What is a liver shunt in dogs?
A portosystemic shunt (PSS) or liver shunt is a condition where the normal flow of blood, to and through the liver, is markedly reduced or absent. Normally, blood returning from the puppy's digestive tract is routed to the liver through the portal vein. The blood flows through the liver and then exits the liver and joins the venous blood flowing back to the heart. A liver shunt is a blood vessel that connects the portal vein with the main systemic blood stream. This causes the blood to bypass the liver. Without adequate blood flow to the liver, the puppy's body cannot thrive.
How do liver shunts occur?
Liver shunts in dogs occur as a result of a birth defect (congenitally). A liver shunt can be intrahepatic, when blood is diverted in a vessel within the liver, or it can be extrahepatic when blood is diverted in a vessel around the outside of the liver.
To better understand congenital intrahepatic and extrahepatic liver shunts in dogs, it is important to appreciation the development and anatomy of the blood vessels near the fetal liver. Because the liver in a fetus is underdeveloped and unable to function, the fetal blood supply by-passes the liver through a special blood vessel that transports blood around the developing liver rather than to and through it. This special blood vessel is called the ductus venosus.
To make up for the fact that the fetal liver is not functional, the fetus's blood is carried from its body to the mother's and back again through the umbilical cord, which is made up of the umbilical artery, the umbilical vein and the placenta. The placenta is where the fetal blood and the mother's blood interact; although they never actually comingle. Here nutrients from the mother's system are passed to the fetus and waste products from the fetus are taken up by the mother and processed through her kidneys and liver. The mother's liver, then, serves as the fetus's liver since the fetal liver is not yet capable of performing this organ's many important functions.
When the puppy is born, the umbilical cord is severed and is no longer functional. Shortly after birth, the ductus venosus contracts, constricts and closes. Once this vessel is closed off, the newborn's blood is forced to pass through its now developed liver. If the ductus venosus fails to close, then a portion of blood will continue to be shunted around the liver through the still patent ductus venosus. This would be an example of a congenital intrahepatic liver shunt. In some cases, an anomalous vessel will form connecting the portal vein with a vein that carries blood away from the liver. This anomalous vessel remains patent even after the ductus venosus has closed and continues to shunt blood around the liver. This would be an example of a congenital extrahepatic portosystemic shunt.
What are the results of a liver shunt in dogs?
When blood is shunted around the liver rather than to and through it, the liver is not able to perform its many important tasks and therefore, metabolic wastes such as ammonia, reach unhealthy levels in the animal and threaten the dog's health.
The degree to which blood is shunted around the liver is dependent on the size of the shunting blood vessel. Liver shunts may be large allowing a large amount of blood to bypass the liver, or they may be partially closed allowing only small amounts of blood to shunt around the liver. The extent of blood shunting varies with every dog.
What are the symptoms of liver shunts in dogs?
The symptoms of liver shunts vary and are directly related to the extent of blood by-passing the liver. If the liver is receiving and processing 95% or greater of the dog's blood, the symptoms may be few, if any. As the amount of blood by-passing the liver increases, the symptoms of this condition will become more pronounced.
The following symptoms may be evident in puppies at only a few weeks of age:
- Poor growth rates
- Increased thirst
- Increased urination
- Walking in circles
Dogs with less severe liver shunts may not exhibit any clinical signs until the puppy is much older, even up to one year of age.
What are the risks of a liver shunt?
All liver shunts, whether mild or severe, are considered serious. Even dogs with mild liver shunts exhibit greater symptoms as they increase in body size. The larger the puppy grows, the more metabolic wastes are produced, and therefore, the more the liver is needed. Most affected dogs will not live a normal life expectancy unless the abnormality is corrected.
How do you diagnose a liver shunt in dogs?
Diagnosis of liver shunts requires sophisticated testing that may include:
- Survey radiographs (x-rays)
- Radiographic intravenous dye studies of the liver
- Laboratory blood analysis including bile acid tests and tests for ammonia
How are liver shunts treated in dogs?
The best and preferred treatment for liver shunts is to identify the abnormal blood vessel(s) and surgically close them; eliminating the shunt and returning normal blood flow to the liver. This is a complicated surgery. A minority of dogs can be treated with medical management that includes a protein restricted diet and the administration of certain medications are often beneficial. Restricted protein diets help reduce the production of toxic wastes such as, ammonia. The older a dog is at the time he starts showing symptoms, the more likely medical management can have some success.
What is the prognosis for puppies with liver shunts?
The prognosis for a dog with a PSS depends on the size and location of the shunting vessel(s). Owners and veterinarians should thoroughly discuss the seriousness, expense, and expected outcome associated with this condition. The cost and results of surgically correcting liver shunts are variable and depend on the anatomical location of the shunt, the degree of shunting and the age of the dog.