The portal vein is formed by the union of the superior mesenteric and splenic veins. The pressure within it is normally 5-8 mmHg with only a small gradient across the liver to the hepatic vein in which blood is returned to the heart via the inferior vena cava. Portal hypertension can be classified according to the site of obstruction:
PREHEPATIC due to blockage of the portal vein before the liver
INTRAHEPATIC due to distortion of the liver architecture, which can be presinusoidal, e.g. in schistosomiasis, or postsinusoidal, e.g. in cirrhosis
POSTHEPATIC due to venous blockage outside the liver (rare)
As portal pressure rises above 10-12 mmHg the compliant venous system dilates and collaterals with the systemic venous system occur.
The main sites of the collaterals are at the gastro-oesophageal junction, the rectum, the left renal vein, the diaphragm,
the retroperitoneum and the anterior abdominal wall via the umbilical vein.
The collaterals at the gastro-oesophageal junction (varices) are superficial in position and tend to rupture.
Portosystemic anastomoses at other sites seldom give rise to symptoms. Rectal varices are frequently found (30%) if carefully looked for and can be differentiated from haemorrhoids, which are lower in the anal canal.
Portal vascular resistance is increased due to the deposition of collagen in the space of Disse and possibly hepatocyte enlargement. This increased resistance leads to portal hypertension and opening of portosystemic anastomoses in both precirrhotic and cirrhotic livers. Patients with cirrhosis have a hyperdynamic circulation. This is now thought to be due to the release of the mediators nitric oxide and glucagon (due to endotoxaemia) which leads to peripheral and splanchnic vasodilatation. This effect is followed by plasma volume expansion due to sodium retention (see ascites) and this has a significant effect in maintaining portal hypertension. The relevant roles of an impaired autonomic nervous system or of humoral factors, e.g. prostaglandins or serotonin, remain unclear.
The commonest cause is cirrhosis. Other causes include the following.
Extrahepatic blockage is due to portal vein thrombosis. The cause is often unidentifiable but some are due to portal vein occlusion secondary to congenital portal venous abnormalities or neonatal sepsis of the umbilical vein. Patients usually present with bleeding, often at a young age. They have normal liver function and, because of this, their prognosis following bleeding is excellent. The portal vein blockage can be identified by ultrasound or Doppler imaging. A splenectomy should never be performed, as it may be possible to perform a splenorenal shunt in adult life. Treatment is usually with repeated sclerotherapy.
Although cirrhosis is the commonest intrahepatic cause of portal hypertension, other causes include:
NON-CIRRHOTIC PORTAL HYPERTENSION or subtle change liver disease. Patients present with portal hypertension and variceal bleeding but without cirrhosis.
Histologically, the liver shows mild portal tract fibrosis.
The aetiology is unknown, but arsenic, vinyl chloride and other toxic agents have been implicated in some cases. A similar disease is found frequently in India. The liver lesion does not progress and the prognosis is therefore good.
SCHISTOSOMIASIS with extensive pipe-stem fibrosis is a common cause worldwide.
OTHER CAUSES include nodular regenerative hyperplasia, congenital hepatic fibrosis and partial nodular transformation. This latter condition is very rare and there is dispute about its existence.
Prolonged severe heart failure with tricuspid incompetence and constrictive pericarditis can both lead to portal hypertension. The Budd-Chiari syndrome is described.
Patients with portal hypertension are often asymptomatic and the only clinical evidence of portal hypertension is splenomegaly.
Presenting features are:
• Haematemesis or melaena from rupture of gastrooesophageal varices
• Ascites and/or
Approximately 70% of patients with cirrhosis will develop gastro-oesophageal varices but only one-third of these will bleed from them. Bleeding is likely to occur with large varices, high pressure and in severe liver disease.