Definitively diagnosing hepatic vascular disease
Vascular imaging is the only method to definitively diagnose hepatic vascular disease. It is imperative that a specific diagnosis is made in patients with suspected hepatic vascular disease so appropriate therapy can be pursued.
DISEASE CLASSIFICATIONThe consensus of the World Small Animal Veterinary Association Liver Standardization Group divides prehepatic vascular anomalies into three general categories1 :
1. Congenital intrahepatic and extrahepatic portosystemic shunts
2. Primary hypoplasia of the portal vein, which may or may not result in portal hypertension
3. Disorders of outflow.2
Most congenital intrahepatic and extrahepatic portosystemic shunts are a single vessel connecting the portal venous system to the systemic circulation. Several types of communication exist and have been described elsewhere.2,3
About 25% to 33% of portosystemic shunts are intrahepatic and occur most often in large-breed dogs.2 Predisposed breeds include Irish wolfhounds, Labrador retrievers, golden retrievers, and Australian cattle dogs. The remaining portosystemic shunts are extrahepatic and occur in small-breed dogs. Predisposed breeds include Yorkshire terriers, Maltese, Shih Tzus, and pugs.
Cats most often have extrahepatic portosystemic shunts, and domestic shorthaired, Persian, Himalayan, and Siamese cats are most commonly affected. A sex predilection toward males may exist in cats.
The clinical signs observed with congenital portosystemic shunts depend on the degree of shunting and are primarily associated with the central nervous, gastrointestinal, and urinary systems.2 Ptyalism is a hallmark of portosystemic shunts in cats.4 Detailed explanations of the clinical signs observed with cases of portosystemic shunting can be found in internal medicine textbooks and review articles.2,4,5
PRIMARY HYPOPLASIA OF THE PORTAL VEIN
Primary hypoplasia of the portal vein may occur as a single anomaly or concurrently with a macroscopic portosystemic shunt. It is characterized by microscopic malformations of the hepatic vasculature.2 Primary hypoplasia of the portal vein without portal hypertension was previously called microvascular dysplasia, and Cairn and Yorkshire terrier breeds are overrepresented.
Primary hypoplasia of the portal vein with portal hypertension has been diagnosed in a variety of breeds, but Doberman pinschers are overrepresented.2,6 It is diagnosed when a patent but hypoplastic portal vein is present with a noncirrhotic liver. Other terms used in the literature include idiopathic noncirrhotic portal hypertension, hepatoportal fibrosis, and veno-occlusive disease. Clinical signs of primary hypoplasia of the portal vein are similar to those observed with portosystemic shunts, with the exception of ascites, which occurs only in cases of primary hypoplasia of the portal vein with portal hypertension. Acquired extrahepatic portosystemic shunts develop as a result of portal hypertension.
HEPATIC ARTERIOVENOUS MALFORMATION
Hepatic arteriovenous malformation is a rare vascular anomaly in which multiple aberrant communications occur among branches of the hepatic artery and portal vein. Hepatic arteriovenous malformation is usually a congenital disorder, has been diagnosed in dogs and cats, and was formerly called arteriovenous fistula. Abnormally high pressure results within the portal system, and a reversal of blood flow occurs, which is termed hepatofugal blood flow (blood flow away from the liver).7 The resultant portal hypertension leads to acquired extrahepatic portosystemic shunts. Clinical signs are similar to those of portosystemic shunts, and ascites is observed in 75% of cases.2