7.2 Coarctation of the Aorta & Interrupted Aortic Arch

Coarctation of the Aorta: Pathophysiology and Prenatal Diagnosis

Coarctation of the aorta is a congenital malformation characterized by a discrete narrowing of the aortic lumen, classically located at the aortic isthmus, which is the segment of the aorta immediately adjacent to the insertion of the ductus arteriosus. In the fetal circulation, only a small fraction of the combined cardiac output (roughly 10%) normally crosses the aortic isthmus, making significant prenatal narrowing difficult to detect definitively. However, identifying fetuses at high risk for postnatal coarctation is crucial, as severe cases may result in rapid hemodynamic collapse once the ductus arteriosus closes after birth.

Echocardiographic Clues and the False-Positive Dilemma

The prenatal diagnosis of coarctation is notoriously challenging and relies heavily on indirect clues rather than direct visualization of the narrowing itself. The fetal heart normally exhibits a mild degree of right ventricular dominance in the third trimester. In fetuses with evolving coarctation, this asymmetry is significantly exaggerated.

Key prenatal sonographic clues include:

• Ventricular Disproportion (RV > LV): The most sensitive, though non-specific, indicator is a noticeable discrepancy in ventricular size in the four-chamber view, with the right ventricle appearing significantly wider and often longer than the left ventricle. Left ventricular function may appear slightly diminished.
• Great Vessel Disproportion (PA > Ao): Similarly, in the outflow tract views, the main pulmonary artery will appear significantly larger than the ascending aorta.
• The Three-Vessel and Trachea View (3VT): This view may reveal a hypoplastic or narrowed transverse aortic arch compared to the ductus arteriosus. The characteristic "V" shape formed by the aortic arch and ductus arteriosus joining at the descending aorta may appear asymmetrical, with the aortic side being much thinner.
• Direct Visualization of a "Shelf": In optimal sagittal views of the aortic arch, a posterior infolding or "shelf" of tissue at the isthmus may occasionally be visualized.
• Doppler Interrogation: While high velocities across the isthmus might be seen, they are not always present. Continuous forward flow throughout diastole in the aortic arch, often termed a "sawtooth" pattern, can indicate an obstruction downstream.

The High False-Positive Rate: It is essential to understand that many fetuses presenting with RV dominance and PA/Ao size discrepancy do not have coarctation postnatally. Mild asymmetries frequently resolve. Therefore, the diagnosis requires serial evaluation and cautious counseling. Any fetus exhibiting these signs should be delivered in a center capable of administering PGE1 and performing postnatal echocardiography.

Interrupted Aortic Arch (IAA)

Interrupted Aortic Arch (IAA) is a more severe and morphologically distinct anomaly characterized by a complete luminal discontinuity between the ascending aorta and the descending aorta. The descending aorta is supplied entirely by the right ventricle via the ductus arteriosus. IAA is heavily dependent on a patent ductus arteriosus for lower body perfusion postnatally and represents a surgical emergency.

Anatomical Classification of IAA

IAA is classified into three types based on the precise anatomical location of the interruption. This classification system, developed by Celoria and Patton, is crucial for both surgical planning and genetic risk assessment.

1. Type A: The interruption occurs distal to the left subclavian artery. This is essentially the most extreme form of coarctation. The arch vessels branch normally (innominate, left common carotid, left subclavian). It accounts for about 30-40% of cases.
2. Type B: The interruption occurs between the left common carotid artery and the left subclavian artery. The left subclavian artery arises directly from the descending aorta (supplied by the ductus arteriosus). This is the most common type (accounting for over 50% of cases) and has a profound genetic association.
3. Type C: The interruption occurs between the innominate artery and the left common carotid artery. This is the rarest form (less than 5% of cases).

Echocardiographic Diagnosis of IAA

Prenatal diagnosis relies on obtaining a clear sagittal view of the aortic arch. Instead of the normal "candy cane" appearance giving rise to three head and neck vessels, the ascending aorta travels straight superiorly, giving rise to one or two branches before terminating abruptly. The descending aorta is seen separately, arising entirely as a continuation of the ductus arteriosus (resembling a "hockey stick").

The three-vessel and trachea view (3VT) is highly informative. Instead of a "V" shape, the view may reveal a straight, large ductus arteriosus joining the descending aorta, with the transverse aortic arch appearing truncated, absent, or unusually positioned. A large subarterial or malalignment ventricular septal defect (VSD) is almost universally present in IAA and is a key associated finding that should prompt a careful search for arch continuity.

Genetic Associations and Clinical Implications

IAA Type B has an exceptionally strong association with 22q11.2 deletion syndrome (also known as DiGeorge syndrome or Velocardiofacial syndrome). Up to 50-80% of fetuses diagnosed with IAA Type B will test positive for this microdeletion. This genetic link is so robust that an amniocentesis for chromosomal microarray analysis is strongly recommended whenever an interrupted arch, particularly Type B, is suspected.

Management of IAA involves immediate administration of Prostaglandin E1 at birth to maintain systemic circulation. Surgical repair is performed in the neonatal period, typically involving a primary anastomosis of the ascending and descending aorta along with closure of the associated VSD. The prognosis relies heavily on the success of the surgical repair and the presence and severity of associated genetic or extracardiac anomalies.