LIVER SHUNTS

 

 

 

Liver shunts cause serious and sometimes fatal outcomes in dogs. A liver shunt, or a portosystemic shunt, is a normal fetal blood vessel that in the womb bypasses liver tissue, allowing the mother's system to filter out toxins for the developing baby. In some animals however, the shunt remains open after the animal is born compromising its liver function, slowing growth, and eventually resulting in death of many affected animals. Congenital portosystemic shunts may be repaired with traditional surgical approaches but a technique developed at the University of Tennessee several years ago implants an ameroid constrictor, a tiny C-shaped piece of metal ring. The constrictor fits around the shunt causing it to slowly shut down over several weeks. Dogs receiving this method of surgical repair generally have a shorter surgery and fewer postoperative complications than traditional methods.

What is a portosystemic shunt?

In animals with a portosystemic shunt (PPS) 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 behavioral 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 congenital 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:

Extra-hepatic PSS: small breeds, especially the Yorkshire Terrier, Miniature Schnauzer and less commonly, Cairn Terriers and Maltese.

Intra-hepatic PSS: large and giant breeds, especially the Irish Wolfhound and less commonly, Australian Cattle Dogs, Golden Retrievers, and 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 and 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 behavioral 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 re-circulated 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.

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.

 

 

 
 
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