Surgery for Extrahepatic Liver Shunts

09/18/2009

Breeders of toy dogs often know all too well about liver shunts, or portosystemic vascular anomaly (PSVA). A condition common in toy and small breeds, liver shunts occur when dogs develop an abnormal blood vessel, the shunt, connecting the portal vein to the vena cava, the large vein in the abdomen that carries blood from the back of the legs to the heart. The condition causes a short-circuiting of blood to the heart, detouring circulation around the liver. As a result, the liver cannot cleanse blood or add important nutrients.

A healthy liver is critical to proper metabolism and filtering of toxins. In normal dogs, toxins are transported from the gastrointestinal system to the liver via the portal vein. Once inside the liver, the portal vein divides into many small veins like a tree branch, spreading throughout the liver tissue and allowing toxins to be removed before circulating in the body. In dogs with a PSVA defect, unfiltered blood from the gastrointestinal system bypasses the liver and circulates throughout the body. The accumulation of toxic material, particularly ammonia, a byproduct of protein digestion, can trigger a host of neurological signs and unusual behavior in as little as 15 to 45 minutes after eating, particularly following a high-protein meat or fish-based meal.   

Toy and small breeds most commonly develop extrahepatic shunts located outside the liver, whereas large-breed dogs are more likely to have intrahepatic shunts inside the liver. Extrahepatic shunts are easier to correct surgically than those inside the liver.

While some dogs show no signs of PSVA, others appear less active or lethargic. Clinical signs are usually observed before an animal is 1 year old and are sometimes seen in puppies as they are weaned. Shunts often are recognized when a veterinarian measures serum bile concentration during treatment for an unrelated illness or after prolonged recovery from surgery due to the liver’s inability to metabolize certain anesthetics.

Affected dogs usually have small red blood cells, low blood urea nitrogen (BUN), low cholesterol and mildly increased liver enzymes (AST and ALT). Bile acid testing is the most informative test since dogs with PSVA are unable to clear bile acids from the systemic circulation. Bile acids are produced by the liver and secreted into bile to aid in digestion. They function like detergents in the liver breaking down food for absorption, and then are resorbed into the portal vein.

Among new interventional imaging techniques used to diagnose PSVA is MRI, or magnetic resonance imaging. Medical management of PSVA involves reducing ammonia and other toxins circulating in the body through a prescription restricted-protein canine food that is easily digested, rich in antioxidants and vitamins, and lower in copper and iron. Many dogs respond well when fed a proper diet.

Reducing ammonia and other toxins circulating in the body is the aim. Lactulose, a carbohydrate digested by certain intestinal bacteria, helps to acidify the large intestine and promote defecation, aiding in the elimination of ammonia and other toxins. Certain antibiotics may be prescribed to help decrease toxin-producing bacteria in the intestines.

Surgery is performed on dogs that do not respond well to a therapeutic diet or medical management. “Surgery for extrahepatic PSVA is highly successful,” Weisse says. “Diet and medication are tried first. These help to make a dog stable and reduce clinical signs, but they do not redirect the blood flow to the liver.”

Though surgery for extrahepatic PSVA is the best chance for long life for affected dogs, it is potentially dangerous. It involves application of an ameroid constrictor, a metal ring lined by a material that slowly expands and causes shunt narrowing over three to 14 days postoperatively. 

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