An arteriovenous fistula is an abnormal passage connecting an artery (which carries oxygenated blood) and a vein (which carries deoxygenated blood). When this connection occurs in the blood vessels, the capillary circulation that delivers oxygenated blood to tissues is all but bypassed, leaving tissues without adequate oxygen. When this occurs, the heart works harder to try to make up for the loss of oxygen in tissues, which can eventually lead to heart failure.
In dogs and cats, arteriovenous fistulas are rare. They can be congenital, meaning that they are present at birth, but more often than not arteriovenous fistulas are acquired. Some causes of acquired arteriovenous fistulas are:
Damaged blood vessels (deep penetrating wounds or blunt trauma)
Arteriovenous fistulas can occur anywhere in the body, including major organs, but common sites are the limbs, paws, ears, flank and tongue. Clinical signs would correlate with the location of the fistula. For instance, an arteriovenous fistula in a limb would deprive the tissues of oxygen and lead to their death. You may notice painful swelling, warmth in the affected area, and lameness.
If a major organ is the site of an arteriovenous fistula, signs will be more specific to the organ and its function. If the fistula is located in the liver, you may see abdominal distention, while a fistula in the central nervous system can cause seizures or paralysis.
If heart failure occurs as a result of an overburdened heart, additional clinical signs will be seen. Coughing, exercise intolerance, and respiratory difficulties are all common signs of heart failure.
Arteriovenous fistulas may be suspected on physical exam alone, especially in the case of peripheral lesions like those found in the limbs, ears, or tongue. Clinical signs in these cases are easy to see. Regardless of the location, your veterinarian will probably want to do blood work to check for metabolic disturbances and might want to x-ray your pet to look for an enlarged heart. For definitive diagnosis, your pet will need an ultrasound or angiography, and you might be referred to a specialty practice for this.
Arteriovenous fistulas should be treated if possible, as they are painful and will continue to grow over time (not to mention the risk for heart failure). Traditionally, surgery was performed to separate the two vessels. This surgery can be quite complicated and many general practitioners are not comfortable performing it. Often, arteriovenous fistulas will recur, or new fistulas will appear following surgery. Another option for the treatment of arteriovenous fistulas is transcatheter embolization to block the vessels. This procedure is non-invasive and has a good success rate.
Luckily for our pets, arteriovenous fistulas are rare. But they do occur, so be aware; painful swellings that crop up after injuries should be checked out by your veterinarian.