USMLE – Portal Hypertension
Portal hypertension is characterized by prolonged elevation of the portal venous pressure (normally 2-5 mmHg). Patients developing clinical features or complications of portal hypertension usually have portal venous pressures above 12 mmHg.
Etiology and pathogenesis
Portal venous pressure is determined by the portal blood flow and by the portal vascular resistance. Increased vascular resistance is usually the main factor producing portal hypertension, irrespective of its cause, and consequently the causes of portal hypertension are classified in accordance with the main sites of obstruction to blood flow in the portal venous system.
Extrahepatic portal vein obstruction is frequently the cause of portal hypertension in childhood and adolescence, while cirrhosis causes 90% or more of portal hypertension in adults in Western countries. Schistosomiasis is the most common cause of portal hypertension world-wide but it is infrequent outside endemic areas. Increased portal vascular resistance leads to a gradual reduction in the now of portal blood to the liver and simultaneously to the development of collateral vessels, allowing portal blood to bypass the liver and enter the systemic circulation directly. Increased portal blood now may contribute to portal hypertension but is not the dominant factor. Collateral vessel formation is widespread but occurs particularly in the gastrointestinal tract, especially the esophagus, stomach and rectum, in the anterior abdominal wall, and in the renal, lumbar, ovarian and testicular vasculature. Normally, virtually all the portal blood flows through the liver but, as collateral vessel formation progresses, half or more, and occasionally almost all, of the portal blood now can be shunted directly to the systemic circulation.
The clinical features of portal hypertension result principally from portal venous congestion and from collateral vessel formation. Splenomegaly is a cardinal finding and a diagnosis of portal hypertension is unlikely when splenomegaly cannot be detected clinically or by ultrasonography. The spleen is rarely enlarged more than 5 cm below the left costal margin in adults, but more marked splenomegaly can occur in childhood and adolescence. Hypersplenism is common and frequently results in thrombocytopenia. Platelet counts are usually around 100 x 10-9/1; values below 50 x 10-9/1 are rare. Leucopenia occurs occasionally, but anemia can hardly ever be attributed to hypersplenism. Collateral vessels may be visible on the anterior abdominal wall and occasionally several radiate from the umbilicus to form a caput medusa. Rarely, a large umbilical collateral vessel has a blood now sufficient to give a venous hum on auscultation (Cruveilhier-Baumgarten syndrome). The most important collateral vessels occur in the esophagus and stomach, where they can cause severe bleeding. Rectal varices also cause bleeding and are often mistaken for hemorrhoids, which are no more common in portal hypertension than in the general population. Fetor hepaticus results from portasystemic shunting of blood which allows mercaptans to pass directly to the lungs.
Radiological and endoscopic examination of the upper gastrointestinal tract can show varices. This establishes the presence of portal hypertension but not its cause. Imaging, particularly ultrasonography, can show features of portal hypertension, such as splenomegaly and collateral vessels, and can sometimes indicate the cause, such as liver disease or portal vein thrombosis. Portal venography demonstrates the site and often the cause of portal venous obstruction and is performed prior to surgical intervention. Portal venous pressure measurements are rarely needed but can be used to confirm portal hypertension and to differentiate sinusoidal and presinusoidal forms.
Gastrointestinal bleeding from varices or from congestive gastropathy is the main complication. Hypersplenism is rarely severe enough to be clinically significant and portal hypertension is only one factor contributing to the development of ascites, renal failure and hepatic encephalopathy.