Any consideration of the pathophysiology and etiology of acute liver failure (ALF) has to start with a description of the terminology that has been used in describing this very rare clinical syndrome. The first attempt at a formal definition described fulminant hepatic failure as 'a potentially reversible condition, the consequence of severe liver injury, in which the onset of hepatic encephalopathy was within eight weeks of the first symptoms of illness, and in the absence of pre-existing liver disease'. Even rarer are the cases described as late-onset hepatic failure in which hepatic encephalopathy develops between 8 and 24 weeks after the onset of jaundice. The latter is more readily identified than encephalopathy by both patients and clinicians, and allows more accurate timing of illness duration. Workers in France have proposed a different set of definitions, although also based on the interval from the onset of jaundice to the development of encephalopathy. Patients with encephalopathy developing within 2 weeks and between 2 and 12 weeks from the onset of jaundice are categorized as fulminant and subfulminant hepatic failure respectively (Benhamou 1991).

It has become clear that the rate of onset of illness is an important indicator of the likely prognosis. Paradoxically, it is the group of patients with the most rapid onset of encephalopathy who have the best chance of spontaneous recovery. To take account of this, we have recently proposed a new classification ( Ta.bJ.eJ..) in which acute liver failure is taken as an umbrella term, with subgroups of hyperacute, acute, and subacute to reflect different clinical patterns of illness, etiology, and, most importantly, prognosis (O,'Grady.efa/ 1993). In this classification acute liver failure becomes the core term, with 'hyper' and 'sub' being used as prefixes to separate the two cohorts at opposite ends of the clinical spectrum. Hyperacute liver failure is used to describe those patients who develop encephalopathy within 7 days of the onset of jaundice. The majority of cases in this group have acute acetaminophen (paracetamol) hepatotoxicity, but a proportion of those with viral hepatitis A or B may also present with cerebral edema. Survival in a group of 81 cases from this unit was 36 per cent. Acute and subacute hepatic failure includes those with a jaundice to encephalopathy time of 8 to 28 days. There was a similar high incidence of cerebral edema but the survival was lower (7 per cent of a cohort of 89 patients). In subacute hepatic failure, with a jaundice to encephalopathy time of 5 to 26 weeks, the outcome was also poor (14 per cent) although the frequency of cerebral edema was low (,o,',,Gia.d.y... MM: 1.993).

Table 1 New classification of ALF based on time from first sign of jaundice to appearance of encephalopathy, with frequency of different etiologies in the three groups

In addition to the defining state, namely hepatic encephalopathy, severe coagulopathy and jaundice develop together with renal impairment and multiorgan failure. These clinical features may not all occur in every case and they are not specific to the etiology of ALF, although patterns of illness may be discerned that are to some extent indicative of the etiology. Thus those with acetaminophen overdose most often present with severe coagulopathy and signs of encephalopathy and the patient may not be jaundiced, whereas those with non-A non-B hepatitis are usually deeply jaundiced at presentation and are less likely to develop cerebral edema. Most of the damage to the liver has already been done by the time of presentation; histopathological examination of the liver shows collapsed hepatic parenchyma with few surviving hepatocytes and, depending on the time after onset, areas of regeneration.

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