Abstract
Vascular rings and pulmonary artery sling refer to congenital anatomic anomalies of the great vessels that cause compression of the trachea and esophagus. Congenital tracheal stenosis and/or tracheomalacia is often present as well. Successful management of these patients requires close collaboration across a wide array of pediatric subspecialties.
Key Words
Vascular Ring, Pulmonary Artery Sling, Double Aortic Arch, Tracheal Reconstruction, Congenital Tracheal Stenosis
Clinical Background
The term vascular ring refers to a spectrum of congenital aortic arch anomalies that can cause compressive symptoms of the aerodigestive tract. “True” or “complete” vascular rings completely encircle the trachea and esophagus, whereas “incomplete” vascular rings do not. The term itself was first coined in 1945 by Gross after successful repair of a double aortic arch in a 4-year-old patient, although the anatomy of a double aortic arch had been described as early as 1737 by Hommell. Numerous variations have been described, and several authors have grouped vascular rings under various schema. An early report from the Mayo Clinic divided vascular rings into seven types (types A through G) and is of historical interest. Backer and Mavroudis proposed a simplified grouping system with the following subcategories:
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Double aortic arch
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Dominant right arch
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Dominant left arch
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Balanced or codominant arches
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Right arch with left ligamentum
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With retroesophageal or “aberrant” left subclavian artery
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With mirror-image branching
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Innominate artery compression
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Pulmonary artery sling (PAS)
Left aortic arch with aberrant right subclavian artery, although not always a true vascular ring (dependent on a right-sided ligamentum), is often included in the discussion of vascular rings because the management of symptomatic patients follows many of the same principles. Left aortic arch with aberrant right subclavian artery is the second most common aortic arch anomaly and is present in 0.5% to 2% of humans. The most common aortic arch anomaly is bovine arch, in which all four vessels arise from a common trunk. Symptoms—either respiratory distress or dysphagia—are caused by extrinsic compression of the trachea and/or esophagus by the vascular ring, although respiratory symptoms in patients with vascular rings may also be compounded by intrinsic airway disease such as tracheomalacia or congenital tracheal stenosis (CTS) with complete tracheal rings. CTS has a particularly strong association with PAS. Because of the potential complicating features of tracheal pathology, a good understanding of the pathophysiology and management of pediatric tracheal disease is extremely useful in approaching the patient with a vascular ring who has respiratory symptoms. All symptomatic patients with vascular ring should be considered for surgical correction. In fact, it is estimated that over 70% of patients with a vascular ring eventually become symptomatic. The principles of surgical correction are ligation and division of the vascular ring to relieve extrinsic compression, preservation of distal blood flow, and tracheal repair or reconstruction as necessary. Incidentally diagnosed individuals should be considered on a case-by-case basis, but generally patients with complete vascular rings should be offered repair, whereas asymptomatic patients with incomplete rings may be initially observed. Knowledge of the natural history of unrepaired vascular rings is limited to a few small series of patients with mild or no symptoms.
Prevalence and Associations
The estimated prevalence of true vascular rings is 0.05% to 0.1% of the general population. In a study by Yu and colleagues, the authors performed two-dimensional echocardiography on 186,213 newly registered school-aged children as part of a pre–sports participation screening program. They diagnosed 1088 patients with vascular ring, the majority (992) having a right aortic arch with an aberrant left subclavian artery. Similarly, in a review of 18,347 pregnant women whose fetuses were screened for congenital heart disease over a 10-year period in Israel, 18 cases (0.1%) of vascular rings were diagnosed and confirmed postnatally. In a large series of 4850 consecutive autopsies in England, there were 10 reported vascular rings that had not been diagnosed premortem. The most common genetic anomaly associated with vascular rings is 22q11 microdeletion, although others include 3q29 duplication, and 16p13.3 deletion. Specifically, haploinsufficiency of the genes CRKL, TBX1, and ERK2, which are located on chromosome 22, has been implicated experimentally in abnormal neural crest cell migration, leading to abnormal development of the aortic arch and great vessels. Intracardiac defects associated with vascular rings include tetralogy of Fallot, ventricular septal defect, and truncus arteriosis. Associated extracardiac anomalies include DiGeorge syndrome, given the association with 22q11 microdeletion, and also tracheomalacia, aortic coarctation, interrupted aortic arch, Pierre Robin sequence, brain ventriculomegaly, and cleft lip and palate.
Embryology
Initially, cranial neural crest cells migrate within the human embryo to form six paired pharyngeal arches, as well as paired left and right dorsal aortae and ventral aortae, and their systematic regression and remodeling contribute to the mature human aorta ( Fig. 44.1 ). The paired right and left dorsal aortae are present at approximately 3 weeks of intrauterine growth. Typically the left dorsal aorta persists to form a left-sided descending aorta, and the right dorsal aorta largely regresses but does contribute to the mature right subclavian artery. Abnormal regression patterns of the pharyngeal arches result in vascular ring anatomy. The first and second arches regress to form part of minor facial arteries and do not contribute to the mature aorta. The third aortic arch forms the common and proximal internal carotid artery. The fourth arches become the innominate artery on the right and the ascending aorta and transverse arch on the left. The fifth arch regresses, and the sixth arch becomes the pulmonary artery and the ductus arteriosus on the left. Complete rings result from a failure of regression of a paired aortic arch, resulting in two mature aortic structures on either side of the aerodigestive tract. The “sidedness” of the aortic arch is determined by its relationship to the trachea, whereas the “sidedness” of the descending aorta is determined by its relationship to the vertebral bodies.
Presentation and Diagnosis
Patients with vascular rings range in presentation from the incidentally diagnosed asymptomatic patient to those with life-threatening respiratory failure and/or intractable feeding intolerance. Anterior anatomic variations (e.g., innominate artery compression) tend to cause respiratory symptoms, whereas posterior pathology (e.g., aberrant subclavian artery) tends to cause dysphagia. Complete vascular rings (e.g., balanced double aortic arch) tend to cause symptoms of both tracheal and esophageal compression. Respiratory symptoms include apparent life-threatening event (ALTEs), in which a previously asymptomatic child may suffer a respiratory arrest without any significant prodrome. Other respiratory symptoms range in severity from cough (“seal bark” has been used to describe the cough associated with vascular ring), stridor, recurrent pneumonias, hyperinflation, migrating atelectasis, and ventilator dependence. The more severe cases will tend to present within the first few months of life. Apneic events are more common in patients with innominate artery compression and tracheomalacia. Symptoms of esophageal compression tend to present later, classically when the infant progresses from liquid to more solid foods. The term dysphagia lusoria (“from an unknown source”) has been used to describe the dysphagia due to compression from an aberrant subclavian artery. The range of dysphagia is broad, from a neonate who is intolerant of enteral nutrition to a nearly asymptomatic child who has learned to slowly and thoroughly chew his or her food.
A variety of imaging modalities are used in the workup of patients with vascular rings. On fetal ultrasonography, vascular rings can be diagnosed by the presence of a U -shaped configuration of the aorta and ductus arteriosus, with the trachea located in between. Initial respiratory symptoms will usually prompt a chest radiograph, on which right-sided or double aortic arch may be noted. Dysphagia may prompt a contrast esophagram, which will classically demonstrate posterior indentation of the esophagus when due to a vascular ring, and anterior esophageal indentation when due to a PAS. Once the diagnosis of a vascular ring has been made in a symptomatic patient, high-quality cross-sectional imaging should be strongly considered to aid in operative planning. Computed tomography (CT) also has the advantage of providing more detailed rendering of the trachea and mainstem bronchi, which can be especially helpful in cases with associated CTS and tracheomalacia. However, intravascular contrast agents will not opacify atretic portions of a nondominant side of a ring. Magnetic resonance (MR) imaging, on the other hand, will better display soft tissue, such as atretic portions of a nondominant arch or the ligamentum. Cardiac gating is usually not required and can be performed with respiratory pauses or quiet breathing. Whereas CT and MR imaging can delineate important subtleties that will influence operative planning, their use should also take into account the clinical status of the patient, specifically that in a critically ill neonate the risk of a contrast load for CT or difficulties with time and sedation required in the MR scanner may outweigh the benefit of the additional anatomic detail this affords. Catheter angiography is of historic interest and is currently rarely used in the workup of patients with vascular rings. Echocardiography is extremely useful to evaluate for the presence of intracardiac defects.
Bronchoscopy, both preoperatively and postoperatively, plays an important role in the management of the child with a vascular ring. This can confirm the diagnosis of tracheal stenosis and tracheomalacia and can accurately document the length of involved trachea. Bronchoscopy can also rule out other causes of respiratory distress, such as subglottic stenosis, and it can also determine whether the bronchi have an abnormal branching pattern. However, severely hypoplastic tracheas may not be able to be traversed at all, and moderately stenotic tracheas may not be safely traversed without compromising ventilation. Inflammation and edema following bronchoscopy can also compromise an already stenotic trachea. External compression patterns seen with bronchoscopy may also give a clue as to the type of vascular ring. Innominate artery compression is usually discrete, and compression is noted anterior and to the right. Double aortic arch tends to cause a more complex pattern of distal tracheal compression and may be seen affecting the carina as well.
Specific Anatomic Variations of Vascular Rings
Double Aortic Arch
In double aortic arch the right aortic arch gives rise to the right common carotid and right subclavian arteries, and the left arch gives off the left common carotid and left subclavian arteries ( Fig. 44.2 ). The arches then converge posteriorly to form one descending thoracic aorta. The ring created encircles both the trachea and esophagus. Either arch may be the largest or “dominant” vessel. Right-dominant arch is more common, accounting for approximately 75% of cases. Left-dominant aortic arch accounts for approximately 20% of cases, with the remaining 5% of cases having right and left aortic arches of equal caliber. Surgical correction requires the division of the nondominant vessel. Division of the dominant vessel may result in the development of a pressure gradient between the ascending and descending aorta. It is often useful to measure simultaneous pressure gradients intraoperatively to ensure division of the correct vessel and avoidance of coarctation. An additional pitfall is injury to the recurrent laryngeal nerve. The right recurrent laryngeal nerve passes underneath the right aortic arch itself rather than the right subclavian artery.
Right Aortic Arch With Left Ligamentum
In right aortic arch variants of vascular ring the aortic arch is positioned to the right of the trachea and the left-sided ligamentum arteriosum either attached to the descending aorta or the left subclavian artery, thereby “completing” the vascular ring ( Fig. 44.3 ). There are two main variants of right aortic arch: (1) Right aortic arch with retroesophageal left subclavian and (2) right aortic arch with mirror-image branching. In right aortic arch with retroesophageal left subclavian artery—which accounts for approximately 65% of cases—the left subclavian is the most posterior artery on the arch and crosses midline by passing posterior to the esophagus. When a bulbous, aneurysmal deformity of the origin of an aberrant subclavian artery is present, it is termed a Kommerell diverticulum, after the German radiologist who first described the anatomy. It is a remnant of the fourth aortic arch and, as such, can be present in a right or left aortic arch and can cause compression of the aerodigestive tract similar to those caused by other vascular rings. In addition to the dysphagia that it causes, there is also risk of further aneurysmal degeneration and rupture. If resection of a Kommerell diverticulum is performed, the subclavian artery can be reimplanted to the ipsilateral common carotid in an end-to-side fashion to avoid upper extremity ischemia. The aberrant subclavian artery can often be sacrificed in the neonatal period without reimplantation, but in older children reimplantation is required to avoid limb ischemia. Mirror-image branching is almost universally associated with congenital heart defects, including tetralogy of Fallot, transposition of the great arteries, truncus arteriosus, and tricuspid atresia.
