Portosystemic shunts can be congenital or acquired with congenital PSS commonly comprising a single communicating vessel between the portal venous circulation and the systemic circulation via the caudal vena cava or azygos vein. Of congenital shunts, 66–75% are extrahepatic. Intrahepatic portosystemic shunts are most commonly identified in larger breeds of dog (though we have also seen a number of terriers with intrahepatic shunts through our clinic).