Hepatic vascular disease comprises a group of anomalies that result in abnormal blood flow either within or around the liver.
The most common anomalies are congenital and acquired portosystemic shunts that form single or multiple direct connections
between the portal vasculature and the systemic circulation. Clinicopathologic findings can be similar among the different
vascular diseases, and more than one anomaly can occur within the same patient.
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.
The 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