Portosystemic Shunts

By | 2011-08-22

Contents

1. What is a portosystemic shunt?

A portosystemic shunt is an abnormal vessel that connects the portal vein to a systemic vein. The most common locations for portosystemic shunts are a patent ductus venosus or a connection between the portal vein and caudal vena cava or azygous vein. Single extraheptic shunts are most common in small-breed dogs and cats, whereas single intrahepatic shunts are most common in large-breed dogs.

2. What is the difference between congenital and acquired portosystemic shunts?

Most acquired shunts are multiple and extrahepatic. Acquired shunts develop because of sustained portal hypertension from chronic liver disease and cirrhosis. Congenital portosystemic shunts are usually single and may be intra- or extrahepatic. The most common intrahepatic portosystemic shunt is a patent ductus venosus.

3. Are certain breeds associated with portosystemic shunts?

Congenital portosystemic shunts may occur in any breed of dog but are common in miniature schnauzers, miniature poodles, Yorkshire terriers, dachshunds, Doberman pinschers, golden retrievers, Labrador retrievers, and Irish setters. There are affected lines in miniature schnauzers, Irish wolfhounds, Old English sheepdogs, and Cairn terriers. Mixed breed cats are more commonly affected than purebred cats, but Himalayans and Persians seem to overrepresented as purebreds. Acquired portosystemic shunts are secondary to chronic hepatic disease and so may occur in any breed.

4. Where are most portosystemic shunts located?

Single extrahepatic shunts most commonly connect the portal vein (or the left gastric or splenic vein) with the caudal vena cava cranial to the phrenicoabdominal vein. Single intrahepatic shunts can be a communication of the portal vein to the caudal vena cava which is a failure of the ductus venosus to close. Shunts in the right medial or lateral liver lobes occur with an unknown pathogenesis.

5. Why do patients with portosystemic shunts have decreased liver function?

Portal venous blood is important because it brings hepatotropic growth factors and insulin to the liver. If insulin bypasses the liver in a shunt, significant quantities are utilized by other organs and the liver receives less benefit. Portal venous blood flow is important for normal liver development as well as glycogen storage, hypertrophy, hyperplasia, and regeneration. Congenital portosystemic shunts are often associated with hepatic atrophy, hypoplasia, and dysfunction.

6. What are the most common clinical signs of portosystemic shunts?

Failure to thrive and failure to gain weight are appropriately common. Most clinical signs are referable to hepatic encephalopathy, which is defined as clinical signs of neurologic dysfunction secondary to hepatic disease. Signs include ataxia, stupor, lethargy, unusual behavior, disorientation, blindness, and seizures. Some animals display anorexia, vomiting, and diarrhea. Polyuria and polydipsia may be present. Some animals have ammonium biurate urolithiasis, which may result in pollakiuria, hematuria, stranguria, or obstruction. Increased production of saliva (ptyalism) and abdominal distention due to ascites occur in some animals. Ptyalism is more common in cats.

7. What causes hepatic encephalopathy associated with portosystemic shunts?

Products of bacterial metabolism in the intestine, such as ammonia, short-chain fatty acids (SCFAs), mercaptans, gamma-aminobutyric acid (GABA), and endogenous benzodiazepines have been suggested as mediators of hepatic encephalopathy. In addition, the ratio of aromatic amino acids to branched-chain amino acids is often increased in patients with portosystemic shunts. The aromatic amino acids may act as false neurotransmitters. Phenylalanine and tyrosine may act as weak neurotransmitters in the presynaptic neurons of the CNS. Tryptophan causes increased production of serotonin, which is a potent inhibitory neurotransmitter. The GABA receptor has binding sites for barbiturates, benzodiazepines, and substances with similar chemical structure to benzodiazepines. These agents may be responsible for depression of the CNS in hepatic encephalopathy.

8. What factors may precipitate an hepatic encephalopathy crisis?

A protein rich meal, gastrointestinal bleeding associated with parastites, ulcers or drug therapy; administration of methionine- containing urinary acidifiers; or lipotropic agents may precipitate a crisis. Blood transfusions with stored blood may also contribute to a crisis as the ammonia levels can be high in the stored blood.

9. How is hepatic encephalopathy treated?

The animal should be evaluated for hypoglycemia immediately and treated appropriately if it is present. Appropriate fluid therapy based on acid-base and electrolyte status (see chapter 81) should be initiated to correct abnormalities. LRS should be avoided. Hypoglycemia, alkalosis, hypokalemia, and gastrointestinal bleeding should be identified and corrected. Ammonia concentration and production should be decreased by administering lactulose and neomycin (10-20 mg/kg orally every 6 hr) if a swallow response is present. Oral metronidazole may be used at a dose of 10 mg/kg every 8 hr in place of neomycin. If the animal is comatose, 20-30 ml/kg of lactulose diluted 1:2 with water or a 1:10 dilution of povidone-iodine solution may be given as an enema. Seizures may be treated initially with elimination of ammonia by enemas as listed above. Oral loading doses of potassium bromide may be useful. If seizures cannot be controlled, IV propofol as a constant rate infusion may be necessary, but respiratory support may be needed. Some animals with hepatic encephalopathy have difficulty in metabolizing benzodiazepines such as diazepam, which should be avoided. If these drugs do not control seizures, intravenous phenobarbital may be titrated slowly to effect. Patients often have decreased clearance of barbiturates.

10. What routine blood work and urinalysis abnormalities suggest portosystemic shunts?

Microcytosis is a consistent abnormality of complete blood cell count in animals with portosystemic shunts. Some animals manifest acid-base, electrolyte, and glucose disturbances (hypoglycemia). Because of vomiting and dehydration, prerenal azotemia may be present. There is no consistent finding with regard to alanine aminotransferase (ALT), aspartate aminotransferase (AST), and serum alkaline phosphatase (ALP); activities of these enzymes may be elevated, decreased, or normal in patients with portosystemic shunts. Hypoalbuminemia is common, as are coagulopathies. Some animals have isothenuric urine due to medullary wash-out; ammonium biurate crystals may be identified on microscopic examination of urine sediment.

11. What are the best ways to diagnose a portosystemic shunt?

Elevated serum pre- and postprandial bile acids in a young animal with signs of hepatic encephalopathy and stunted growth are consistent with but not diagnostic for portosystemic shunts. A nuclear medicine scan using transcolonic sodium pertechnetate Tc99m demonstrates radioactivity in the heart before the liver in an animal with portosystemic shunt. Nuclear medicine is rapid, noninvasive, and safe to the animal. The disadvantages are that the animal is radioactive for 24 hours, studies can be performed only by specially trained personnel, exact location of the shunt cannot be determined, and cases of hepatic microvascular dysplasia, which have shunting within the liver (as in Cairn terriers), may give false-negative results. When nuclear medicine facilities are unavailable, positive contrast portography may demonstrate the anomalous vessel. Portography, however, is technically demanding and invasive. Furthermore, a second surgical procedure is required to repair the shunt because of an otherwise dangerously long period of anesthesia. The major advantage of positive contrast portography is that it definitively locates the shunt.

12. What is the best way to manage a patient with portosystemic shunt?

Although medical management may be beneficial, surgical ligation of the shunt is optimal. In one study, animals that receive total ligation, even if it had to be done in two or more surgeries, showed more clinical improvement than patients with incomplete shunt ligation. In general, cats do not do as well with medical therapy.

13. Describe the preoperative management of a patient with portosystemic shunt.

In animals displaying hepatic encephalopathy, it is important to correct acid-base and electrolyte disturbances before surgery. Measures to control hepatic encephalopathy also should be performed before surgery, including a low protein diet, oral lactulose, and neomycin or metronidazole. A moderately protein-restricted diet with the bulk of calories coming from carbohydrates and fat is optimal. Vegetable and dairy proteins are better tolerated than meat and egg proteins. With each patient, the protein level should be increased to the maximum tolerated. Psyllium at 1-3 teaspoons per day has been advocated to help tolerance of proteins. Some have recommended supplementation with vitamins A, B, C, E, and K. Medical stabilization for 1-2 weeks before surgery is recommended for all patients with portosystemic shunts. A preoperative coagulation screen should be performed, and crossmatched fresh whole blood should be available. Fresh frozen plasma transfusions may be necessary for hypoalbuminemic patients. Most surgeons administer a broad-spectrum antibiotic (e.g., first-generation cephalosporin) intravenously before and during surgery.

14. What considerations must be given to drug therapy and anesthetic use in patients with portosystemic shunts?

Because liver function decreases in patients with portosystemic shunts, drugs that are potentially hepatotoxic should be avoided. In addition, hepatic clearance of drugs and anesthetic agents may be delayed.

15. What parameters should be monitored postoperatively in patients with portosystemic shunts?

After surgery, many patients with portosystemic shunts are hypoglycemic, hypothermic, and hypoalbuminemic. A postoperative database should include body weight, temperature, packed cell volume, total solids, and glucose. Additional useful information is provided by electrolytes and albumin. Maintaining hydration status and perfusion with a balanced electrolyte solution is important. Mucous membrane color, capillary refill time, pulse rate and quality, and temperature should be assessed, and the patient should be monitored for seizures. In addition, serial measurement of abdominal circumference is helpful because a number of patients develop portal hypertension and ascites postoperatively.

16. What are common postsurgical complications?

Sepsis, seizures, and portal hypertension are the most critical complications that may develop postoperatively, although pancreatitis and intussusceptions have been reported. Gastrointestinal hemorrhage also may result, which can precipitate a hepatic encephalopathy crisis. Animals with seizures should be treated with appropriate measures to normalize acid-base and electrolyte balance. Sepsis should be treated aggressively.

17. What are common signs of postoperative portal hypertension?

Portal hypertension most commonly results in abdominal distention secondary to ascites. In some cases, portal hypertension is subclinical and ascites resolves in several days. Some patients develop abdominal distention, pain, and hypovolemia; others have abdominal distention with severe pain, hypovolemia, cardiovascular collapse, hemorrhagic diarrhea, and septic or endotoxic shock.

18. How should postoperative portal hypertension be treated?

If the animal develops abdominal distention with no clinical signs of pain or discomfort, continued medical therapy is indicated. Most animals with pain and abdominal distention stabilize with colloid fluid therapy. Patients with severe pain, abdominal distention, bloody diarrhea, and cardiovascular shock should be treated for shock with fluids, stabilized as much as possible, and taken for exploratory surgery to remove the ligature or thrombus that has probably developed in a partially attenuated portosystemic shunt.

19. Why may a patient with portosystemic shunt become septic postoperatively?

A patient with portosystemic shunt may develop septic peritonitis postoperatively because of bacteremia in the portal vein. The monocyte-phagocyte system in the liver may not be fully functional. Sepsis may develop as a result of inadequate filtering of portal blood by the liver before the blood reaches the systemic circulation.

20. What is hepatic microvascular dysplasia?

Hepatic microvascular dysplasia is a congenital disorder with histologic vascular abnormalities that resemble those seen in portosystemic shunts.

21. Are there breed predispositions for hepatic microvascular dysplasia?

Cairn and Yorkshire terriers are most commonly affected with hepatic microvascular dysplasia. However, many other breeds, including dachshund, poodle, Shih Tzu, Lhasa Apso, cocker spaniel, and West Highland White terrier may be affected.

22. What are the clinical signs of hepatic microvascular dysplasia?

Clinical signs are not consistently seen, but in severe cases they are quite similar to those seen with portosystemic shunts. Hyperammonemia and ammonium biurate cystalluria rarely develop. A dog may have hepatic microvascular dysplasia with elevated bile acids but be sick for another cause.

23. When should hepatic microvascular dysplasia be considered as a differential diagnosis?

Hepatic microvascular dysplasia should be considered in a patient with clinical signs consistent with a portosystemic shunt, increased bile acid concentration, and consistent liver biopsy results. Scintigraphy is consistently normal.

24. What is the treatment for hepatic microvascular dysplasia?

Treatment should not be done if the patient is subclinical. If signs of hepatic encephalopathy are present, treatment is indicated as for patients with portosystemic shunts. It is unknown at this time whether subclinical patients will develop signs of disease.