Division, or partitioning, of one of the atrial chambers is a rare malformation. Severe symptoms, or even death, may ensue if it is untreated. When recognised, and treated surgically, life expectancy should be normal. Either the morphologically right, or the morphologically left, atrium can be divided by fibromuscular partitions. Division of the morphologically left atrium is by far the most common, and the most important, type. When used in isolation, the term cor triatriatum almost always refers to the divided left atrium. 1 For those who prefer classical terminology, it is more accurate to describe either cor triatriatum sinister or cor triatriatum dexter . For those who prefer the vernacular, the lesions can be described simply as divided left or right atriums. Division of one or other atrium was the rarest lesion, bar one, in the files of the Hospital for Sick Children, Toronto, when they were used for the classical textbook emanating from that institution. 2 Despite their rarity, the malformations are sufficiently well circumscribed anatomically to warrant their own chapter in our book.
DIVIDED MORPHOLOGICALLY LEFT ATRIUM
Morphology and Morphogenesis
Church 3 is usually credited as being the first to describe division of the left atrium, albeit that it was Borst 4 who coined the term cor triatriatum . Since then, there have been several classifications, not all of which are restricted to cases that represent a congenitally partitioned left atrium. James, 5 for example, included examples of totally anomalous pulmonary venous connection to the coronary sinus in his grouping, together with aneurysms of the atrial septum, neither of which would now be considered as examples of divided left atrium. Thilenius and his colleagues 6 proposed a much expanded concept, which included rare cases reported in the literature with atresia of the mouth of the coronary sinus and anomalous pulmonary venous connection to a midline additional chamber, which then drained to the morphologically right atrium. They also described a further solitary case, with mitral atresia, in which there was a midline atrial compartment that received neither systemic nor pulmonary veins. As they indicated, it is as well to be aware of these variants. There is little doubt, however, that almost all cases encountered in clinical practice will be of the classical type. It is this anomaly, typically called cor triatriatum sinister , with which we shall be concerned in the remainder of this chapter.
In the typical lesion, an obliquely orientated fibromuscular partition divides the morphologically left atrium into a proximal compartment which is connected to the pulmonary veins, and a distal portion in communication with the atrial appendage and the mitral valvar vestibule ( Fig. 26-1 ). These components of the left atrium have been described in various ways, such as superior and inferior , upper and lower , common pulmonary venous and left atrial , and accessory and left atrial chambers. In this respect, the terms proximal and distal have themselves been used with opposite meanings, according to the fashion in which they are perceived. We agree with Thilenius and his colleagues 6 that it is sensible to name the chambers according to the direction of flow of blood.
Although all the examples falling within the characteristic pattern have the oblique dividing partition, there is still scope for considerable anatomical variability. The significant variations are the size of the communication between the proximal and distal compartments, the site of an interatrial communication, if present, and the connection of the pulmonary veins, although any other associated malformation can co-exist. The communication between the compartments can vary from being non-restrictive, which is rare, to the more usual arrangement in which there is a pinhole meatus ( Fig. 26-2 ). Usually the communication is single, but multiple orifices can be found. The atrial septum is intact in up to half of patients. When present, a septal defect is almost always within the oval fossa. It most frequently communicates with the distal compartment (see Figs. 26-1 through 26-3 ). In the series reported by Thilenius and his colleagues, 6 nonetheless, about one-quarter of their classical cases were found with the oval fossa in communication with the proximal atrial component. Almost always the atrial communication is within the oval fossa, albeit that rare cases have been described where the communication with the distal chamber is an atrioventricular rather than an atrial septal defect. 6 One case of divided left atrium has also been found in the setting of an atrioventricular septal defect with intact septal structures. 7 Just as the morphology of the interatrial communications can vary, so can their size. While the pulmonary veins usually connect to the distal atrial compartment, many types of partially anomalous venous connection are described, and it such associations that, for the most part, account for the exceedingly complex categorisation designed by Thilenius and his colleagues. 6 We prefer to account for these uncommon associated lesions in descriptive fashion. As emphasised, any other lesion can co-exist with a divided left atrium, including such malformations as mitral atresia, 8 and discordant atrioventricular connections. 8 The dividing partition itself is made up of a double layer of myocardium, arguably giving clues as to the morphogenesis of the lesion. 9,10 The right side of the heart is usually considerably hypertrophied when the left atrium is partitioned. In keeping with this, as with other cases of obstructed pulmonary venous return, there are marked changes within the lungs, 11 with varicose dilation of the alveolar capillaries, concentric medial thickening of the pulmonary arterioles with luminal narrowing. There can also be prominent dilation of the pleural lymphatics. 12 Division of the left atrium was conventionally explained on the basis of failure of absorption of the common pulmonary vein into the left atrium. This malincorporation hypothesis was then replaced by the entrapment concept. 10 This concept was based on the belief that the primary pulmonary vein, during its development, was caught in a vice between the left sinus horn and the rest of the systemic venous sinus. The concept accounts well for the classical anomaly. It less readily explains the variant in which the atrial septal defect, unequivocally at the oval fossa, communicates with the proximal chamber. It is possible, however, to combine concepts of malseptation with the entrapment hypothesis to account for these variants. As Thilenius and his colleagues rightly commented, 6 all these theories are of necessity speculative.
Incidence and Aetiology
As already discussed, division of the left atrium represented less than 0.1% of the case load in the files of the Hospital for Sick Children, Toronto. 2 Only four examples were found in nearly 4000 catheterisations performed at the Royal Brompton Hospital from 1970 through 1982. 13 Males are affected more frequently than females. 6,14 There has been no observed aetiology for the lesion, although it is suggested 15 that the partition may be induced during development by persistence of the left superior caval vein. Although superficially attractive, it is difficult to reconcile this hypothesis with the fact that so few patients with persistent left caval veins also have a divided left atrium.
Presentation and Clinical Features
The age and mode of presentation relate to the tightness of the communication between the proximal and distal chambers. Presentation with symptoms usually implies a small communication, but symptoms will be amplified in the presence of a left to right shunt. Usually, patients then present in infancy, or early childhood, with dyspnoea and frequent respiratory infections, although presentation with congestive heart failure, unexplained pulmonary hypertension, and haemoptysis in adult life is well-described. The cases presenting earlier do so with tachypnoea with or without cyanosis, as in totally anomalous pulmonary venous connection. When cyanosis is present, this usually is the consequence of a right-to-left shunt from the right atrium through an oval foramen or atrial septal defect to the distal compartment of the divided left atrium. Partially anomalous pulmonary venous connection, when present, removes the chance of cyanosis, and may alleviate the obstructive symptoms. 16
The clinical signs are dominated by evidence of pulmonary venous congestion and pulmonary hypertension. The child will be pale and sweating, with tachycardia, and extreme breathlessness on feeding, but usually will remain fully saturated. There is a right ventricular heave and, on auscultation, the pulmonary component of the second sound is almost always accentuated. No murmurs may be heard, although an apical diastolic murmur may simulate mitral stenosis. A soft, blowing systolic murmur may represent secondary tricuspid regurgitation in severe disease. Occasionally the apical murmur is continuous, or the early diastolic murmur of pulmonary incompetence may be heard secondary to pulmonary hypertension. The signs of congestive heart failure are found when this supervenes, including crepitations in the lungs and a palpable liver.
Investigations
The chest radiograph usually reveals cardiomegaly and shows the presence of pulmonary venous obstruction. The so-called staghorn, or butterfly wing, sign is seen because of prominent venous engorgement of the upper pulmonary veins. Pulmonary arterial hypertension is reflected by a prominent pulmonary knob.
Right ventricular hypertrophy is almost invariably present on the electrocardiogram. The frontal mean QRS axis is usually between +120 and +140 degrees. Apart from broad P waves, which are sometimes present as a consequence of right atrial hypertrophy, the rhythm and remainder of the electrocardiographic pattern are usually within normal limits.
Cross sectional echocardiography ( Fig. 26-3 ) is the definitive investigation, permitting direct visualisation of the obstructive partition, and showing the site of interatrial defects, if present. The technique should also permit the recognition of any associated malformations. Doppler studies will then help clarify the situation. The position and size of the communication between the distal and proximal chambers are often best delineated by colour flow mapping ( Fig. 26-4 ), while spectral Doppler studies will define the pressure difference between the two ( Fig. 26-5 ).
