More commonly, especially in chronic liver disease, hepatic encephalopathy is triggered by an additional cause, and identifying these triggers can be important to treat the episode effectively. Hepatic encephalopathy may also occur after the creation of a transjugular intrahepatic portosystemic shunt (TIPS). This is used in the treatment of refractory ascites, bleeding from oesophageal varices and hepatorenal syndrome. TIPS-related encephalopathy occurs in about 30% of cases, with the risk being higher in those with previous episodes of encephalopathy, higher age, female sex, and liver disease due to causes other than alcohol. Pathogenesis
There are various explanations why liver dysfunction or portosystemic shunting might lead to encephalopathy. In healthy subjects, nitrogen-containing compounds from the intestine, generated by gut bacteria from food, are transported by the portal vein to the liver, where 80–90% are metabolised through the urea cycle and/or excreted immediately. This process is impaired in all subtypes of hepatic encephalopathy, either because the hepatocytes (liver cells) are incapable of metabolising the waste products or because portal venous blood bypasses the liver through collateral circulation or a medically constructed shunt. Nitrogenous waste products accumulate in the systemic circulation (hence the older term "portosystemic encephalopathy"). The most important waste product is ammonia (NH3). This small molecule crosses the blood–brain barrier and is absorbed and metabolised by the astrocytes, a population of cells in the brain that constitutes 30% of the cerebral cortex. Astrocytes use ammonia when synthesising glutamine from glutamate. The increased levels of glutamine lead to an increase in osmotic pressure in the astrocytes, which become swollen.
There is increased activity of the inhibitory γ-aminobutyric acid (GABA) system and the energy supply to other brain cells is decreased. This can be thought of as an example of brain edema of the "cytotoxic" type.Despite numerous studies demonstrating the central role of ammonia, ammonia levels do not always correlate with the severity of the encephalopathy; it is suspected that this means that more ammonia has already been absorbed into the brain in those with severe symptoms whose serum levels are relatively low. Other waste products implicated in hepatic encephalopathy include mercaptans (substances containing a thiol group), short-chain fatty acids, and phenol.Numerous other abnormalities have been described in hepatic encephalopathy, although their relative contribution to the disease state is uncertain. Loss of glutamate transporter gene expression (especially EAAT 2) has been attributed to acute liver failure. Benzodiazepine-like compounds have been detected at increased levels as well as abnormalities in the GABA neurotransmission system. An imbalance between aromatic amino acids (phenylalanine, tryptophan and tyrosine) and branched-chain amino acids (leucine, isoleucine and valine) has been described; this would lead to the generation of false neurotransmitters (such octopamine and 2-hydroxyphenethylamine). Dysregulation of the serotonin system, too, has been reported. Depletion of zinc and accumulation of manganese may play a role. Inflammation elsewhere in the body may precipitate encephalopathy through the action of cytokines and bacterial lipopolysaccharide on astrocytes.