Coarctation of the aorta refers to narrowing of a segment of the aorta along the aortic arch, usually near the origin of the ductus arteriosus (Figure 51-1A and 51-1B). The narrowed segment of the aorta can be of any length and can be preductal, juxtaductal, or postductal in location. The area of narrowing can be caused by a discrete soft tissue shelf, or can be due to hypoplasia of a segment of the arch, leading to complete aortic arch interruption. Previously, coarctation was divided into infantile and adult forms. The infantile form classically occurred in a preductal location, was associated with other cardiac malformations, and was more likely to be associated with neonatal congestive heart failure. The adult form classically occurred in juxtaductal or postductal locations and was associated with a better prognosis. This distinction now seems to be imprecise and is of little relevance, with both forms of coarctation presenting at various ages.

The pathogenesis of coarctation is unclear. It may be due to a developmental defect of the aortic arch, possibly because of failure of connection of the fourth and sixth branchial arches to the descending aorta. Another possible mechanism is the development of abnormal blood flow patterns in utero, leading to decreased aortic-arch flow and increased flow in the pulmonary artery and ductus arteriosus. The consequence of this imbalance in flow may be the development of relative hypoplasia of the aortic arch. A third possible mechanism for the pathogenesis of coarctation is the presence of aberrant ductal tissue in the arch, leading to narrowing of the aortic arch after closure of the ductus (Whitley and Perry, 1990).

Associated cardiac malformations are present in up to 90% of cases of coarctation, including bicuspid aortic valve, aortic stenosis (see Chapter 50) or insufficiency, septal defects (see Chapters 43 and 44), transposition of the great vessels (see Chapter 55), double outlet right ventricle (see Chapter 53), postnatal patent ductus arteriosus, and truncus arteriosus (see Chapter 54) (Romero et al., 1988). In one series of 68 cases of prenatally diagnosed coarctation of the aorta, 43% had a ventricular septal defect, and 15% had a bicuspid aortic valve (Paladini et al., 2004). Additional noncardiac malformations are present in 40% of cases (Paladini et al., 2004). A strong association exists between Turner syndrome and coarctation, with 10% of Turner syndrome patients being affected (see Chapter 134). Overall, 35% of cases of aortic coarctation will have an abnormal karyotype (Paladini et al., 2004).

Coarctation of the aorta is present in approximately 2.5 per 1000, or 4 per 10,000, livebirths (Hoffman and Kaplan, 2002). While there is a significant male preponderance for coarctation in the entire population, there appears to be an equal incidence in both genders at birth. Coarctation accounts for up to 8% of all cases of congenital heart disease.

Prenatal diagnosis of coarctation of the aorta using ultrasound examination is controversial. Some believe that it cannot be reliably diagnosed prenatally because in most cases the hemodynamic complications depend on closure of the ductus. Although it is true that coarctation of the aorta can be entirely a postnatal event, several series of prenatally diagnosed cases of coarctation have been documented (Allan et al., 1988; Sharland et al., 1994). However, in a population-based study of prenatal diagnosis of congenital heart disease from Australia, correct identification of coarctation prenatally was possible in only 26% of cases (Chew et al., 2007). This study ranked coarctation as the second most difficult form of congenital heart disease to accurately diagnose prenatally, after simple transposition of the great arteries (see Chapter 55).

An increased nuchal translucency measurement may be an early marker for aortic coarctation (Moselhi and Thilaganathan, 1996). Pathologic examination of fetuses with increased nuchal translucency thickness has demonstrated a high prevalence of cardiac defects and abnormalities of the great arteries. This has been shown in both trisomic and chromosomally normal fetuses (Hyett et al., 1995, 1996). In fetuses with trisomy 21, the aortic valve and ascending aorta are wider than in normal fetuses, while the aortic isthmus is narrower. In chromosomally normal fetuses, the aorta is narrowed at both the level of the isthmus and immediately above the aortic valve.

Later in gestation, prenatal visualization of a shelf or discrete area of narrowing in the aortic arch is extremely difficult. However, if there is complete hypoplasia or arch interruption, Doppler evaluation may demonstrate a disturbance of flow (Figure 51-2). Prestenotic or poststenotic dilation of the aortic arch may also be present. Growth curves for the transverse aortic arch at various gestational ages have been published, with the suggestion that cases of coarctation are associated with measurements at or below the third percentile (Hornberger et al., 1992). However, no data are available on the reliability or sensitivity of these techniques for the prenatal diagnosis of coarctation.

Relative enlargement of the right ventricle and pulmonary artery, when compared with the left ventricle and aorta, may be more easily detectable signs consistent with coarctation. The combination of ventricular disproportion and great vessel disproportion probably represents a more sensitive marker for coarctation (Rosenthal, 2005). In one series of prenatally diagnosed dilation of the right ventricle and pulmonary artery, 18 of 24 fetuses had coarctation (Allan et al., 1988). Doppler visualization of reduced aortic blood flow was present in most cases of coarctation, but was also present in some normal fetuses. Discrepancy in the size of the ventricles, caused by relative hypoplasia of the left heart structures, has a poor positive predictive value for coarctation, with the false-positive rate being higher after 34 weeks of gestation (Brown et al., 1997). Overall, 62% of cases of coarctation may have ventricular discrepancy recognizable on fetal echocardiograms. The higher false-positive rate at later gestational ages may be due to the normal relative increase in right heart size as gestation advances (Sharland and Allan, 1992). At present, no prenatal sonographic signs—taken either individually or in combination—can reliably distinguish true cases of coarctation from false-positive results (Sharland et al., 1994).