum are shown in Figure 36.6 [14].

Pathophysiology

The pathophysiology is determined by the size of the fenestration in the shelf between the pulmonary chamber and the remainder of the left atrium, by the size of communication, if present, with the right atrium, and by the effects of associated lesions. In the usual pattern, in which all four veins drain into the pulmonary chamber, which in turn drains into the remainder of the left atrium via the obstructing fenestration, pulmonary venous obstruction with pulmonary hypertension will be present. A large fenestration may be occasionally nonobstructive. The presence of an interatrial communication between the pulmonary chamber and the right atrium can lead to significant left‐to‐right shunting.

Clinical Presentation

The severity of symptoms and the age of presentation are dependent on the size of the communication between the pulmonary venous chamber and either the remainder of the left atrium or the right atrium. If the communication between the pulmonary and vestibular chambers is less than 3 mm, and no other communications are present, the age of presentation will usually be in the first year of life. The clinical picture will then be of pulmonary venous obstruction, with low cardiac output and severe pulmonary hypertension. Adult presentation with severe obstruction is rare, but has been reported [15]. Tachypnea, failure to thrive, and outright respiratory failure can be presenting symptoms when pulmonary venous obstruction is present. Signs of pulmonary hypertension include a long pulmonary component of the second heart sound, right ventricular heave, and pulmonary systolic ejection click. The electrocardiogram shows evidence of right ventricular strain. The chest radiograph shows cardiac enlargement and a “ground glass” appearance, characteristic of pulmonary venous obstruction. A nonobstructive communication between the pulmonary and vestibular chambers may be clinically silent and may not become apparent until adulthood [16]. Communication to the right atrium can result in right atrial and ventricular dilation. The clinical presentation can then mimic totally anomalous pulmonary venous connection.

Adults patients have unique and different clinical presentations that warrant separate consideration. Incidental diagnosis in children is quite rare; however, as many as 18% of reported adult cases in a recent systematic review (n=171 cases) were asymptomatic at the time of diagnosis [17]

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