Complex Conotruncal Anomalies

Conotruncal Development

Imaging of patients with conotruncal defects is aided by an understanding of cardiac development. The heart starts to form in the third week of gestation and is more or less fully formed by 8 weeks. Mesodermal precardiac cells migrate to form the cardiac crescents (i.e., primary heart fields) in anterior lateral plate mesoderm, and they are then brought together to form a primary linear heart tube by ventral closure of the embryo. Cells of the second heart field continue to proliferate outside the heart and are added to the heart tube over the course of embryogenesis, contributing to the atria, the right ventricle (RV), and the outflow tract.

Cardiac neural crest cells migrate into the developing heart in the fifth to sixth weeks and are essential for septation of the outflow, formation of the semilunar valves, and patterning of the aortic arches. Once formed, the heart tube grows and elongates by the addition of cells from the second heart field. The ends of the heart tube are relatively fixed by the pericardial sac so that as it elongates, it must bend or loop. In most hearts, the loop falls to the right ( d -loop). Further elongation pushes the midportion of the tube (i.e., future ventricles) inferior or caudal to the inflow, resulting in the normal relationship between the atria and ventricles. Further growth pushes the outflow medially and is associated with outflow rotation; both processes are essential for normal alignment of the outflow. The proximal part of the outflow is incorporated into the RV, shortening the outflow in association with further rotation.

While this remodeling is occurring, the outflow is undergoing septation under the influence of cardiac neural crest cells. Septation proceeds from distal to proximal, culminating in formation and muscularization of the infundibular or muscular outflow septum, which inserts onto the superior endocardial cushion at the rightward rim of the outflow foramen, walling the aorta into the left ventricle (LV) by means of the outflow foramen and the pulmonary artery (PA) directly into the RV. Failure of any of these processes, including outflow elongation, rotation, shortening, and septation, results in a conotruncal anomaly. ^,

Nomenclature for Congenital Heart Defects

Congenital heart defects are complex anomalies, and a segmental approach is used for their analysis and description. The heart is composed of several parts, or segments. In the segmental approach, the cardiac anatomy is assessed first by dividing the heart into three distinct segments, based on 10 embryologic regions, which are analyzed separately before formulating a comprehensive diagnosis. The principal segments are the atria, the ventricles, and the great arteries, which are joined together by the atrioventricular (AV) canal and the conus or infundibulum.

Segmental classification of congenital heart disease (CHD) uses specific set notation to describe cardiac anatomy. The notation system, which is a series of three letters separated by commas and enclosed within curly brackets (e.g., {S,D,L}), is used to describe the visceroatrial situs, the orientation of the ventricular loop, and the positions and relationships of the great vessels. For example, {S,D,S} describes the normal anatomic configuration, in which the right atrium (RA) and largest hepatic lobe are on the patient’s right side and the left atrium, stomach, and spleen are on the left side (situs s olitus [S]); the ventricular loop is curved rightward (dextro- or d -loop [D]); and the aortic valve is posterior to and to the right of the pulmonary valve (situs s olitus [S]).

In practice, the situs, or organization, of the three main segments is determined first. For example, in the normal heart, the RV is right sided with right-hand chirality and is organized inflow-to-outflow from right to left; the LV is left sided with left-hand chirality and is organized inflow-to-outflow from left to right. Second, the segmental alignments are determined (i.e., what drains into what). For example, in the normal heart, the RA is aligned with (drains into) the RV and the LV with the aorta. Third, the segmental connections (i.e., the ways in which adjacent segments are physically connected to each other) are described. For example, in the normal heart, the PA is connected to the RV by a complete muscular conus or infundibulum, whereas the aorta is connected to the LV by aortic-mitral fibrous continuity (i.e., without a complete conus).

Alignment and connection are different concepts, and both are important, especially in complex defects. Fig. 44.1 is a diagrammatic summary of the types of situs, alignment, and connection seen in a variety of congenital heart defects.

Genetics Considerations

Conotruncal defects are associated with several chromosomal abnormalities, most notably a deletion at chromosome 22q11 (i.e., DiGeorge syndrome). Echocardiographic clues to this association in patients with a conotruncal defect include an associated right aortic arch or aberrant subclavian artery and the absence of thymic tissue in infants.

Many adults living with conotruncal defects may not have undergone testing for DiGeorge syndrome. This condition is important to recognize because a variety of psychiatric disorders and disabilities in cognitive function may be present and untreated. Comparative genomic hybridization was used to match DNA copy variations in 60 patients with conotruncal defects with the data from genomic databases. The study revealed that 38% of subjects had some genetic imbalance. These results emphasize the growing importance of genome-wide assays for patients with CHD.

Aortopathy in Conotruncal Anomalies

It should not be surprising that aortopathy is a common finding in patients with conotruncal anomalies because the arterial walls are derived from cardiac neural crest and second heart field cells, either or both of which can be abnormal in these defects. For example, marked histologic abnormalities have been documented in the aortic root and ascending aorta that are present from infancy in patients with TOF. Aortic dilation is common in adults with repaired TOF, and risk factors include older age, male gender, right aortic arch, pulmonary atresia, prior aortopulmonary shunt, and 22q11 deletion. ^, Up to 25% of adults with repaired TOF have an aortic root diameter larger than 4 cm but only 6.6% have indexed aortic values above the expected limits. An aortic root dimension of 5.5 cm has been suggested as the threshold for consideration of aortic repair, ^, but there is no universally accepted size that triggers the decision for surgery. Only four cases of aortic dissection have been reported in the literature on TOF, suggesting that the risk of dissection is low.

Similarly, 50% of children with d -loop TGA have aortic root dilation 10 years after an arterial switch operation; however, this dilation does not appear to be progressive. In a single-center retrospective review of 76 patients with TA, the truncal root was dilated in all but three patients, with a mean truncal root Z score of 5.1 ± 2.3. Despite the high prevalence of dilated ascending aorta in patients with conotruncal anomalies, aortic dissection is rare.

Echocardiography laboratories have various imaging protocols for aortic measurements that must be taken into account when comparing serial studies. The current multimodality imaging guidelines for the thoracic aorta in adults advocate obtaining aortic root measurements at end-diastole with a leading edge–to–leading edge technique, whereas imaging guidelines for pediatric echocardiograms, which are often used in echocardiography laboratories specializing in CHD, suggest obtaining aortic root measurements in mid-systole with an inner edge–to–inner edge technique.