THE MERCK VETERINARY MANUAL
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Acquired Portosystemic Shunts in Small Animals

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APSS are caused by portal hypertension secondary to chronic liver disease due to development of severe, diffusely remodeled architecture; fibrosis or cirrhosis; congenital intrahepatic portal vein atresia; hepatic arteriovenous fistula; veno-occlusive lesions; portal venous thrombi; or polycystic liver disease in cats. The main body of the portal vein lacks valves and is normally maintained at pressures <5 mm Hg. High-pressure retrograde flow driven by the hepatic arterial circulation in the portal triad leads to formation of APSS as blood follows the path of least resistance. APSS develop as multiple nests of tortuous veins unite the portal vasculature with the abdominal vena cava.

The most common sites of APSS are caudal to the left kidney, in the region of the colorectal vasculature, and associated with vessels of the spleen. Nests of small tortuous vessels can usually be identified during ultrasound examination using Doppler color-flow. Surgical exploration for shunt ligation should not be done in animals with suspected PSVA associated with APSS because finding APSS confirms the presence of portal hypertension. However, liver biopsies should be collected from these patients to determine the underlying cause.

Clinical signs include episodic HE, PU/PD, vomiting, diarrhea (sometimes bloody), and abdominal effusion. Laboratory abnormalities consistent with a primary underlying hepatic disease can be seen in addition to markers of shunting (RBC microcytosis, low BUN and creatinine, hypocholesterolemia, and ammonium biurate crystalluria). Hyperbilirubinemia may or may not be present, depending on the underlying cause. Ligation of multiple APSS is contraindicated, because this is a compensatory response to portal hypertension. Banding of the vena cava to reduce the extent of shunting is ill advised. Medical treatment to minimize signs of HE and sodium restriction combined with diuretics are used to control abdominal effusion. Dogs and cats with APSS can live several years or longer uneventfully with appropriate medical and nutritional support.

Last full review/revision March 2012 by Sharon A. Center, DVM, DACVIM

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