PORTOSYSTEMIC SHUNTS

Article Written by Dr. Daniel A. Degner, Board-certified Veterinary Surgeon (DACVS)

Key Points

A portosystemic shunt, is an abnormal vessel that allows blood to bypass the liver. As a result the blood is not cleansed by one of the bodies filters: the liver. Generally the bile acids levels (after feeding a meal) in patients having a shunt are higher than 100. Surgery is the best treatment for a shunt. The overall success rate is about 85%.

Shunt anatomy and physiology

In the fetus a shunt, called the patent ductus venosus, is present and bypasses blood away from the liver to the placenta so that the mother can cleanse the blood for the fetus.
Once the fetus is born the shunt closes within three days after birth and the puppy's liver must clean the blood. Sometimes the shunt does not close off. A portosystemic shunt, therefore is an abnormal vessel that allows blood to bypass the liver. As a result the blood is not cleansed by one of the bodies filters: the liver. These dogs also have much less blood that flows to the liver which causes the liver to remain small. There are many variations of congential portosystemic shunts that are found at surgery, however there two main groups: shunts located in the liver (intrahepatic shunt) and shunts located outside of the liver tissue (extrahepatic shunt). The intrahepatic shunt is most commonly found in large breed dogs and extrahepatic shunts are seen in small breeds. Above is a diagram showing the blood flow from the intestines, through the shunt (labeled) , then to the heart. Most of the blood does not flow to the liver due to the resistance of the filters in the liver, thus the blood is not cleansed by the liver

Clinical signs

Abnormal behavior after eating
Pacing and aimless wandering
Pressing the head against the wall
Episodes of apparent blindness
Seizures
Poor weight gain
Stunted growth
Excessive sleeping and lethargy
Straining to urinate due to bladder stone formation
Patient with a shunt may have many clinical signs and some have only a single clinical sign
Some dogs do not show signs until they are older

Diagnosis

A variety of blood tests can be used to help to support a diagnosis of a portosystemic shunt.
Bile acid test are always elevated. Generally the bile acids levels (after feeding a meal) in patients having a shunt are higher than 100. Other diseases such as microvascular dysplasia, generalized liver disease, and acquired shunts due to liver cirrhosis can also cause elevated bile acids tests. Sometimes the shunt can be identified with ultrasound imaging. Nuclear scintigraphy is a fairly reliable test to ascertain whether a shunt is present, but may not be able to differentiate acquired from congenital shunts. In most dogs we are able to identify the portosystemic shunt at the time of surgery. Sometimes multiple acquired shunts, caused by end-stage liver disease are found; unfortunately no surgical therapy is useful to treat this (other than liver transplant, which is not practical in dogs). If the shunt cannot be found at the time of surgery, dye is injected into one of the veins going to the liver and x-rays are taken (portogram). This will show the blood supply of the liver and the offending shunt, if it is present.

Treatment

If possible, we prefer to have the patient as stable as possible prior to surgery. This involves having your pet on a low protein diet and administering prescribed medication. Antibiotics are used as bacteria, which are normally removed by the liver, by pass the liver and result in bacteria circulating in the blood. Lactulose is a medication which traps toxins such as ammonia in the stool. It also decreases the transit time of the stool so that toxins are expelled quicker (thus the pet will defecate more often). Low protein diet should be fed in order to decrease poisons that affect the brain. Surgery is the best treatment for a shunt. Usually at the time of surgery the shunt can be identified (arrow showing large vessel) as is seen in photo below.

For pets that have a shunt that is located outside of the liver, an ameroid constrictor ring is placed around the vessel (in photo below see metal ring). This device slowly closes the shunt over a period of 6 weeks.If the shunt is located in the liver the surgery is much more complex. Because these shunts are usually found in large breed dogs, the shunt likewise is frequently very large. We have successfully used large ameroid constrictors for this purpose, but in some cases two surgeries are needed.

Potential complications

Complications can include high blood pressure (portal hypertension) of the vessels going to the liver, which results in fluid accumulation in the belly. If a high level of portal hypertension is present after the shunt is tied off, the pet will die.
About 15 % of the dogs having surgery will develop small-acquired shunts that basically function like the congenital shunt and thus the pet may need medical treatment for life.
Infection is an uncommon but possible complication.
Seizures may occur in the first 3 to 5 days after surgery.
Seizures can be caused by low blood sugar levels, which is easily treated
Seizures can be caused by the imbalances of the chemicals within the brain as the blood is being cleansed. These patients may die if they are not responsive to medication.

Postop care

After surgery your pet will still need to eat a low protein diet. Once bile acid levels (blood test) normalize a regular diet can be fed.
Lactulose and antibiotics are continued for about 10 days after surgery.

Prognosis

The overall success rate is about 85%. Usually the pet will start to feel better with 10 to 14 days after surgery

If you are concerned about the health of your pet you should contact your veterinary surgeon at all times.

My thanks to Dr. Daniel A. Degner for allowing me to use his material, original can be viewed at the Vet Surgery Central website.