Acquired Portosystemic Shunts in Small Animals
Acquired portosystemic shunts (APSSs) form secondary to portal hypertension caused by 1) chronic liver disease (fibrosis, regenerative nodules), 2) congenital severe portal vein atresia, 3) acquired damage to the fine branches of the intrahepatic portal vein (noncirrhotic portal hypertension), 4) hepatic arteriovenous malformations, 5) congenital hepatic fibrosis associated with polycystic liver malformations (ductal plate malformations), 6) portal vein thrombosis or stricture, or 7) outflow obstruction through the hepatic vein/venules (thrombosis [Budd-Chiari syndrome], hepatic vein injury [veno-occlusive syndrome]). The main body of the portal vein lacks valves and maintains a blood pressure of <8 mmHg. Any disorder diminishing hepatic portal perfusion results in a hepatic arterial buffer response that increases hepatic arterial perfusion. High-pressure retrograde arterial flow into portal vasculature leads to formation of APSSs as blood follows the path of least resistance to the vena cava. In animals with extrahepatic portal atresia or portal thrombi, splanchnic circulatory hypertension leads to an APSS. In animals with occluded hepatic venular outflow or fibrotic hepatic remodeling, sinusoidal and postsinusoidal hypertension also results in retrograde flow of blood into the valveless portal system. APSSs represent nests of tortuous veins uniting portal vasculature with the abdominal vena cava.
The most common sites of APSSs 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. Although esophageal varicoceles are most common in people, this location does not predominate in animals. Surgical exploration for shunt ligation should not be done in animals with suspected PSVAs associated with APSSs, because finding an APSS confirms the presence of portal hypertension. Liver biopsies are needed to determine the underlying cause of portal hypertension.
Clinical signs of APSSs 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, ammonium biurate crystalluria, and subnormal protein C activity). Hyperbilirubinemia may be present, depending on the underlying cause. Ligation of multiple APSSs 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 along with sodium restriction and combination diuretic therapy are used to control abdominal effusion. Dogs and cats with APSSs can live several years without clinical signs, some having a normal lifespan uneventfully when provided with appropriate medical and nutritional support.