Vascular Changes

Thrombosis of the Portal Vein
According to Virchow’s triad there are three main
mechanisms leading to a thrombosis: hypercoagulability,
stasis and injury to the vascular endothelium. Obstruction of the portal vein may be intermittent, as can be shown in ultrasonographic and histologic studies. Recanalization is quite rare once the thrombus formation has reached the smaller portal branches.

Obstruction of Smaller Portal Branches

The type of portal hypertension caused by obliteration of the smaller portal branches sometimes occurs in systemic vasculitis or rheumatic disease and may be found prior to a manifest cirrhosis in primary sclerosing cholangitis (PSC), primary biliary cholangitis (PBC) or sarcoidosis. An infection with schistosoma – with eggs of the parasite causing a chronic inflammatory reaction – may lead to thrombosis and fibrosis. Acute thrombosis of a small portal vein causes a so-called pseudo-infarction
(Zahn infarction), while thrombosis of larger branches may induce more diffuse atrophy with subsequent regenerative hyperplasia.

Budd-Chiari Syndrome

The combination of portal hypertension and hepatomegaly
caused by an obstruction of venous drainage was first described by Budd in 1845. This 
obstruction may be located intrahepatically in the small hepatic veins or extrahepatically in the larger veins or the inferior caval vein. The type of obliterative endophlebitis of small hepatic veins described by Chiari is called “veno-occlusive disease”.A principal clinical symptom is slowly increasing portal hypertension. Only a few patients develop a fulminant disease with acute liver failure, hepatic encephalopathy and coagulopathy, and this arises from the sudden obstruction of all larger hepatic veins. This may be caused by any coagulation disorder which predisposes subjects to a thrombosis, or by a growing neoplasm, a hypertrophied
caudate lobe or membrane formation in the inferior caval vein.Treatment options include anticoagulation, resection of a mechanical obstruction or porto-systemic shunting.The last approach may involve liver transplantation. In acute obstruction

image

Cut section of a liver from a patient with “Budd-Chiari syndrome” demonstrating thrombus formation in a large hepatic
vein (arrow)

Veno-Occlusive Disease (VOD)
A fibrotic obstruction of the small (< 1 mm) liver veins leads to hepatomegaly with congestion of the sinusoids. If longstanding, this results in liver fibrosis.
Causative agents include pyrrolizidine alkaloids, which are found in exotic teas and as contamination in cereals. Azathioprin and cysteamin are also known to induce VOD, while whole body irradiation, together with intensive chemotherapy, is thought to lead to injury of the endothelium of small hepatic veins.Acute symptoms include sudden onset of pain with hepatomegaly and ascites.
More than 50% of all bone marrow-transplanted patients show signs of VOD. Typical symptoms in these cases are weight gain, jaundice, thrombocytopenia and early stage liver failure. A similar disease disease is seen in patients treated with Dacarbazine, although histologically there is a thrombosis, rather than fibrotic obstruction, probably induced by an allergic reaction.

Lobular or Segmental Atrophy Thrombosis of the portal or hepatic veins may lead to focal atrophy with subsequent compensatory hypertrophy of the neighboring segments. This is especially common in patients with cirrhotic livers.
Syphilis and metastatic carcinoma are known to cause so-called hepar lobatum. Segmental atrophy may also be due to a congenital anomaly Infarction / Ischemia Usually, an obstruction of the hepatic artery does not induce hepatic ischemia, as the blood flow via the portal vein is sufficient to guarantee parenchymal supply. Large infarcts are only possible if both vessels have a diminished flow, as occurs in shock, chronic portal vein thrombosis or cirrhotic livers. Here, the increased portal tension induces a reduction of blood flow in the portal vessels. Intravasal coagulation may also cause liver infarctions.

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