Portosystemic Shunt in Dogs

What is a liver shunt?

A liver shunt is abnormal blood circulation in which blood detours around the liver instead of going through it. Normally, a large vein, called the portal vein, collects blood from the gastrointestinal system, pancreas, and spleen and delivers it to the liver. The liver then filters out toxins and metabolizes nutrients before sending them to the rest of the body.

With a liver shunt, blood bypasses this important step and instead moves directly into the general circulation. The portal vein also supplies nutrients and oxygen to the liver, helping it survive and function. Dogs with a liver shunt generally have a small liver with reduced function, since it is deprived of this blood supply.

In most cases, a liver shunt is caused by a birth defect, in which case it is called a congenital portosystemic shunt. In some cases, multiple small shunts form because of severe liver disease, such as cirrhosis. These are referred to as acquired portosystemic shunts.

How does a congenital portosystemic shunt develop?

All mammalian fetuses have a large shunt called the ductus venosus that carries blood quickly through the fetal liver to the heart. A congenital portosystemic shunt develops in the following circumstances: 

• The ductus venosus fails to collapse at birth and remains intact and open after the fetus no longer needs it. 
• A blood vessel outside the liver develops abnormally and remains open after the ductus venosus closes. 

What are the clinical signs of a liver shunt?

The most common clinical signs include stunted growth, poor muscle development, abnormal behaviors such as disorientation, staring into space, circling or head pressing, and seizures. Less common symptoms include drinking or urinating too much, vomiting, and diarrhea. 

Dogs with a liver shunt often take a long time to recover from anesthesia. Behavioral clinical signs may only occur after eating high-protein meals. Some dogs do not show signs until they are older, when they develop urinary problems such as recurrent kidney or bladder infections or stones.

How is a liver shunt diagnosed?

Diagnosis is based on medical history and clinical signs. Common diagnostic tests include:

• Complete blood count (CBC) and serum chemistries. Typical abnormal findings include mild anemia or smaller than normal red blood cells (microcytosis), low blood urea nitrogen (BUN) and albumin, and increases in liver enzymes (AST, ALT).
• Urinalysis. Urine may be dilute (low urine specific gravity) or there may be evidence of infection. The urine may contain small, spiky crystals known as ammonium biurate crystals.
• Bile Acid Test. Most dogs with liver shunts have elevated bile acids. If the bile acids are mildly increased, or if the dog seems clinically normal despite abnormal test results, the tests will often be repeated in three to four weeks.

Additional diagnostic tests may include:

• Ultrasound with Doppler flow analysis (this helps to better define blood flow between the gut and liver).
• Computed tomography (CT) scan or nuclear scintigraphy, which is a nuclear scan that measures blood flow through the liver.
• Portovenography, which is an X-ray showing the blood vessels supplying and/or bypassing the liver, using radio-opaque dye injected directly into the portal vein.
• Magnetic resonance imaging (MRI).
• Exploratory surgery (laparotomy).

Certain breeds, such as Yorkshire terriers, Old English sheepdogs, Irish wolfhounds, Cairn terriers, and beagles have an increased incidence of portosystemic shunts. Small-breed dogs usually have extrahepatic shunts (abnormal blood vessels outside the liver) while larger breeds have intrahepatic shunts (abnormal blood vessels inside the liver). Extrahepatic shunts are easier to surgically repair than intrahepatic shunts.

What are bile acids?

Bile acids are produced in the liver and stored in the gall bladder between meals. Normally, they are released into the intestines to help break down and absorb fats, and then are reabsorbed by the portal system and enter the liver. Once in the liver, bile acids are removed from circulation and stored again, until needed. Dogs with liver shunts have increased bile acid concentrations in the blood because the liver does not get a chance to remove and store these chemicals after they are reabsorbed.

Tests that measure bile acids in the blood are used to screen for liver shunts. To perform this screening test, two blood samples are usually taken. The first sample is taken after fasting (pre-prandial). The second sample is usually taken two hours after being fed (post-prandial). The technique may vary based on the patient and on your veterinarian’s preferences.

How is a portosystemic shunt treated?

Dogs with portosystemic shunts are usually stabilized with special diets and medications, which attempt to reduce the amount of toxins produced and absorbed in the large intestines. Dogs that are severely ill may require intravenous fluids to stabilize blood sugar, an enema to remove intestinal toxins before they are absorbed, and medications such as diazepam (Valium®) to stop seizures.

The most common medical treatments include:

• Diet change. The optimal diet for a portosystemic shunt has not been firmly established. It was thought that restricting protein was ideal, but new approaches focus less on reducing protein in the diet. Feeding a highly digestible protein has been recommended, as have diets that are soy-based rather than meat-based. More research is needed. Restriction of manganese and copper is also recommended.
• Lactulose. Administering lactulose changes the pH in the large intestines, which decreases the absorption of ammonia and other toxins and makes the intestinal environment unfavorable for toxin-producing bacteria.
• Antibiotics. In some cases, antibiotics are used to alter the bacterial population in the intestines, and to reduce intestinal bacterial overgrowth.

What is the prognosis for a dog diagnosed with a portosystemic shunt?

Most dogs improve almost immediately with proper diet and medication. About one-third of the dogs treated medically will live a relatively long life. Unfortunately, over half of the dogs treated medically are euthanized within 10 months of diagnosis because of uncontrollable neurological signs such as seizures, behavioral changes, or progressive liver damage.

Dogs that do well with long-term medical management are usually older at the time of diagnosis, have more normal blood test values, and have less severe clinical signs. Dogs with a single shunt, especially one that is extrahepatic, have an excellent prognosis if it is surgically corrected.

What does portosystemic shunt surgery involve?

Most surgeons use a device such as an ameroid constrictor that slowly closes the shunt. The ameroid constrictor is a metal band with an inner ring of casein, a protein found in milk. In the abdomen, the inner ring absorbs normal abdominal fluid and gradually swells, pressing on the shunt and encouraging it to scar shut. Shunts usually close within three to four weeks after ameroid constrictor placement.

Other surgical treatments include 1) cellophane bands that induce inflammation, gradually closing the shunt with scar tissue and 2) intravascular (into the vein) occlusive, clot-inducing devices.

How successful is surgical treatment?

Surgery provides the best chance for a long, healthy life in most dogs with extrahepatic shunts, with a survival rate of 84%- 95%. Many dogs are clinically normal within four to eight weeks following surgery. A small percentage of dogs will develop multiple acquired shunts and must be managed with a protein-restricted diet and lactulose for life.

What is involved in the post-surgical management of portosystemic shunt patients?

It is important to feed your dog a protein-restricted diet for a minimum of six to eight weeks. After blood test values return to normal, your dog may return to a high-quality maintenance diet. Lactulose is usually given for several weeks after surgery.

The liver will begin to grow as the shunt closes and will often be normal in size and function within two to four months. Blood tests will be repeated at regular intervals to evaluate liver function.