Causes of Congenital Portosystemic Shunts
Congenital portosystemic shunts (PSS) are embryonic blood vessels that do not regress normally before or just after birth. These veins “shunt” blood from the gastrointestinal tract and spleen around the liver, which causes toxins, nutrients and other substances that normally would be filtered, metabolized or modified by the liver to remain in circulation. Many of these substances, especially ammonia, are highly toxic to other tissues, especially to the central nervous system. The severe neurological symptoms that frequently are associated with portosystemic shunts in dogs are caused by a condition called hepatic encephalopathy. When blood does not flow normally through the liver, it is not detoxified as it should be, and the metabolism of fats and other essential nutrients is disturbed. As a result, the blood in circulation accumulates abnormal amounts of ammonia and other substances, which in turn adversely affect the brain and cause it to become spongy and damaged. Other organs, especially the intestines and kidneys, are also damaged. The abnormal shunting of blood around the liver also causes low portal blood pressure.
The exact reasons for congenital portosystemic shunts are not known. They appear to be an anomaly of prenatal development and may be caused by some insult to the puppies in utero. However, because portosystemic shunts seem to be more prevalent in certain breeds, it is thought that there may be a genetic component to them as well.
Preventing Liver Shunts
Because the cause of congenital portosystemic shunts is not known, there is no documented way to prevent development of this disorder. However, most authorities recommend that affected dogs (those who are identified as having a PSS) not be used as part of a responsible breeding program.
Congenital portosystemic shunts are the most common congenital portovascular disorder – or disorder of portal circulation - in domestic dogs. “Portal circulation” refers to the normal flow of blood from the gastrointestinal tract and the spleen through the large portal vein into the liver.