Tal Geva Boston Children’s Hospital; Harvard Medical School, Boston, MA, USA Before the advent of surgical treatment for patients with congenital heart disease (CHD), physicians generally regarded these conditions as hopeless. In 1936, Abbott, who worked at McGill University, published the first systematic classification of CHD based on a study 1000 heart specimens [1]. Two years later in 1938, Gross, who worked at Boston Children’s Hospital, performed the first successful cardiovascular operation – ligation of a patent ductus arteriosus in a 7.5‐year‐old girl [2]. This breakthrough procedure was followed by Blalock and Taussig’s landmark establishment of a systemic‐to‐pulmonary artery shunt to alleviate cyanosis in tetralogy of Fallot [3]; resection of aortic coarctation by Gross and Hufnagel and by Crafoord and Nylin; and the first open heart surgery to close an atrial septal defect by Gibbon in 1953 [4]. These and other landmark advances in the treatment of patients with CHD during the 1940s and 1950s inspired interest in the study of CHD morphology and stimulated the development of several taxonomies for its classification. The highly variable spectrum of congenital anomalies of the cardiovascular system presents a challenge for those who care for these patients. A comprehensive classification scheme of CHD based on clear and internally consistent nomenclature is, therefore, essential for diagnosis, management, research, and education. The goals of any CHD taxonomy are: This chapter describes a segment‐by‐segment approach, known as the segmental approach to congenital heart disease, to the classification and nomenclature of congenital anomalies of the heart and great vessels. Inspired by Lev, Van Praagh and his colleagues originally proposed this classification system in the 1960s and early 1970s [5–7] and have since refined it [8,9]. Importantly, notable modifications and alternative taxonomies have been proposed by pathologists, geneticists, embryologists, cardiologists, cardiovascular surgeons, radiologists, and others [9–22]. In particular, many practitioners favor the nomenclature advocated by Anderson and colleagues [23]. However, despite numerous efforts by individuals and professional groups to advocate the use of a single classification system, none has achieved uniform acceptance. A potentially more successful approach, currently being developed, is to acknowledge the major taxonomies and to create a system that cross‐links individual malformations and therapeutic procedures to common codes [24–26]. Although this chapter emphasizes the taxonomy advocated by Van Praagh et al., readers should also familiarize themselves with Anderson’s nomenclature and, most importantly, maintain a clear and consistent method of communication within their institutions. Analysis of CHD is based upon an understanding of the developmental, morphologic, and segmental anatomy of the heart and great vessels. The cardiac segments are the anatomic and embryologic “building blocks” that form the mammalian heart (Figure 3.1). The three main segments are: (i) veins and atria; (ii) ventricles; and (iii) great arteries. There are two connecting segments between the main segments: (i) the atrioventricular (AV) canal; and (ii) the conus or infundibulum. The AV canal consists of the AV valves (the mitral and tricuspid valves in normally formed hearts) and the atrioventricular septum. The infundibulum (or conus) is the connecting segment between the ventricles and the great arteries. In normally formed hearts, the infundibulum consists of a circumferential subpulmonary myocardium shaped like a prolate cone (as viewed externally; hence conus) or a funnel (as viewed internally; hence infundibulum). The normal subpulmonary infundibular myocardium separates the pulmonary and tricuspid valves. The normal subaortic infundibulum consists mostly of a conal septum (the myocardium that separates the left and right ventricular outflow tracts). The posterior aspect of the subaortic infundibular free wall is normally absent, resulting in fibrous continuity between the left and noncoronary aortic valve leaflets and the anterior leaflet of the mitral valve. Figure 3.1 Anatomic segments of the heart. The three main cardiac segments are the atria, ventricles, and great arteries. There are two connecting segments: the atrioventricular (AV) canal (including the AV valves, the interatrial and interventricular components of the AV canal septum), and the conus (or infundibulum). The connecting segments may be viewed as multidirectional joints between the main cardiac segments allowing infinite possibilities of AV and ventriculoarterial alignments. In the segmental approach to congenital heart disease it is essential that each cardiac segment is analyzed separately and independently of adjacent segments. The fundamental principle of segmental analysis of CHD is to analyze each of the aforementioned components of the heart in a sequential step‐by‐step fashion. First, the anatomic pattern (i.e., situs) of the abdominal and thoracic organs is defined and the position of the heart is described. Then, each of the main cardiac segments is examined, described, and assigned a designation based upon its unique morphologic features, independent of its neighboring segments (Figure 3.2). For example, each ventricle is defined according to its intrinsic morphology and not by the entering AV valve or the exiting great artery. Analysis of the main cardiac chambers – veins and atria, ventricles, and great arteries – involves two steps. First, the identity of the chamber or great vessel is determined based upon its morphology and intrinsic myocardial architecture. Second, the situs of the cardiac segment is determined. In the case of the atria, their situs may be solitus (normal), inversus (mirror image of solitus), or ambiguous (indeterminate, or lacking unique anatomic features that identify the atria). Once the three main cardiac segments are characterized according to their unique morphologic features, the connecting segments are evaluated and defined. Finally, a complete set of diagnoses is formulated by combining the five cardiac segments and all associated cardiovascular anomalies as described in the following section. The following 10 steps are taken as part of the segmental analysis of CHD. Before analyzing intracardiac anatomy, the situs of the thoracic and abdominal organs is determined to provide an “anatomic framework” for further analysis (Figure 3.3). Normally, the visceral organs are “lateralized.” In other words, the pattern of anatomic organization of the abdominal organs, tracheobronchial tree, and lungs is asymmetric. In situs solitus (normal arrangement), the spleen, pancreas, stomach, and sigmoid colon are left‐sided, and the liver, cecum, and appendix are right‐sided. The left lung comprises two lobes and the left mainstem bronchus is longer, more horizontal, and hyparterial (courses inferior to the left pulmonary artery) (Figure 3.4). The right lung comprises three lobes and the right mainstem bronchus is shorter and eparterial (courses posterior to the right pulmonary artery). In visceral situs inversus, the spatial organization of the abdominal and thoracic organs is the mirror image of normal. In other words, there is complete left–right reversal of the position and orientation of the organs. It is worth noting that in visceral situs inversus the pattern of anatomic organization is asymmetric, similar to situs solitus but in mirror image. Ciliary disorders are often associated with situs inversus. In situs ambiguous, the spatial position and orientation of the abdominal and thoracic organs are abnormally symmetric and inconsistent. For example, the spleen may be absent, the liver is often midline, both lungs may have two or three lobes, and the bronchi may be similar to each other in length and orientation. Situs ambiguous is typically associated with heterotaxy syndrome, a condition characterized by partial or complete lack of lateralization of the visceral organs leading to an abnormal degree of symmetry, anomalies of the spleen (e.g., polysplenia, asplenia), CHD, and extracardiac anomalies [27–29]. In many patients with heterotaxy syndrome, visceral situs cannot be clearly designated as solitus or inversus, hence the term situs ambiguous is used. However, the anatomic organization of the visceral organs in these patients is often partially lateralized, allowing for determination of a “predominant situs.” For example, when the stomach is right‐sided and the inferior vena cava is left‐sided, the predominant abdominal situs is inversus even though the liver may be midline. Figure 3.2 The 10 steps of the segmental approach to the diagnosis of congenital heart disease. Before analyzing intracardiac anatomy, the situs of the thoracic and abdominal organs and cardiac position within the thorax are determined to provide an “anatomic framework” for further analysis (steps 1 and 2). When describing cardiac anatomy, the three following principles apply. (i) Each cardiac segment must be described in terms of its own unique anatomic features and not according to those of adjacent segments (steps 3–9). For example, the left ventricle is determined according to its internal morphology, particularly its smooth superior septal surface, and not according to the AV valve that connects it with the atria (usually it is the mitral valve but it may be both mitral and tricuspid valves, as in a double‐inlet left ventricle, or it may be a common AV valve or even a tricuspid valve). (ii) For each cardiac segment, both its situs and connections must be described specifically and not inferred from each other. (iii) Associated malformations (step 10) may be described in order of their hemodynamic importance or in an anatomic order (progressing from the venous entry to the arterial exit of the heart). DORV, double‐outlet right ventricle; TGA, transposition of the great arteries. Figure 3.3 Thoracoabdominal situs. (Top) Morphology of the tracheobronchial tree. In situs solitus, the right mainstem bronchus is short and eparterial (its branch for the right upper lobe is over the second branch of the right pulmonary artery) and the left mainstem bronchus is longer and hyparterial (it courses underneath the left pulmonary artery). In situs inversus, there is mirror imaging of the anatomy seen in situs solitus. In situs ambiguous, the bronchi can have a bilaterally right or bilaterally left morphology. Bilaterally hyparterial left bronchial morphology is often seen in patients with heterotaxy syndrome and polysplenia, whereas bilaterally eparterial right bronchial morphology is often seen in patients with heterotaxy syndrome and asplenia. (Middle) Lung lobation. A bilobed left lung and a trilobed right lung are typical in situs solitus. In situs inversus, the right lung is bilobed and the left lung is trilobed. As with the tracheobronchial tree, in situs ambiguous the lungs may be bilaterally bilobed or bilaterally trilobed. (Bottom) In visceral situs solitus the liver is right‐sided and the stomach and spleen are left‐sided. Incomplete lateralization of the abdominal organs with a midline liver and stomach may be seen in patients with heterotaxy syndrome. Splenic anomalies (asplenia, polysplenia, hyposplenia, and a single right‐sided spleen) and complex cardiac anomalies are frequent. In patients with heterotaxy syndrome and visceral situs ambiguous, the disposition of the abdominal situs predicts atrial situs less reliably then bronchial anatomy. Figure 3.4 Relationships between the mainstem bronchi and the pulmonary arteries. In normal anatomy, the right pulmonary artery courses anterior to the right mainstem bronchus (the bronchus is said to be eparterial) and left pulmonary artery courses over the left mainstem bronchus (the bronchus is said to be hyparterial). In the majority of patients with asplenia syndrome both mainstem bronchi are eparterial and in most patients with polysplenia syndrome both mainstem bronchi are hyparterial. The position of the heart within the thorax can be described as levocardia, mesocardia, or dextrocardia (Figure 3.5). This designation is based on the spatial location of the majority of the cardiac mass relative to a sagittal plane that crosses the thoracic midline from the sternum to the spine. In addition, the orientation of the long axis of the heart (the orientation of the axis connecting the AV junction and the ventricular apex) should be described. Although the position of the heart within the thorax and the orientation of the base‐to‐apex axis are often concordant (e.g., levocardia and a leftward pointing apex), occasional exceptions occur (e.g., dextrocardia and a leftward pointing apex). Figure 3.5 Cardiac position within the thorax. In levocardia, the heart is predominantly in the left hemithorax. In dextrocardia, the heart is predominantly in the right hemithorax. In mesocardia, the heart is midline and the apex typically points anteriorly or inferiorly. The orientation of the apex should be explicitly described because the position of the heart within the thorax and the orientation of the apex may not be concordant (e.g., dextrocardia with a leftward pointing apex). In levocardia, the heart is positioned predominantly in the left hemithorax. In dextrocardia, the heart is predominantly in the right hemithorax. In mesocardia, the heart is midline with approximately equal proportions of the cardiac mass on each side of the sternum. The base‐to‐apex axis in mesocardia is usually oriented anteriorly or sometimes inferiorly. The terms primary and secondary dextrocardia are used to distinguish between cardiac malposition related primarily to cardiac anomalies and secondary to noncardiac anomalies. Primary dextrocardia is defined as a condition in which the heart is in the right hemithorax in association with a structural congenital heart defect. In primary dextrocardia the apex usually points to the right. Secondary dextrocardia is a condition in which the heart is either “pushed” or “pulled” toward the right hemithorax due to extracardiac abnormalities. Examples of circumstances where the heart is “pushed” to the right hemithorax include left‐sided tension pneumothorax, left congenital lobar emphysema, and left‐sided diaphragmatic hernia. Conditions where the heart is pulled toward the right hemithorax include hypoplasia or agenesis of the right lung. In secondary dextrocardia the cardiac apex may point to the left or anteriorly. Leftward malposition of the heart occurs in patients with right diaphragmatic hernia or hypoplasia or agenesis of the left lung. In the latter condition, the heart is displaced toward the left superior hemithorax and the base‐to‐apex orientation points toward the left axilla. In addition to malposition of the heart within the thorax, the heart can rarely be partially or completely exteriorized, a condition termed ectopia cordis. The extent of the midline defect allows the heart to be partially or completely outside the thoracic cavity varies. A known association of anomalies, of which ectopia cordis is a component, is termed pentalogy of Cantrell. This group of defects includes: deficiency of the anterior diaphragm; a midline supraumbilical abdominal wall defect; a defect in the diaphragmatic pericardium; congenital cardiac abnormalities; and a defect of the lower sternum [30]. At this stage of the segmental analysis, the three main segments and the two connecting segments are analyzed individually (steps 4–9 in Figure 3.2). The first step in determining atrial situs is to identify the atria according to their morphologic characteristics (Table 3.1, Figure 3.6
CHAPTER 3
Segmental Approach to Congenital Heart Disease
Introduction
Segmental analysis of congenital heart disease
Step‐by‐step segmental analysis
Thoracoabdominal situs
Cardiac position
Segment‐by‐segment analysis of cardiac anatomy
Atrial situs
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