Anatomic Variations

A number of congenital malformations affect the thoracic aorta. Many of these affect the aortic arch and its branches.

Double Aortic Arch

Double aortic arch is one of the two most common forms of vascular ring. It is a class of congenital anomaly of the aortic arch system in which the trachea and esophagus are completely encircled by connected segments of the aortic arch and its branches. Although various forms of double aortic arch exist, the common defining feature is that both left and right aortic arches are present.

The development of double aortic arch begins early in the course of embryonic morphogenesis. Six pairs of pharyngeal arch arteries develop in conjunction with the branchial pouches. The first through sixth arches appear in a more or less sequential fashion, with left-to-right symmetry, and constitute the primitive vascular supply to the bra-chiocephalic structures, running from the aortic sac to the paired dorsal aortas. As normal cardiovascular morphogenesis proceeds, patterned regression and persistence of the various arches and right-sided dorsal aorta occur, ultimately resulting in the mature configuration of the thoracic aorta and its branches. The third, fourth, and sixth arches, along with the seventh intersegmental arteries and the left dorsal aorta, are the primary contributors to the normal aortic arch and its major thoracic branches [8].

The segments of the bilateral aortic arch system that normally regress include the distal portion of the sixth arch and the right-sided dorsal aorta. Normally, the left fourth arch becomes the aortic arch, the right fourth arch contributes to the innominate artery, the distal left sixth arch becomes the ductus arteriosus, the proximal sixth arches bilaterally contribute to the proximal branch pulmonary arteries, the left dorsal aorta becomes the descending thoracic aorta, and the dorsal intersegmental arteries bilaterally become the subclavian arteries.

Vascular rings are formed when this process of regression and persistence does not occur normally, and the resulting vascular anatomy completely encircles the trachea and esophagus. Other forms of aortic arch anomaly may occur in which a vascular ring is not present, such as right aortic arch and aberrant left subclavian artery. A double aortic arch is formed when both fourth arches and both dorsal aortas remain present.Various forms of double aortic arch exist. Both arches may be patent, or an atretic (but persistent) segment may exist at one of several locations in either arch. When both arches are patent, the right or left arch may be larger, or they may be similar in size. A cervical arch on either side, variable laterality of the descending thoracic aorta, coarctation of the major arch, and/or discontinuity of the central pulmonary arteries may be present. In more than 75% of patients with double aortic arch, the right arch is dominant [9].

Double aortic arch usually occurs without associated cardiovascular anomalies. Ventricular septal defect and tetralogy of Fallot are probably the most common associated defects, although truncus arteriosus, transposition of the great arteries, pulmonary atresia, and complex univentric-ular defects sometimes occur in conjunction with a double arch [10].

Right Aortic Arch

A right aortic arch is formed when the right dorsal aorta remains patent and either the left fourth arch or the left dorsal aorta regress abnormally. Two primary forms of vascular ring with a right aortic arch exist, while two other forms are much less common.

In the most frequent form of vascular ring with a right aortic arch, an aberrant origin of the left subclavian artery from a retroesophageal divertic-ulum (diverticulum of Kommerell) is present, which originates as the last branch of the aortic arch (distal to the right subclavian artery). The ring is completed by a left-sided ductus arteriosus (or its remnant ligamentum arteriosum) passing from the aberrant left subclavian artery to the proximal left pulmonary artery. The retroesophageal diver-ticulum is distinguished from the aberrant left sub-clavian artery by its larger caliber. Although the course of the descending thoracic aorta varies, it typically crosses gradually to the left of the vertebral column to pass through the diaphragm in the usual location of the aortic hiatus [11].

In the second major type of vascular ring with a right aortic arch, the brachiocephalic vessels originate from the arch in mirror-image fashion with the left innominate artery the first branch followed by the right common carotid and subclavian arteries. A left-sided ductus arteriosus or ligamen-

tum arteriosum passes between the descending aorta and the proximal left pulmonary artery. In contrast to right aortic arch with aberrant left sub-clavian artery from a retroesophageal diverticu-lum, the descending aorta usually crosses to the left side of midline proximally in its course, although in rare cases, it remains to the right of mid-line until reaching the lower portion of the thorax [11,12].

Anomalies Involving Aortic Branches

Anomalies concerning the branches of the aortic arch may involve the position and/or the number of primary branches. The branches, instead of arising from the highest part of the arch, may develop from the start of the arch or the upper part of the ascending aorta. Alternatively, the distance between the branches at their origins may be increased or diminished. The most frequent change in this respect is an anomalous proximity of the left carotid toward the innominate artery.

The number of primary branches may be reduced to one, or more commonly two; the left carotid arising from the innominate artery, or (more rarely) the carotid and subclavian arteries of the left side arising from a left innominate artery. In other cases the number of branches may be increased to four, for example, with the right carotid and subclavian arteries arising directly from the aorta while the innominate artery is absent. In most of these latter cases the right subcla-vian artery is found to arise from the left end of the arch; in other cases it is the second or third branch rather than the first. Another common form in which there are four primary branches is that in which the left vertebral artery arises from the arch of the aorta between the left carotid and subclavian arteries.

Lastly, the number of trunks from the arch may be increased to five; in this instance, the external and internal carotids arise separately from the arch, the common carotid being absent on one or both sides. In a few cases six branches have been found. This condition sees both vertebral arteries arise from the arch [13].

When the aorta arches over to the right side, the three branches have an arrangement which is the reverse of that which is usual; the innominate artery is a left one, and the right carotid and sub-clavian arteries arise separately. In other cases in which the aorta takes its usual course, the two carotids may be joined in a common trunk. In such cases the subclavians arise separately from the arch, with the right subclavian generally arising from the left end of the arch.

In some instances other arteries derive from the arch of the aorta. Of these the most common are the bronchial, one or both [11,12].

Another common developmental abnormality is coarctation of the aorta. It is a relatively common type of congenital heart defect which is often inherited and found mostly in males. Often this defect goes undiagnosed until early adulthood. In over 30% of patients affected by coarctation, a bicuspid aortic valve is also present [ 14]. Further details about congenital cardiovascular malformations are found in Chapter VIII "MRA in pediatric patients".

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