The aortic arch is the portion of the aorta that runs between the ascending and descending aorta, and from which the right brachiocephalic (innominate), left common carotid, and left subclavian arteries originate. Normally, the aortic arch lies obliquely on the left side of the trachea and esophagus, and above the proximal portion of the left main bronchus . Some authors consider aortic arch anomalies as congenital abnormalities of the position and/or branching pattern of the aortic arch [1, 2], and anatomically, the definition of left or right aortic arch refers to which bronchus is crossed by the arch, not to the side of the midline where the aortic root ascends [3, 4].
To understand the development of the aortic arch, the hypothetical double aortic arch model introduced by the cardiac pathologist Jesse E. Edwards in 1948 is very useful . In this model, the ascending aorta divides into two arches, one to the right of the trachea and esophagus, and the other to the left, which converge posteriorly to form the descending aorta. The normal anatomy of the aortic arch (left aortic arch) is derived from regression of the right aortic arch distal to the origin of the right subclavian artery. Most of the aortic arch anomalies results from either abnormal persistence of a part or parts that should have regressed, and/or from abnormal regression of a part or parts that should have persisted .
A right aortic arch (RAA) refers to an aortic arch that is located on the right side of the trachea over the proximal part of the right main bronchus . Its incidence is 1 per 1000 in the general population , and possibly higher in fetuses with a cardiac abnormalities . There are different types of right aortic arch, the primary ones with mirror-image branching patterns and others with aberrant left subclavian or brachiocephalic arteries. The double aortic arch presents a right aortic arch in addition to the normal arch located to the left of the trachea.
In the four-chamber view, the descending aorta is more centrally located anterior to the spine, but visualization of the three-vessel and trachea view in the upper part of the mediastinum is essential for the identification of a right aortic arch . In fetal life, the bronchi cannot be easily seen, so the diagnosis is made by visualizing the transverse part of the aortic arch with respect to the trachea . The position of the aortic arch relative to the trachea can also be evaluated in a coronal plane through the tracheal bifurcation , but this plane may be difficult to obtain. Diagnosis may be easier in prenatal life than after birth, since postnatally the ductus arteriosus closes forming the ligamentum arteriosum, and is difficult to identify without flow on color Doppler.
A right aortic arch with mirror image branching is caused by abnormal regression of the left aortic arch distal to the origin of the left subclavian artery, and a persisting right or left ductus arteriosus. Predominant persistence of one or the other varies according to the literature consulted . A mirror image of the normal arrangement is produced, with the left brachiocephalic artery (dividing into the left subclavian and left common carotid arteries), the right common carotid artery and the right subclavian artery. The ultrasound findings will depend on the laterality of the ductus arteriosus . In cases of a persistent left-sided ductus arteriosus, the aortic arch is located on the right of the trachea forming a U shape and a vascular ring. When the right ductus arteriosus persists (true mirror-image), ultrasound shows what Abuhamad and Chaoui  call the “right V sign”: the aortic arch and the ductus arteriosus converge in a V shape to the right of the trachea, without the formation of a vascular ring. For diagnosis, it is key to look at the position of the trachea, which is located not behind the superior vena cava, but on the other side of the V, to the left of the ductus arteriosus. This sign has been described as early as 11 weeks + 6 days .
Recently, Chiappa et al  have identified up to 7 types of ductus arteriosus in right aortic arch (types 1-7), based on site of origin, alignment to major body planes, and relationship with the trachea. The types that present with a right ductus arteriosus are type 4 (the previously described, right‑sided, mirror‑image V, transverse) and type 5 (right‑sided, s‑shaped, transverse). In type 5, the pulmonary trunk and the first part of the ductus arteriosus run to the right, anteriorly to the trachea, bending posteriorly to join the descending aorta on the right anterior corner of the spine. This view resembles a capital letter “H” or an inverted lowercase “h”, depending on whether the left pulmonary artery is included in the section plane or not. Several studies have described the prenatal diagnosis of right aortic arch with right “H” shaped ductus arteriosus (type 5) [12-14].
Right aortic arch is often associated with congenital heart defects, especially in the subgroup of mirror-image branching with left ductus arteriosus, where an incidence >90% has been reported [6, 15-17]. The most common associated congenital heart disease is tetralogy of Fallot , followed by pulmonary atresia with a ventricular septal defect, a common arterial trunk, and a double-outlet right ventricle. When the ductus is on the right side, there are usually no associated cardiac malformations [12-14, 18], although in type 5 ductus arteriosus, a case of asymptomatic hypoplasia of left pulmonary artery has been described postnatally .
Right aortic arch with mirror-image branching is also associated with the 22q11.2 microdeletion (incidence between 13.3% and 25%), especially in cases affected by conotruncal congenital heart disease or extracardiac malformations [15, 19]. Karyotyping should be recommended when a right aortic arch coexists with structural congenital heart defects. Identifying the thymus and assessing its size in the anterior three-vessel and trachea view can be of great help in ruling out the 22q11.2 microdeletion .
Regarding the differential diagnosis, right aortic arch must be differentiated from isomerisms and situs inversus, in which the aorta is located on the right side throughout its entire trajectory, not only in the upper portion . In complete situs inversus, the right V sign can be confused with normal anatomy if the right and left sidedness of the fetal thorax have been not systematically assessed by the examiner .
The precise diagnosis of the type of right aortic arch is more important than the differential diagnosis with other entities, given its prognostic implications. Right aortic arch with aberrant left subclavian or innominate artery is diagnosed in the presence of the “U-sign”: a loose vascular ring formed by the aortic arch to the right of the trachea, the ductus arteriosus and the pulmonary trunk to the left, and the aberrant left subclavian artery behind the trachea. In the double aortic arch, careful examination of the three-vessel-trachea view allows identification of a transverse aortic arch that bifurcates into the right and left aortic arches, between which are the trachea and esophagus. The left aortic arch may be narrower than the right arch, making it difficult to visualize .
The prognosis of right aortic arch depends on the associated cardiac and extracardiac malformations as well as the presence of chromosomal anomalies. In right aortic arch with right ductus arteriosus, the absence of a vascular ring nullifies the possibility of compressive symptoms after birth [2, 7, 15].
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