A portosystemic shunt (PSS) is an abnormal connection between the portal circulation and the systemic circulation. In many cases, these connections occur between the splenic, phrenic, cranial mesenteric, caudal mesenteric, gastric, or gastroduodenal veins and the caudal vena cava or azygos vein.
Congenital shunts occur when the fetal circulatory system fails to mature normally, similar to a patent ductus arteriosus or persistent right aortic arch. Intrahepatic congenital shunts are most commonly observed in large breed dogs, while extrahepatic congenital shunts are typically seen in small breed dogs (although there are certainly exceptions in both cases). Single congenital shunts can often be treated surgically.
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