Hepatic encephalopathy and cerebral edetna in fultninant liver failure

RF BUTIERWORTH. Hepatic encephalopathy and cerebral edema in fulmi, nant liver failure. Can J Gastroenterol 1993;7(7):547,549. Hepatic encephalopathy and cerebral edema are major complications of fulminant liver failure. Currently available evidence suggests that neurotransmission failure, rnther than primary brain energy failure, is responsible for these disorders. Brain ammonia rises to millimolar concentrations in experimental hepatic encephalopathy, and exposure of brain preparations to ammonia results in edema and alterations of excitatory and inhibitory neurotransmission. Specific neurotransmitter systems implicated in the pathogenesis of hepatic encephalopathy in fulrninant liver failure include the excitatory amino acids glutamate and aspartate. There is little convincing evidence for a parhogenetic role for the brain gamma-aminobutyric acid (GABA) system in this disorder. While there is some evidence for accumulation of endogenous substances that bind to the GABArelated benzodiazepine receptor in brain in some patients with fulminant Liver failure, the pathophysiological relevance of these findings remains to be established. The most rational means of prevention of the cerebral consequences of fulminant liver failure remains the lowering of blood ammonia.

Encephalopathie hepatique et oedeme cerebral dans l'insuffisance hepatique fulminante RESUME: L'encephalopathie hepatique et l'oedeme cerebral sont deux complications importantes de l'insuffisance hepatique fulminante.Les resultats disponibles a l'heure actuelle mettent en cause une insuffisance au plan de la neurotransmission plutot que du fonctionnement energetique primaire du cerveau.L'ammoniaque cerebral s'elcve a des concentrations millimolaires clans l'encephalopathie hepatique experimentale et )'exposition de preparations cerebrales a l'ammoniaque donne lieu a de l'oedeme et a des alterations de la neurotransmission qu'il s'agisse de stimulation ou d'inhibition.Les systemes neurotransmetteurs specifiques en jeu clans la pathogenese de l'encephalopathie associated with fulminant liver failure is a clinical syndrome resulting from severe inflammatory and/or necrotic Ii ver disease ( of rapid onset).The neurological disorder progresses from altered mental status to coma, generally within hours or days.Death frequently results from brain herniation caused by increased intracranial pressure as a result of massive brain edema.

Neuroscience Research
Available evidence suggests Lhat neurotransmission failure -rather than decreased energy metabolism -is the primary cause of HE in fulminant liver failure.Recent Pathogenetic mechanisms proposed to explain the central nervous system (CNS) complications of fulminant liver disease include the following: ammonia neurotoxicity, brain amino acid imbalance and the action of 'endogenous' benzodiazepines.
Brain ammonia concentrations are consistently in the 1 to 5 mM range in the brains of experimental animals at coma stages of HE resulting from fulmi-BU1TERWORTH hepatique liee a l'insuffisance hepatique fulminante incluent les acides amines excitateurs glutamate et aspartate.ll y a peu de preuves convaincantes a l'effet du role pathogene du systeme de l'acide gamma-aminobutyrique c~bral (AGA B) clans cette maladie.Si quelques indices portent a croire a une accumu• lation de substances endogenes qui se fixent aux recepteurs de la benzodi82pine liee a l'AGAB au n iveau du cerveau chez certains patients atteints d'insuffisance hepatique fulminante, la portee physiopathologique de ces resultats reste a explorer.La mesure preventive la plus rationnelle des sequelles cerebrates de l'insuffisance hepatique fulminante est d'abaisser l'ammoniaque sanguin.nant liver failure.Brain is devo id of an effective urea cycle so that ammonia removal by brain relies almost exclusively on glutamine synthesis.G lutamine concentrations are significantly increased in autopsied brain samples from patients who d ied in fulminant liver failure, suggesting exposure to high levels of ammonia; cerebrospinal fl uid (CSF) glutamine concentrations correlate well with the severity of neurological impairment in patients with fulminant liver failure.Millimolar concentrations of ammonia (equivalent concentrations to those encountered in brain in fulminant liver failure) affect both inhibitory and excitatory neurotransmission (2 ).In add ition, ischemic liver failure in the rat results in increased brain water content and in parallel increases of brain ammonia, the latter reaching concentrations in excess of 4 mM at coma stages of encephalopathy (3 ).Addition of 5 mM ammo nia to cortical bra in slices in vitro resul ts in significant swelling.Taken together, these findings suggest a role for ammonia in the pathogenesis of brain edema in fulminant liver failure.Whether edema results fro m a direct action of ammonia or fro m the action of an ammonia metabo lite such as glucamine remains to be established.
Several studies in the past decade suggest that HE in fulminant liver failure could result fro m alterations of neuroactive amino acids.In 1982 it was proposed that gut-derived gamma-aminobutyric acid (GABA), by virtue of its decreased removal by the liver and its entry into brain via a permeable bloodbrain barrier, might contribute to the neural inhibition characteristic of HE ( 4 ).Evidence in favo ur of this hypothesis was derived from stud ies in an ex-perimental animal model of fulminant liver failure derived by treatment with the hepatotoxin galactosamine.However, early findings of alterations of G ABA-related enzymes and binding sites observed in galactosamine-induced liver failure were not confi rmed by other investigators working either with the same experimental models or with other preparations.Rats with thioacetamide-induced liver failure and HE were found to have unaltered brain G ABA receptor affinities and densities (5); similar negative findings were reported in experimental ischemic liver failure.GABA levels are unchanged in autopsied brain tissue from patients with fulminant liver failure (6), and in the CSF and brains of rats with ischemic liver failure (7) .
In contrast, there is mounting evidence to suggest that the excitatory amino acids glutamate and aspartate may be modified in HE associated with fulminant liver failure.Using a novel indwelling cistema magna catheter technique, Swain et al (7 ) demonstrated increased CSF concentrations of these amino acids in ischemic liver failure in the rat.Brain concentrations of glutamate and aspartate concomitantly decreased as neurological status worsened.Similar changes were observed in thioacetamide-induced liver failure.The loss of these potent neuroexcitatory amino acids in brain in experimen tal fulminant liver failure affords a possible explanation for the generalized neural inhibition characteristic of this condition.However, the precise mechanism(s) implicated re• main to be established.
Benzodiazepines exert their CNS effects by interacting with binding sites on the GABA-benzodiazepine receptor complex.In the 1980s, two reports demonstrated increased densities of GABA and benzodiazepine binding sites in the brains of animals with galactosamine-induced fulminant liver failure.In 1985, a report described favourable results using the antagonist Ro 15-1788 (flumazenil) in the treatment of a patient with H E resulting from fulminant liver failure (8).It was proposed that the reversal of HE by flumazenil was mediated by the drug preventing the action of 'endogenous benzodiazepines' in the brain of this patient.Following up on this initial report, benzodiazepine receptor antagonists were found to induce a transient decrease in the clinical severity of HE caused by galactosamine-induced liver failure in the rabbit.O n the other hand, HE resulting from liver ischemia in both the rat and the rabbit did not respond to beruodiazepine antagonists.Initial findings of beneficial effects of flumazenil in the thioacetamide rat model of fulminant liver failure were not substan tiated in a subsequent study (9).Elevated brain concentrations of benzodiazepines have been described in autopsied brain tissue from approximately 30% of patients with fulminant liver fail ure resulting from acetaminophen overdose (10).A necdotal reports and uncontro lled stud ies in human fu lminant liver failure continue to yield conflicting results (8,11,1 2).The d ifficulty with any such studies lies in the problems in ensuring the rigorous exclusion of patients previously exposed to benzodiazepine med ication.
In conclusion , the precise pathogenetic factors responsible for HE and cerebral edema in fulminant liver failure contin ue to elude us.New find ings of a possible role for ammon ia (or one of its metabolites) in brain edema, coupled with the known effects of ammonia on CNS inhibit ion and excitation, suggest that the most rational means of prevent ion of the neuro logical complications of fulminant liver failure remains the lowering of blood ammonia in these patients.The efficacy of benzodiazepine receptor antagon ists in the treatment of HE in fulminant liver failure awaits the results of adequately controlled clinical trials.

Stem Cells International
Unit, Hopiral Saint-Luc (University of Monrreal) , Montreal, Quebec Correspondence and reprints : Dr RF Butterworth, Neuroscience Research Unit , H6piral Saint-Luc (University of Montreal), Monrreal , Quebec H2X 3)4 CAN J GAs-IBOENTEROL VOL 7 No 7 SEl"'TEMBER/0crOBER 1993 H EPATIC ENCEPHALOPATHY (HE) clearly demonstrated that in experimental animal models of fulminant liver failure, high energy phosphates are maintained at normal levels in the brains of severely encephalopathic animals (1).