Other vascular abnormalities include hepatic arteriovenous fistulas, hepatic venous outflow obstruction, (veno-occlusive disease, Budd-Chiari syndrome), and portal venous thromboembolism. These are relatively uncommon compared with PSVA and MVD and other acquired hepatic disorders.
Hepatic Arteriovenous Fistula
An arteriovenous (AV) fistula is an intrahepatic connection between the high-pressure hepatic arterial system and the low-pressure portal venous system. High-pressure arterialized blood flows retrograde into the portal vasculature, causing intrahepatic and extrahepatic portal hypertension, ascites, and formation of APSS. These may be congenital or, less commonly, acquired from trauma or neoplasia. Clinical signs of congenital AV fistulas initially manifest in young animals and include HE, abdominal effusion, inappetence, vomiting, and diarrhea (often bloody). A murmur from turbulent blood flow may be audible over the affected liver lobe.
Laboratory abnormalities are identical to those associated with PSVA. While ascites is a distinguishing feature; dogs with severe congenital portal atresia also develop abdominal effusion. Abdominal ultrasonography can easily identify an intrahepatic AV fistula and associated APSS. Definitive images require contrast angiography via the celiac or anterior mesenteric artery or multisector CT.
Lobectomy is recommended if one lobe is involved. Biopsy of the liver from sites distant to the vascular malformation (other liver lobes) is imperative, because many of these dogs have widespread intrahepatic vascular malformation. Surgery has a dismal prognosis for cure because of the widespread distribution of microscopic vascular malformations. Recent description of intravascular acrylamide injection as an alternative salvage procedure also has high risk for post-procedure complications and poor outcome.
Hepatic Vein Outflow Obstruction
Hepatic venous outflow obstruction can result from cardiac or pericardial disorders causing passive congestion of the caudal vena cava (eg, right heart failure, pericardial disease, congenital defects, cardiac tumors), obstruction of the caudal vena cava (eg, postcaval syndrome associated with heartworm disease, congenital “kinking” of the caudal vena cava, thrombosis or neoplasia of the caudal vena cava, diaphragmatic hernia compressing the caudal vena cava), or obstruction of the efferent hepatic venous system (eg, liver lobe torsion, compression by an hepatic mass, idiopathic postsinusoidal venous obstruction associated with extensive fibrosis, severe occlusive or obstructive extramedullary hematopoiesis, or aberrant physiologic hepatic venule constriction associated with PSVA and MVD in small-breed dogs).
Clinical features of occlusive disorders include hepatomegaly (unless cause is associated with PSVA or MVD), ascites, formation of multiple portosystemic shunts, and signs suggestive of the underlying primary disorders. Disorders simply causing passive congestion are associated with hepatomegaly, modest increases in liver enzymes, normal bile acid concentrations, and formation of a modified transudate. Laboratory abnormalities of veno-occlusive disease (hepatic venule inflammatory occlusion) or a Budd-Chiari syndrome (thrombosis of the hepatic venule or vena cava) reflect portosystemic shunting (eg, high TSBA concentrations, hypocholesterolemia, low Protein C activity), mild to moderate increases in hepatic transaminases, and variable total bilirubin and albumin concentrations. A modified transudative abdominal effusion is common.
Thoracic and abdominal radiographs help distinguish cardiac from other causal disorders and may disclose kinking or impingement of the caudal diaphragmatic region of the vena cava. Cardiac ultrasonography can assist in identifying disorders that cause passive congestion (eg, differentiate between pericardial disease, cardiac tumors, congenital disease, or intrathoracic masses compressing the caudal vena cava). Abdominal ultrasonography reveals distention of the hepatic venules in passive congestion and diminished hepatic venule size in animals with a veno-occlusive or Budd-Chiari type lesion. The latter processes are associated with hepatic dysfunction and APSS. Treatment and prognosis depend on the underlying disease.
Last full review/revision March 2012 by Sharon A. Center, DVM, DACVIM