Coarctation of the aorta
Coarctation of the aorta (CoA) is estimated to account for 4–6% of CHDs in newborns, with a reported prevalence of approximately 4 per 10 000 live births28. Congenital coarctation of the aorta increases impedance to the left ventricular outflow, but this condition is usually well tolerated by the fetus because the foramen ovale and ductus arteriosus equalize intracardiac and great arteries pressures and shunts. Nevertheless, it potentially harbors life-threatening risks if coarctation is not suspected and the arterial duct closes spontaneously in postnatal life. This lesion was first described by Morgagni in 1761 and further delineated by Meckel in 1827 with a report on a sudden death of a 35-year-old peasant who showed a ruptured right atrium beside an exceedingly thinned aortic lumen with the diameter of a blade of a straw right beneath the ligamentum aortae during autopsy29-31. Coarctation of the aorta shows a considerable variability in its anatomic, physiologic and clinical characteristics, and chromosomal and extracardiac anomalies are commonly described. Morphologically, CoA encompasses a marked narrowing of the ascending aorta and the aortic arch, typically affecting the isthmic region (between the left subclavian artery and the ductus arteriosus). Although often found to be a discrete anatomic anomaly, coarctation may also consist of a long-segment stenosis associated with tubular hypoplasia of the entire transverse aortic arch (as shown in fig. 1). The embryologic origin is still not well understood, but CoA is suggested to most likely result from a manifestation of an abnormality in the development of the left fourth and sixth aortic arches during embryogenesis. There are two pathophysiological concepts - according to a hemodynamic theory, intracardiac lesions, which diminish the volume of the left ventricular outflow, promote the development of coarctation in the fetus by reducing flow through the aortic isthmus. In contrast, the ductus tissue theory assumes that migration of smooth muscle cells into the aorta leads to subsequent aortic constriction postnatally. The latter concept is largely based on histological findings of differential elastic fiber formation between the aorta and ductal tissue32. Bahado-Singh et al. published on fetal epigenomic dysregulation of genes involved in both heart development and postnatal cardiovascular diseases potentially associated with the pathogenesis of CoA33.
Grayscale ultrasound reveals different degrees of ventricular disproportions and a larger pulmonary artery than the aorta suspicious for CoA34,35. Figure 1 clearly depicts these anatomic changes – an RV-LV discrepancy seen in the 4CV/5CV views and different diameters of the pulmonary artery compared to the cross-section of the aorta displayed in the short-axis view of the great vessels/RVOT. However, most strikingly, the three vessels and trachea view (3VT) shows a hypoplastic transverse aortic arch, confirming the small ascending aorta seen in the 5CV/LVOT views. Hornberger et al. described the ratio of right-to-left ventricular width to be 1.19 in normal fetuses and greater than 1.69 in those with CoA36. However, an increased right-to-left ventricular ratio might also be found in other cardiac conditions such as HLH, DORV, APVS, CDH or Ebstein’s anomaly. The volumetric approach described herein allows for detailed reconstruction of additional planes in order to rule out these differential diagnoses. Color Doppler interrogation may provide additional information in all diagnostic planes, as described by Yeo and Romero37.
There is a general consensus that a normal appearance of the sagittal section of the aortic arch on fetal echocardiography constitutes the major exclusion criterion for CoA. In the case of coarctation, this view might depict a posterior shelf or ridge in the aortic wall in addition to a markedly narrowed aorta (as seen in figure 1). Arya et al. proposed novel fetal echocardiographic morphometric measures for objective antenatal assessment of the aortic arch and its branches and found significant differences in the LCSA, AAo-DAo angle, and Tao-DAo angle between normal fetuses and those with CoA19. Similar findings were published by other groups of researchers who found that a smaller mitral valve diameter, MV to TV ratio, IDD, and isthmus-ductal angle (IDA), carotid-subclavian artery index (CS) or distal arch index (DAI) were associated with the development of CoA after birth38-40. In contrast to HLH, the apex is still formed by the LV, and the myocardial contractility seems to be normal in CoA. However, DeVore et al. described the 4CV view to be more spherical in shape, increased in area and width, and decreased in length during speckle tracking analysis41. A recent metaanalysis stated that adoption of multiparametric diagnostic models integrating different ultrasound signs may assist examiners in identifying fetuses with CoA. Interestingly, a persistent left superior vena cava, a ventricular septal defect, and a bicuspid aortic valve were not associated with an increased risk for this anomaly42. Acknowledging that these parameters might be helpful in the prenatal diagnosis of coarctation, one has to admit that precise and reproducible assessment is strongly operator dependent and inevitably needs different cardiac planes for an exact calculation of the aforementioned objective measures. This might partly explain the considerably high number of both false-positive and false-negative diagnoses and why CoA remains the most commonly missed ductal-dependent lesion to date.
From a functional perspective, recent data on cardiovascular adaptation during neonatal life suggest that, despite biventricular remodeling and cardiac output redistribution, fetuses with CoA have no significant myocardial dysfunction43. Regarding the postnatal prognosis of CoA, the variables associated with intervention comprised smaller AAo and transverse arch size, earlier gestational age at diagnosis, and the additional finding of a higher peak AAo Doppler44. Confirmed cases of CoA must be followed closely over the lifetimes of affected newborns for complications such as recoarctation, aortic aneurysm, persistent hypertension, and changes in any associated cardiac defects45.