Introduction
Division, or partitioning, of one of the atrial chambers is a rare malformation. Severe symptoms, or even death, may result if this condition is untreated. When recognized and treated surgically, life expectancy should be normal. Either the morphologically right or the morphologically left atrium can be divided by fibromuscular partitions. Clinically significant division of the morphologically left atrium is by far the most common type, and the most important. Frequently described as “cor triatriatum,” hearts having such lesions do not, in fact, possess three atrial chambers. It is therefore simpler to describe divided left or right atriums. Despite their rarity, the malformations are sufficiently well circumscribed anatomically to warrant their own chapter.
Divided Morphologically Left Atrium
Morphology and Morphogenesis
Church is usually credited as being the first to describe division of the left atrium, although Borst coined the term “cor triatriatum.” Subsequent suggested classifications have not all been restricted to cases representing congenital partitioning of the left atrium. One system, for example, included examples of totally anomalous pulmonary venous connection to the coronary sinus, together with aneurysms of the atrial septum. The system proposed by Thilenius and colleagues, which represents the largest review, included rare cases reported with atresia of the mouth of the coronary sinus, along with examples of anomalous pulmonary venous connection to a midline additional chamber, which then drained to the morphologically right atrium. Almost all cases encountered in clinical practice will be of the “classic” type, with intracavitary division of the left atrium. This anomaly, also called “cor triatriatum sinister,” is the focus of this chapter.
In the typical lesion, an obliquely orientated fibromuscular partition divides the morphologically left atrium into a compartment connected to the pulmonary veins, and a second component in communication with the atrial appendage and the mitral valvar vestibule ( Fig. 30.1 ). The components have been described in various ways, with the terms “proximal” and “distal” themselves being used with opposite meanings, according to the fashion in which they are perceived. Such problems can be circumvented simply by describing the pulmonary venous and the vestibular components.
Even if the characteristic feature is the obliquity of the dividing partition, there is still considerable anatomic variability. The significant variations are the size of the communication between the pulmonary venous and vestibular compartments, and the site of an interatrial communication, if present. Other malformations can coexist. It is rare for the communication between the compartments to be nonrestrictive by the time it is discovered. In most examples gaining clinical attention, the area of the communication is significantly smaller than the area of the mitral valve, and it is sometimes barely more than a pinhole ( Fig. 30.2 ). The communication is usually single, but multiple connections are sometimes seen. The atrial septum is intact in up to half of patients. When present, a septal defect is almost always within the oval fossa, communicating most frequently with the pulmonary venous compartment (see Fig. 30.2 ). On occasion, the oval fossa can be in communication with the vestibular component. In rare cases, the communication with the vestibular chamber can be an atrioventricular rather than an atrial septal defect. Although some of the pulmonary veins usually connect to the divided atrium, justifying description of the pulmonary venous compartment, various partially anomalous venous connections have been described. The anomalous pulmonary venous connections, if they exist, are best accounted for in descriptive fashion along with other rare associated malformations, such as mitral atresia or discordant atrioventricular connections.
The characteristic dividing partition is a double layer of myocardium, which has been interpreted to provide a clue to the morphogenesis of the lesion. The right side of the heart is usually considerably hypertrophied because of associated pulmonary hypertension. As with other cases of obstructed pulmonary venous return, there are also marked changes within the lungs and the pleural lymphatics. In terms of morphogenesis, the lesion was initially explained on the basis of failure of absorption of the common pulmonary vein into the left atrium. This “malincorporation” hypothesis was replaced by the “entrapment” concept, which accounts well for the classic anomaly but less readily explains the variant in which the oval fossa is in communication with the vestibular chamber. At present, both theories remain speculative.
Incidence and Etiology
Only four examples of divided left atrium were found in nearly 4000 catheterizations performed in patients with congenitally malformed hearts at the Royal Brompton Hospital from 1970 through 1982. Males are affected more frequently than females. It has been suggested that the partition could be induced during development by persistence of the left superior caval vein, but this does not fit with the fact that so few patients with persistent left caval veins also have divided left atriums.
Presentation and Clinical Features
The age and mode of presentation relate to the size of the communication between the pulmonary venous and vestibular chambers. Presentation with symptoms usually implies a small communication, but symptoms will be amplified in the presence of a left-to-right shunt, for example a patent arterial duct. Patients usually present in infancy or early childhood with dyspnea and frequent respiratory infections, although presentation in adult life with congestive heart failure, unexplained pulmonary hypertension, and hemoptysis is well described. When present, cyanosis is usually the consequence of a right-to-left shunt from the right atrium through an oval foramen or atrial septal defect to the vestibular compartment of the divided left atrium. The clinical signs are dominated by evidence of pulmonary venous congestion and pulmonary hypertension. The child with severe obstruction will be pale and sweating, with tachycardia and extreme breathlessness on feeding, but usually will remain fully saturated. There may be 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 reflect secondary tricuspid regurgitation. 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 crackles in the lungs and an enlarged liver.
Investigations
The chest radiograph usually reveals cardiomegaly and shows the presence of pulmonary venous congestion. Pulmonary arterial hypertension is reflected by a prominent pulmonary artery in older patients.
Right ventricular hypertrophy is almost invariably present on the electrocardiogram. The frontal mean QRS axis is usually between +120 and +140 degrees. There may be evidence of left or right atrial hypertrophy and rarely right ventricular hypertrophy if there is significant pulmonary hypertension.
Cross-sectional echocardiography ( Fig. 30.3 ) is the definitive investigation, permitting direct visualization 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 help clarify the situation. The position and size of the communication between the compartments of the divided atrium are often best delineated by color flow mapping ( Fig. 30.4 ), while spectral Doppler studies define the pressure difference between the two ( Fig. 30.5 ). If there is any doubt regarding the disposition of the pulmonary veins or other associated abnormalities, cardiac MRI may help refine the diagnosis. Invasive investigation is now rarely performed for diagnosis, but assessment of pulmonary artery pressure and resistance may be indicated in adults when there is concern for possible irreversible pulmonary hypertension, although this occurrence must be exceptionally rare.
