Persistent Right Aortic Arch and Other Vascular Ring Anomolies
As mammalian embryos develop in the womb they pass through a stage of development where they have six loops or arches of blood vessels arising from the heart, very similar to the definitive gills seen in fish. As the embryo matures towards birth these vascular arches follow an orderly progression of fusing or regressing to result in the definitive vascular anatomy of the adult animal. Sometimes there’s a mistake, and the wrong arch regresses or a wrong arch persists, or both. These anomalies usually do not affect blood flow to organs or compromise either the heart or blood pressure. However, there are structures that lie over the heart (trachea and esophagus) that can become partially or completely encircled and compressed by these vessels if the proper developmental progression fails. A
vascular ring anomaly
(VRA) is defined as a compression, usually of the esophagus, by one of these birth defects. Depending on the type of anomaly, this compression may be partial (just one side of the esophagus is squeezed) or complete (the esophagus is completely encircled between the vessels and the base of the heart). The most common variant of VRA is termed a
persistent right aortic arch (PRAA). In this defect, the aorta arises from the embryo’s right fourth aortic arch instead of the normal left fourth arch.
Because there is a normal remnant of another aortic arch (the left sixth arch) that connects the aorta to the pulmonary artery, and this band (called the ligamentum arteriosum) is on the left side, dogs with PRAA end up with this band crossing from the left side to the right side, completely trapping the esophagus.
- What are the signs of PRAA? As mentioned above, there are usually no cardiovascular effects of vascular ring anomalies. But the entrapment of the esophagus causes problems swallowing, since solid food cannot pass through the strictured area. The result is regurgitation, or the passive retrograde movement of food back up into the neck and often out the mouth. It is distinguished from vomiting, which is an active process wherein there are strong abdominal contractions trying to expel food from the stomach. Dogs with PRAA may appear distressed as food backs up, and drooling may be profuse. Often there will be an obvious bulge in the neck as food is trapped there.
- How is PRAA diagnosed? Because this is a congenital problem (present at birth), signs usually become evident as soon as a puppy or kitten begins to eat solid food with weaning. Most animals will regurgitate immediately (or within a short time) after eating a meal. Besides this very classic history, there are other tests, such as a barium x-ray, that can assist the veterinarian in making the diagnosis. Very uncommonly, dogs with PRAA may not show these signs until older, some as old as several years of age. The reason for the delayed onset of signs is not known, but these cases are exceptionally rare.
- How is PRAA treated? We usually do not need to do major vascular surgery to treat these patients. Since the cause for the esophageal constriction is the ligamentum arteriosum that connects the pulmonary artery on the left with the aorta on the right, and this band is usually not patent (i.e., it has no blood flow in it after birth), the surgery involves identifying and severing the ligamentum arteriosum. In addition, there may be a build up of restricting scar tissue around the espophagus where it was strictured, and this is relieved by carefully dissecting these bands of scar tissue. Surgery can be done either through an open incision between the ribs (thoracotomy), or in some cases with minimally invasive techniques using a thoracoscope. Both types of surgery are usually well-tolerated.
- When should surgery be performed? It is CRITICAL
to operate as soon as possible after the condition has been diagnosed. The stricture of the esophagus results in stretching of that portion of the esophagus in front of the stricture. This can lead to rapid loss of function due to injury to the muscles and nerves of the esophageal wall. Delaying surgery makes it more likely that the animal will have a permanent zone of megaesophagus (dilated, non-functional esophagus), with resultant life-long difficulties swallowing and regurgitation despite surgical treatment. This means that most animals with PRAA have surgery at around 6-8 weeks of age (the age of weaning, when signs usually first appear).
- Are there problems operating on patients so young? There are challenges to both anesthesia and surgery because of the young age of these patients, and their small size. But these are challenges most surgical specialists have been trained to handle successfully. The biggest worry is that these patients may be malnourished (since they have been unable to get solid food down), and they are also prone to developing aspiration pneumonia as a consequence of inadvertent inhalation of food that has been regurgitated into the pharynx. These problems, if present, are treated by placing a temporary feeding tube directly into the stomach, and antibiotics and other interventions. for any pneumonia. Surgery may be briefly delayed, but usually not more than a few days.
- What is the success of surgery for PRAA? The biggest determinant of success is how rapidly the problem is diagnosed and treated. Longer delays in either makes permanent megaesophagus more likely, with a concomitant need for lifelong management. Even with rapid recognition and treatment, some dogs, though only 6 weeks of age when treated, will still have abnormal esophageal function. This is usually quite manageable for most owners. Some of the modifications that might be needed include careful selection of food consistency (some dogs will be able to eat normally, while others may have difficulty with certain preparations such as “burger” type diets, and some may require gruel feeding), training the dog to eat from an upright position with the front end of the dog elevated, so as to allow gravity to help propel food into the stomach, and dividing daily intake into several smaller meals.
- Can I use my pet for breeding if it has a VRA? The exact mechanism that causes birth defects such as VRA/PRAA is not always known, but it is presumed that there is some component that is heritable. We see PRAA in certain breeds more frequently than others, further suggesting a genetic basis. For this reason, animals with any congenital defect that is thought to have a heritable basis should be neutered and not allowed to breed.
- Besides PRAA, what other types of VRA are seen, and how are they treated?
In clinical practice, about 95% of the VRA’s we diagnose are PRAA. When another type is suspected or found, the treatment varies. The underlying goal remains the same (relieving any compression of the esophagus or trachea), but for certain anomalies (such as “double aortic arch”, where the aorta has two origins, one on the left and one on the right), it may require vascular surgery to divide or otherwise manage the offending vessel(s).