Portosystemic Shunts:
Congenital and Acquired

Congenital Portosystemic Shunts

Pathogenesis:  During normal embryological development, vitelline veins and the cardiac veins draining the trunk of the fetus anastomose through several different temporary vessels.  The fetus is dependent on the hepatic system of the dam, and has no use for its own until after parturition.  Occasionally, one or more of these anastomoses remain after birth, and blood is continuously shunted around the liver.  Without the filtering and metabolizing capabilities of the liver, ammonia and other toxins absorbed from the gastrointestinal tract through the portal vein are not completely removed.  Animals with portosystemic shunts may, therefore, exhibit signs of hepatic encephalopathy due to the effects of neurotoxins like ammonia and false neurotransmitters in the central nervous system.  Greater than 80% of the blood must be shunted around the liver for obvious clinical signs to be observed.  In most cases, the blood will go directly from the portal vein to the caudal vena cava or the azygous vein.  In dogs, incidences of persistent ductus venosus, have been reported to cause the same syndrome.  In these cases the ductus venosus shunts blood within the liver parenchyma, bypassing the capillary plexus among the hepatocytes.  

Normal portogram: Contrast dye injected into a portal vein demonstrates the normal portal vasculature in the liver lobe to which the vein drains. Click on either image for a larger view.	Congenital Portosystemic Shunt Portogram: Dye injected into a portal vein is shunted directly through the liver via (in this case) a patent ductus venosus.  The dye rapidly enters the circulation and "lights up" the kidney and other structures.

Gross Pathology:  

• Finding a shunt. There can be one or multiple shunts, which can be either extra- or intrahepatic. 

• Microhepatica characterizes many livers in animals with a portosystemic shunt because the liver is never able to develop fully. It is often difficult to palpate.  

• Fatty changes within the parenchyma may give the liver a lighter color and softer texture.  

Histopathology:  

• Lobular hepatocellular atrophy is the most consistent cellular change in portosystemic shunts.  

• The portal veins and tributaries will be inconspicuous.  

• There is often arterial duplication.  

• Milder changes include lipidosis and vacuolar change.  

• Minimal necrosis and inflammation.  

Species Affected:  

• Cats are most commonly affected with congenital shunts.  Males under three years of age and of mixed breeding, Persian, or Himalayan descent appear to be at increased risk. 

• Dogs, calves, and horses also experience congenital portosystemic shunts.  

Clinical Signs:  Clinical signs are similar to those seen with hepatic encephalopathy.  

• The most common clinical signs are: chronically ill, exhibiting ill-thrift,  reduced growth rates, and poor hair coats.  

• Neurologic abnormalities:  impaired vision, circling, ataxia, gate abnormalities, behavior changes, seizures, and head pressing.   Signs may wax and wane.

• Gastrointestinal signs:  vomiting, diarrhea, and anorexia.  Are intermittent.

• A frequent clinical sign in cats is pytalism.  

• Some animals are also reported to be PU/PD, and have intermittent fever. 

• Because the signs are often mild and come and go, some animals may not be diagnosed until they are anesthetized for spay or neuter operations, whereupon they will have prolonged recovery from anesthesia.

Clinical Pathology:  Classic clinical pathologic changes will include: 

• Elevated post prandial serum bile acids 

• Post-challenge hyperammonemia 

• Decreased BUN 

• Microcytosis and poikilocytosis

• Decreased urine specific gravity  

• Liver enzymes will be normal or only mildly increased in the serum because hepatocellular damage is limited to hypoxia and slight cholestasis.

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Acquired Portosystemic Shunts

Pathogenesis:  Usually referred to as multiple acquired portosystemic shunts, these circumventive routes are extrahepatic, collateral vessels that develop as a compensatory response to portal hypertension.  Every normal dog and cat has the potential to develop acquired shunts as these are rudimentary microvascular pathways established in development.  When the portal hypertension exceeds the systemic venous (or vena caval) pressure, these pathways undergo hyperplasia to allow blood to skirt around the hepatic parenchyma and dump into the caudal vena cava. 

    Portal hypertension is usually associated with severe, chronic, diffuse liver pathology.  When liver pathology restricts blood flow through the parenchyma, portal hypertension begins to climb, and shunts develop as a means to skirt the constricted vasculature of the liver.  Examples of liver disease that could potentially cause portal hypertension include:   chronic hepatitis, lobular dissecting hepatitis, idiopathic hepatic fibrosis, and cirrhosis.  Other, more uncommon causes of multiple acquired PSS include:  chronic partial occlusion of the portal vein due to neoplasia, extraluminal compression or thrombosis, and surgical ligation of congenital portosystemic shunts.

Hepatic Cirrhosis:  This dog had ascites due to portal hypertension.  The hypertension in the portal system was caused by restricted blood flow through the liver due to hepatic fibrosis and abnormal hepatic architecture. In a case like this, a shunt may form in response to the high blood pressure built up behind the liver. The shunt would have much lower pressure, and thus be a "path of least resistance".

Gross Pathology:   Grossly the liver will resemble the underlying disease process.  

• Fibrosis and nodular hyperplasia are likely to be encountered, as this is an end stage result of chronic liver disease (as in cirrhosis).  

• Ascites due to portal hypertension is common.

Histopathology:  Liver biopsies are helpful in detecting the underlying disease process.  Changes will include:  

• Prominent smooth muscular sphincters 

• Dilated central and portal veins 

• Cellular atrophy and variable fibrosis 

• Absence of inflammation, variable arteriolar hyperplasia 

• Bile duct proliferation  

• Portal fibrosis

Species Affected:  Any species with the capacity for development of chronic liver disease may develop multiple acquired portosystemic shunts.  It is most commonly seen in dogs and cats.

Clinical signs:  Clinical signs will be variable depending upon the underlying disease process.  Generally clinical features will reflect portal hypertension (ascites) and portosystemic shunting (hepatic encephalopathy), plus signs of chronic hepatic disease.

Clinical Pathology:  Besides findings referable to the underlying disorder, microcytosis, and increased fasting serum bile acids are commonly encountered.

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