Hepatic encephalopathy and brain edema in acute hepatic failure: does glutamate play a role?

Glycine and hyperammonemia: potential target for the treatment of hepatic encephalopathy

Hepatic encephalopathy (HE) is a neuropsychiatric disorder caused by hepatic dysfunction. Numerous studies dictate that ammonia plays an important role in the pathogenesis of HE, and hyperammonemia can lead to alterations in amino acid homeostasis. Glutamine and glycine are both ammoniagenic amino acids that are increased in liver failure. Modulating the levels of glutamine and glycine has shown to reduce ammonia concentration in hyperammonemia. Ornithine Phenylacetate (OP) has consistently been shown to reduce arterial ammonia levels in liver failure by modulating glutamine levels. In addition to this, OP has also been found to modulate glycine concentration providing an additional ammonia removing effect. Data support that glycine also serves an important role in N-methyl D-aspartate (NMDA) receptor mediated neurotransmission in HE. This potential important role for glycine in the pathogenesis of HE merits further investigations.

The effect of bilirubin on the excitability of mitral cells in the olfactory bulb of the rat

Olfactory dysfunction is a common clinical phenomenon observed in various liver diseases. Previous studies have shown a correlation between smell disorders and bilirubin levels in patients with hepatic diseases. Bilirubin is a well-known neurotoxin; however, its effect on neurons in the main olfactory bulb (MOB), the first relay in the olfactory system, has not been examined. We investigated the effect of bilirubin (>3 μM) on mitral cells (MCs), the principal output neurons of the MOB. Bilirubin increased the frequency of spontaneous firing and the frequency but not the amplitude of spontaneous excitatory postsynaptic currents (sEPSCs). TTX completely blocked sEPSCs in almost all of the cells tested. Bilirubin activity was partially blocked by N-methyl-D-aspartate (NMDA) and α-amino-3-hydroxy-5-methyl-4-isoxazolepro pionic acid (AMPA) receptor antagonists. Furthermore, we found that bilirubin increased the frequency of intrinsic firing independent of synaptic transmission in MCs. Our findings suggest that bilirubin enhances glutamatergic transmission and strengthens intrinsic firing independent of synaptic transmission, all of which cause hyperexcitability in MCs. Our findings provide the basis for further investigation into the mechanisms underlying olfactory dysfunction that are often observed in patients with severe liver disease.

Mechanisms of Acute Liver Failure

Acute liver failure (ALF) is characterized by the sudden onset of liver failure in a patient without evidence of chronic liver disease. This definition is important, as it differentiates patients with ALF from patients who suffer from liver failure owing to end-stage chronic liver disease [1].

Effects of CA1 glutamatergic systems upon memory impairments in cholestatic rats

BACKGROUND

Bile duct ligation (BDL) is shown to induce cholestasis-related liver function impairments as well as consequent cognitive dysfunctions (i.e. impaired learning and memory formation). Glutamatergic neurotransmission plays an important role in hippocampal modulation of learning and memory function. The present study aimed to investigate the possible involvement of dorsal hippocampal (CA1) glutamatergic systems upon cholestasis-induced amnesia.

METHOD

Cholestasis was induced in male Wistar rats through double-ligation of the main bile duct (at two points) and transection of the interposed segment. Step-through passive avoidance test was employed to examine rats' learning and memory function. All drugs were injected into CA1 region of the hippocampus.

RESULTS

our results indicated a decrease in memory retrieval following cholestasis (11, 17 and 24 days post BDL). Only subthreshold doses of N-methyl-d-aspartate (NMDA; 0.125 and 0.25 μg/μl) but not its effective dose (0.5 μg/μl), restored the cholestasis-induced amnesia in step-through passive avoidance test, 11, 17 and 24 days post BDL. This effect was blocked by the subthreshold dose of D-[1]-2-amino-7-phosphonoheptanoic acid (D-AP7, NMDA receptor antagonist; 0.0625 μg/μl, intra-CA1) at 0.125 μg/μl and 0.25 μg/μl doses of NMDA. Moreover, our data revealed that only effective doses of D-AP7 (0.125 and 0.25 μg/μl, intra-CA1) potentiate memory impairments in 11 days after BDL. It was noted that none of applied drugs/doses exerted an effect on memory acquisition and locomotors activity, 10 and 12 days post laparotomy, respectively.

CONCLUSION

Our findings suggest the potential involvement of CA1 glutamatergic system(s) in cholestasis-induced memory deficits.

Pathogenesis of hepatic encephalopathy: lessons from nitrogen challenges in man

Induction of hyperammonaemia with nitrogen challenge in man can be used to study the pathogenesis and treatment of hepatic encephalopathy complicating cirrhosis. Initially 20 g of glutamine was given orally as a flavored solution which resulted in doubling of blood ammonia concentration and this was associated with a deterioration in performance of the choice reaction time. The effect could have been due to a direct effect of glutamine rather than the ammonia generated so in subsequent experiments a glutamine free mixture of amino acids resembling the composition of haemoglobin was used (gastrointestinal bleeding is a known precipitant of hepatic encephalopathy). In Child grade B and C patients, 2-3 h after 54 g, slowing of the EEG was observed. The cerebral effects of induced hyperammonaemia were studied with diffusion weighted imaging and MR spectroscopy after giving 54 g of a mixture of threonine, serine and glycine when apparent diffusion coefficient increased. Also the change in ammonia levels correlated with the change in cerebral glutamine levels (r = 0.78, p = 0.002) suggesting intra cerebral formation of glutamine from ammonia and this may have accounted for the fall in cerebral myoinositol concentrations observed. Finally a colonic source for ammonia was confirmed by administering urea using colon coated capsules when ammonia concentrations slowly increased from 5 h after administration and rapidly after 10 h. In two patients the hyperammonaemia was ameliorated by pre treatment with Rifaximin 1200 mg per day for 1 week. Nitrogen challenge studies are thus a valuable model for studying new treatments for hepatic encephalopathy without the need to simultaneously treat precipitating factors.

Brain edema in acute liver failure and chronic liver disease: similarities and differences

Hepatic encephalopathy (HE) is a complex neuropsychiatric syndrome that typically develops as a result of acute liver failure or chronic liver disease. Brain edema is a common feature associated with HE. In acute liver failure, brain edema contributes to an increase in intracranial pressure, which can fatally lead to brain stem herniation. In chronic liver disease, intracranial hypertension is rarely observed, even though brain edema may be present. This discrepancy in the development of intracranial hypertension in acute liver failure versus chronic liver disease suggests that brain edema plays a different role in relation to the onset of HE. Furthermore, the pathophysiological mechanisms involved in the development of brain edema in acute liver failure and chronic liver disease are dissimilar. This review explores the types of brain edema, the cells, and pathogenic factors involved in its development, while emphasizing the differences in acute liver failure versus chronic liver disease. The implications of brain edema developing as a neuropathological consequence of HE, or as a cause of HE, are also discussed.

Brain imaging and hepatic encephalopathy

Novel imaging techniques allow the investigation of structural and functional neuropathology of hepatic encephalopathy in greater detail, but limited techniques are applicable to the clinic. Computed tomography and magnetic resonance imaging (MRI) can rule out other diagnoses and, in MRI, give diagnostic features in widely available sequences. An internationally accepted diagnostic framework that includes an objective imaging test to replace or augment psychometry remains elusive. Quantitative MRI is likely to be the best candidate to become this test. The utility of MR and nuclear medical techniques to the clinic and results from recent research are described in this article.

Motor-evoked potentials in awake rats are a valid method of assessing hepatic encephalopathy and of studying its pathogenesis

Experimental models of hepatic encephalopathy (HE) are limited by difficulties in objectively monitoring neuronal function. There are few models that examine a well-defined neuronal pathway and lack the confounding effects of anesthetics. Motor-evoked potentials (MEPs) assess the function of the motor tract, which has been shown to be impaired in patients with cirrhosis. MEPs were elicited by cranial stimulation (central) and compound motor action potential by sciatic nerve stimulation (peripheral) in several models of HE in the rat. The experiments were performed using subcutaneous electrodes without anesthetics. Brain water content was assessed by gravimetry, brain metabolites were measured by magnetic resonance spectroscopy, and amino acids in microdialysates from the frontal cortex were analyzed by high-performance liquid chromatography. Abnormalities of MEP were observed in acute liver failure (ALF) induced by hepatic devascularization in relation to the progression of neurological manifestations. Similar disturbances were seen in rats with portocaval anastomosis after the administration of blood or lipopolysaccharide, but were absent in rats with biliary duct ligation. Hypothermia (≤35°C) and mannitol prevented the development of brain edema in acute liver failure, but only hypothermia avoided the decrease in the amplitude of MEP. Disturbances of MEP caused by the administration of blood into the gastrointestinal tract in rats with portocaval anastomosis were associated with an increase in ammonia, glutamine, and glutamate in brain microdialysate.

CONCLUSION

Assessment of MEP in awake rats is a valid method to monitor HE in models of ALF and precipitated HE. This method shows the lack of efficacy of mannitol, a therapy that decreases brain edema, and relates disturbances of the function of the motor tract to ammonia and its metabolites.

The role of magnetic resonance imaging and spectroscopy in hepatic encephalopathy

Hepatic encephalopathy (HE) is a diverse manifestation of acute and chronic liver failure, ranging from cognitive impairment, only detectable on psychometric evaluation through to confusion, coma and death from cerebral oedema. While there is widespread acceptance of its importance, there is little consensus on how best to diagnose and monitor HE. Clinical descriptions, psychometric testing, electroencephalography and magnetic resonance (MR) imaging (and lately, MR spectroscopy) have all been proposed. MR techniques, in contrast to other modalities, have the benefit of objectivity and of being able to interrogate the brain directly with respect to changes in brain size, function and the metabolic disturbances thought to underlie HE, particularly in the context of astrocyte swelling. Modern clinical MRI scanners with multinuclear MR spectroscopy capabilities and brain mapping software can demonstrate structural and functional cellular changes using volumetric MRI, magnetization transfer MRI, diffusion-weighting MRI, functional MRI with oxygenation measurements and in vivo and in vitro (1)H and (31)P MR spectroscopy. This review describes the relative merits of these techniques and provides guidance on the directions for future research and translation into clinical practice.

Organic osmolytes in hyponatremia and ammonia toxicity

Hyperammonemia (HA) is a major and commonly observed feature of hepatic encephalopathy. Furthermore, hyponatremia is an important pathogenetic factor in patients with hepatic encephalopathy. Both conditions have some features in common, such as the release of organic osmolytes, which might be an adaptive mechanism against cell swelling. However, the consequence of a possible relationship between osmoregulatory response in hyperammonemia and hyponatremia is not completely understood. This review gives a short introduction into the pathogenesis of hepatic encephalopathy and hyponatremia. For a comparison of both pathological events, some basics on cellular osmo- and volume regulation are explained, in particular as the mechanisms involved in the adaption of the cell to volume changes can be different under both pathological conditions. The role of brain glutamine and organic osmolytes in hyponatremia and hyperammonemia and their combination are discussed based on findings in experimental animal models, and finally on data obtained from primary astrocytes in culture. The observations that the decrease of brain organic osmolytes in astrocytes not adequately compensate for an increased intracellular osmolarity caused by glutamine are consistent with results obtained after chronic hyponatremia in rats, in which the release of osmolytes does not protect from ammonia-induced brain edema. Furthermore, a decrease in intracellular osmolarity is attributed both to the release and a reduced de novo synthesis of amino acids.

New concepts in the mechanism of ammonia-induced astrocyte swelling

It is generally accepted that astrocyte swelling forms the major anatomic substrate of the edema associated with acute liver failure (ALF) and that ammonia represents a major etiological factor in its causation. The mechanisms leading to such swelling, however, remain elusive. Recent studies have invoked the role of oxidative stress in the mechanism of hepatic encephalopathy (HE), as well as in the brain edema related to ALF. This article summarizes the evidence for oxidative stress as a major pathogenetic factor in HE/ALF and discusses mechanisms that are triggered by oxidative stress, including the induction of the mitochondrial permeability transition (MPT) and activation of signaling kinases. We propose that a cascade of events initiated by ammonia-induced oxidative stress results in cell volume dysregulation leading to cell swelling/brain edema. Blockade of this cascade may provide novel therapies for the brain edema associated with ALF.

Acute liver failure in children

OBJECTIVE

To review the incidence, etiologies, pathophysiology, and treatment of acute liver failure (ALF) in children. Emphasis will be placed on the initial management of the multiple organ system involvement of ALF.

METHOD

MEDLINE search from 1970 to March 2005 was performed. Search headings were as follows: acute liver failure, fulminant liver failure, pediatric liver failure, hepatic encephalopathy, and liver transplantation. Studies written in English were selected. Pediatric studies were emphasized. Adult studies were referenced if there were no pediatric studies available in regard to a specific aspect of liver failure.

CONCLUSIONS

Pediatric acute liver failure is a rare but life-threatening disease. The common etiologies differ for given age groups. Management includes treating specific causes and supporting multiple organ system failure. Commonly associated disorders that require initial recognition and treatment include energy production deficiencies (hypoglycemia), coagulation abnormalities, immune system dysfunctions, encephalopathy, and cerebral edema. Criteria used to determine the need for liver transplant are reviewed as well as the difficulties associated with predicting which patients will meet these criteria and how rapidly liver transplant will become the only option. Finally, experimental procedures that may provide additional time for the liver to recover are briefly reported.

Mechanisms of Acute Liver Failure

Acute liver failure is characterized by the sudden onset of liver failure in a patient without evidence of chronic liver disease. This definition is important, as it differentiates patients with acute liver failure from patients who suffer from liver failure owing to end-stage chronic liver disease.

Therapy of intracranial hypertension in patients with fulminant hepatic failure

Severe intracranial hypertension (IH) in the setting of fulminant hepatic failure (FHF) carries a high mortality and is a challenging disease for the critical care provider. Despite considerable improvements in the understanding of the pathophysiology of cerebral edema during liver failure, therapeutic maneuvers that are currently available to treat this disease are limited. Orthotopic liver transplantation is currently the only definitive therapeutic strategy that improves outcomes in patients with FHF. However, many patients die prior to the availability of donor organs, often because of cerebral herniation. Currently, two important theories prevail in the understanding of the pathophysiology of IH during FHF. Ammonia and glutamine causes cytotoxic cerebral injury while cerebral vasodilation caused by loss of autoregulation increases intracranial pressure (ICP) and predisposes to herniation. Although ammonia-reducing strategies are limited in humans, modulation of cerebral blood flow seems promising, at least during the early stages of hepatic encephalopathy. ICP monitoring, transcranial Doppler, and jugular venous oximetry offer valuable information regarding intracranial dynamics. Induced hypothermia, hypertonic saline, propofol sedation, and indomethacin are some of the newer therapies that have been shown to improve survival in patients with severe IH. In this article, we review the pathophysiology of IH in patients with FHF and outline various therapeutic strategies currently available in managing these patients in the critical care setting.

The pathophysiology of hepatic encephalopathy

Hepatic encephalopathy (HE) is a complicated disorder, the pathophysiology of which remains to be fully understood. This article reviews the current main theories including the potential involvement of ammonia, gamma-aminobutyric acid (GABA)/benzodiazipines and false neurotransmitters. Each theory is critically examined with the evidence for each reviewed carefully, and the potential relationship of ammonia to the remaining two theories explored. Known preciptating factors of HE are also considered as evidence. The conclusions drawn from the evidence provided indicate the large role played by ammonia and suggest that this may be the key to understanding HE as science progresses.

Amino acid challenge in patients with cirrhosis and control subjects: ammonia, plasma amino acid and EEG changes

BACKGROUND/AIMS

The pathogenesis of hepatic encephalopathy (HE) is controversial. We have therefore studied the effect of induced hyperammonaemia in man.

PATIENTS AND METHODS

108 g of an amino acid mixture was given orally to 18 cirrhotics and 11 control subjects and changes in blood ammonia, EEG and plasma amino acids were observed.

RESULTS

Basal (39+/-6 versus 14+/-2 micromol/l) and 120-min post amino acid (77+/-10 versus 27+/-4) blood ammonia concentrations in cirrhotics were significantly increased compared to healthy controls (p < 0.001). Associated with these changes there was a significant increase in the ratio of slow-to-fast wave activity indicating EEG slowing (+0.41+/-0.16; N=13 versus -0.05+/-0.08; N=8; p=0.036). As expected in cirrhotics, basal valine and leucine concentrations were decreased while phenylalanine, tyrosine and methionine were significantly increased. Although the basal molar ratio of branched chain amino acids to the aromatic amino acids phenylalanine and tyrosine was significantly decreased in cirrhotics (1.5+/-0.2 versus 3.2+/-0.2; p < 0.0001), after the challenge when EEG changes were apparent in cirrhotics, the ratio significantly increased (p < 0.005) in both groups to 2.7+/-0.3 versus 4.1+/-0.3 (p=0.002). In the combined groups, there were significant correlations between EEG ratio change and the 120-min blood ammonia concentration (r=0.498; p=0.022).

CONCLUSION

The alterations in plasma amino acid patterns do not support a specific role for any of the amino acid groups in the pathogenesis of hepatic encephalopathy. They are however more in keeping with the direct or indirect role of ammonia.

Olfactory deficits in patients affected by minimal hepatic encephalopathy: a pilot study

Minimal hepatic encephalopathy (MHE) is the earliest stage of hepatic encephalopathy and is associated with changes in cognitive functions, in electrophysiological parameters, and in cerebral neurochemical/neurotransmitter homeostasis. MHE can be observed in patients with cirrhosis who have no clinical evidence of hepatic encephalopathy (HE). At present, no data are available on a possible olfactory dysfunction in such a syndrome, although the pathophysiology of HE may alter olfactory functions since some of the neurotransmitters impaired in the syndrome are involved in the transmission of olfactory information. In the present paper, we performed a preliminary study aimed at detecting whether identification and recognition odor memory is altered in patients with MHE. Twelve patients diagnosed as MHE on the basis of their scores at the portosystemic encephalopathy (PSE)-syndrome test battery, and 12 age-matched controls were studied. Consistent with the hypothesis, patients performed significantly worse than controls for both odor identification and recognition tasks. In addition, a significant correlation between the two olfactory tests and the PSE-syndrome test score was found. This pattern supports the notion that olfactory alterations related to cognitive dysfunction in patients with MHE may be linked to the pathophysiology of HE.

Hypothermia in acute liver failure

The development of encephalopathy in patients with acute liver injury defines the occurrence of liver failure. The encephalopathy of acute liver failure is characterized by brain edema which manifests clinically as increased intracranial pressure. Despite the best available medical therapies a significant proportion of patients with acute liver failure die due to brain herniation. The present review explores the experimental and clinical data to define the role of hypothermia as a treatment modality for increased intracranial pressure in patients with acute liver failure.

Management of patients with fulminant hepatic failure and brain edema

Cerebral edema in acute liver failure is associated with a poor prognosis. Optimization of cerebral perfusion pressure and blood flow plays a key role in contemporary management of these patients. However, understanding of the pathophysiology of brain edema is required for optimal patient management. This review explains the relationships between cerebral perfusion and edema and summarizes therapies that are currently used in patients with acute liver failure to prevent and reduce intracranial pressure.

Taurine reduces ammonia- and N-methyl-D-aspartate-induced accumulation of cyclic GMP and hydroxyl radicals in microdialysates of the rat striatum

Acute ammonia neurotoxicity caused by intraperitoneal administration of ammonium salts is mediated by overactivation of N-methyl-D-aspartate (NMDA) receptors, with ensuing generation of free radicals and extracellular accumulation of cyclic GMP (cGMP) arising from stimulation of nitric oxide (NO) synthesis. In this study, infusion of ammonium chloride or NMDA into the striata of rats via microdialysis probes increased the contents of cyclic GMP and hydroxyl radicals in the microdialysates. Co-infusion of taurine virtually abolished both the ammonia- and NMDA-induced accumulation of cGMP. Taurine also attenuated accumulation of hydroxyl radicals evoked by either treatment. This result is the first evidence of a potential of taurine to attenuate the effects of NMDA receptor overactivation by ammonia in vivo and points to the inhibition of the NMDA receptor-mediated NO synthesis as a possible mechanism of its neuroprotective action. Taurine or its blood-brain barrier penetrating analogues may be applicable in treatment of ammonia-induced neurological deficits.

Cerebral microdialysis in patients with fulminant hepatic failure

Fulminant hepatic failure (FHF) is often complicated by high intracranial pressure (ICP) and fatal brain damage. In this study, we determined if a rise in [glutamate]ec and [lactate]ec preceded surges of high ICP in patients with FHF (median age, 42; range, 20-55 years; 7 women; 3 men) by inserting a microdialysis catheter into the brain-cortex together with an ICP catheter. The microdialysis catheter was perfused with artificial cerebrospinal-fluid at a rate of 0.3 microL/min. Dialysate was collected approximately every 30 minutes or when ICP increased. A total of 352 microdialysis samples were collected during a median of 3 days and allowed for approximately 1,760 bedside analyses of the collected dialysate. In 5 patients that later developed surges of high ICP, the initial values of [glutamate]ec and [lactate]ec were 2 to 5 times higher compared with patients with normal ICP. [Glutamate]ec then tended to vanish with time in both groups of patients. An increase in [glutamate]ec did not precede high ICP in any of the cases. In contrast, [lactate]ec was high throughout the study in the high ICP group and increased further before surges of high ICP. We conclude that in patients with FHF, cerebral [glutamate]ec and [lactate]ec are elevated. However, the elevated [glutamate]ec is not correlated to high ICP. In contrast, elevations in [lactate]ec preceded surges of high ICP. In conclusion, accelerated glycolysis with lactate accumulation is implicated in vasodilatation and high ICP in patients with FHF. The data suggest that bedside cerebral microdialysis is a valuable tool in monitoring patients with FHF and severe hyperammonemia.

Ammonia-induced heme oxygenase-1 expression in cultured rat astrocytes and rat brain in vivo

Ammonia is a key factor in the pathogenesis of hepatic encephalopathy (HE), which is a major complication in acute and chronic liver failure and other hyperammonemic states. The molecular mechanisms underlying ammonia neurotoxicity and the functional consequences of ammonia on gene expression in astrocytes are incompletely understood. Using cDNA array hybridization technique we identified ammonia as a trigger of heme oxygenase-1 (HO-1) mRNA levels in cultured rat astrocytes. As shown by Northern and Western blot analysis, HO-1 mRNA levels were upregulated by ammonia (0.1-5 mmol/L) after 24 h and protein expression after 72 h in astrocytes. These ammonia effects on HO-1 are probably triggered to a minor extent by ammonia-induced glutamine synthesis or by astrocyte swelling, because HO-1 expression was not inhibited by the glutamine synthetase inhibitor methionine sulfoximine (which abrogated ammonia-induced cell swelling in cultured astrocytes), and ammonia-induced HO-1 expression could only partly be mimicked by hypoosmotic astrocyte swelling. Hypoosmotic (205 mOsm/L) exposure of astrocytes led even to a decrease in HO-1 mRNA levels within 4 h, whereas hyperosmotic (405 mOsm/L) exposure increased HO-1 mRNA expression. After 24 h, hypoosmolarity slightly raised HO-1 mRNA expression. Taurine and melatonin diminished ammonia-induced HO-1 mRNA or protein expression, whereas other antioxidants (dimethylthiourea, butylated hydroxytoluene, N-acetylcysteine, and reduced glutathione) increased HO-1 mRNA levels under ammonia-free conditions. An in vivo relevance is suggested by the finding that increased HO-1 expression occurs in the brain cortex from acutely ammonia-intoxicated rats. It is concluded that ammonia-induced HO-1 expression may contribute to cerebral hyperemia in hyperammonic states.

Mild hypothermia in the prevention of brain edema in acute liver failure: mechanisms and clinical prospects

Mild hypothermia (32 degrees C-35 degrees C) reduces intracranial pressure in patients with acute liver failure and may offer an effective adjunct therapy in the management of these patients. Studies in experimental animals suggest that this beneficial effect of hypothermia is the result of a decrease in blood-brain ammonia transfer resulting in improvement in brain energy metabolism and normalization of glutamatergic synaptic regulation. Improvement in brain energy metabolism by hypothermia may result from a reduction in ammonia-induced decrease of brain glucose (pyruvate) oxidation. Restoration of normal glutamatergic synaptic regulation by hypothermia may be the consequence of the removal of ammonia-induced decreases in expression of astrocytic glutamate transporters resulting in normal glutamate neurotransmitter inactivation in brain. Randomized controlled clinical trials of hypothermia are required to further evaluate its clinical impact.

Suppression of ammonia-induced swelling by aspartate but not by ornithine in primary cultures of rat astrocytes

Cerebral edema with a rise in intracranial pressure is the hallmark of fulminant hepatic failure (FHF) and acute hyperammonemic (HA) states and is characterized by a poor survival rate. Astrocytes are the cells in brain which are swollen in these conditions. Several hypotheses have been proposed to explain the mechanism of cerebral edema in FHF and treatment strategies have evolved based on these putative mechanisms. Treatment with a mixture of ornithine and aspartate has been proven to be clinically beneficial as it reduces edema and improves the neurological status. It has been suggested that these two amino acids generate the glutamate required for the synthesis of glutamine and that they also enhance urea synthesis in surviving hepatocytes in FHF and HA. Presently, we report that of these two amino acids, only aspartate is effective in suppressing ammonia-induced swelling in primary cultures of astrocytes, while ornithine is ineffective. These results are discussed in relation to the metabolism of aspartate and ornithine in astrocytes, with an emphasis on glutamine synthesis and the malate-aspartate shuttle (MAS). We propose that the ability of aspartate to generate glutamate in the cytosol for glutamine synthesis and oxaloacetate in mitochondria to support the citric acid cycle play a role in its ability to reduce ammonia-induced swelling in astrocytes.

Prolonged unilateral vasodilatation and brain edema in fulminant hepatic failure, associated with symptomatic seizure

We report prolonged unilateral vasodilatation and hemispheric brain edema in a 49-year-old man with fulminant hepatic failure (FHF). The patient presented with a tonic-clonic seizure caused by a hypertensive subcortical hemorrhage in the left parietal lobe. Serial computed tomography (CT) scans showed progressive darkening of the ipsilateral hemisphere, suggesting hemispheric cerebral infarction, but the patient did not show clinical signs of deterioration. Brain magnetic resonance angiography showed dilation of the large arteries of the left hemisphere. Evaluation of cerebral blood flow 7 days postictus with single photon emission CT revealed marked ipsilateral hyperperfusion. The darkening of the hemisphere was brain edema elicited by hyperperfusion. Brain edema was reversible, disappearing 14 days postictus. Hemispheric brain edema was caused by unilateral cerebral vasodilatation and resultant hyperperfusion. Although brain edema is a major complication in FHF patients and cerebral hyperperfusion is responsible for edema formation, CT findings of these patients almost invariably show a bilateral lesion. Unilateral vasodilatation and subsequent hemispheric hyperperfusion may be due to overproduction of vasodilators, already abundant in the brains of patients with severe hepatic failure, by seizure activity.

Amino acid challenge in patients with cirrhosis: a model for the assessment of treatments for hepatic encephalopathy

BACKGROUND/AIMS

To mimic episodic hepatic encephalopathy after gastrointestinal bleeding under controlled conditions, cirrhotic patients were challenged with an amino acid mixture of comparable composition to haemoglobin.

METHODS

Basal EEG, psychometric score (HE test), reaction times and venous blood ammonia were recorded. Following a 54 or 108 gm oral amino acid challenge, blood ammonia levels and EEG were recorded at 30-min intervals, and psychometric testing was repeated at 180 min. Ten controls (57 +/- 2) and 31 cirrhotics (52 +/- 2) of which 21 were Child's grade A or B and 10 grade C underwent the challenge. Nine had a transjugular intrahepatic porta-systemic shunt in situ.

RESULTS

Seventeen patients had abnormal baseline HE scores. Basal blood ammonia and reaction time A were significantly greater in patients (52 +/- 5 micromol/l and 478 +/- 20 ms, respectively) than controls (19 +/- 2 micromol/l and 372 +/- 14 ms) (P < 0.001). Following the challenge, in patients with advanced liver disease (Child's grade B and C) the slowing of reaction time A (+85 +/- 38 and +71 +/- 31 ms, respectively; P < 0.03) and EEG (ratio of slow to fast wave activity +0.31 +/- 0.12 and +0.58 +/- 0.19; P < 0.02) were significantly greater than in controls (-3.3 +/- 8 ms and 0.00 +/- 0.03, respectively). Patients with an abnormal basal HE score had the most pronounced changes (reaction time A +110 +/- 39 ms, P < 0.01, EEG +0.52 +/- 13, P < 0.01, respectively). The change in EEG ratio correlated with the dose of amino acid administered (r = 0.96; P < 0.008).

CONCLUSION

The amino acid challenge constitutes a reproducible human model of episodic, Type C hepatic encephalopathy unaffected by the complications usually encountered in clinical practice.

Hepatic encephalopathy

In the last decade, a significant amount of research has been devoted to the pathogenesis and treatment of hepatic encephalopathy (HE). Non-invasive neuroimaging techniques such as magnetic resonance imaging and spectroscopy have become important research tools. The search for a suitable animal model of HE associated with cirrhosis is still ongoing. Moreover, consensus terminology and diagnostic criteria for HE in humans are badly needed.

Alzheimer-type I astrogliopathy (AIA) and its implications for dynamic plasticity of astroglia: a historical review of the significance of AIA

Alzheimer-type I astrogliopathy (AIA) is an uncommon neuropathological phenomenon encountered in Wilson's disease and less often in acquired hepatic encephalopathy. Since its first description in 1912 it has received little attention. However, after 1971, when the nature of its morphogenesis began to be recognized and it was shown that it could be reproduced experimentally, its significance has been increasingly appreciated. Two intriguing characteristics of the dynamic plasticity of astroglia were revealed from the studies of the inter-relationships between AIA and Alzheimer-type II astrogliopathy (AIIA); normal astroglia and AIIA; and reactive astrogliosis and AIIA, namely, the compensatory "rebound" phenomenon of Alzheimer astrogliopathy, and a dual cellular origin for reactive astrogliosis taking place in both normal and dystrophic astrocytes. More recently the presence of AIA and AIIA has been reported in a case of anoxic encephalopathy, and also in a case of Marchiafava-Bignami's disease. In this review, dependable criteria for the identification of the pathological features of AIA are discussed and emphasized. Both types of Alzheimer astrogliopathy may be used as pathologic markers with specific morphological and immunocytochemical characteristics to study in detail the disturbances of metabolic interactions between the astrocyte-neuron coupling and the exact mechanisms of the dynamic plasticity of astroglia.

Extracellular concentrations of taurine, glutamate, and aspartate in the cerebral cortex of rats at the asymptomatic stage of thioacetamide-induced hepatic failure: modulation by ketamine anesthesia

Subclinical hepatic encephalopathy (SHE) was produced in rats by two intraperitoneal injections of TAA at 24 h intervals and the animals were examined 21 days later. Concentrations of the neuroactive amino acids taurine (Tau), glutamate (Glu) and aspartate (Asp), were measured in the cerebral cortical microdialysates of thioacetamide (TAA)-treated and untreated control rats. During microdialysis some animals were awake while others were anesthetized with ketamine plus xylazine. There was no difference in the water content of cerebral cortical slices isolated from control and SHE rats, indicating a recovery from cerebral cortical edema that accompanies the acute, clinical phase of hepatic encephalopathy in this model. When microdialysis was carried out in awake rats, dialysate concentrations of all the three amino acids were 30% to 50% higher in SHE rats than in control rats. Ketamine anesthesia caused a 2.2% increase of water content of cerebral cortical slices and increased Asp, Glu, and Tau concentration in microdialysates of control rats. In SHE rats, ketamine anesthesia produced a similar degree of cerebral edema, however, it did not alter Asp and Glu concentrations in the microdialysates. These data may reflect on one hand a neuropathological process of excitotoxic neuronal damage related to increased Glu and Asp, on the other hand neuroprotection from neuronal swelling indicated by Tau redistribution in the cerebral cortex. The reduction of the effects of SHE on Glu and Asp content in ketamine-anesthesized rats is likely to be due to interference of ketamine with the NMDA receptor-mediated component of the SHE-evoked excitatory neurotransmitter efflux and/or reuptake of the two amino acids. By contrast, the SHE-related increase of Tau content was not affected by ketamine anesthesia, indicating that the mechanism(s) underlying SHE-evoked accumulation of Tau must be different from the mechanism causing release of excitatory amino acids. The results with ketamine advocate caution when using this anesthetic in studies employing the cerebral microdialysis technique for measurement of extracellular amino acids.

Subclinical seizure activity and prophylactic phenytoin infusion in acute liver failure: a controlled clinical trial

Subclinical seizure activity in the patient with encephalopathy and on ventilation with acute liver failure (ALF) is a poorly recognized entity. Its importance lies in the likely exacerbation of cerebral hypoxia and the contribution of such seizure activity to the development of cerebral edema. The aim of the present study was to document the frequency of subclinical seizure activity in a cohort of patients with ALF by using a cerebral function and activity monitor that allows continuous recording of electroencephalogram activity at the bedside and to determine whether the prophylactic administration of the antiepileptic agent phenytoin would reduce its occurrence. Forty-two patients were enrolled in a controlled clinical trial: 20 patients were given phenytoin and 22 acted as controls. Subclinical seizure activity was recorded in 3 and 10 patients, respectively, of the treated and control groups. Pupillary abnormalities indicative of seizure activity and/or raised intracranial pressure (ICP) were also seen less frequently in the phenytoin-treated group compared with the controls (5 and 11 patients, respectively). Autopsy examinations available in 19 patients showed signs of cerebral edema in only 2 (22%) of the phenytoin-treated patients compared with 7 (70%) of the controls (P <.033). Based on these findings, we recommend that patients with ALF, on reaching the stage of grade III or IV encephalopathy, should be routinely monitored for subclinical seizure activity. In this study, prophylaxis with phenytoin reduced the frequency of such seizure activity and its effects, and proved to be safe with the regimen used.

Pathophysiology of septic encephalopathy: a review

OBJECTIVES

Encephalopathy is a common complication of sepsis. This review describes the different pathologic mechanisms that may be involved in its etiology.

DATA SOURCES

The studies described here were derived from the database PubMed (http:¿¿www.nlm.nih.gov) and from references identified in the bibliographies of pertinent articles and books. The citations are largely confined to English language articles between 1966 and 1998. Older publications were used if they were of historical significance.

STUDY SELECTION

All investigations in which any aspect of septic encephalopathy was reported were included. This selection encompasses clinical, animal, and in vitro cell culture work.

DATA EXTRACTION

The literature cited was published in peer-reviewed clinical or basic science journals or in books.

DATA SYNTHESIS

Contradictions between the results of published studies are discussed.

CONCLUSIONS

The most immediate and serious complication of septic encephalopathy is impaired consciousness, for which the patient may require ventilation. The etiology of septic encephalopathy involves reduced cerebral blood flow and oxygen extraction by the brain, cerebral edema, and disruption of the blood-brain barrier that may arise from the action of inflammatory mediators on the cerebrovascular endothelium, abnormal neurotransmitter composition of the reticular activating system, impaired astrocyte function, and neuronal degeneration. Currently, there is no treatment.

Mild hypothermia delays the onset of coma and prevents brain edema and extracellular brain glutamate accumulation in rats with acute liver failure

Mild hypothermia is effective in the prevention of brain edema associated with cerebral ischemia and traumatic brain injury. Brain edema is also a serious complication of acute liver failure (ALF). To assess the effectiveness of hypothermia in ALF, groups of rats were subjected to hepatic devascularization (portacaval anastomosis, followed 48 hours later by hepatic artery ligation), and body temperatures were maintained at either 35 degrees C (hypothermic) or 37 degrees C (normothermic). Mild hypothermia resulted in a significant delay in the onset of severe encephalopathy and in reduction of brain water content compared with normothermic ALF rats (control [n = 8] 80.22%; ALF-37 degrees C [n = 8] 81.74%; ALF-35 degrees C [n = 8] 80.48% [P <.01 compared with ALF-37 degrees C]). This protective effect was accompanied by a significant reduction of cerebrospinal fluid (CSF) (but not plasma) ammonia concentrations (CSF ammonia: control: 0.05 mg/dL; ALF-37 degrees C: 1.01 mg/dL; ALF-35 degrees C: 0.07 mg/dL, P <.01 compared with ALF-37 degrees C). In vivo cerebral microdialysis studies revealed that mild hypothermia resulted in a significant reduction of extracellular glutamate concentrations in the brains of rats with ALF (control: 1. 06 micromol/L; ALF-37 degrees C: 2.74 micromol/L; ALF-35 degrees C: 1.49 micromol/L [P <.01 compared with ALF-37 degrees C]). These findings suggest that: 1) mild hypothermia is an effective approach to the prevention of the central nervous system consequences of experimental ALF; and that 2) the beneficial effect of hypothermia is mediated via mechanisms involving reduced blood-brain transfer of ammonia and/or reduction of extracellular brain glutamate concentrations. Mild hypothermia may be an effective approach to delay the onset of brain edema in patients with ALF awaiting liver transplantation.

Cerebral blood flow and the development of ammonia-induced brain edema in rats after portacaval anastomosis

Two mechanisms may account for brain edema in fulminant hepatic failure: the osmotic effects of brain glutamine, a product of ammonia detoxification, and a change of cerebral blood flow (CBF). We have shown brain edema, a marked increase in brain glutamine, and a selective rise in CBF in rats after portacaval anastomosis receiving an ammonia infusion. In this study, we inhibited the activity of glutamine synthetase with methionine-sulfoximine (MSO) and examined ammonia levels, brain water and CBF. Four groups received either a continuous ammonium acetate or control infusion; half of the animals had been pretreated with MSO or vehicle. The ammonia group exhibited brain edema (79.97 +/- 0.04 vs. 81.11 +/- 0. 13% water), an increase in cerebrospinal fluid (CSF) glutamine (1.29 +/- 0.21 vs. 2.84 +/- 0.39 mmol/L) and CBF (63 +/- 11 vs. 266 +/- 45 mL/min/100 g brain). When MSO was added to the ammonia infusion, ammonia levels rose further (928 +/- 51 vs. 1,293 +/- 145 mmol/L, P <.05) but CSF glutamine decreased (2.84 +/- 0.39 vs. 1.61 +/- 0.2 mmol/L, P <.01). Brain edema (80.48 +/- 0.11%) and cerebral hyperemia (140 +/- 25 mL/min/100 g brain) were significantly ameliorated in the ammonia plus MSO group. Brain output of circulating nitric oxide (NO(x)) was increased in the ammonia-infused group but normalized in the ammonia plus MSO group. In this model, the rise of CBF reflects intracranial events that occur after glutamine synthesis. Activation of nitric oxide synthase in the brain could account for these findings.

Acute liver failure

Severe hepatitis A infection is an infrequent but well-recognized cause of acute liver failure that can now be effectively prevented with vaccination against hepatitis A virus. Bromfenac and troglitazone hepatotoxicity as well as various herbal remedies are some of the newly identified causes of acute liver failure. The recently identified transfusion-transmitted virus has been implicated in some cases of idiopathic acute liver failure whereas hepatitis G virus does not appear to be a causative agent. Recognizing, monitoring, and treating patients with life-threatening cerebral edema remain critically important but difficult aspects of the clinical care of acute liver failure. Hypothermia and N-acetylcysteine are promising experimental approaches to cerebral edema but emergency liver transplantation is the only proven means of improving patient survival. Although recent changes in organ allocation may reduce waiting time to transplantation, more reliable and validated markers of liver regeneration and prognosis are needed to triage patients. The potential application and limitations of novel technologies including bioartificial liver devices and auxiliary liver transplantation continue to evolve from pioneering work in animal models and human subjects.

Changes in the extracellular profiles of neuroactive amino acids in the rat striatum at the asymptomatic stage of hepatic failure

Rats were treated with a hepatotoxin thioacetamide (TAA) and examined 21 days later, when they showed moderate fatty metamorphosis of the liver and morphological changes in brain indicative of excitotoxic neuronal damage, but no evident biochemical or neurophysiological symptoms of hepatic encephalopathy (HE). High-performance liquid chromatography (HPLC) analysis of extracellular amino acids in striatal microdialysates of TAA-treated rats revealed a significant increase in the excitatory amino acids glutamate (Glu) and aspartate (Asp) and their amino acid metabolites glutamine (Gln) and alanine (Ala). Microdialysis in the presence of 50 mM K+ triggered in TAA-treated rats an accumulation of Asp and Glu, and diminished the accumulation of Gln. These effects were virtually absent in control rats. None of the treatments affected the accumulation of the nontransmitter amino acid leucine (Leu). The above changes mirror those previously described in symptomatic HE and are likely to contribute to excitotoxic damage. The basal microdialysate content of taurine (Tau), an amino acid with antioxidant and volume regulatory properties, was 60% lower in TAA-treated rats than in control rats despite its increased blood-to-brain transport. The decrease in extracellular Tau may thus reflect Tau redistribution to adjacent central nervous system (CNS) cells manifesting a cell-protective response. Stimulation with 50 mM K+ increased extracellular Tau in control rats by 182% and in TAA-treated rats by 322%. Stimulation with 100 microM N-methyl-D-aspartate (NMDA) increased extracellular Tau in control rats by 27 % and in TAA-treated rats by as much as 250%. The increase of K+- or NMDA-dependent Tau release may reflect improved cell volume regulation and neuroprotection and contribute to attenuation of neurologic symptoms in rats with liver failure.

Cerebral herniation in patients with acute liver failure is correlated with arterial ammonia concentration

Cerebral edema leading to cerebral herniation (CH) is a common cause of death in acute liver failure (ALF). Animal studies have related ammonia with this complication. During liver failure, hepatic ammonia removal can be expected to determine the arterial ammonia level. In patients with ALF, we examined the hypotheses that high arterial ammonia is related to later death by CH, and that impaired removal in the hepatic circulation is related to high arterial ammonia. Twenty-two patients with ALF were studied retrospectively. In addition, prospective studies with liver vein catheterization were performed after development of hepatic encephalopathy (HE) in 22 patients with ALF and 9 with acute on chronic liver disease (AOCLD). Cerebral arterial-venous ammonia difference was studied in 13 patients with ALF. In all patients with ALF (n = 44), those who developed CH (n = 14) had higher arterial plasma ammonia than the non-CH (n = 30) patients (230 +/- 58 vs. 118 +/- 48 micromol/L; P <. 001). In contrast, galactose elimination capacity, bilirubin, creatinine, and prothrombin time were not different (NS). Cerebral arterial-venous differences increased with increasing arterial ammonia (P <.001). Arterial plasma ammonia was lower than hepatic venous in ALF (148 +/- 73 vs. 203 +/- 108 micromol/L; P <.001). In contrast, arterial plasma ammonia was higher than hepatic venous in patients with AOCLD (91 +/- 26 vs. 66 +/- 18 micromol/L; P <.05). Net ammonia release from the hepatic-splanchnic region was 6.5 +/- 6. 4 mmol/h in ALF, and arterial ammonia increased with increasing release. In contrast, there was a net hepatic-splanchnic removal of ammonia (2.8 +/- 3.3 mmol/h) in patients with AOCLD. We interpret these data that in ALF in humans, vast amounts of ammonia escape hepatic metabolism, leading to high arterial ammonia concentrations, which in turn is associated with increased cerebral ammonia uptake and CH.

Astroglial dysfunction in hepatic encephalopathy

While the pathogenesis of hepatic encephalopathy (HE) remains elusive, there is considerable evidence pointing to a key role of ammonia-induced dysfunction of astrocytes in this condition. Deficits in the ability of astrocytes to take up glutamate from the extracellular space may lead to abnormal glutamatergic neurotransmission. Furthermore, excessive stimulation of neuronal and glial glutamate receptors by elevated extracellular levels of glutamate may lead to excitotoxicity and greater glial dysfunction. Ammonia also causes upregulation of astroglial peripheral-type benzodiazepine receptors (PBRs) which is associated with increased production of neurosteroids. These neurosteroids have potent positive modulatory effects on the neuronal GABA(A) receptor which, combined with an ammonia-induced astroglial defect in GABA uptake, may result in enhanced GABAergic tone. Brain edema, associated with fulminant hepatic failure, may also result from astroglial abnormalities as the edema appears to be principally caused by swelling of these cells. Increased amounts of glutamine in astrocytes resulting from elevated brain ammonia levels may be a factor in this swelling. Other osmolytes such as glutathione may also be involved. Glial swelling may also result from NH4+ - and K+ -mediated membrane depolarization as well as by the actions of PBR agonists and neurosteroids. These findings show that an ammonia-induced gliopathy is a major factor in the pathogenesis of HE.

Hepatocyte liver-assist systems--a clinical update

Liver failure is a serious problem that affects thousands of people in the United States each year. Other than liver transplantation, a supportive therapy has been unavailable for patients with liver failure that is refractory to medical treatment. An apparent solution to this problem is a hepatocyte liver-assist system. Such a system is composed of mammalian hepatocytes loaded in a mechanical apparatus, such as a hollow fiber cartridge. During extracorporeal perfusion of the system, the hepatocytes provide metabolic function to the patient with liver failure. At least two extracorporeal hepatocyte systems have shown promise in human clinical trials of acute liver failure. In fact, one system has gained approval from the Food and Drug Administration for testing in a randomized multicenter clinical trial. In this article, key issues of clinical testing are reviewed, and major contributions and questions that remain unresolved are emphasized.

Oral L-ornithine-L-aspartate therapy of chronic hepatic encephalopathy: results of a placebo-controlled double-blind study

BACKGROUND/AIMS

In the current state of knowledge of the pathophysiology of hepatic encephalopathy, a reduction in hyperammonemia is the most important evidence of effective treatment. Therefore, the therapeutic efficacy of oral L-ornithine-L-aspartate, which improves impaired ammonia detoxification, was investigated in patients with cirrhosis, hyperammonemia and stable, overt, chronic hepatic encephalopathy, and in subclinical hepatic encephalopathy in a randomized, double-blind, placebo-controlled clinical trial.

METHODS

Oral L-ornithine-L-aspartate was administered three times daily at fixed times for 14 consecutive days in a total dose of 18 g per day. The design was chosen to prevent an increase in ammonia induced by a protein meal of 0.25 g/kg body weight, given at the start of the daily treatment period. Efficacy variables were: fasting and postprandial ammonia concentration, Number-Connection-Test time, mental state grades, and a Portosystemic Encephalopathy Index. Analyses were based on the total study sample of 32 placebo- and 34 L-ornithine-L-aspartate-treated patients as well as on the subgroup samples in the overt (20 placebo- and 23 L-ornithine-L-aspartate-treated) and subclinical hepatic encephalopathy (12 placebo- and 11 L-ornithine-L-aspartate-treated) patients.

RESULTS

Number Connection Test performance times (p<0.01) as well as fasting (p<0.01) and postprandial (p<0.05) venous blood ammonia concentrations in the L-ornithine-L-aspartate-treated group showed improvement in comparison to placebo. Also, the mental state grade (p<0.05) and the Portosystemic Encephalopathy Index (p<0.01), improved to a much greater degree in the L-ornithine-L-aspartate group than in the placebo group. Adverse events were observed in neither the placebo nor the L-ornithine-L-aspartate-treated patients.

CONCLUSION

Oral L-ornithine-L-aspartate is a safe, well-tolerated treatment with a good compliance rate and a beneficial therapeutic effect in patients with cirrhosis and stable, overt, chronic hepatic encephalopathy.