VASCULAR RINGS

 

·        Vascular rings are uncommon anomalies and make up less than 1% of all congenital cardiac defects.

·        They occur with about equal frequency in both sexes. No geographical or racial predominance exists.

The most common types of complete vascular rings are:

1.      Double aortic arch

2.      Right aortic arch with left ligamentum arteriosum.

     These two make up 85-95% of the cases.

3.      Other rare (<1%) complete vascular rings include:

§         Right aortic arch with mirror-image branching

§         Left ligamentum arteriosum and left aortic arch with retroesophageal right subclavian artery, right-sided descending aorta, and right ligamentum arteriosum.

Other named anomalies that produce symptoms but do not form a complete anatomic vascular ring make up the remainder and include the abnormally placed or anomalous innominate artery and the retroesophageal right subclavian artery with left-sided aorta and left ligamentum arteriosum.

 

EMBRYOLOGY

Early in the course of embryonic morphogenesis, 6 pairs of pharyngeal arch arteries develop in conjunction with the branchial pouches. The first through sixth arches appear in more or less sequential fashion, with left-right symmetry, and constitute the primitive vascular supply to the brachiocephalic structures, running from the aortic sac to the paired dorsal aortas. As normal cardiovascular morphogenesis proceeds, a 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.

In normal development, each primitive arch either progresses into a functional vascular structure or involutes as follows:

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 occur in which a vascular ring is not present.) 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.

 

TYPES OF VASCULAR RINGS
  1. Right subclavian artery arising anomaly, where this artery starts after the origin of the left subclavian, as the fourth branch of the aortic artery.    The vessel usually passes behind the esophagus and trachea, and more frequently, courses with no symptoms.
  2. Right aortic arch. When the left fourth aortic arch disappears in the embryo between the fifth and sixth week (normally is the right one which disappears), the aortic arch will be rightward of the trachea with esophagus passing over the right bronchus and then behind the esophagus. The majority of these patients are asymptomatic. Few of them may present with dysphagia and/or dyspnea.    

·        Right aortic arch–associated abnormalities.

o       In cases of individuals in whom the left fourth branchial arch involutes and the right remains, a right aortic arch is present. Right aortic arch occurs less frequently than 1 in 100,000 times in the general population and may exist in the absence of any other anomalies. Its presence is suggestive of the existence of an associated anomaly. About 30% of patients with tetralogy of Fallot have an associated right aortic arch.

o       Persistence of the right arch with involution of the left creates a situation in which the origins of the left subclavian artery and ductus arteriosus can vary. Several of these configurations can produce a vascular ring.

o       Right aortic arch with aberrant left subclavian artery and left ligamentum arteriosum.

§         In these cases, the right arch first gives off the left carotid artery, which travels anterior to the trachea. It then gives off the right carotid, followed by the right subclavian artery, and, lastly, the left subclavian artery, which courses in a retroesophageal position and gives rise to the ligamentum arteriosum from its base. The position that the ligamentum occupies between this site and the left or main pulmonary artery completes the ring.

o       Right aortic arch with mirror-image branching and retroesophageal ligamentum arteriosum.

§         In these cases, only partial resorption of the distal left fourth arch occurs. The first brachiocephalic vessel originating from the right arch is a left innominate artery, which, in turn, branches into a left carotid and left subclavian artery. These course anterior to the trachea.

o       Following these, a right carotid artery and then a right subclavian artery arise. The ligamentum arteriosum is the final structure arising from the arch in this sequence. It originates from an area called Kommerell diverticulum, which represents the nonresorbed remnant of the left fourth arch and is situated at the point of merger between the right arch and the proximal descending thoracic aorta. The ligamentum passes leftward and behind the esophagus and then travels anteriorly to join with the left pulmonary artery and complete the ring. More commonly, in cases of right aortic arch with mirror-image branching, the ligamentum arteriosum travels from the mirror-image innominate or left subclavian artery to the left pulmonary artery. A complete ring is not present in these cases.

  1. Vascular anomalies associated with a left sided aortic arch. Two extremely rare complete rings occur in the presence of a left aortic arch, and both are associated with a right-sided descending thoracic aorta.

o       Left aortic arch with right descending aorta and right ligamentum arteriosum.

§         The first arch vessel to exit the left aortic arch is the right common carotid, which passes anterior to the trachea. The left carotid is next, followed by the left subclavian artery. The right subclavian artery arises more distally as a branch of the proximal right-sided descending aorta. The ligamentum arteriosum arises from the base of the right subclavian artery or a nearby diverticulum and travels to the right pulmonary artery.

o       Left aortic arch, right descending aorta, and atretic right aortic arch.

§         The brachiocephalic vessels arise from the left-sided arch in a normal arrangement. The left arch passes behind the esophagus to join a right-sided descending aorta. An atretic right arch is present and completes the ring.

  1. Double aortic arch. Where both the left and right aortic arch persist in the embryo. This real vascular ring many times produces syptoms of dysphagia and dyspnea with frequent respiratory track infections. In double aortic arch, usually each carotid artery and subclavian artery arises from its respective arch and no brachiocephalic artery exists. The classic double aortic arch anatomy develops when involution of the distal right fourth arch does not take place. The fourth right and left arches both persist and join the left-sided descending thoracic aorta. In about 30% of cases of double aortic arch, the small, or less dominant, of the arches is atretic but remains in continuity with the descending aorta, maintaining the complete ring.
  2. Brachiocephalic artery with anomalous origin. More often the origin of this artery is abnormally displaced toward the left, passing in front of the trachea which may compress the structure and produce dyspnea.
  3. Left carotid anomalous origin. Where its origin is abnormally displaced toward the right, may produce compressive subocclusion of the trachea.
  4. Interruption of the aortic arch. Many variants are described as portrayed in the illustration in this chapter. The lack of continuity between the aortic arch and the descending aorta constitutes the main anomaly but PDA exists in many of these patients connecting the pulmonary artery with the descending aorta. VSD is also present in the majority of the cases. Not infrequently, complete transposition or both arteries arising from right ventricle can be encountered.
  5. Arch abnormalities producing compression symptoms without an anatomic ring.

·        Anomalous innominate artery.

o       The actual prevalence of this abnormality is widely debated. This is because, in as many as 90% of cases in which symptomatic tracheal compression is produced by the innominate artery, the vessel is noted angiographically to have a normal origin from the aorta. When an anatomic abnormality is noted in these cases, the innominate artery appears to originate from a more distal and leftward position on the arch than normal. As it takes its course from left to right, it crosses the trachea anteriorly and in doing so may produce compression of the trachea.

·        Retroesophageal right subclavian artery with left aortic arch and left ligamentum arteriosum.

o       This is the most common of the arch vessel anomalies, occurring in about 0.5% of the population. In these cases, the right subclavian artery does not arise from an innominate trunk with the right carotid artery but originates as the last brachiocephalic branch from the descending aorta and takes a retroesophageal route to its destination (see Image 6). A normally positioned ligamentum arteriosum is present on the left. If a right ligamentum arteriosum were present instead of one on the left, its course would proceed from the base of this anomalous right subclavian artery to the right pulmonary artery and a complete ring would exist. Instead, no true vascular ring is present in these cases.

  1. Anomalous left pulmonary artery or pulmonary sling.

·        This abnormality occurs when the left main pulmonary artery arises as a branch of the right pulmonary artery instead of originating from the main pulmonary artery. This is believed to be an abnormality related to sixth aortic arch development. In this anomaly, the left pulmonary artery leaves the right and courses in a position cephalad to the right mainstem bronchus, proceeding around the right side of the trachea. It then travels between the trachea and esophagus as it goes to the left lung (see Image 7). This lesion is often associated with hypoplasia and other abnormalities of the tracheal and bronchial cartilages.

 

 

 

ASSOCIATED CARDIOVASCULAR ANOMALIES

Vascular rings, including those with a right aortic arch, usually occur without associated cardiovascular anomalies. Ventricular septal defect is the most common associated anomaly, though various others have been reported as well.

ASSOCIATED SYNDROMES AND NON-CARDIAC CONDITIONS

Vascular ring with a right aortic arch is associated with a band 22q11 deletion in approximately 20% of patients. Band 22q11 deletion is responsible for DiGeorge, velocardiofacial, and conotruncal anomaly face syndromes, which often are referred to by the unified terms CATCH-22 syndrome or chromosome 22q11 deletion syndrome. In patients with vascular rings, the frequency of phenotypes satisfying the clinical criteria for these various syndromes is not known. The important point, rather, is that vascular rings with a right aortic arch may be associated with band 22q11 deletion, which has a variety of other possible manifestations. These include, but are not limited to, palatal abnormalities, laryngotracheal anomalies, speech and learning delay, characteristic facial features, hypocalcemia, abnormalities of T-cell–mediated immune function, and neurologic defects.

Occasionally, patients with right aortic arch may have anomalies consistent with either VACTERL (vertebral, anal, cardiac, tracheal, esophageal, renal, limb) or CHARGE (posterior coloboma, heart defect, choanal atresia, retardation, genital, ear) associations. One of the more important noncardiac features that sometimes is found in association with right aortic arch is esophageal atresia, insofar as an undiagnosed arch anomaly may complicate repair of the esophageal atresia, which usually is recognized earlier than the right aortic arch.