Portosystemic Shunts: Overview - The Merck Veterinary Manual

Portosystemic Shunts: Overview

The most common circulatory anomaly of the liver in both dogs and cats is the portosystemic shunt (PSS). A PSS is a connection between the portal vessels and systemic circulation that diverts blood flow, in varying degrees, from the liver. Decreased blood flow results in liver atrophy and subsequent dysfunction, decreasing liver metabolism of neurotoxins. Clinical signs of hepatic encephalopathy are frequently noted and can be most severe postprandially, especially after a high-protein meal.

Congenital portosystemic shunts, dog

Congenital PSS are seen primarily in purebred dogs, including Miniature Schnauzers, Yorkshire Terriers, Cairn Terriers, Maltese, Scottish Terriers, Pugs, Irish Wolfhounds, Golden Retrievers, Labrador Retrievers, German Shepherds, and Poodles. In cats, congenital PSS are seen more frequently in mixed breeds, but Himalayans and Persians are affected more commonly than other purebreds. Cats and small-breed dogs usually have extrahepatic shunts, whereas large-breed dogs have intrahepatic shunts. Extrahepatic shunts arise from the portal vein, left gastric vein, or splenic vein and connect to the caudal vena cava (most common), the azygous vein, or other systemic vessels. Congenital intrahepatic shunts usually are due to failure of the fetal ductus venosus to close at birth.

Clinical Findings and Diagnosis:

Animals with congenital PSS are often smaller than littermates, show failure to thrive, and can have other congenital abnormalities (eg, cryptorchidism in dogs and cats, heart murmurs in cats). Male cats may be more prone to congenital shunts than females. Clinical signs are usually seen by 6 mo of age in cats and between 6 mo and 1 yr of age in dogs. If clinical signs are mild, diagnosis may be not be made until the animal is several years old. Hepatic encephalopathy is the most common clinical sign. Other clinical signs include vomiting, diarrhea, pica, nausea, and anorexia. Polyuria and polydipsia are common in dogs but not in cats. Hematuria, pollakiuria, stranguria, or urethral obstruction due to ammonia biurate urolithiasis have been reported. Hypersalivation is a common clinical sign in cats; blindness and excessive vocalization have also been reported.

Abnormalities in laboratory data include 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), normal bilirubin, hyposthenuria or isothenuria, and ammonia urate crystals in the urine. Cats can have a microcytosis without anemia, decreased creatinine and BUN levels, and increased ALT and AP. After a prolonged fast, both the fasting bile acid and resting ammonia values may be normal, but postprandial bile acids and ammonia tolerance test will be abnormal.

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:

The treatment of choice for single congenital PSS is surgical attenuation or ligation. Whether ligation is total or partial depends on portal pressures (should be <20 cm H₂O). If only partial ligation can be achieved at the time of the first surgery, then additional surgery may be indicated if clinical signs do not resolve. The most common postsurgical complication is acute portal hypertension, which manifests as accumulation of abdominal effusion, bloody diarrhea, abdominal pain, ileus, endotoxic shock, and cardiovascular collapse. If this develops, immediate medical treatment for shock and removal of the ligature is necessary. Postsurgical seizures have been reported. Acquired shunts can develop after surgery if the portal vein is unable to dilate and compensate for the increased portal hypertension induced by surgical ligation.

Acute portal hypertension can be prevented if an ameroid constrictor band is used to attenuate a single extrahepatic shunt. The band is impregnated with cellulose that absorbs abdominal fluid; as the cellulose swells, the shunt is slowly ligated over an extended period of time.

In cases in which either surgery is not possible or clinical signs are minor, medical management can be used. Although medical management can lessen clinical signs initially, signs eventually worsen because liver atrophy in congenital PSS is progressive.

Overall, the prognosis is good if complete correction can be achieved before 1 yr of age in dogs with single extrahepatic congenital PSS. The prognosis is less favorable with partial correction, multiple shunts, and intrahepatic shunts. Poor prognosis after 2 yr of age is attributed to progressive liver atrophy. Overall prognosis postsurgically for cats with congenital shunts is not as favorable as for dogs. Cats appear to be more likely to develop multiple shunts after ligation. Delay in diagnosis and treatment may partially explain the difference.

Acquired PSS are caused by portal hypertension. Acquired shunts are usually seen in older animals, more frequently in dogs than in cats, and are usually multiple. Acquired shunts develop to prevent fatal portal hypertension, which develops as a result of chronic, severe, diffuse intrahepatic disease (eg, chronic hepatitis, cirrhosis, and hepatic fibrosis). The vessels involved are connections between the splenic and mesenteric veins through the renal veins, gonadal veins, or the venous sinuses within the spinal cord to the caudal vena cava. These vessels are fetal vasculature that open as a compensatory mechanism to shunt blood to the lower pressure systemic circulation as a response to portal hypertension. During acquired PSS, these vessels become tortuous. Possible causes of portal hypertension in younger dogs include hepatic arteriovenous fistulas, hepatoportal vascular hypoplasia veno-occlusive disease in Cocker Spaniels, or portal vascular atresia.

Clinical signs include polydypsia, vomiting, and diarrhea. Ascites is a common finding in acquired portosystemic shunts but is rarely seen in congenital shunts unless hypoalbuminemia is severe (<.15 g/dL). Laboratory abnormalities consistent with the primary underlying hepatic disease can be seen, including microcytosis, hyperbilirubinemia, decreased BUN, and increased AP and ALT. Ligation of multiple acquired PSS is contraindicated because this is a compensatory response, and correction can lead to acute portal hypertension. However, banding of the caudal vena cava can increase pressure within the vena cava to be slightly above portal pressure and reduce the signs of hepatic encephalopathy. Medical treatment of the underlying disease along with banding of the caudal vena cava can result in a favorable prognosis for some animals with acquired PSS.