Portosystemic Shunt

During development of the canine fetus, blood bypasses the liver, going directly back to the heart via a shunt. In normal development, this shunt closes off just after birth, and blood from the intestines is then sent through the liver to be detoxified before being routed back to the heart. If this shunt does not close, the pup develops a congenital portosystemic shunt.

Acquired portosystemic shunts are seen much later in life, developing as a response to portal hypertension. This, in turn, results from liver diseases such as chronic hepatitis or cirrhosis. It probably isn't hereditary.

Symptoms

While the shunt is usually present at birth, it takes some time for effects to be seen. Most dogs are diagnosed between six months and two years of age. In puppies, symptoms are subtle, but a pup may be smaller than littermates and show a failure to thrive, or a general impression of less than perfect health.

Later signs can include vomiting, diarrhea, pica (eating nonfood items), anorexia, and polydipsia and polyuria (drinking and urinating more than normal). Fluid can leak from the portal system into the abdominal cavity — the dog will appear pot-bellied. Because the liver plays a significant role in metabolism of drugs, a dog with a portosystemic shunt, when anesthetized for spay or neuter surgery, can take an uncommonly long time to wake up from the anesthetic.

Failure of the liver to clear toxins from the blood can result in detrimental effects on the brain. After a meal, when a large amount of by-products of digestion are circulating, the dog may stumble, walk in circles, or stand with his head pressed against the wall or the furniture. Seizures can occur.

Because the liver isn't metabolizing ammonia and urates effectively, compounds concentrate in the urine and cause stones in the urinary system. The stones result in bloody urine, straining to urinate, more frequent urination, abdominal pain, and possibly vomiting.

Diagnosis

As with many health problems, the first step is blood work. Blood work results showing elevated liver enzymes, lowered red blood cell count, low blood urea nitrogen (BUN), and low blood glucose indicate a possible portosystemic shunt. Urinalysis showing ammonium biurate crystals increases the probability.

If other tests will have to be done after the blood tests anyway, why should you bother with the blood tests?

Blood tests are relatively easy, inexpensive, and nonspecific, so they can quickly give a strong indication of what might be happening. Certain diseases can be ruled in or out on the basis of blood tests, saving you from more stressful and expensive testing that might be unnecessary.

Two more specific blood tests can then be used. Blood ammonia levels are checked. A high result indicates possibility of a shunt. The bile acid level blood test requires two samples: the first after the dog has not eaten for twelve hours, and the second two hours after a meal. If portosystemic shunt is present, the sample after eating will show elevated levels of bile acids in the blood.

If all indications point toward a shunt, an ultrasound may be done if an experienced diagnostician is available — it can be tricky to find and see the shunt this way. Alternatively a minor surgery called a portogram can be done. Through a small incision, dye is injected into a vein draining the small intestine, and a series of x-rays is taken. The radiographs will show either a normal circuit of blood through the liver or blood being carried past the liver in a shunt.

Treatment

Surgery is required, but some dogs may need to be medically managed firstto improve their general health to prepare to undergo the surgery. The diet may be changed to a low-protein choice, antibiotics might be given to decrease bacteria in the digestive tract, and lactulose, a laxative, can help decrease absorption of ammonia and other toxins.

The aim of surgery is to close off the shunt. Unfortunately, this can't be done all at once because the underdeveloped liver can't adapt to the sudden increase in blood flow that would occur. The sudden inflow of so much blood leads to portal hypertension, and can cause death within hours. So the shunt can only be partially closed.

One option is to partially close the shunt, wait a couple of months to give the liver time to adapt, and then perform a second surgery to further (maybe completely) close the shunt. Obviously, multiple surgeries increase the risk of surgical complications. The other option is to place a device called an ameroid constrictor around the shunt vessel. An outer metal ring holds the inner dried casein material against the shunt. As the casein beings to hydrate in the body, it swells, and because of the metal ring outside, it can only swell inward. At the same time, it inflames the vessel, creating swelling. It can take anywhere from two weeks to three months for the constriction and swelling to close off the shunt. The danger is in closing too quickly and creating portal hypertension.

For several months following either surgery, the dog remains on the special diet, antibiotics, and a laxative. Exercise must be strictly limited, and the dog should stay in clean areas only.

Photography by Cheryl A. Ertelt

Paying close attention to your dog's health aids in the prevention of common congenital and hereditary diseases.

Prognosis

If the condition is a single shunt that can be completely corrected — the shunt closed off — before the dog reaches a year of age, chances are good that the dog can lead a fairly normal life. After two years of age, progressive liver atrophy can make recovery impossible, and the dog can only be medically managed during a significantly shortened lifespan.

The surgery, though absolutely necessary, carries a substantial risk, with the mortality rate given as anywhere up to 20 percent. Some dogs develop constant seizures after surgery. Some dogs with corrected congenital portosystemic shunt develop acquired multiple portosystemic shunts later in life.

This is not an easy problem to resolve. If buying from a breeder, be sure that no relatives of the pups suffer from portosystemic shunt.

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