Hepatic encephalopathy describes a broad range of neuropsychiatric abnormalities caused by advanced hepatic insufficiency or portosystemic shunting.[1,2,3] The likelihood of developing hepatic encephalopathy correlates with the severity of the liver disease. Hepatic encephalopathy is broadly classified as either overt hepatic encephalopathy (neurologic and neuropsychiatric abnormalities detected with bedside examination and bedside tests) or minimal hepatic encephalopathy (normal mental status and normal neurologic examination in conjunction with abnormalities on psychometric testing).[4] Overt hepatic encephalopathy will occur in approximately 30 to 40% of individuals with cirrhosis at some point during their illness.[2] Individuals with cirrhosis who undergo transjugular intrahepatic portosystemic shunts (TIPS) also frequently develop overt hepatic encephalopathy, with an estimated incidence of 10 to 50%.[3,5] This risk may be decreased with newer stent designs and smaller diameter shunts, but the risk is still substantial.[6] Minimal hepatic encephalopathy is estimated to develop in more than 80% of persons with cirrhosis. The onset of hepatic encephalopathy in a person with cirrhosis signals a poor prognosis and reduced survival, especially if liver transplantation is not performed.[7,8,9]
Pathogenesis
Although hepatic encephalopathy is not a single clinical entity and precise details of its pathogenesis remain incompletely understood, there is a consensus that elevated levels of ammonia play a central role in this disorder, primarily by acting as a neurotoxin that generates astrocyte swelling.[1] As part of the normal physiologic process, colonic bacteria and gut mucosal enzymes break down dietary proteins, which results in the release of ammonia from the gut into the portal circulation.[3,4] Normally, the ammonia is converted to urea in the liver. In many persons with liver failure or portosystemic shunting, the ammonia released into the portal circulation does not get adequately eliminated by the liver and it accumulates at high levels in the systemic circulation.[1,4] The circulating ammonia results in substantial levels of ammonia crossing the blood-brain barrier where rapid conversion to glutamine occurs by astrocytes; in the brain, astrocytes are the only cells that convert ammonia to glutamine.[1,4] Within astrocytes, glutamine levels accumulate, acting as an osmolyte to draw water inside the cell, which causes astrocyte swelling. The end result of the high circulating levels of ammonia is cerebral edema and intracranial hypertension.[4] Other factors, such as oxidative stress, neurosteroids, systemic inflammation, increased bile acids, impaired lactate metabolism, and altered blood-brain barrier permeability likely contribute in the process of hepatic encephalopathy.[1,4,10]
Nomenclature
In 1998, a consensus group at the 11th World Congress of Gastroenterology in Vienna proposed a standardized nomenclature for hepatic encephalopathy based on the type of hepatic abnormality, the severity of the manifestations, and the frequency (episodic or persistent) (Figure 1).[11] The International Society for Hepatic Encephalopathy and Nitrogen Metabolism (ISHEN) consensus identifies disorientation or asterixis as the beginning of overt hepatic encephalopathy (grade II through IV), which consists of neurological and psychiatric abnormalities that can be detected by bedside clinical tests, whereas covert hepatic encephalopathy (minimal and grade 1) can only be distinguished by specific psychometric tests, as these individuals have normal mental and neurological status on clinical examination.[4,12]