Other Hepatic Vascular Disorders in Small Animals

A hepatic arteriovenous (AV) malformation is a direct 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 APSS formation. These may be congenital or, less commonly, acquired from trauma, biopsy lesions, or neoplasia. Clinical signs of a congenital AV malformation initially manifest in young animals and include HE, abdominal effusion, inappetence, vomiting, and diarrhea (often bloody). A murmur or bruit associated with turbulent blood flow may be audible over the affected liver lobe. Rarely, an intrahepatic AV malformation represents a variant of an intrahepatic PSVA.

Laboratory abnormalities are identical to those associated with more common PSVAs (see Portosystemic Vascular Anomalies) and additionally may disclose erythrocyte changes reflecting turbulence or a shearing effect (ie, schistocytes). Although ascites is a feature distinguishing an AV malformation from a PSVA, dogs with severe congenital portal atresia also may develop abdominal effusion (if they do not manifest a PSVA). Abdominal ultrasonography can easily identify an intrahepatic AV malformation (pulsing flow) and the associated APSS on color-flow Doppler interrogation. Definitive imaging requires contrast angiography via the celiac or anterior mesenteric artery (former "gold standard") or multisector contrast angiographic CT (current "gold standard").

Multiple AV connections are often evident within an affected liver lobe. Although surgical lobectomy or ligation of the nutrient artery is the conventional treatment, many affected dogs have other microvascular malformations causing intrahepatic shunting along with APSSs thwarting efficacy of lesion ablation. Biopsy of the liver from sites distant to the AV malformation (other liver lobes) is imperative to detect other intrahepatic vascular malformations. Surgical management has a dismal prognosis for cure because of the widespread intrahepatic distribution of microscopic vascular malformations and the presence of functional APSSs. Intravascular acrylamide injection using an interventional radiographic approach is an alternative salvage procedure. Unfortunately, this also cannot guarantee clinical cure because of the complex effect of this malformation on hepatic microvasculature. Limited outcome data for this approach describes treatment failures, some procedural complications (unintended embolization of nontargeted vasculature), and need for chronic medical management of HE.

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 causing tamponade, congenital defects [cor triatriatum dexter], cardiac tumors), obstruction of the caudal vena cava (eg, postcaval syndrome associated with heartworm disease, congenital “kinking” of the caudal vena cava, vascular or neoplastic thrombosis 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 a hepatic mass, idiopathic postsinusoidal venous obstruction secondary to acquired fibrosis or hepatic venular destruction, severe occlusive or obstructive extramedullary hematopoiesis, or hepatic venule obstruction by occlusive lipogranulomas (subset of dogs with extrahepatic PSVAs or MVD).

Clinical features of occlusive disorders include hepatomegaly (unless cause is associated with PSVAs or MVD), ascites, formation of APSSs, and signs suggestive of the underlying primary disorders. Simple passive congestion leads to hepatomegaly, modest increases in liver enzymes, normal bile acid concentrations, and an abdominal effusion characterized as a modified transudate. Laboratory abnormalities of veno-occlusive disease (hepatic venule occlusion) or a Budd-Chiari syndrome (thrombosis of the hepatic vein or vena cava) reflect portosystemic shunting (eg, high TSBA concentrations, hypocholesterolemia, and usually 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; these may disclose kinking or impingement of the caudal diaphragmatic region of the vena cava. Cardiac ultrasonography helps identify causes of passive congestion (eg, differentiate between pericardial disease, cardiac tumors, congenital malformations, or intrathoracic masses compressing the caudal vena cava). Abdominal ultrasonography discloses hepatic venular distention in passive congestion and diminished hepatic venule size in veno-occlusive or Budd-Chiari syndromes associated with hepatic dysfunction and APSSs. Treatment and prognosis depend on the underlying disease.