related terms: hepatic encephalopathy, congenital portosystemic shunt, extrahepatic or intrahepatic portosystemic shunt

What is a portosystemic shunt?

In animals with a portosystemic shunt (PSS) there is abnormal blood flow in the liver. Blood should flow from the digestive tract to the liver via the portal system into the blood vessels of the liver, and then to the caudal vena cava which is the large blood vessel carrying blood back to the heart. In a portosystemic shunt, as the name implies, portal blood by-passes the liver and goes directly to the systemic venous circulation (caudal vena cava). One important function of the liver is to clear toxins, many of which are by-products of protein digestion, from the blood. In PSS, these toxins are not cleared, and circulate in the body. This causes the clinical signs associated with PSS, many of which are neurological. The complex of neurological and behavioural signs caused by liver dysfunction is called hepatic encephalopathy.

Portosystemic shunts may be acquired secondary to another disease, or they may be congenital - that is the animal is born with a shunt. A congenital shunt usually occurs as a single abnormal blood vessel that is a remnant of normal embryonic development. These shunts are defined as intra-hepatic or extra-hepatic depending on the location of the blood vessel in relation to the liver.

Most animals with congenital portosystemic shunts show clinical signs before 6 months of age. Where signs are subtle, the condition may not be diagnosed until much later.

How are congential portosystemic shunts inherited?

The mode of inheritance is not known.

What breeds are affected by congenital portosystemic shunts?

There is a breed predisposition to congenital PSS in the following breeds:

extrahepatic PSS: small breeds, especially the Yorkshire terrier, miniature schnauzer and, less commonly, Cairn terriers, Maltese.

intrahepatic PSS: large and giant breeds, especially the Irish wolfhound and, less commonly, Australian cattle dogs, golden retrievers, Labrador retrievers. Approximately one third of large dogs with shunts have extra-hepatic shunts.

For many breeds and many disorders, the studies to determine the mode of inheritance or the frequency in the breed have not been carried out, or are inconclusive. We have listed breeds for which there is a general consensus among those investigating in this field and among veterinary practitioners, that the condition is significant in this breed.

What does a congenital portosystemic shunt mean to your dog & you?

If your dog has a congenital portosystemic shunt, you will likely see signs of this while he or she is a young puppy. These signs are generally associated with the central nervous system, the gastrointestinal tract, or the urinary tract. Most consistently, there are signs of hepatic encephalopathy - neurological and behavioural evidence of diffuse brain disease due to liver dysfunction. These signs can be quite vague and may include loss of appetite, depression, lethargy, weakness, poor balance, disorientation, blindness, seizures, and coma. The signs may wax and wane, and may worsen after eating a protein-rich meal.

Your pup may appear to be growing very slowly. Other non-specific gastrointestinal signs can include intermittent loss of appetite, vomiting and diarrhea.

Failure of the liver to clear ammonia means that there will be increased excretion in the urine. This commonly leads to urolithiasis - kidney, bladder or urethral calculi or stones due to the build-up of mineral salts. Your dog may have blood in the urine, or difficulty or pain in urinating.

The first sign of PSS in a dog may be a prolonged recovery from anesthesia, or excessive sedation after treatment with some medications. This occurs because the drugs are not metabolized as they would normally be by the liver, but instead are recirculated in the body.

How is a congenital portosystemic shunt diagnosed?

Generally the diagnosis of congenital PSS is suspected based on the history, clinical signs, and laboratory features. Typically an affected dog is young, of a breed with a predisposition for PSS, with clinical signs and laboratory findings relating to liver dysfunction. A special radiographic tool, contrast portal radiography, is the best way to confirm the diagnosis. Contrast dye is injected into one of the blood vessels going into the liver. In a normal liver, the contrast material disperses into the many blood vessels in the liver, but in congenital PSS, a large portion of the contrast bypasses those vessels and goes directly to the caudal vena cava, the large blood vessel that carries blood to the heart. Contrast radiography also helps in assessing the chances of successfully tying off the shunt surgically. The more contrast that is apparent in the liver, the higher the likelihood of success. Contrast radiography will also identify whether the shunt is intra- or extra-hepatic.

FOR THE VETERINARIAN:

    • HEMATOLOGY: often unremarkable. Subtle but important clues may include microcytosis, target cells, and/or a mild nonregenerative anemia.
    • BIOCHEMISTRY: mild abnormalities suggestive of hepatic dysfunction are seen, such as hypoproteinemia, hypoglobulinemia, hypoalbuminemia, hypoglycemia, normal to mild increases in serum liver enzymes. Postprandial serum bile acids (SBA) are consistently elevated, and the ammonia tolerance test is consistently abnormal in dogs with PSS. (See note re Irish wolfhounds below.)
    • URINALYSIS: dogs with polyuria and polydipsia often are isosthenuric or hyposthenuric. Ammonium biurate crystals in the urine sediment are an important and common finding. Where urolithiasis occurs, there may also be hematuria, proteinuria and pyuria.
    • PLAIN RADIOGRAPHY: commonly see a small liver.
    • ADDITIONAL RADIOGRAPHIC TECHNIQUES: such as ultrasonography, contrast portography, and rectal portal scintigraphy can provide information about the presence, location, and type of shunt .
    • IRISH WOLFHOUNDS:
      • · A typical hereditary intrahepatic portosystemic shunt occurs in 2-3% of Irish wolfhounds. This causes high blood concentrations of ammonia, and clinical signs of hepatic encephalopathy.
      • · Less dramatic elevations of blood ammonia are common in young Irish wolfhounds (transient metabolic hyperammonaemia). This is not associated with a portosystemic shunt or with any clinical signs. It appears to be a transient juvenile defect in ammonia metabolism that is present in almost all Irish wolfhounds, and resolves with age, causing no problems for the dogs.
      • · In Irish wolfhounds, measurement of post-prandial bile acid concentration is the preferred means to differentiate normal pups from those with a PSS. The fasting ammonia test is not reliable, since there is considerable overlap between normal and affected pups. (See references below for more information on screening Irish wolfhound pups for PSS).

How is a congenital portosystemic shunt treated?

Many of the clinical signs associated with PSS can be improved by medical management. Since circulating ammonia is one of the main causes of hepatic encephalopathy, and ammonia is derived from eating meat, a low-protein diet is an essential component of medical therapy for PSS.

While medical treatment will improve the clinical signs temporarily, surgery to tie off the shunt is required to correct the condition for the long term. Ligation (tying-off) of the shunt may be partial or complete. An intra-hepatic shunt is generally technically more difficult to correct than an extra-hepatic one. Your veterinarian may suggest your dog go to a referral centre, both because the procedure is best performed by an experienced surgeon and because close monitoring is required post-operatively to watch for signs of portal hypertension. This potentially serious complication may develop within 24 hours of surgery, due to the increase in blood flow in the liver once the blood is no longer moving through the shunt.

With single extra-hepatic shunts, there is a very good prognosis for a normal life for your dog after surgery. Dogs with intra-hepatic shunts may still have some biochemical abnormalities following surgery but in general, clinically, show much improvement.

Breeding advice

Affected individuals and their parents should not be used for breeding. Siblings should only be used after careful screening. If any affected offspring are born, breeding of the parents should be discontinued.

Reliable screening can be carried out on Irish wolfhound pups at 6 to 8 weeks of age, ie. before they leave the breeder. See resources listed below for more information.

FOR MORE INFORMATION ABOUT THIS DISORDER, PLEASE SEE YOUR VETERINARIAN.

Resources

Kerr, M.G. and van Doorn, T. 1999. Mass screening of Irish wolfhound puppies for portosystemic shunts by the dynamic bile acid test. Veterinary Record 144:693-696

Copyright © 1998 Canine Inherited Disorders Database. All rights reserved.
Revised: October 30, 2001.