Portal-vein hypoplasia, also referred to as microvascular dysplasia (MVD), is a confusing syndrome associated with abnormal
microscopic hepatic portal circulation. Initially, the condition has been referred to as hepatic microvascular dysplasia.
Hepatic portal-vein hypoplasia has been suggested as better terminology that may better reflect the cause of this condition.
It is believed that the primary defect in affected dogs is the result of hypoplastic small intrahepatic portal veins.
This condition is thought to be a defect in embryologic development of the portal veins. With paucity in size or presence
of portal veins, there is a resultant increased arterial blood flow in attempt to maintain hepatic sinusoidal blood flow.
The hepatic arteries become torturous and abundant in the triad. Sinusoidal hypertension occurs under this high-pressure system.
Lymphatic and venous dilation results with opening up of embryologic sinusoidal vessels and, thus, acquired shunts develop
to transport some of the blood to the central vein, bypassing the sinusoidal hepatocytes.
This results in abnormal hepatic parenchymal perfusion and lack of normal trophic factors bathing the sinusoids, causing hepatic
atrophy. With portal shunting of blood, increased iron uptake occurs that results in hepatic iron granuloma formation. Ascites
or portal hypertension generally does not occur in this condition. Angiography or transcolonic portal scintigraphy fails to
demonstrate macroscopic shunting in this condition.
Often a needle biopsy is not sufficient to provide enough portal areas to make the diagnosis, and consequently a wedge or
laparoscopic biopsy may be necessary. All dogs have abnormal serum bile acid concentrations (usually moderate elevations)
and variable serum liver enzymes. Therapy is symptomatic and includes management of hepatic encephalopathy. Evidence of oxidative
damage to livers of shunt dogs provides evidence for antioxidant supplementation. The long-term prognosis is uncertain because
of lack of experience with this relatively new disease.
Portal-vein hypoplasia and secondary portal hypertension
Portal vein hypoplasia with portal hypertension (also known as idiopathic noncirrhotic portal hypertension or congenital hepatic
fibrosis) and ascites occurs as a fibrosis variant. It is generally accepted that dogs having congenital portosystemic vascular
anomalies with a single intrahepatic or extrahepatic shunting vessel have signs associated with hepatic encephalopathy but
do not have portal hypertension or ascites. However, there is a subgroup of dogs with portal-vein hypoplasia that have moderate
to marked fibrosis of the portal tracts, sometimes resulting in portal to portal fibrosis and a varying proliferation of arterioles
and bile ductules, particularly at the periphery of the portal area.
Ascites, portal hypertension and secondary acquired portosystemic shunts occur. The hepatic pathology is void of inflammatory
infiltrates. There are increased amounts of hepatic iron deposited in the liver. The fibrosis and bile-duct replication may
be a non-specific reaction from increased growth factors promoting arterial proliferation. This condition is observed in dogs
under 2.5 years of age and there is no breed prevalence; however, Doberman Pinschers, Cocker Spaniels and Rottweilers may
be over-represented. The clinical presentation is similar to dogs having either congenital intrahepatic or extrahepatic shunts
except most dogs have ascites. The serum liver enzymes generally are increased with a hypoalbuminemia and very high serum
bile acid concentrations. Work-up of these dogs fails to identify a single shunting vessel, but rather these cases have marked
portal hypertension associated with multiple acquired portosystemic shunts. These dogs present with ascites and signs of hepatic
encephalopathy. Ultrasound study often is helpful, showing microhepatica, hepatofugal portal blood flow and multiple abnormal
extrahepatic collateral shunts. Portal contrast studies indicate acquired portal shunts and pressure measurements document
portal hypertension. The prognosis for this condition generally is guarded, but some dogs are reported to have a prolonged
survival using anti-fibrotic agents and hepatic encephalopathy therapy.
Hepatic vacuolar hepatopathies
Diffuse vacuolar hepatopathies can be very frustrating in determining the underlying cause. When hepatocytes become injured,
one response is for them to swell and become vacuolated. Hepatocellular vacuoles distending the cytosolic compartment may
contain fat, glycogen, intracellular water (edema), or other metabolic wastes or intermediates. Vacuolar hepatopathies may
occur in conjunction with hydropic degeneration in which there is cytosolic swelling but the hepatocytes are devoid of distinct
vacuoles. A number of conditions can be responsible for vacuolar hepatopathies.
Glucocorticoid (steroid) hepatopathies
Glucocorticoid (steroid) hepatopathies occur in the dog secondary to exogenous or endogenous glucocorticoids. The vacuolar
lesions contain glycogen that is not easily differentiated from other vacuolar contents without specialized histopathologic
processing. The development of steroid hepatopathies is linked with marked increases in serum ALP. The glucocorticoid-associated
ALP (G-ALP) is unique to the dog and dogs having steroid hepatopathies are associated with a large component of G-ALP.