Portosystemic shunts (PSS) are vascular anomalies that divert blood from the abdominal viscera to the heart, bypassing the hepatic sinusoids and carrying intestinal absorption products directly to the systemic circulation. Portosystemic shunts can be classified as extrahepatic or intrahepatic, single or multiple, and congenital or acquired. Types of single congenital portovascular anomalies include intrahepatic portocaval shunts (i.e. patent ductus venosus) and extrahepatic portocaval or portal-azygos shunts. In a small percentage of dogs, the prehepatic portal vein is also congenitally absent.
Single intrahepatic and extrahepatic PSS are usually diagnosed in immature animals. No sex predilection is evident. Single intrahepatic PSS are found primarily in large breed dogs such as Irish Wolfhound, Old English sheepdog, Golden and Labrador Retrievers, and Samoyed, and in medium-sized breeds such as Australian shepherds and Australian Cattle dogs. Single extrahepatic PSS occur primarily in small breed dogs such as the Yorkshire terrier, Schnauzer, poodle, Maltese, Shih Tzu, and dachshund. These shunts may connect the portal vein with the caudal vena cava directly, or may originate from a portal tributary such as the left gastric vein. Cats may have extrahepatic or intrahepatic shunts. Congenital shunts are hereditary in Maltese and Irish Wolfhounds and are thought to be hereditary in Yorkies.
Multiple extrahepatic PSS occur with chronic increased hepatic portal venous resistance and portal pressure. These shunts are frequently found around the kidneys. Breed predisposition for multiple PSS include the German Shepherd and Doberman pinscher.