The normal venous anatomy of the abdomen consists of a portal vein and the caudal vena cava. The portal vein drains all of the blood from the digestive organs of the abdomen including the spleen. This blood is then delivered to the liver where the nutrients absorbed from digestion are made into building bocks for the body to use. The harmful products from digestion such as toxins and bacteria are removed from the blood by the liver. The caudal vena cava drains blood from the rear limbs and all non-digestive organs in the abdomen. This blood is delivered straight to the right side of the heart and pumped to the lungs for re-oxygenation.

Portosystemic shunts are aberrant vessels that allow the unfiltered blood from digestive organs to directly communicate with systemic venous circulation. This means that all of the harmful contaminants from the digestive organs can reach the brain and allow for detrimental consequences. The major offender is usually ammonia, which can cause bizarre behavioral changes to occur. Symptoms range from seizures, blindness, drooling, head pressing, lethargy, vomiting or failure to thrive. Usually these symptoms are associated with a meal, but that is not always the case. Routine blood work can reveal evidence that a shunt is present also. High liver enzymes, bile acids and white blood cell counts as well as low blood urea nitrogen, albumin and red blood cell counts are common.

There are three different types of shunts: Single extra hepatic, multiple extra hepatic and intrahepatic. Single extra hepatic are most frequently found in small breed dogs such as Yorkies, Maltese, Pugs, Poodles, etc. See Figure 1.

These are single aberrant vessels that connect the portal vein or one of its tributaries to a systemic vein such as the caudal cava or azygous vein. Dogs or cats with this type of shunt are usually diagnosed early in life (< 1 ½ years of age) and are most commonly poor doers. Multiple extra hepatic shunts are uncommon. These shunts can be present at birth or can be the product of liver disease. Single intrahepatic shunts are most commonly found in large breed dogs such as Labrador retrievers. These dogs are most likely diagnosed early in life also. The shunt vessel is found entirely in the liver tissue it self in these case and this make correction more difficult. Multiple extra hepatic shunts are more rare and are usually associated with severe liver disease.

Medical therapy is usually initiated on diagnosis of the shunt. This consists of a low protein diet, lactulose and if necessary antibiotic therapy. The low protein diet decreased the amount of ammonia that can be produced. Lactulose is a laxative that works not speed the digested food through the intestines, but it also make ammonia more difficult to be absorb through the intestines into the blood stream. Antibiotics decrease the numbers of bacteria in the abdomen, which thereby decrease toxin production.

Definitive therapy for portosystemic shunt is dependent on surgery. The advent of the ameroid constrictor has revolutionized surgical correction of shunts. The ameroid ring is a stainless steel ring with a very small section missing. On the internal diameter so the ring is a material that slowly swell with exposure to liquid. Once the shunt vessel is located and isolated the ring is placed around the shunt. This ring slowly swells shut over a period of around 60 days. This process slowly occludes the shunt and forced the blood flow back into the portal vein and into the liver. The success rate for single extra hepatic shunts is very high and the potential to have a normal pet is probable.