Vascular Ring Anomalies

Vascular ring anomalies are developmental abnormalities of the great vessels that result in a complete or incomplete ring of vessels encircling the trachea and esophagus. Congenital anomalies of the great vessels have been reported in 20% of dogs, but most variants are not clinically significant [1]. The most common vascular ring anomaly in both human beings and dogs is aberrant right subclavian artery, although this is frequently not associated with clinical signs [1]. The most common clinical vascular ring anomaly is persistent right aortic arch (PRAA) with a left ligamentum arteriosum, an anomaly that accounts for 95% of canine clinical cases [2]. A patent ductus arteriosus is present in approximately 10% of dogs with a PRAA. Dogs with vascular ring anomalies may also have a persistent left cranial vena cava or left hemiazygous vein, which, although not clinically significant, may complicate the surgical approach.

Embryology of the Great Vessels

In the embryo, the paired dorsal and ventral aortas are separated by the foregut and pulmonary bud. The six pairs of aortic arches, corresponding to the six brachial arches, connect the dorsal and ventral aortas, and encircle the foregut. As the embryo develops, selective involution and reconnection of the vessels results in the formation of the definitive cardiovascular system and the release of the esophagus and trachea.

During normal embryonic development, the ventral aortas fuse caudally to form the heart and the dorsal aortas fuse caudally to form the descending aorta. The first, second and fifth aortic arches involute early in development. The paired ventral aortas between the third and fourth arches develop into the common carotid arteries and the corresponding sections of the dorsal aortas between the third and fourth arches disappear. The paired ventral aortas that served as the origin of the first two aortic arches develop into the external carotid arteries and the third aortic arches and the cranial dorsal aortas form the internal carotid arteries.

The left ventral aortic root of the fourth arch and the fourth left aortic arch develop into the adult aortic arch, which connects to the fused caudal dorsal aortas that form the adult descending aorta. The right ventral aortic root of the fourth arch becomes the brachiocephalic trunk and the right fourth aortic arch forms the right subclavian artery. The left subclavian artery arises from the left seventh intersegmental artery. The sixth aortic arches form the pulmonary arteries. The left aortic arch retains a connection to the fused dorsal aortas that serves as the left ductus arteriosus in the embryo and becomes the left ligamentum arteriosum after birth. The right ductus arteriosum involutes to release the esophagus and trachea.

Abnormal development of the third aortic arches will not produce a vascular ring anomaly, whereas abnormal development of the fourth and sixth aortic arches can produce a vascular ring anomaly. Complete vascular rings are formed when the right and left portions of the sixth aortic arches are retained, whereas abnormal development of the fourth aortic arches can result in an aberrant right or left subclavian artery and an incomplete vascular ring.

Types of Vascular Ring Anomalies

Several types of vascular ring anomalies have been described in dogs.

Persistent Right Aortic Arch

Persistent right aortic arch and left ligamentum arteriosum

The most common clinical vascular ring anomaly in which the aortic arch develops from the fourth right aortic arch and the ligamentum arteriosum connects the left pulmonary artery to the descending aorta, forming a complete ring around the esophagus.

Persistent right aortic arch, right ligamentum arteriosum, and aberrant left subclavian artery

The aberrant left subclavian artery courses over the esophagus and passes ventrally toward the ventral thoracic inlet [3]. The aberrant left subclavian artery can cause esophageal obstruction because it acts as a band, causing an incomplete vascular ring.

Persistent right aortic arch, left ligamentum arteriosum, and aberrant left subclavian artery

This anomaly forms two strictures, a complete vascular ring associated with the left ligamentum arteriosum and a more cranial incomplete vascular ring caused by the aberrant subclavian artery [4].

Persistent right aortic arch and right ligamentum arteriosum

This is the mirror image of normal anatomy and does not result in the formation of a vascular ring. However, this anomaly has been associated with cranial thoracic esophageal dilation and regurgitation [5]. In dogs with a PRAA and left ligamentum arteriosum, fibrous, periesophageal bands are often present under the ligamentum arteriosum; it was speculated that, although the left ligamentum arteriosum had involuted, similar bands may have been present.

Double Aortic Arch

This results from the persistence of both the right and left fourth aortic arches [6]. This anomaly can result in significant tracheal stenosis.

Left Aortic Arch

Left aortic arch and right ligamentum arteriosum

This is the mirror image of PRAA with a left ligamentum arteriosum [7]. Although it is uncommon, it is of particular significance because it cannot be corrected via a left lateral thoracotomy.

Left aortic arch, left ligamentum arteriosum, and aberrant right subclavian artery

This represents normal anatomy of the aortic arch and ligamentum arteriosum, with an aberrant right subclavian artery that passes dorsal to the esophagus, causing an incomplete vascular ring [8].

Left aortic arch, right ligamentum arteriosum, and aberrant right subclavian artery

This anomaly forms two strictures, a complete vascular ring associated with the right ligamentum arteriosum and a more cranial incomplete vascular ring caused by the aberrant subclavian artery [9].

Vascular ring anomalies are reported less frequently in cats. Vascular ring anomalies that have been described in the cat include PRAA and left ligamentum arteriosum [10], left aortic arch and right ligamentum arteriosum [11], double aortic arch [12] and PRAA, right ligamentum arteriosum and aberrant left subclavian artery [13].

Diagnosis

Vascular ring anomalies are reported in a variety of breeds; German shepherds and Irish setters have an increased risk compared with the general population [14]. The condition can affect multiple animals in the same litter; breeding of affected animals should be strongly discouraged. Most affected animals are considered normal until weaning, with the development of postprandial regurgitation after the introduction of solid food. In one study, 20% and 80% of dogs were diagnosed before 2 and 6 months of age, respectively,14 though some animals are not diagnosed until later in life.

Clinical signs of vascular ring anomalies are caused primarily by a partial esophageal obstruction. The usual presenting complaint is regurgitation of undigested food. Affected animals are often smaller than their littermates and appear malnourished despite a ravenous appetite. Respiratory signs can develop secondary to aspiration pneumonia or tracheal compression in animals with a double aortic arch. Signs of cardiac disease are uncommon, except for an association between vascular ring anomalies and patent ductus arteriosus.

The diagnosis of a vascular ring anomaly can be confirmed by survey and positive-contrast radiography. On survey radiographs, the esophagus cranial to the heart may be dilated with air, fluid, or ingesta. On the ventrodorsal projection, it may be possible to identify the descending aorta on the right side of the esophagus in animals with a PRAA. Radiographic signs of aspiration pneumonia may be present. Positive-contrast radiography will confirm the presence of esophageal obstruction at the level of the base of the heart, with dilation of the esophagus cranial to the obstruction; the esophagus caudal to the obstruction is usually undilated. Angiography is rarely performed, but can be used to help determine the type of vascular malformation. Endoscopy can be used to rule out other causes of esophageal obstruction and may show a right-sided aortic pulse in animals with a PRAA.

Treatment

Long-term medical management is generally unrewarding as the degree of esophageal dilation usually increases with age. Preoperative medical management should be used in malnourished animals and patients with aspiration pneumonia. The aim of surgery is to divide the vascular ring by transecting the ligamentum arteriosum and the periesophageal fibrous bands that form under it. Aberrant subclavian arteries are ligated and divided. With double aortic arches, one branch is usually dominant, and the contralateral branch can be divided and oversewn.

Prognosis

The most common problem after surgery is persistent regurgitation. A loss of normal neuromuscular function occurs in the dilated section of the esophagus cranial to the vascular ring, and, although some reduction in esophageal dilation and improvement in motility can be expected following surgery, normal esophageal motility will not be regained. Factors that may affect prognosis include the degree of esophageal constriction and dilation, the severity of debilitation, the presence and severity of aspiration pneumonia before correction, and the medical management before and after surgery [15].

The prognosis for surgical correction of vascular ring anomalies has improved. In 1981, Shires and Lui reported an excellent outcome (no regurgitation) in 9% of dogs, a good outcome (intermittent regurgitation) in 67%, and a poor outcome in 25% [14]. In 1997, Muldoon et al., using a similar classification scheme, reported the outcome as excellent in 92% and good in 8% [16].