Persistence of the right aortic arch with atresia of the left occurs in approximately 1/2200 persons without congenital heart disease. The incidence in certain congenital anomalies such as Tetralogy of Fallot and persistent truncus arteriosus can be as high as 50%. Most persons with right aortic arch are completely asymptomatic and come to medical attention because the right arch is misdiagnosed on chest radiographs as a mediastinal mass. There are two common types of right arch: those with mirror-image branching of the great vessels (Type I) and those with aberrant origin of the left subclavian artery (Type II).
In the Type I right arch, the first vessel arising from the arch is a left brachiocephalic artery, followed in order by the right common carotid artery and the right subclavian artery. This anomaly results from atresia of the embryonic double arch distal to the left subclavian artery. These rarely cause symptoms of any kind, but are almost always associated with congenital heart disease.
In the Type II arch, the order of origination of the great vessels is left common carotid, right common carotid, right subclavian artery, and left sub-
clavian artery. In this case the left subclavian artery arises from the proximal portion of the descending aorta, just below the origin of the ductus arteriosus. Because the ligamentum is almost always left-sided, there is the potential for a vascular ring, although it is uncommon for it to be clinically significant.
With both types of right arch, the arch (by definition) passes to the right side of the trachea. In most cases the descending aorta is also on the right, not crossing over to the diaphragmatic hiatus until just above the diaphragm. Occasionally the arch will cross behind the esophagus at the level of the transverse arch, resulting in the so-called retroesophageal right arch. These patients usually have an aberrant left subclavian artery and may be more likely to complain of dysphagia than those in whom the aorta descends on the right.
The CT appearance of the Type I arch is the exact mirror image of the typical left arch. With Type II right arch the first branch of the aorta, the left common carotid, tends to be smaller than the typical brachiocephalic artery. The aberrant left subclavian artery, arises from the aorta just below the arch and courses posterior to the esophagus before ascending to resume its normal course (Fig. 3).
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