Management of Portosystemic Shunts
Portosystemic Vascular Anomalies (PSVA) or Portosystemic Shunts (PSS) are anomalous vessels that allow normal portal blood to pass directly into the systemic circulation without first passing through the liver. PSS is dog and cats can either be congenital or acquired. The congenital form is the most common. Congenital PSS are anomalous vessels that usually occur as single intrahepatic or extrahepatic shunts and are not secondary to portal hypertension. Congenital PSS commonly occur in miniature and toy-breed dogs such as Yorkshire Terriers, Lhasa Apso, Pekingese, Poodles, and Miniature Schnauzers. In cats, Persians and Himalayans appear to be at increased risk. Acquired PSS form in response to portal hypertension. These shunts are typically multiple extrahepatic shunts that connect the portal system and the caudal vena cava. Single intrahepatic PSS is a communication between the portal vein and the caudal vena cava. These shunts can be classified as left, central, or right divisional. The pathogenesis of intrahepatic PSS that occur in the right medial (central divisional) or right lateral (right divisional) liver lobes is unknown. A left divisional shunt (via the left hepatic vein) is consistent with patent ductus venosus. Single intrahepatic PSS are most common in large breed dogs.
Many affected animals have a history of stunted growth, failure to gain weight compared with unaffected littermates, or weight loss. A history of prolonged recovery after general anesthesia or excessive sedation after treatment with tranquilizers, anticonvulsants, or organophosphates can be attributed to impaired hepatic metabolism of these substances. Clinical signs of hepatic encephalopathy (HE) predominate on history and physical examination because of inadequate hepatic clearance of enterically derived toxins such as ammonia, mercaptans, short-chain fatty acid, gamma-aminobutyric acid, and endogenous benzodiazepines. Decreased hepatic blood flow and lack of hepatotropic factors such as insulin, glucagons, and nutrients result in hepatic atrophy. The most consistent signs of HE are often subtle, such as anorexia, depression, and lethargy. Other common findings indicative of diffuse cerebral disease include episodic weakness, ataxia, head pressing, disorientation, circling, pacing, behavioral changes, amaurotic blindness, seizures, and coma. Hypersalivation is a prominent sign in cats but also occurs in dogs. Signs of HE may be exacerbated by a protein-rich meal; gastrointestinal bleeding associated with parasites, ulcers, or drug therapy; or administration of methionine-containing urinary acidifiers or lipotropic agents. Gastrointestinal signs of intermittent anorexia, vomiting, and diarrhea are common nonspecific features of hepatic dysfunction and are not necessarily accompanied by overt signs of HE. Urate urolithiasis, an important complication of PSS, occurs because of increased urinary excretion of ammonia and uric acid. Renal, cystic, or urethral calculi may occur.
Routine hematologic findings are often unremarkable in dogs and cats with congenital PSS. Hematologic findings include erythrocytic microcytosis, target cells, poikilocytosis (especially in cats), and mild nonregenerative anemia. These red blood cell changes can be subtle but important diagnostic clues in an otherwise normal CBC. The cause of microcytosis is not known; however, decreased serum iron concentration, normal to increased ferritin concentration, and accumulation of stainable iron in the liver suggest that microcytosis is associated with abnormal iron metabolism rather than absolute iron deficiency.
Biochemical findings are suggestive of hepatocellular dysfunction. These include hypoproteinemia, hypoalbuminemia, hypoglobulinemia, hypoglycemia, decreased BUN, and hypocholesterolemia. Hypoglycemia, especially after a prolonged fast, is most likely in affected toy breeds of dogs. Potential mechanisms for hypoglycemia include decreased hepatic glycogen stores, decreased insulin catabolism, and endotoxemia. The total serum bilirubin concentration is typically normal. The serum liver enzyme activity (ALP, ALT, and AST) is normal to mildly (two or three times) increased consistent with a lesion of hepatic atrophy and minimal hepatocellular injury or intrahepatic cholestasis. Coagulation tests in dogs may show increased partial thromboplastin times and hypofibrinogenemia, but clinical evidence of a bleeding problem is rare.
Isosthenuria or hyposthenuria is frequently detected, by urinalysis of dogs that are polyuric and polydipsic. Ammonium biurate crystals are a common finding on urine sediment examination and are an important clue to underlying liver disease in dogs and cats. If urolithiasis is a complication of congenital PSS, additional findings may include hematuria, proteinuria, and pyuria.
Serum bile acid concentrations should be determined to document hepatic dysfunction in dogs and cats suspected to have congenital PSS. The fasting SBA is often increased but can be normal, because during prolonged fasting, the liver may eventually clear the bile acids from the systemic circulation. Postprandial SBA is consistently abnormal and is a good screening test for animals suspected to have PSS. Postprandial SBA concentrations typically exceed 100 umol/L. If postprandial SBA concentrations are consistently in the normal range, a diagnosis of congenital PSS is unlikely. Hyperammonemia is a common finding in dogs and cats with PSS, although a fasting blood ammonia concentration may be normal. The ammonia tolerance test is consistently abnormal and is equal in sensitivity to postprandial SBA in detecting hepatic dysfunction associated with congenital PSS.
Survey abdominal radiographs are often obtained for animals with suspected PSS to evaluate for microhepatica or presence of urinary calculi and to investigate other causes of gastrointestinal or urinary tract signs. Microhepatica is a common finding on survey abdominal radiographs of dogs with congenital PSS. Ammonium urate calculi are not usually visible on survey radiographs unless they also contain substantial amounts of magnesium and phosphate. Additional radiographic imaging techniques, such as ultrasonography, contrast portography, or transcolonic portal scintigraphy, can provide important information about the presence, location, and type-of PSS. Although ultrasonography and transcolonic portal scintigraphy have the advantage of being noninvasive, contrast portography is still considered the "gold standard" for the anatomic evaluation of the portal vasculature.
Medical management of HE in dogs and cats with congenital PSS is indicated before anesthesia and definitive surgical correction. A diet that is moderately protein restricted with the bulk of calories derived from carbohydrates, fat, and dairy (cottage cheese, yogurt) proteins are preferred. Meat and egg proteins are poorly tolerated. The recommended dietary protein intake on a dry matter basis for patients with HE is 18 to 22 per cent (dogs) and 30 to 35 percent (cats). The protein content of the diet should be increased to the maximum amount tolerated without signs of HE. Dietary supplementation with soluble fiber (psyllium 1 to 3 teaspoons per day) appears to be beneficial in managing HE by mechanisms similar to those with lactulose and may allow higher levels of dietary protein to be tolerated. Lactulose, a non-metabolizable disaccharide, acidifies colonic contents (causing ammonia trapping), shortens the intestinal transit time, alters colonic flora, promotes incorporation of ammonia into bacterial proteins, and reduces production of potentially toxic short-chain fatty acids (SCFA) by producing the nontoxic SCFA acetate. The dose is 0.1 to 0.22 mL/lb by mouth every 8 to 12 hours to achieve two or three soft stools per day. It can be safely given on a long-term basis. Antibiotics such as neomycin (10 mg/lb by mouth every 8 to 12 hours) or metronidazole (4 mg/lb by mouth every 12 hours) are commonly used on a short-term basis to alter the urease-producing intestinal bacterial population. Systemic antibiotics such as amoxicillin or ampicillin are also effective.
When severe CNS depression or coma prevents oral administration of lactulose and neomycin, these drugs are administered by enema. Acute decompensation of HE requires fluid therapy for correction of dehydration, correction of electrolyte and acid-base imbalances, and maintenance of blood glucose. Lactated Ringer's solution should be avoided. Precipitating causes of HE such as hypoglycemia, gastrointestinal bleeding, hypokalemia, and alkalosis should be identified and corrected whenever possible. Benzodiazepines, sedatives, and tranquilizers should be avoided. In addition to routine management of HE, control of seizures with anticonvulsant therapy (potassium bromide or phenobarbital) is indicated before general anesthesia and surgery.
The short-term response to therapy for HE in dogs with congenital PSS is often dramatic. Most dogs are clinically normal with therapy, even before surgical shunt ligation. The response of cats to medical management of HE may not be as rewarding. If surgical shunt correction is not feasible or is declined by the owner, long-term medical management can adequately control clinical signs for as long as 2 to 4 years in some dogs. However, most dogs managed medically on a long-term basis are not clinically normal and eventually have refractory neurologic signs. Medical therapy does not reverse the progressive hepatic atrophy and associated alterations in carbohydrate, lipid, and protein metabolism.
The treatment of choice for dogs and cats with a congenital PSS is surgical attenuation or ligation of the anomalous vessel. Single intrahepatic shunts are technically more difficult to correct than single extrahepatic shunts. Total surgical ligation of a single congenital PSS is preferred; however, in many cases only partial ligation of the shunt can be safely performed because of the risk of portal hypertension (PH). PH occurs because the intrahepatic vasculature cannot accommodate the additional volume of portal blood that is diverted back to the liver after total occlusion of the shunt vessel. Many animals with partial suture ligation of a single extrahepatic PSS eventually have complete closure of their shunt, as assessed by transcolonic scintigraphy. However, recurrence of clinical signs (41 to 50 per cent of dogs) is more likely if a partial rather than complete ligation has been performed. A liver biopsy specimen is also taken at the time of surgery to rule out presence of other disease processes such as microvascular dysplasia.
Use of an ameroid constrictor for gradual occlusion of single extrahepatic PSS has been described. The ameroid constrictor is a specialized device consisting of hydrophillic casein material in a stainless steel ring. The device is surgically placed around the shunt, and as fluid is absorbed the lumen of the ring becomes progressively smaller, causing shunt occlusion. Advantages of this procedure include gradual progressive occlusion of the shunt over a 30- to 60-day period (thus preventing acute postoperative PH), decreased surgical and anesthesia time, and lack of need to monitor portal pressures during surgery. This technique appears preferable to suture ligation for single extrahepatic PSS and makes the surgical issue of partial versus complete shunt ligation obsolete. Suture ligation is still indicated for most intrahepatic PSSs because ameroid constrictors may not be available in large enough sizes and surgical access to the shunt is more difficult. Successful use of transvenous coil embolization for gradual occlusion of a patent ductus venosus under radiographic guidance has also been described.
When suture attenuation or ligation is performed, PH may occur 2 to 24 hours after surgery. Signs of acute severe PH include abdominal distention and pain, bloody diarrhea, ileus, endotoxic shock, and peracute cardiovascular collapse. In the postoperative period, ascites may be exacerbated by severe hypoalbuminemia or overzealous fluid therapy. Moderate to severe ascites may also occur postoperatively. Ascites usually resolves within 14 to 21 days after surgery. Sustained PH that is not immediately lifethreatening can result in the development of multiple acquired PSSs after 1 to 2 months. On occasion, seizures and status epilepticus are a complication of surgical shunt ligation. The use of an ameroid constrictor appears to prevent the likelihood of this complication. Dogs older than 18 months of age may be at increased risk. The pathogenesis is obscure, but seizures do not appear to be caused by simple hypoglycemia or HE. It is possible that the brain may have adapted to an altered metabolism. Sudden withdrawal of the anticonvulsant effects of endogenous benzodiazepines (produced in the gut) after ligation of the PSS has been hypothesized. The prognosis for recovery from this complication is poor.
Routine postoperative management consists of systemic antibiotics and fluid therapy. Oral lactulose and neomycin (or metronidazole) and a protein-restricted diet are usually continued for at least 4 to 8 weeks or longer, depending on the individual patient's clinical response. On a long-term basis, many dogs are clinically normal and do not require a protein-restricted diet or medications for HE, especially if total shunt ligation has been performed. After shunt ligation, hepatic regeneration and an increase in liver blood flow result in liver enlargement and reversal of histopathologic abnormalities. Indicators of hepatic function such as SBA concentrations often improve but do not usually return to normal, even in dogs that become clinically normal. Persistent hepatic dysfunction may be related to coexisting hepatic microvascular dyplasia and persistent microscopic shunting of portal blood. In one study, there was no correlation between follow-up SBA concentrations and the clinical response.
The prognosis in dogs for resolution of signs after total surgical ligation of the shunt is excellent if the dog survives the immediate postoperative period. In dogs with partial shunt ligation, the prognosis is not as good. Although clinical signs may resolve after surgery and the response appears favorable in the first few years, long-term follow-up (more than 3 years) suggests that signs recur in 40 to 50 percent of dogs with partial shunt legations. On the basis of this information, dogs who have previously undergone a partial ligation should be reevaluated by transcolonic scintigraphy. If shunting persists, surgical exploration to perform complete suture ligation or ameroid constrictor placement is indicated.
The response to surgical correction of a congenital PSS in cats appears to be less encouraging than in dogs. With partial shunt ligation, clinical improvement is usually noted after surgery, but relapse of clinical signs is common. Persistent seizures and blindness are also more likely to occur when partial rather than total ligation is performed. Total shunt ligation may not be possible because of the high likelihood of severe intrahepatic portal atresia and associated PH. The development of multiple acquired PSS after surgery appears to be more likely in cats than in dogs.
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Published on November 22, 2011.