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Portosystemic shunts are defined as a connection between the portal vessels and systemic circulation that diverts blood flow, in varying degrees, from the liver. They are categorized as either intrahepatic or extrahepatic shunts, and invariably result in poor hepatic hemoperfusion, with secondary fatty infiltration and iron accumulation within hepatocytes (lipogranulomas).
Decreased blood flow results in liver atrophy and subsequent dysfunction, decreasing liver metabolism of neurotoxins.
Extrahepatic shunts are the most frequent form of PSS and may arise from any part of the portal system. Most commonly, they originate from the main portal vein trunk, gastro-splenic or gastro-duodenal branches. Extrahepatic shunts usually drain into the posterior caval vein or the hemiazygos vein, but they can also empty into the hepatic veins, renal vein, phrenicoabdominal vein, internal thoracic vein or the thoracic posterior caval vein.
Although most extrahepatic congenital PSS are single vascular anomalies, multiple (mostly double) CPSS are occasionally diagnosed.
Portosystemic shunts can also be an acquired condition as a result of portal hypertension due to chronic hepatitis, congenital hepatic fibrosis, hepatic arteriovenous fistulas, hepatoportal vascular hypoplasia veno-occlusive disease in Cocker Spaniels or portal vascular atresia.
A number of breeds are predisposed, such as the Cairn Terrier, Miniature Schnauzer, Yorkshire Terrier, Maltese, Scottish Terrier, Pug, Irish Wolfhound, Golden Retriever, Labrador Retriever, German Shepherd and Poodle.
In the Irish Wolfhound, portosystemic shunts commonly occur due to delayed closure of the ductus venosus, resulting in an intrahepatic portosystemic shunt. The incidence of this condition in this breed is approximately 2.1 –3.4% and is due to a polygenic mode of inheritance.
Clinically-affected dogs are usually smaller than litter mates and may have concurrent abnormalities such as cryptorchidism. Symptoms first appear at about 6 months of age with mental depression due to hepatic encephalopathy, vomiting, diarrhea, blindness, pica, cystitis, polyuria and polydipsia.
A tentative diagnosis can be established on presenting clinical signs, and radiographic evidence of reduced liver size. Ultrasonographic, CT, MRI and transplenic portal scintigraphy imaging will assist a stronger suspicion of primary congenital liver disease.
Blood tests usually reveal a microcytic, nonregenerative anemia, poikilocytosis, target cells, hypoproteinemia, hypoalbuminemia, hypoglycemia (especially toy-breed dogs), low BUN, hypocholesterolemia, normal to mildly increased enzymes (ALT, AST, and AP), elevated c-reactive protein, reduced gastrin and normal bilirubin. After a prolonged fast, bile acids may rise from normal range of 15 - 30 to as high as 180 μmol/L.
Microhepatica and renomegaly are usually noted on abdominal radiographs. Ultrasonography is a useful noninvasive tool for identifying the shunt, determining if the shunt is intrahepatic or extrahepatic, and identifying radiolucent uroliths in the kidneys or bladder. Contrast portography is a more invasive method to identify the shunt but is the best way to evaluate portal vessel anatomy. Rectal portal scintigraphy is noninvasive but is not widely available and cannot differentiate between intrahepatic and extrahepatic shunts or determine location of the shunt. Liver biopsy is indicated in shunt repair or if multiple shunts are noted to determine the primary underlying disease.
Treatment in most cases begins with medical therapy, with nutritional support (low-protein diet) and drugs such as lactulose (to acidify the gastrointestinal tract ), neomycin (to minimize gastrointestinal microflora production of ammonia) and S-Adenosylmethionine. Although surgical intervention is associated with a better chance of long-term survival, medical management provides an acceptable first-line option.
In case where the disease progresses, surgical attenuation or ligation of the shunt with an ameroid constrictor is required. Portacaval shunts are usually occluded at their termination at the caudal vena cava and portoazygous shunts can be occluded at the abdominal side of the diaphragm. An intravascular Amplatzer vascular plug has also shown promise as a noninvasive alternative procedure in dogs.
Correction of the shunt results in increased liver mass, development of intrahepatic portal vasculature and improved liver function. 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.
Postoperative complications are common, including portal hypertension, postligation seizures, heart failure, bacterial hepatitis and pyelonephritis. Seizures are commonly controlled with use of phenobarbital or levetiracetam.
The prognosis is good if complete correction can be achieved before 1 year of age and the prognosis is less favorable with partial correction, multiple shunts, and intrahepatic shunts.
- Pet MD
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