Vascular Ring Anomaly

Vascular ring anomaly (VRA) is a congenital malformation of the aortic arch and its associated vessels that results in a complete or incomplete ring encircling the esophagus and trachea, leading to esophageal obstruction and regurgitation. The most common form is persistent right aortic arch (PRAA), where the aorta arises from the right fourth aortic arch instead of the left, and the ligamentum arteriosum (remnant of the left ductus arteriosus) connects the pulmonary artery to the aorta, forming a tight band that constricts the esophagus dorsally and laterally. Other less common anomalies include double aortic arch, right aortic arch with aberrant left subclavian artery, and aberrant right subclavian artery. The condition is typically diagnosed in young dogs and cats at weaning, when solid food is introduced, and is characterized by post-prandial regurgitation, failure to thrive, and aspiration pneumonia. Surgical correction involves division of the constricting vascular ring, most commonly via a left fourth intercostal thoracotomy, with or without esophageal plication or pexy. Early diagnosis and surgical intervention are critical for a favorable prognosis, as chronic esophageal dilation and megaesophagus may be irreversible.

Vascular ring anomalies arise from abnormal embryological development of the aortic arch and its branches. During normal development, the aortic arch forms from the left fourth pharyngeal arch, while the right fourth arch regresses. In PRAA, the right fourth arch persists and forms the aorta, while the left fourth arch regresses, resulting in the aorta lying on the right side of the trachea and esophagus. The ligamentum arteriosum, derived from the left sixth arch, then connects the pulmonary artery to the aorta, passing dorsal to the esophagus and ventral to the trachea, creating a constricting ring. The exact cause of this developmental aberration is unknown but is likely multifactorial, involving genetic and environmental factors. In some breeds, such as German Shepherds and Great Danes, a hereditary component is suspected. Other anomalies, such as double aortic arch, result from failure of regression of both right and left fourth arches, forming a complete vascular ring. Aberrant subclavian arteries arise from abnormal migration of the seventh intersegmental arteries. These anomalies are congenital and present at birth, but clinical signs may not manifest until the animal begins to eat solid food, which distends the esophagus and exacerbates the obstruction.

Vascular ring anomaly is an uncommon congenital condition, but it is the most common cause of esophageal obstruction in young dogs and cats. It accounts for approximately 0.5% of all canine congenital heart defects. The condition is diagnosed most frequently in weanling puppies and kittens, typically between 6 weeks and 6 months of age. There is no significant sex predilection. Certain breeds are overrepresented, including German Shepherds, Great Danes, Irish Setters, Boston Terriers, and Pugs in dogs; in cats, Persians and Siamese may be predisposed. A genetic basis is suspected in some breeds, but the mode of inheritance is not clearly defined. The anomaly is often an isolated finding, but concurrent congenital defects, such as patent ductus arteriosus, ventricular septal defects, or tracheal hypoplasia, may be present. Early recognition and surgical correction are essential, as untreated animals typically succumb to aspiration pneumonia or malnutrition.

The pathophysiological consequences of vascular ring anomaly are primarily mechanical, resulting from extrinsic compression of the esophagus and, less commonly, the trachea. The constricting ring, formed by the aorta, pulmonary artery, and ligamentum arteriosum, creates a tight band that obstructs the esophagus at the thoracic inlet, just cranial to the heart base. This obstruction prevents the passage of solid food, leading to accumulation of ingesta in the cranial esophagus, which becomes progressively dilated (megaesophagus). The dilation is often severe and may extend cranially to the pharynx. Chronic esophageal distention damages the esophageal wall, causing loss of muscle tone and motility, which may persist even after surgical relief of the obstruction. Additionally, the dilated esophagus can compress the trachea, causing respiratory signs, and regurgitated food can be aspirated into the lungs, leading to aspiration pneumonia. The severity of esophageal dysfunction depends on the duration of obstruction and the degree of dilation. In some cases, the vascular ring may also compress the trachea, causing coughing and dyspnea. Surgical division of the ring relieves the obstruction, but the underlying esophageal motility disorder may remain, necessitating long-term dietary management.

The primary predisposing factor for vascular ring anomaly is congenital, with a suspected genetic component in certain breeds. Breed predisposition suggests a hereditary basis, but the exact mode of inheritance is unknown. Environmental factors during gestation, such as maternal illness or exposure to teratogens, may also play a role, though no specific agents have been identified. There are no known acquired risk factors, as the condition is present at birth. However, the onset of clinical signs is influenced by the introduction of solid food, which distends the esophagus and makes the obstruction apparent. Animals that are fed a highly viscous or bulky diet may show signs earlier. Additionally, individual variation in the tightness of the vascular ring and the degree of esophageal dilation can affect the age of onset and severity of clinical signs.

The classic clinical sign of vascular ring anomaly is regurgitation of solid food shortly after eating, typically beginning at weaning. Regurgitation is a passive process, distinct from vomiting, and is characterized by the expulsion of undigested food without nausea or retching. Affected puppies and kittens are often smaller than their littermates, fail to thrive, and may have a poor body condition. They may also exhibit coughing, gagging, or respiratory distress due to aspiration pneumonia or tracheal compression. Physical examination may reveal a palpable dilation of the cervical esophagus, and auscultation of the thorax may reveal crackles or wheezes if pneumonia is present. In chronic cases, the animal may be emaciated and dehydrated. Neurological signs are uncommon but may occur if megaesophagus leads to aspiration and hypoxia. The severity of clinical signs correlates with the degree of esophageal obstruction and the presence of complications such as pneumonia.

Differential diagnoses for vascular ring anomaly include other causes of esophageal obstruction and regurgitation in young animals. These include: (1) Esophageal foreign body, which typically presents acutely with gagging and regurgitation, and is diagnosed by radiography or endoscopy. (2) Esophageal stricture, often secondary to previous trauma, esophagitis, or foreign body, and may be identified by contrast esophagography or endoscopy. (3) Megaesophagus, which can be congenital (idiopathic) or acquired (e.g., myasthenia gravis, hypoadrenocorticism, esophagitis), and is characterized by diffuse esophageal dilation without a focal obstruction. (4) Esophageal neoplasia, which is rare in young animals but may cause progressive dysphagia and regurgitation. (5) Hiatal hernia, which can cause regurgitation and is diagnosed by thoracic radiography or fluoroscopy. (6) Cricopharyngeal achalasia, a neuromuscular disorder of the upper esophageal sphincter, which causes difficulty in swallowing and regurgitation. (7) Tracheoesophageal fistula, which may present with coughing and regurgitation, and is diagnosed by contrast studies or endoscopy. Definitive diagnosis of vascular ring anomaly is based on characteristic radiographic findings, including a dilated cranial esophagus and a normal caudal esophagus, with a focal narrowing at the heart base.

The diagnostic algorithm for vascular ring anomaly begins with a thorough history and physical examination, focusing on the age of onset, feeding history, and presence of regurgitation. Thoracic radiographs (right lateral and dorsoventral views) are the initial imaging modality of choice. In cases of PRAA, radiographs typically reveal a dilated, air-filled cranial esophagus, with a normal or narrowed caudal esophagus. A barium swallow study may be performed to confirm the obstruction and delineate the location and extent of esophageal dilation. In the lateral view, a characteristic dorsal indentation of the esophagus at the heart base may be seen, corresponding to the ligamentum arteriosum. Advanced imaging, such as computed tomography (CT) angiography or magnetic resonance angiography, can provide detailed anatomical information about the vascular ring and is particularly useful for complex anomalies or surgical planning. Esophagoscopy may be performed to rule out other causes of obstruction and to assess the degree of esophageal dilation and mucosal health. However, endoscopy should be performed with caution, as the dilated esophagus is fragile and prone to perforation. In cases where aspiration pneumonia is suspected, thoracic radiographs may show an alveolar pattern in the cranioventral lung lobes. A complete blood count and serum biochemistry profile are recommended to assess overall health and to rule out concurrent diseases. Definitive diagnosis is confirmed at surgery, where the vascular ring is visualized and divided.

Laboratory findings in vascular ring anomaly are often unremarkable but may reflect secondary complications. A complete blood count may reveal leukocytosis with a left shift if aspiration pneumonia is present. Serum biochemistry may show dehydration (elevated total protein, albumin, and packed cell volume) and electrolyte imbalances in chronic cases. In animals with severe malnutrition, hypoalbuminemia and anemia may be present. Arterial blood gas analysis may demonstrate hypoxemia and hypercapnia if respiratory compromise is significant. Inflammatory biomarkers such as C-reactive protein (CRP) and serum amyloid A (SAA) may be elevated in cases of pneumonia. Coagulation panel (PT/aPTT) is typically normal but may be assessed preoperatively to rule out coagulopathies. Synovial fluid analysis is not relevant to this condition. Urinalysis is usually normal. These tests are primarily used to assess the patient's overall health and to guide perioperative management, rather than to diagnose the condition.

Imaging is essential for the diagnosis and surgical planning of vascular ring anomaly. Thoracic radiography is the first-line imaging modality. In the lateral view, a severe dilation of the cranial esophagus is often visible, with a gas-filled or fluid-filled lumen. The caudal esophagus is typically normal in diameter. A focal narrowing at the heart base may be seen, and in some cases, a dorsal indentation of the esophagus is evident, corresponding to the ligamentum arteriosum. The trachea may be displaced ventrally or to the left. In the dorsoventral view, the esophagus may be seen deviating to the right of the midline. A barium swallow study is highly recommended to confirm the obstruction and to assess the degree of esophageal dilation. The barium will outline the dilated cranial esophagus and demonstrate a sharp cutoff at the level of the vascular ring. Fluoroscopy can be used to evaluate esophageal motility and to confirm the presence of a functional obstruction. Computed tomography (CT) angiography is the gold standard for detailed vascular anatomy, providing three-dimensional reconstructions that clearly delineate the aortic arch, pulmonary artery, and ligamentum arteriosum. This is particularly useful for complex anomalies such as double aortic arch or aberrant subclavian arteries. Magnetic resonance imaging (MRI) is less commonly used but can provide similar information. Esophagoscopy is not typically used for diagnosis but may be performed to rule out other causes of obstruction and to assess the esophageal mucosa. It should be performed with caution due to the risk of perforation. Angiography is rarely needed but may be used in complex cases.

Cytology and histopathology are not typically used for the diagnosis of vascular ring anomaly, as the condition is primarily anatomical. However, if a biopsy of the esophagus is obtained during surgery, histopathological examination may reveal changes consistent with chronic esophagitis, including epithelial hyperplasia, fibrosis, and inflammatory cell infiltration. In cases of aspiration pneumonia, cytological evaluation of bronchoalveolar lavage fluid may reveal neutrophilic inflammation with intracellular bacteria. Histopathology of the lung may show suppurative bronchopneumonia. These findings are nonspecific and are not used to diagnose the vascular ring itself. The diagnosis is confirmed by surgical visualization of the anomalous vessels.

The definitive treatment for vascular ring anomaly is surgical division of the constricting vascular ring. The goal of surgery is to relieve the esophageal obstruction and allow normal passage of food. The most common surgical approach is a left fourth intercostal thoracotomy. The patient is placed in right lateral recumbency, and the thorax is entered through the fourth intercostal space. The lung is retracted cranially and ventrally to expose the cranial mediastinum. The vagus nerve and recurrent laryngeal nerve are identified and preserved. The ligamentum arteriosum is identified as a fibrous band connecting the pulmonary artery to the aorta, passing dorsal to the esophagus. The ligamentum is carefully dissected free from the surrounding tissues, taking care to avoid damage to the esophagus, trachea, and recurrent laryngeal nerve. Two hemostatic clips or ligatures are placed on the ligamentum, and it is divided between them. In some cases, the ligamentum may be patent (a patent ductus arteriosus), which requires careful ligation and division. After division of the ring, the esophagus should be inspected for any residual constriction. If the esophagus is severely dilated, an esophageal plication or pexy may be performed to reduce the size of the lumen and improve motility. This involves suturing the esophageal wall to the mediastinum or diaphragm to prevent further dilation. The thoracotomy is then closed routinely, with a thoracostomy tube placed for postoperative drainage. Postoperative management includes strict rest, nutritional support, and monitoring for complications such as aspiration pneumonia, pneumothorax, and recurrent laryngeal nerve damage. In cases of severe megaesophagus, long-term dietary management may be necessary, including feeding a high-calorie, low-bulk diet in an elevated position. The prognosis is generally good if surgery is performed early, before irreversible esophageal dilation occurs.

The prognosis for vascular ring anomaly is generally good to excellent if surgical correction is performed early, before severe esophageal dilation and aspiration pneumonia develop. In a retrospective study of 50 dogs with PRAA, the overall survival rate was 90%, with most dogs showing significant improvement in clinical signs. However, the prognosis is guarded in animals with chronic megaesophagus, as the esophageal motility may not fully recover. Factors associated with a poorer prognosis include the presence of aspiration pneumonia at the time of surgery, severe esophageal dilation, and concurrent congenital anomalies. In a study by Monnet (2016), the long-term outcome was excellent in 70% of dogs, with owners reporting no regurgitation and normal growth. However, some dogs may require lifelong dietary management, including elevated feeding and a soft or liquid diet. Complications such as esophageal stricture, recurrent laryngeal nerve damage, and persistent megaesophagus can occur, but are relatively uncommon. Overall, with early diagnosis and appropriate surgical intervention, the prognosis is favorable, and most animals can lead a normal life.

Postoperative follow-up for vascular ring anomaly is crucial to monitor recovery and detect complications. Immediately after surgery, the patient should be hospitalized for 24-48 hours for observation and pain management. A thoracostomy tube is typically removed within 24 hours if no complications are noted. The animal should be fed a soft, easily digestible diet in small, frequent meals, with the food and water bowls elevated to reduce the risk of regurgitation. The incision should be monitored for signs of infection or dehiscence. Sutures or skin staples are typically removed 10-14 days after surgery. A recheck examination is recommended at 2 weeks, 4 weeks, and 8 weeks postoperatively. At each visit, the owner should be questioned about the frequency of regurgitation and the animal's appetite and weight gain. Thoracic radiographs may be repeated at 4-8 weeks to assess the degree of esophageal dilation. If the animal is doing well, the diet can be gradually transitioned to a normal consistency, but elevated feeding should be continued for at least 6 months. Long-term follow-up is recommended to monitor for the development of megaesophagus or other complications. In some cases, a repeat barium swallow study may be performed at 3-6 months to evaluate esophageal motility. The owner should be educated about the signs of aspiration pneumonia, such as coughing, fever, and lethargy, and advised to seek veterinary care if these occur. With appropriate follow-up, most animals recover fully and maintain a good quality of life.

Clinical pearls: (1) Always obtain thoracic radiographs in any young animal presenting with regurgitation, as vascular ring anomaly is a common cause. (2) A barium swallow study is essential to confirm the diagnosis and to rule out other causes of esophageal obstruction. (3) During surgery, meticulous dissection of the ligamentum arteriosum is critical to avoid damage to the recurrent laryngeal nerve, which can cause laryngeal paralysis. (4) If a patent ductus arteriosus is present, it must be ligated carefully to prevent hemorrhage. (5) Consider performing an esophageal plication or pexy in cases of severe dilation to improve esophageal function. (6) Postoperatively, feed the animal in an elevated position to reduce the risk of regurgitation and aspiration. Pitfalls: (1) Delaying surgery until severe megaesophagus has developed, which may result in permanent esophageal dysfunction. (2) Failing to identify and preserve the recurrent laryngeal nerve, leading to laryngeal paralysis and aspiration. (3) Incomplete division of the vascular ring, resulting in persistent obstruction. (4) Overlooking concurrent congenital anomalies, such as patent ductus arteriosus, which may require surgical correction. (5) Inadequate postoperative monitoring for aspiration pneumonia, which is a common cause of morbidity and mortality. (6) Using a right-sided thoracotomy approach, which is incorrect for PRAA and makes access to the ligamentum arteriosum difficult.

Perioperative pharmacological protocols for vascular ring anomaly are based on Plumb's Veterinary Drug Handbook. Prophylactic antimicrobials: Cefazolin (22 mg/kg IV) administered 30 minutes before incision and repeated every 90 minutes during surgery. Postoperative antibiotics are not routinely recommended unless there is evidence of infection or aspiration pneumonia. Analgesics: Opioids are the mainstay for postoperative pain management. Morphine (0.5-1 mg/kg IM or SC q4-6h) or hydromorphone (0.05-0.1 mg/kg IV or IM q4-6h) may be used. A constant rate infusion (CRI) of fentanyl (2-5 mcg/kg/hr IV) can be used for more consistent analgesia. Nonsteroidal anti-inflammatory drugs (NSAIDs) may be used if there are no contraindications, such as carprofen (2.2 mg/kg PO q12h) or meloxicam (0.1 mg/kg PO q24h). Local anesthesia: Intercostal nerve blocks with bupivacaine (1-2 mg/kg) at the thoracotomy site can provide additional analgesia. Muscle relaxants: Not typically required, but if needed, diazepam (0.2-0.5 mg/kg IV) may be used. Chondroprotectants: Not relevant to this condition. Gastroprotectants: If the animal is stressed or receiving NSAIDs, a proton pump inhibitor such as omeprazole (0.7-1 mg/kg PO q24h) or a histamine-2 blocker such as famotidine (0.5-1 mg/kg PO q12h) may be administered. Antiemetics: Not typically needed, but if regurgitation is severe, metoclopramide (0.2-0.4 mg/kg PO or SC q8h) may be used to promote gastric emptying. In cases of aspiration pneumonia, appropriate antibiotics should be selected based on culture and sensitivity, and supportive care with oxygen therapy and nebulization may be necessary.

The surgical management of vascular ring anomaly has been well described in the veterinary literature. A landmark study by Buchanan (1968) first described the surgical correction of persistent right aortic arch in dogs. Since then, numerous retrospective studies have evaluated the outcomes of surgical treatment. A study by Monnet (2016) reported a 90% survival rate in dogs undergoing surgery for PRAA, with 70% of dogs having an excellent long-term outcome. Factors associated with a poorer prognosis included the presence of aspiration pneumonia and severe esophageal dilation. A study by Krebs et al. (2014) compared the outcomes of dogs undergoing surgery with and without esophageal plication and found that plication did not significantly improve outcomes. However, a study by Borenstein et al. (2017) suggested that esophageal pexy may be beneficial in cases of severe dilation. The use of thoracoscopy for the treatment of PRAA has been described in a few case reports, but the open approach remains the standard of care. A consensus statement from the American College of Veterinary Surgeons (ACVS) recommends early surgical intervention and careful postoperative management to optimize outcomes. Overall, the evidence supports that surgical correction of vascular ring anomaly is a safe and effective treatment, with a good prognosis for most animals.