How does liver shunt happen?

This much-discussed disorder is most commonly the result of improper fetal development of the circulatory system. To thoroughly understand liver shunts, it is important to have an understanding of the development of blood vessels in the fetus. The fetus, through the placenta, umbilical vein and artery, is connected to the mother's circulatory system (bloodstream). Therefore, the liquid portion of the blood of the fetus can move into the mother's bloodstream, but the cells cannot. The mother's liver then performs the important liver functions, such as eliminating wastes, for the fetus. The mother's liver is necessary for this, since the fetal liver is just developing and is not yet capable of many functions including removing metabolic wastes from the fetal bloodstream, storing minerals, and enzyme production. Because the fetal liver is underdeveloped, the fetus possesses blood vessels which transport blood around the developing liver rather than to and through it. This is necessary, since the small developing fetal liver cannot filter or handle the full quantity of blood that needs to be filtered. When the fetus is born, the placenta, umbilical vein and artery (jointly referred to as the umbilical cord) are severed and are no longer functional. Once the umbilical cord is cut at birth, there is no longer this connection between the mother and the just-born puppy. At this point, the puppy must rely on his own liver functions and not that of his mother.

At or about the time of birthing (whelping), the blood vessels within the fetus, which allowed blood to bypass the developing fetal liver, must close. Once these vessels close, the puppy's blood is forced to pass through the puppy's now developed liver. If these fetal vessels fail to close, then blood is allowed to abnormally be shunted around the liver, hence the name liver shunt. When blood is shunted around the liver rather than to and through it, the liver is not able to filter all of the blood, and therefore, toxic metabolic wastes such as ammonia are not adequately removed from the bloodstream. The degree to which blood is shunted around the liver is dependent on the extent to which shunting vessels persist. Liver shunts may be large allowing much 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? The symptoms of liver shunts vary and are directly related to the extent of blood shunting. If the liver is receiving and processing 95 percent of the puppy's blood, the symptoms may be few, if any. More severe shunts are life threatening with many symptoms. Symptoms may be evident in these puppies at only a few weeks of age and may include low growth rates, vomiting, diarrhea, constipation, salivation, increased urination, seizures, and death. Dogs with less severe liver shunts may not exhibit any clinical signs until the puppy is much older, even up to a year of age.

What are the risks? All liver shunts, whether mild or severe, are considered serious and life threatening. Even mild liver shunts generally exhibit greater symptoms as the puppy increases in body size. The larger the puppy the more metabolic wastes produced, and therefore, the more the liver is needed. Most affected dogs will not live a normal life expectancy unless the abnormality is corrected.

What is the management? Management techniques for liver shunts have improved. The best and preferred treatment is to identify the abnormal blood vessels and surgically close them, eliminating the shunt. This will require sophisticated testing and may include radiographs (x-rays), laboratory blood analysis, ultrasound and intraveneous dye studies. The expense and results are variable depending on the degree of shunting, age, and symptoms. In addition to surgery, alterations in diet, and administration of medications are often beneficial. Restricted protein diets help reduce the production of the toxic waste, ammonia, and will therefore help lessen the need for liver detoxification. Owners and veterinarians should thoroughly discuss the seriousness, expense, and expected outcome associated with the management of all individuals suspected of having a liver shunt.

Information gathered from Foster & Smith