Blood normally flows from the digestive tract to the liver via the portal vein system into the blood vessels of the liver, and then to the caudal vena cava, which is a large blood vessel carrying blood back to the heart. With a portosystemic shunt, as the name implies, portal blood by-passes the liver and goes directly to the systemic venous circulation (caudal vena cava). One important function of the liver is to clear toxins, many of which are by-products of protein digestion, carried by the blood from the digestive tract. When there is a portosystemic shunt, these toxins are not cleared, and circulate in the body.
Portosystemic shunts in dogs (does not occur in cats) may be acquired, may be secondary to another disease, or they may be congenital (present at birth). A congenital shunt usually occurs as a single abnormal blood vessel that is a remnant of normal embryonic development. These shunts are characterized as intra-hepatic (within the liver) or extra-hepatic (outside the liver) depending on the location of the shunt in relation to the liver.
Most animals with congenital portosystemic shunts show clinical symptoms before six months of age. In cases where symptoms are subtle, the condition may not be diagnosed until much later. The clinical symptoms associated with portsystemic shunt are due to the build up of aforementioned toxins, most of which are neurological in nature. The patient may be disoriented, experience ataxia (stumbling gait), and/or experience siezures (sometimes severe) . The complex of neurological and behavioural signs caused by liver dysfunction is called hepatic encephalopathy. More subtle symptoms include poor to non-existent appetite, ill thrift, stunted growth, poor hair coat, and thin body condition. Symptoms may be always present in varying degrees, or may wax and wane, often worsening immiedately following meals.
Portosystemic shunts are diagnosed by a combination of: history, signalment, physical examination, small liver visualized on routine abdominal x-rays, and biochemical adnormalities (low albumin, elevated bile acids). However, a specialized imaging technique called contrast portal radiography is the best way to confirm the diagnosis.
Surgical ligation (tieing off) of the shunt is the most effective treatment. Most extrahepatic shunts carry a good prognosis with surgery, while intrahepatic shunts offer variable results, as their ligation is more technically challenging. In some cases, intraheptic shunts are not ammenable to surgical ligation due to being multiple and/or anatomically impossible to expose for ligation.
When financial considerations or medical/anatomical factors concerning the portosystemic shunt prohibit surgical ligation, variable success can be attained with medical management. This consists of a low protein diet that keeps toxic digestive byproducts to a minimum, as well as the use of anti-oxidants and other biological sponges to absorb accumulated toxins.