Pathogenesis of intracranial hypertension in acute liver failure: inflammation, ammonia and cerebral blood flow

Acute liver failure in dogs and cats

OBJECTIVE

To define acute liver failure (ALF), review the human and veterinary literature, and discuss the etiologies and current concepts in diagnostic and treatment options for ALF in veterinary and human medicine.

ETIOLOGY

In veterinary medicine ALF is most commonly caused by hepatotoxin exposure, infectious agents, inflammatory diseases, trauma, and hypoxic injury.

DIAGNOSIS

A patient may be deemed to be in ALF when there is a progression of acute liver injury with no known previous hepatic disease, the development of hepatic encephalopathy of any grade that occurs within 8 weeks after the onset of hyperbilirubinemia (defined as plasma bilirubin >50 μM/L [>2.9 mg/dL]), and the presence of a coagulopathy. Diagnostic testing to more specifically characterize liver dysfunction or pathology is usually required.

THERAPY

Supportive care to aid the failing liver and compensate for the lost functions of the liver remains the cornerstone of care of patients with ALF. Advanced therapeutic options such as extracorporeal liver assist devices and transplantation are currently available in human medicine.

PROGNOSIS

The prognosis for ALF depends upon the etiology, the degree of liver damage, and the response to therapy. In veterinary medicine, the prognosis is generally poor.

Pathogenesis of hepatic encephalopathy: role of ammonia and systemic inflammation

The syndrome we refer to as Hepatic Encephalopathy (HE) was first characterized by a team of Nobel Prize winning physiologists led by Pavlov and Nencki at the Imperial Institute of Experimental Medicine in Russia in the 1890's. This focused upon the key observation that performing a portocaval shunt, which bypassed nitrogen-rich blood away from the liver, induced elevated blood and brain ammonia concentrations in association with profound neurobehavioral changes. There exists however a spectrum of metabolic encephalopathies attributable to a variety (or even absence) of liver hepatocellular dysfunctions and it is this spectrum rather than a single disease entity that has come to be defined as HE. Differences in the underlying pathophysiology, treatment responses and outcomes can therefore be highly variable between acute and chronic HE. The term also fails to articulate quite how systemic the syndrome of HE can be and how it can be influenced by the gastrointestinal, renal, nervous, or immune systems without any change in background liver function. The pathogenesis of HE therefore encapsulates a complex network of interdependent organ systems which as yet remain poorly characterized. There is nonetheless a growing recognition that there is a complex but influential synergistic relationship between ammonia, inflammation (sterile and non-sterile) and oxidative stress in the pathogenesis HE which develops in an environment of functional immunoparesis in patients with liver dysfunction. Therapeutic strategies are thus moving further away from the traditional specialty of hepatology and more towards novel immune and inflammatory targets which will be discussed in this review.

Neuroinflammation in hepatic encephalopathy: mechanistic aspects

Hepatic encephalopathy (HE) is a major neurological complication of severe liver disease that presents in acute and chronic forms. While elevated brain ammonia level is known to be a major etiological factor in this disorder, recent studies have shown a significant role of neuroinflammation in the pathogenesis of both acute and chronic HE. This review summarizes the involvement of ammonia in the activation of microglia, as well as the means by which ammonia triggers inflammatory responses in these cells. Additionally, the role of ammonia in stimulating inflammatory events in brain endothelial cells (ECs), likely through the activation of the toll-like receptor-4 and the associated production of cytokines, as well as the stimulation of various inflammatory factors in ECs and in astrocytes, are discussed. This review also summarizes the inflammatory mechanisms by which activation of ECs and microglia impact on astrocytes leading to their dysfunction, ultimately contributing to astrocyte swelling/brain edema in acute HE. The role of microglial activation and its contribution to the progression of neurobehavioral abnormalities in chronic HE are also briefly presented. We posit that a better understanding of the inflammatory events associated with acute and chronic HE will uncover novel therapeutic targets useful in the treatment of patients afflicted with HE.

Effects of dexamethasone and cox inhibitors on intracranial pressure and cerebral perfusion in the lipopolysaccharide treated rats with hyperammonemia

Introduction

Systemic inflammation may affect the brain by aggravating the stage of encephalopathy and increasing intracranial pressure (ICP) especially if liver insufficiency with hyperammonemia is present. The aim of this study was to determine if the influence of concomitant hyperammonemia and lipopolysaccharide (LPS) on the brain can be prevented by dexamethasone and cyclooxygenase (COX) inhibitors.

Method

Fifty-four male Wistar rats, 6 in each group, were divided into the following groups: Saline+saline; LPS (2mg/kg)+saline; LPS+indomethacin (10mg/kg); LPS+diclofenac (10mg/kg); LPS+dexamethasone (2mg/kg) in experiment A. Experiment-B included the following groups: LPS+NH3 (140μmol/kg/min)+saline; LPS+NH3+indomethacin; LPS+NH3+diclofenac and LPS+NH3+dexamethasone. ICP was monitored via a catheter placed in cisterna magna and changes in CBF were recorded by laser Doppler flowmetry.

Results

LPS with and without NH3 induced a similar increase in plasma 6-keto-prostaglandin-F_1α (6-keto-PGF_1α) concentration together with a concomitant rise in CBF and ICP. Indomethacin and diclofenac prevented the increase in ICP by LPS alone, and with the addition of NH3 the increase in both CBF and ICP, which was associated with a decrease in 6-keto-PGF_1α. Dexamethasone only reduced the LPS induced increase in ICP but not CBF, and partly the 6-keto-PGF_1α plasma concentration in the combined setup.

Conclusion

These data indicate that activation of cycloooxygenases is of central importance for development of cerebral hyperemia and high ICP during concomitant systemic inflammation and hyperammonemia.

Magnetic resonance of the brain in chronic and acute liver failure

Brain alterations such as hepatic encephalopathy or brain edema are usually associated with liver failure. The mechanisms that lead to the generation of edema seem to be different depending on the course of liver failure (acute, chronic or acute-on-chronic liver failure). Several neuroimaging methods allow a non-invasive assessment of brain alterations in liver failure. Magnetic resonance has gained more interest due to the ability of giving information about cerebral metabolism using spectroscopy, water distribution by diffusion methods or neuronal connectivity by means of resting state magnetic resonance. These techniques have been applied to experimental models and patients with liver failure to elucidate cerebral pathways involved in the pathogenesis of hepatic encephalopathy. In the future, the development of new magnetic resonance implementations will generate handy tools for the study of the brain and get better understanding of the mechanisms that take place in liver failure. This could be useful for the early diagnosis, as well as for the design of new treatments for cerebral complications of liver failure.

The brain in acute on chronic liver failure

Acute-on-chronic liver failure (ACLF) is a newly defined clinical entity with significant morbidity and mortality (~40-90% at 1 year dependent on need for organ support at presentation). It defines a presentation with acute severe liver injury, often with multiorgan dysfunction, on a background of previously known or unknown cirrhosis. In its severest form, it is almost indistinguishable from acute liver failure, as similarly in around 5% may rapidly progress to intracranial hypertension and cerebral oedema culminating in coma and/or death. Our understanding of such cerebral sequelae is currently limited to clinical observation, though our knowledge base is rapidly expanding since recent consensus clinical definition and guidance. Moreover, there are now animal models of ACLF and imaging modalities to better characterize events in the brain that occur with ACLF. However, as yet there has been little in the way of interventional study of this condition which are much needed. In this review we dissect existing clinical and experimental data to better characterise the manifestations of ACLF on the brain and allow for the development of targeted therapy as currently the plethora of existing interventions were designed to treat either the effects of cirrhosis or acute liver injury independently.

Brain edema in acute liver failure: mechanisms and concepts

Brain edema and associated increase in intracranial pressure continue to be lethal complications of acute liver failure (ALF). Abundant evidence suggests that the edema in ALF is largely cytotoxic brought about by swelling of astrocytes. Elevated blood and brain ammonia levels have been strongly implicated in the development of the brain edema. Additionally, inflammation and sepsis have been shown to contribute to the astrocyte swelling/brain edema in the setting of ALF. We posit that ammonia initiates a number of signaling events, including oxidative/nitrative stress (ONS), the mitochondrial permeability transition (mPT), activation of the transcription factor (NF-κB) and signaling kinases, all of which have been shown to contribute to the mechanism of astrocyte swelling. All of these factors also impact ion-transporters, including Na(+), K(+), Cl(-) cotransporter and the sulfonylurea receptor 1, as well as the water channel protein aquaporin-4 resulting in a perturbation of cellular ion and water homeostasis, ultimately resulting in astrocyte swelling/brain edema. All of these events are also potentiated by inflammation. This article reviews contemporary knowledge regarding mechanisms of astrocyte swelling/brain edema formation which hopefully will facilitate the identification of therapeutic targets capable of mitigating the brain edema associated with ALF.

Pharmacotherapy for hyperammonemia

INTRODUCTION

Hepatic encephalopathy (HE) is a serious neuropsychiatric complication that is seen in patients with liver failure. The pathogenesis of HE is not entirely understood, but several hypotheses have emerged and persisted during the years. Despite the many prevalent hypotheses, most of the existing evidence point to ammonia as the main culprit behind primary and secondary symptoms making it the center of potential therapeutic options for the treatment of HE. Most treatments of hyperammonemia target the organs and metabolic processes involved in ammonia detoxification.

AREAS COVERED

This article provides a review of the current targets of therapy as well as the drugs used for hyperammonemia treatment.

EXPERT OPINION

Lactulose and rifaximin have a proven role as measures to use for secondary prophylaxis and are the mainstay of current therapy. The use of molecular adsorbent recirculating system in patients with severe HE has been proven to be efficacious, but through mechanisms that appear to be independent of ammonia. The main challenge that faces the further development of treatments for HE is finding appropriate end points, and the next step would be to provide evidence of the effectiveness of established treatments and define the role of emerging new treatments.

Acute Fulminant Hepatic Failure, Encephalopathy and Early CT Changes

Background:

Acute fulminant hepatic failure (AFHF) is common in tertiary care centres with transplant facilities. Cerebral edema frequently threatens the lives of such patients. We reviewed our cases of AFHF, noting the incidence of cerebral edema with serial CT scans and factors associated with mortality.

Methods:

Patients were captured through HmRI classification of acute liver'hepatic failure. Chart review included tabulation of: demographics, INR; serum bilirubin, creatinine, albumin; in-hospital mortality. Computed tomogram (Ct) scans were re-read with blinding to clinical information and catalogued for changes in sulcal markings, ventricular size and grey-white differentiation (GWD). Inclusion criteria: age equal to or greater than 16 years, encephalopathy, hepatic failure within eight weeks of onset of liver disease, CT scans of head performed.

Results:

Of our 25 cases with AFHF, acetaminophen toxicity was the most common etiology (nine cases). Twelve of the 25 patients (48%) had cerebral edema on CT, including eight of the nine (89%) with acetaminophen toxicity. Decrease in sulcal markings and ventricular size preceded conspicuous alterations in GWD. Fourteen died, including all 12 with cerebral edema, although death was due to herniation in only one patient. None of the hematological or biochemical variables correlated significantly with mortality.

Conclusions:

Acetaminophen toxicity is a common cause of AFHF; this combination has a strong association with cerebral edema. Cerebral edema can be detected in its early stages and followed by baseline and serial CT scans. This facilitates management to prevent fatal brain herniation.

High-volume hemofiltration in children with acute liver failure*

OBJECTIVES

High-volume hemofiltration has shown beneficial effects in severe sepsis and multiple organ failure, improving hemodynamics and fluid balance. Recent studies suggest that acute liver failure shares many pathophysiologic similarities with sepsis. Therefore, we assessed the systemic effects of high-volume hemofiltration in children with acute liver failure.

DESIGN

Retrospective observational cohort study.

PATIENTS

Twenty-two children.

SETTING

Forty-two-bed multidisciplinary pediatric and neonatal ICUs in a tertiary university hospital.

INTERVENTION

We evaluated high-volume hemofiltration therapy as part of standard management of 22 children admitted in our unit for acute liver failure. Fifteen patients had fulminant hepatic failure, three had acute-on-chronic liver disease, and four had primary nonfunction. High-volume hemofiltration was initiated in patients requiring emergency liver transplantation and when hepatic encephalopathy grade higher than 2 and/or hemodynamic instability requiring vasopressors occurred. High-volume hemofiltration was defined by a flow of ultrafiltrate of more than 80 mL/kg/hr. Clinical and biological variables were assessed before and 24 and 48 hours after initiation of high-volume hemofiltration therapy.

MEASUREMENTS AND MAIN RESULTS

High-volume hemofiltration was initiated with a median grade III of hepatic encephalopathy. The median flow of ultrafiltrate was 119 mL/kg/hr (range, 80-384). After 24 hours of high-volume hemofiltration treatment, we observed an increase in mean arterial pressure (p = 0.0002) and a decrease in serum creatinine (p = 0.0002). In half of the patients, the encephalopathy grade decreased. After 48 hours of treatment, mean arterial pressure (p = 0.0005), grade of hepatic encephalopathy (p = 0.04), and serum creatinine (p = 0.0002) improved. Overall mortality was 45.4% (n = 10). Emergency liver transplantation was performed in eight children. Five patients spontaneously recovered liver function.

CONCLUSIONS

High-volume hemofiltration therapy significantly improves hemodynamic stability and neurological status in children with acute liver failure awaiting for emergency liver transplantation.

Peroxisome proliferator-activated receptor α activation attenuates the inflammatory response to protect the liver from acute failure by promoting the autophagy pathway

Peroxisome proliferator-activated receptor α (PPARα) has been reported to induce a potent anti-inflammatory response. Autophagy is a recently recognized rudimentary cellular response to inflammation and injury. The aim of the present study was to test the hypothesis that PPARα activation mediates autophagy to inhibit liver inflammation and protect against acute liver failure (ALF). PPARα expression during ALF and the impact of PPARα activation by Wy-14 643 on the hepatic immune response were studied in a D-galactosamine/lipopolysaccharide-induced mouse model. Autophagy was inhibited by 3-methyladenine or small interfering RNA (siRNA) against Atg7. In both the mouse model and human ALF subjects, PPARα was significantly downregulated in the injured liver. PPARα activation by pretreatment with Wy-14 643 protected against liver injury in mice. The protective effect of PPARα activation relied on the suppression of inflammatory mechanisms through the induction of autophagy. This hypothesis is supported by the following evidence: first, PPARα activation suppressed proinflammatory responses and inhibited phosphorylated NF-κBp65, phosphorylated JNK and phosphorylated ERK pathways in vivo. Second, protection by PPARα activation was due to the induction of autophagy because inhibition of autophagy by 3-methyladenine or Atg7 siRNA reversed liver protection and inflammation. Third, PPARα activation directly induced autophagy in primary macrophages in vitro, which protected cells from a lipopolysaccharide-induced proinflammatory response. Here, for the first time, we have demonstrated that PPARα-mediated induction of autophagy ameliorated liver injury in cases of ALF by attenuating inflammatory responses, indicating a potential therapeutic application for ALF treatment.

Changes of the thioredoxin system, glutathione peroxidase activity and total antioxidant capacity in rat brain cortex during acute liver failure: modulation by L-histidine

Glutathione and thioredoxin are complementary antioxidants in the protection of mammalian tissues against oxidative–nitrosative stress (ONS), and ONS is a principal cause of symptoms of hepatic encephalopathy (HE) associated with acute liver failure (ALF). We compared the activities of the thioredoxin system components: thioredoxin (Trx), thioredoxin reductase (TrxR) and the expression of the thioredoxin-interacting protein, and of the key glutathione metabolizing enzyme, glutathione peroxidase (GPx) in the cerebral cortex of rats with ALF induced by thioacetamide (TAA). ALF increased the Trx and TrxR activity without affecting Trip protein expression, but decreased GPx activity in the brains of TAA-treated rats. The total antioxidant capacity (TAC) of the brain was increased by ALF suggesting that upregulation of the thioredoxin may act towards compensating impaired protection by the glutathione system. Intraperitoneal administration of l-histidine (His), an amino acid that was earlier reported to prevent acute liver failure-induced mitochondrial impairment and brain edema, abrogated most of the acute liver failure-induced changes of both antioxidant systems, and significantly increased TAC of both the control and ALF-affected brain. These observations provide further support for the concept of that His has a potential to serve as a therapeutic antioxidant in HE. Most of the enzyme activity changes evoked by His or ALF were not well correlated with alterations in their expression at the mRNA level, suggesting complex translational or posttranslational mechanisms of their modulation, which deserve further investigations.

Management of hepatic encephalopathy

OPINION STATEMENT

Hepatic encephalopathy management varies depending on the acuity of liver failure. However, in patients with either acute or chronic liver failure five basic steps in management are critical: stabilization, addressing modifiable precipitating factors, lowering blood ammonia, managing elevated intracranial pressure (ICP) (if present), and managing complications of liver failure that can contribute to encephalopathy, particularly hyponatremia. Because liver failure patients are prone to a variety of other medical problems that can lead to encephalopathy (such as coagulopathy associated intracranial hemorrhage, electrolyte disarray, renal failure, hypotension, hypoglycemia, and infection), a thorough history, physical and neurologic examination is mandated in all encephalopathic liver failure patients. There should be a low threshold for brain imaging in patients with focal neurological deficits given the propensity for spontaneous intracranial hemorrhage. In patients with acute liver failure and high grade encephalopathy, identification of the etiology of acute liver failure is essential to guide treatment and antidote administration, particularly in the case of acetaminophen poisoning. Equally critical is management of elevated ICP in acute liver failure. Intracranial hypertension can be treated with hypertonic saline and/or adjustment of the dialysis bath. Placement of an intracranial monitor to guide ICP therapy is risky because of concomitant coagulopathy and remains controversial. Continuous renal replacement therapy may help lower serum ammonia, treat coexisting uremia, and improve symptoms. Liver transplantation is the definitive treatment for patients with acute liver failure and hepatic encephalopathy. In patients with chronic hepatic encephalopathy, lactulose and rifaxamin remain a mainstay of therapy. In these patients, it is essential to identify reversible causes of hepatic encephalopathy such as increased ammonia production and/or decreased clearance (eg, infection, GI bleed, constipation, hypokalemia, dehydration). Chronic hyponatremia should be managed by gradual sodium correction of no more than 8‒12 meq/L per day to avoid central myelinolysis syndrome. Free water restriction and increased dietary sodium are reasonable, cost effective treatment options. Many emerging therapies, both pharmacologic and interventional, are currently being studied to improve management of hepatic encephalopathy.

Pathophysiology of cerebral oedema in acute liver failure

Cerebral oedema is a devastating consequence of acute liver failure (ALF) and may be associated with the development of intracranial hypertension and death. In ALF, some patients may develop cerebral oedema and increased intracranial pressure but progression to life-threatening intracranial hypertension is less frequent than previously described, complicating less than one third of cases who have proceeded to coma since the advent of improved clinical care. The rapid onset of encephalopathy may be dramatic with the development of asterixis, delirium, seizures and coma. Cytotoxic and vasogenic oedema mechanisms have been implicated with a preponderance of experimental data favouring a cytotoxic mechanism. Astrocyte swelling is the most consistent neuropathological finding in humans with ALF and ammonia plays a definitive role in the development of cytotoxic brain oedema. The mechanism(s) by which ammonia induces astrocyte swelling remains unclear but glutamine accumulation within astrocytes has led to the osmolyte hypothesis. Current evidence also supports an alternate ‘Trojan horse’ hypothesis, with glutamine as a carrier of ammonia into mitochondria, where its accumulation results in oxidative stress, energy failure and ultimately astrocyte swelling. Although a complete breakdown of the blood-brain barrier is not evident in human ALF, increased permeation to water and other small molecules such as ammonia has been demonstrated resulting from subtle alterations in the protein composition of paracellular tight junctions. At present, there is no fully efficacious therapy for cerebral oedema other than liver transplantation and this reflects our incomplete knowledge of the precise mechanisms underlying this process which remain largely unknown.

Blocking NMDA receptors delays death in rats with acute liver failure by dual protective mechanisms in kidney and brain

Treatment of patients with acute liver failure (ALF) is unsatisfactory and mortality remains unacceptably high. Blocking NMDA receptors delays or prevents death of rats with ALF. The underlying mechanisms remain unclear. Clarifying these mechanisms will help to design more efficient treatments to increase patient's survival. The aim of this work was to shed light on the mechanisms by which blocking NMDA receptors delays rat's death in ALF. ALF was induced by galactosamine injection. NMDA receptors were blocked by continuous MK-801 administration. Edema and cerebral blood flow were assessed by magnetic resonance. The time course of ammonia levels in brain, muscle, blood, and urine; of glutamine, lactate, and water content in brain; of glomerular filtration rate and kidney damage; and of hepatic encephalopathy (HE) and intracranial pressure was assessed. ALF reduces kidney glomerular filtration rate (GFR) as reflected by reduced inulin clearance. GFR reduction is due to both reduced renal perfusion and kidney tubular damage as reflected by increased Kim-1 in urine and histological analysis. Blocking NMDA receptors delays kidney damage, allowing transient increased GFR and ammonia elimination which delays hyperammonemia and associated changes in brain. Blocking NMDA receptors does not prevent cerebral edema or blood-brain barrier permeability but reduces or prevents changes in cerebral blood flow and brain lactate. The data show that dual protective effects of MK-801 in kidney and brain delay cerebral alterations, HE, intracranial pressure increase and death. NMDA receptors antagonists may increase survival of patients with ALF by providing additional time for liver transplantation or regeneration.

Serum from patients with hepatitis E virus-related acute liver failure induces human liver cell apoptosis

The pathogenesis of acute liver failure has not been fully elucidated. The present study investigated the effects of the serum from patients with hepatitis E virus (HEV)-related acute liver failure on human liver cell survival and apoptosis, and evaluated the protective effects of anti-lipopolysaccharide(LPS) antibody recognizing core polysaccharide against acute liver failure serum-induced apoptosis. Serum was collected from patients with HEV-related acute liver failure. The levels of endotoxin (LPS) in the serum were measured using a quantitative tachypleus amebocyte lysate endotoxin detection kit with a chromogenic endpoint. Serum with a mean concentration of LPS was incubated with L02 human liver cells and the rate of apoptosis was detected by flow cytometry. The apoptotic rate was also evaluated in liver cells incubated with antibody and the HEV-related acute liver failure serum. The results indicated that the concentration of LPS in the serum of patients with HEV-related acute liver failure was 0.26±0.02 EU/ml, which was significantly higher than that of the control group (P<0.05). The rate of apoptosis in the human liver cells induced by acute liver failure serum was 5.83±0.42%, which was significantly increased compared with that in the cells treated with the serum of healthy individuals (P<0.05). The apoptotic rate of the cells incubated with antibody and the acute liver failure serum was 5.53±0.51%, which was lower than that of the cells incubated with acute liver failure serum alone (P>0.05). These results indicate that the serum of patients with HEV-related acute liver failure induces the apoptosis of human liver cells. LPS may be directly involved in the apoptosis of human liver cells. Moreover, the presence of the antibody did not significantly reduce the level of apoptosis of liver cells exposed to HEV-related acute liver failure serum.

Hepatic encephalopathy: effects of liver failure on brain function

Liver failure affects brain function, leading to neurological and psychiatric alterations; such alterations are referred to as hepatic encephalopathy (HE). Early diagnosis of minimal HE reveals an unexpectedly high incidence of mild cognitive impairment and psychomotor slowing in patients with liver cirrhosis - conditions that have serious health, social and economic consequences. The mechanisms responsible for the neurological alterations in HE are beginning to emerge. New therapeutic strategies acting on specific targets in the brain (phosphodiesterase 5, type A GABA receptors, cyclooxygenase and mitogen-activated protein kinase p38) have been shown to restore cognitive and motor function in animal models of chronic HE, and NMDA receptor antagonists have been shown to increase survival in acute liver failure. This article reviews the latest studies aimed at understanding how liver failure affects brain function and potential ways to ameliorate these effects.

Effects of N-acetylcysteine on cytokines in non-acetaminophen acute liver failure: potential mechanism of improvement in transplant-free survival

BACKGROUND

N-Acetylcysteine (NAC) improves transplant-free survival in patients with non-acetaminophen acute liver failure (ALF) when administered in early stages of hepatic encephalopathy. The mechanisms of this benefit are unknown.

AIM

To determine whether NAC improves transplant-free survival in ALF by ameliorating the surge of pro-inflammatory cytokines.

METHODS

Serum samples were obtained from 78 participants of the randomized, ALF Study Group NAC Trial with grade 1 or 2 hepatic encephalopathy on randomization. Concentrations of ten cytokines, chosen to represent a wide array of inflammatory responses, were determined by multiplex enzyme-linked immunosorbent assay ELISA.

RESULTS

In univariate analysis, predictors of transplant-free survival included NAC administration (P = 0.012), admission bilirubin (P = 0.003), international normalized ratio INR (P = 0.0002), grade 1 vs. grade 2 encephalopathy (P = 0.006) and lower admission interleukin (IL)-17 concentrations (P = 0.011). IL-17 levels were higher in patients with grade 2 vs. grade 1 encephalopathy on randomization (P = 0.007) and in those who progressed to grade 3 or grade 4 encephalopathy over the following 7 days (P ≤ 0.01). Stepwise multivariate logistic regression analysis identified only NAC administration and lower IL-17 concentrations as independent predictors of transplant-free survival. In patients with detectable IL-17 concentrations on admission, 78% of those who received NAC vs. 44% of those who received placebo had undetectable levels by day 3-5 (P = 0.042), and the mean decrease in IL-17 concentrations between admission and late samples was significantly greater in patients who received NAC vs. placebo (P = 0.045).

CONCLUSIONS

N-acetylcysteine (NAC) may improve transplant-free survival in patients with non-acetaminophen ALF by ameliorating the production of IL-17, which is associated with progression of hepatic encephalopathy and poor outcome.

Ciclosporin does not attenuate intracranial hypertension in rats with acute hyperammonaemia

AIM: To investigate the neuroprotective potential of ciclosporin during acute liver failure. We evaluated the effect of intrathecally administered ciclosporin on intracranial pressure, brain water content and aquaporin-4 expression in a rat model with acute hyperammonaemia.

METHODS: Twenty-four male Wistar rats with portacaval anastomosis were randomised into four groups receiving ciclosporin or vehicle and ammonia or saline infusion. Ciclosporin or vehicle was given intrathecally prior to the ammonia or saline infusion. The ammonia or saline infusion was given intravenously for 4 h, while intracranial pressure and arterial pressure was recorded. At the end of the experiment, cerebral cortex and cerebellar brain tissue was analysed for water and aquaporin-4 content.

RESULTS: The following intracranial pressures were found at the end of the experiment: ammonia + ciclosporin: 10.0 ± 1.7 mmHg, ammonia + vehicle: 6.8 ± 1.0 mmHg, saline + ciclosporin: 3.1 ± 0.5 mmHg, saline + vehicle: 3.3 ± 0.6 mmHg. Ammonia infusion had a significant effect on intracranial pressure and brain water content, which both were higher in the groups receiving ammonia (P < 0.001, two-way analysis of variance). Treatment with ciclosporin resulted in relevant tissue concentrations of ciclosporin (> 0.2 micromolar) but did not reduce intracranial pressure after 4 h. Furthermore, ciclosporin did not attenuate the increase in cerebral water content, and did not affect aquaporin-4 expression.

CONCLUSION: Intrathecal administration of ciclosporin does not attenuate intracranial hypertension or brain oedema in rats with portacaval anastomosis and 4 h of ammonia infusion.

Brain edema in acute liver failure: role of neurosteroids

Brain edema is a major neurological complication of acute liver failure (ALF) and swelling of astrocytes (cytotoxic brain edema) is the most prominent neuropathological abnormality in this condition. Elevated brain ammonia level has been strongly implicated as an important factor in the mechanism of astrocyte swelling/brain edema in ALF. Recent studies, however, have suggested the possibility of a vasogenic component in the mechanism in ALF. We therefore examined the effect of ammonia on blood-brain barrier (BBB) integrity in an in vitro co-culture model of the BBB (consisting of primary cultures of rat brain endothelial cells and astrocytes). We found a minor degree of endothelial permeability to dextran fluorescein (16.2%) when the co-culture BBB model was exposed to a pathophysiological concentration of ammonia (5mM). By contrast, lipopolysaccharide (LPS), a molecule well-known to disrupt the BBB, resulted in an 87% increase in permeability. Since increased neurosteroid biosynthesis has been reported to occur in brain in ALF, and since neurosteroids are known to protect against BBB breakdown, we examined whether neurosteroids exerted any protective effect on the slight permeability of the BBB after exposure to ammonia. We found that a nanomolar concentration (10nM) of the neurosteroids allopregnanolone (THP) and tetrahydrodeoxycorticosterone (THDOC) significantly reduced the ammonia-induced increase in BBB permeability (69.13 and 58.64%, respectively). On the other hand, we found a marked disruption of the BBB when the co-culture model was exposed to the hepatotoxin azoxymethane (218.4%), but not with other liver toxins commonly used as models of ALF (thioacetamide and galactosamine, showed a 29.3 and 30.67% increase in permeability, respectively). Additionally, THP and THDOC reduced the effect of TAA and galactosamine on BBB permeability, while no BBB protective effect was observed following treatment with azoxymethane. These findings suggest that ammonia does not cause a significant BBB disruption, and that the BBB is intact in the TAA or galactosamine-induced animal models of ALF, likely due to the protective effect of neurosteroids that are synthesized in brain in the setting of ALF. However, caution should be exercised when using azoxymethane as an experimental model of ALF as it caused a severe breakdown of the BBB, and neurosteriods failed to protect against this breakdown.

Clinical impact of arterial ammonia levels in ICU patients with different liver diseases

PURPOSE

Increased arterial ammonia levels are associated with high mortality in patients with acute liver failure (ALF). Data on the prognostic impact of arterial ammonia is lacking in hypoxic hepatitis (HH) and scarce in critically ill patients with cirrhosis.

METHODS

The patient cohort comprised 72 patients with HH, 43 patients with ALF, 100 patients with liver cirrhosis and 45 patients without evidence for liver disease. Arterial ammonia concentrations were assessed on a daily basis in all patients and the results were compared among these four patient groups and between 28-day survivors and 28-day non-survivors overall and in each group.

RESULTS

Overall 28-day mortality rates in patients with HH, ALF and cirrhosis and in the control group were 54, 30, 49 and 27 %, respectively. Peak arterial ammonia levels differed significantly between transplant-free 28-day survivors and non-survivors in the HH and ALF groups (p < 0.01 for both). Multivariate regression identified peak arterial ammonia concentrations as an independent predictor of 28-day mortality or liver transplantation in patients with HH and ALF, respectively (p < 0.01). There was no association between mortality and arterial ammonia in patients with liver cirrhosis and in the control group. Admission arterial ammonia levels were independently linked to hepatic encephalopathy grades 3/4 in patients with HH (p < 0.01), ALF (p < 0.05) and cirrhosis (p < 0.05), respectively.

CONCLUSIONS

Elevated arterial ammonia levels indicate a poor prognosis in acute liver injury and are associated with advanced HE in HH, ALF and cirrhosis. Arterial ammonia levels provide additional information in the risk assessment of critically ill patients with liver disease.

Endothelial-astrocytic interactions in acute liver failure

Brain edema and the subsequent increase in intracranial pressure are major neurological complications of acute liver failure (ALF), and swelling of astrocytes (cytotoxic brain edema) is the most prominent neuropathological abnormality in ALF. Recent studies, however, have suggested the co-existence of cytotoxic and vasogenic mechanisms in the brain edema associated with ALF. This review 1) summarizes the nature of the brain edema in humans and experimental animals with ALF; 2) reviews in vitro studies supporting the presence of cytotoxic brain edema (cell swelling in cultured astrocytes); and 3) documents the role of brain endothelial cells in the development of astrocyte swelling/brain edema in ALF.

Alterations of blood-brain barrier and associated factors in acute liver failure

Brain edema in acute liver failure (ALF) remains lethal. Cytotoxic mechanisms associated with brain edema have been well recognized, but the role of vasogenic mechanisms of brain edema has not been explored. Intact tight junctions (TJs) between brain capillary endothelial cells are critical for normal BBB function. Recent reports found significant alterations in the tight junction elements including occludin and claudin-5, suggesting a vasogenic injury in the blood-brain barrier (BBB) integrity. However, the role of TJ in ALF has not been completely understood. This paper reviews the role of the paracellular tight junction in the increased selective BBB permeability that leads to brain edema in ALF and furthermore explores the effect of systemic inflammatory cytokines on the tight junction dysfunction.

Inflammation and hepatic encephalopathy

Hepatic encephalopathy (HE) is a neuropsychiatric syndrome associated with both acute and chronic liver dysfunction, spanning a spectrum that ranges from mild neuropsychological disturbances to coma. The central role of ammonia in the pathogenesis of HE remains incontrovertible however, there is a robust evidence base indicating the important role of inflammation in exacerbating the neurological effects of HE. Inflammation can arise directly within the brain itself as a result of deranged nitrogen and energy homeostasis, with resultant neuronal, astrocyte and microglial dysfunction. Inflammation may also originate in the peripheral circulation and exert effects on the brain indirectly, via the release of pro-inflammatory mediators which directly signal to the brain via the vagus nerve. This review summarises the data that demonstrate the synergistic relationship of inflammation and ammonia that culminates in the manifestation of HE. Sterile inflammation arising from the inflamed or necrotic liver, circulating endotoxin arising from the gut (bacterial translocation) inducing immune dysfunction, and superimposed sepsis will be comprehensively discussed. Finally, this review will provide an overview of the existing and novel treatments on the horizon which can target the inflammatory response, and how they might translate into clinical practise as therapies in the prophylaxis and treatment of HE.

The values of cerebrovascular pressure reactivity and brain tissue oxygen pressure reactivity in experimental anhepatic liver failure

BACKGROUND

We investigated in a porcine model of anhepatic acute liver failure (ALF), the value of two parameters describing cerebrovascular autoregulatory capacity, pressure reactivity index (PRx) and brain tissue oxygen pressure reactivity (ORx), regarding their power to predict the development of intracranial hypertension.

METHODS

In six pigs, hepatectomy was performed. Only one animal was sham operated. All animals received neuromonitoring including arterial blood pressure, intracranial pressure (ICP), and brain tissue partial oxygen pressure (P(br)O(2)). The average time of neuromonitoring was 31.0 h. Cerebral perfusion pressures (CPP), cerebrovascular pressure reactivity index (PRx) and brain tissue oxygen reactivity index (ORx) were calculated.

RESULTS

Perioperative disturbance of AR improved within 4 h after surgery. From 6 to 16 h post hepatectomy, ICP did slowly increase by 4 mmHg from baseline; CPP remained stable around 40 mmHg. PRx and ORx, however, indicated in this period a progressive loss of AR, reflected in a decrease of P(br)O(2) despite unchanged CPP. Beyond 16 h, ICP rose quickly. At CPP levels below 35 mmHg, P(br)O(2) fell to ischemic levels.

CONCLUSIONS

The loss of cerebrovascular autoregulatory capacity, indicated by a rise of PRx and ORx precedes the final crisis of uncontrollable intracranial hypertension in this animal model by hours. During this phase cerebral blood flow, as reflected in tissue oxygenation, deteriorates despite unchanged CPP. Monitoring of AR during ALF therefore seems to carry the power to identify a risk for development of critical CBF and intracranial hypertension.

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.

Current pathogenetic aspects of hepatic encephalopathy and noncirrhotic hyperammonemic encephalopathy

Hepatic encephalopathy is a medical phenomenon that is described as a neuropsychiatric manifestation of chronic or acute liver disease that is characterized by psychomotor, intellectual and cognitive abnormalities with emotional/affective and behavioral disturbances. This article focuses on the underlying mechanisms of the condition and the differences between hepatic encephalopathy and noncirrhotic hyperammonemic encephalopathy. Hepatic encephalopathy is a serious condition that can cause neurological death with brain edema and intracranial hypertension. It is assumed that approximately 60%-80% of patients with liver cirrhosis develop hepatic encephalopathy. This review explores the complex mechanisms that lead to hepatic encephalopathy. However, noncirrhotic hyperammonemic encephalopathy is not associated with hepatic diseases and has a completely different etiology. Noncirrhotic hyperammonemic encephalopathy is a severe occurrence that is connected with multiple pathogeneses.

Acute-on chronic liver failure

Acute-on-chronic liver failure (ACLF) is an increasingly recognised entity encompassing an acute deterioration of liver function in patients with cirrhosis, which is usually associated with a precipitating event and results in the failure of one or more organs and high short term mortality. Prospective data to define this is lacking but there is a large body of circumstantial evidence suggesting that this condition is a distinct clinical entity. From the pathophysiologic perspective, altered host response to injury and infection play important roles in its development. This review focuses upon the current understanding of this syndrome from the clinical, prognostic and pathophysiologic perspectives and indicates potential biomarkers and therapeutic targets for intervention.

Increased Th1 immune response in SERPINB3 transgenic mice during acute liver failure

Acute liver failure (ALF) is characterized by severe neurological complications, known as acute hepatic encephalopathy, where brain ammonia and inflammatory processes play a dominant role. In experimental models of acute liver failure SERPINB3 was found significantly increased in microglia, the intrinsic immune cells of the central nervous system. The aim of the present study was to investigate the extent of brain tissue damage and the inflammatory milieu in experimental acute liver failure using a SERPINB3-transgenic mouse model. C57BL/6J wild-type and transgenic mice were inoculated with acetaminophen or phosphate-buffered saline and sacrificed 20 h postinjection. Proliferation and apoptotic activity were analyzed in brain tissue by immunohistochemistry and terminal deoxynucleotidyl transferase-mediated dUTP nick-end labeling technique. The expression of cytokines was analysed in brain and liver tissue by real time polymerase chain reaction and in the corresponding serum samples using a Bio-Plex system. Acetaminophen induced a significantly lower body temperature and shorter survival in transgenic than in wild-type mice, despite liver function was similar in both groups. The brain of transgenic mice, expressing SERPINB3 positivity in microglia, showed increased glial cell number, associated to significant lower apoptotic death events, compared with wild-type mice. In mice injected with acetaminophen, remarkably higher values of cytokines mRNA were observed in the liver of both groups, with a trend toward higher values in transgenic animals. In brain tissue similar increase of tumor necrosis factor-αwas detected in transgenic and wild-type mice, while IL-10 mRNA increased only in the wild-type group. A remarkable increase of circulating Th1 cytokines was detected in serum of transgenic mice, while in the wild-type group they remained rather unchanged. These figures were associated with lower levels of granulocyte macropage colony-stimulating factor, despite similar increase of IL-10 values in both groups. In conclusion, in acute liver failure SERPINB3 determines an enhanced inflammatory background, mainly mediated by higher levels of Th1 proinflammatory cytokines.

Serial evaluation of children with ALF with advanced MRI, serum proinflammatory cytokines, thiamine, and cognition assessment

OBJECTIVES

This prospective, sequential study was done to understand changes in cerebral edema (CE) on magnetic resonance imaging and magnetic resonance spectroscopy, liver functions, and neurocognitive testing (NCT) in children with acute liver failure (ALF).

METHODS

A total of 11 ALF and 8 healthy controls were evaluated with advanced magnetic resonance (MR) imaging, blood proinflammatory cytokines (PCs), thiamine levels, liver functions, and NCT. Reevaluation was done at 43.5 ± 26.9 days (first follow-up, n = 8) and 157.3 ± 52.3 days (second follow-up, n = 6) after discharge.

RESULTS

At diagnosis, patients with ALF had vasogenic and cytotoxic CE, raised brain glutamine (23.2 ± 3.4 vs. 15.3 ± 2.7), and serum PCs (tumor necrosis factor [TNF]-α 40.1 ± 8.9 vs. 7.2 ± 2.7  pg/mL, interleukin [IL]-6 29.2 ± 14.4 vs. 4.7 ± 1.2  pg/mL). The mammillary bodies (MBs) were smaller, and brain choline (1.9 ± 0.36 vs. 2.6 ± 0.6) and blood thiamine (55.2 ± 6.7 vs. 81.8 ± 10.2  nmol/L) were lower than controls. At first follow-up, the brain glutamine and CE recovered. Brain choline and MBs volume showed improvement and thiamine levels normalized. Significant reduction in TNF-α and IL-6 was seen. The patients performed poorly on NCT, which normalized at second follow-up. Liver biochemistry and thiamine levels were normal and TNF-α and IL-6 showed further reduction at second follow-up.

CONCLUSIONS

Patients with ALF have CE contributed by raised brain glutamine and PCs. MBs are small because of thiamine deficiency and show recovery in follow-up. CE and brain glutamine recover earlier than normalization of NCT and liver functions. Persistence of raised cytokines up to 6 months after insult suggests possible contribution from liver regeneration.

Glasgow coma score and tumor necrosis factor α as predictive criteria for initial poor graft function

Initial poor graft function (IPGF) is a major factor influencing the clinical outcome after liver transplantation (LT), but there is no reliable method to assess and predict graft dysfunction. To help clinicians determine prognosis in the early postoperative period, individual parameters and complex scoring systems have been suggested, but most of them are inaccurate because of the multifactorial nature of transplantation courses. Therefore, the aim of our study was to retrospectively evaluate predictive criteria for retransplantation. Forty-two patients were enrolled in this study: 18 who experienced primary non-function (PNF) and 24 with delayed graft function (DGF). All of the patients were treated with the Molecular Adsorbent Recirculating System (MARS). They were into 3 subgroups: patients who survived without LT (n = 20; 47.7%); patients who underwent LT (n = 16; 37%), and patients who died before transplantation (n = 6; 14%). Stepwise multivariable logistic regression analysis was performed with the intent to find the risk factors for LT or death after MARS treatment (second analysis). Receiver operating characteristic (ROC) curves were performed on significant variables in the logistic regression model with the intent to individually predict variables for LT or death. After a stepwise multivariable logistic regression analysis enrolling all of the previously reported features only 2 variables, tumor necrosis factor (TFN)-α and Glasgow coma score (GCS) score, were statistically significant. TNF-α was an unique independent risk factor for retransplantation or death after MARS treatment (odds ratio [OR] 1.235; P = .013). Conversely, GCS score was protective against retransplantation or death (OR 0.150; P = .003). Starting from these assumptions, a predictive model was created using these 2 variables. On ROC analysis, the combined score showed an area under the curve greater than that of the 2 variables considered separately. Validating these results with a larger number of patients, we considered these 2 factors as subjective parameters to determine outcomes and the difference between PNF and DGF.

Metabolic encephalopathies in the critical care unit

PURPOSE OF REVIEW

This article summarizes the most common etiologies and approaches to management of metabolic encephalopathy.

RECENT FINDINGS

Metabolic encephalopathy is a frequent occurrence in the intensive care unit setting. Common etiologies include hepatic failure, renal failure, sepsis, electrolyte disarray, and Wernicke encephalopathy. Current treatment paradigms typically focus on supportive care and management of the underlying etiology. Directed therapies that target neurochemical and neurotransmitter pathways that mediate encephalopathy are not currently available and represent an important area for future research. Although commonly thought of as reversible neurologic insults, delirium and encephalopathy have been associated with increased mortality, prolonged length of stay and hospital complications, and worse long-term cognitive and functional outcomes.

SUMMARY

Recognition and treatment of encephalopathy is critical to improving outcomes in critically ill patients.

Transporters and channels in cytotoxic astrocyte swelling

Brain edema is a severe clinical complication in a number of pathologies and is a major cause of increased morbidity and death. The swelling of astrocytes caused by a disruption of water and ion homeostasis, is the primary event contributing to the cytotoxic form of brain edema. Astrocyte cytotoxic swelling ultimately leads to transcapillary fluxes of ions and water into the brain parenchyma. This review focuses on the implication of transporters and channels in cytotoxic astrocyte swelling in hyponatremia, ischemia, trauma and hepatic encephalopathy. Emphasis is put on some salient features of the astrocyte physiology, all related to cell swelling, i.e. predominance of aquaporins, control of K(+) homeostasis and ammonia accumulation during the brain ammonia-detoxifying process.

Reduction in hyperammonaemia by ornithine phenylacetate prevents lipopolysaccharide-induced brain edema and coma in cirrhotic rats

OBJECTIVE

In liver failure, inflammation synergistically exacerbates the deleterious cerebral effects of ammonia. The aims were to test whether treatment with the ammonia-lowering agent ornithine phenylacetate (OP) and/or anti-TNF-α (infliximab) prevent the deleterious brain consequences of lipopolysaccharide (LPS) in cirrhotic rats.

DESIGN

Rats 4 weeks following bile duct-ligation (BDL), sham-operation (sham) and/or 7 days hyperammonemic feed (HD), were randomized to receive LPS (1 mg/kg) or saline, and treatment with either 3 days intraperitoneal injections of OP (0.6 g/kg) and/or infliximab, 10 mg/kg. Animals were sacrificed at coma stages or at 3 h.

RESULTS

In sham rats, both HD and LPS increased brain water, with an increase in ammonia in the former and brain cytokines in the latter but with no effect on consciousness. BDL + HD rats caused significantly higher plasma ammonia, TNF-α and IL-6 levels compared to sham. LPS significantly worsened coma stage, increased brain water and plasma and brain TNF-α. OP significantly delayed LPS-induced progression to coma stages (P < 0.009), reduced arterial ammonia and brain water (P < 0.001 and P < 0.01 respectively), which was associated with a significant reduction in cytokines. Infliximab significantly reduced plasma and brain cytokines, but not brain water. OP + infliximab attenuated increase in brain water and delayed occurrence of coma, which was not different to OP alone. In BDL rats, OP reduced the expression of brain iNOS and NFκB.

CONCLUSION

Reduction in ammonia with OP in cirrhotic rats prevents LPS-induced brain edema and delays coma, suggesting that ammonia may prime the brain to the deleterious effect of LPS, possibly through effects on iNOS and NFκB related mechanisms.

[Acute liver failure. How much diagnostic work-up and therapy does my patient need?]

Acute liver failure is a multisystem disease with predominantly sudden and severe hepatic injury and hepatic encephalopathy caused by apoptotic or necrotic hepatocyte damage. The clinical challenge in patients with acute liver failure is to promptly identify those with poor prognosis and refer them for emergency liver transplantation. This review article highlights the main aspects of decision making in the setting of acute liver failure, summarizes new aspects of its critical care management and gives an overview of sclerosing cholangitis in the critically ill patient, an under-recognized disease entity that can progress to acute liver failure.

Ammonia reduction with ornithine phenylacetate restores brain eNOS activity via the DDAH-ADMA pathway in bile duct-ligated cirrhotic rats

Ammonia is central in the pathogenesis of hepatic encephalopathy, which is associated with dysfunction of the nitric oxide (NO) signaling pathway. Ornithine phenylacetate (OP) reduces hyperammonemia and brain water in cirrhotic animals. This study aimed to determine whether endothelial NO synthase activity is altered in the brain of cirrhotic animals, whether this is associated with changes in the endogenous inhibitor, asymmetric-dimethylarginine (ADMA) and its regulating enzyme, dimethylarginine-dimethylaminohydrolase (DDAH-1), and whether these abnormalities are restored by ammonia reduction using OP. Sprague-Dawley rats were studied 4-wk after bile duct ligation (BDL) (n = 16) or sham operation (n = 8) and treated with placebo or OP (0.6 g/kg). Arterial ammonia, brain water, TNF-α, plasma, and brain ADMA were measured using standard techniques. NOS activity was measured radiometrically, and protein expression for NOS enzymes, ADMA, DDAH-1, 4-hydroxynonenol ((4)HNE), and NADPH oxidase (NOX)-1 were measured by Western blotting. BDL significantly increased arterial ammonia (P < 0.0001), brain water (P < 0.05), and brain TNF-α (P < 0.01). These were reduced significantly by OP treatment. The estimated eNOS component of constitutive NOS activity was significantly lower (P < 0.05) in BDL rat, and this was significantly attenuated in OP-treated animals. Brain ADMA levels were significantly higher and brain DDAH-1 significantly lower in BDL compared with sham (P < 0.01) and restored toward normal following treatment with OP. Brain (4)HNE and NOX-1 protein expression were significantly increased in BDL rat brain, which were significantly decreased following OP administration. We show a marked abnormality of NO regulation in cirrhotic rat brains, which can be restored by reduction in ammonia concentration using OP.

Neurological complications of acute liver failure: pathophysiological basis of current management and emerging therapies

One of the major causes of mortality in patients with acute liver failure (ALF) is the development of hepatic encephalopathy (HE) which is associated with increased intracranial pressure (ICP). High ammonia levels, increased cerebral blood flow and increased inflammatory response have been identified as major contributors to the development of HE and the related brain swelling. The general principles of the management of patients with ALF are straightforward. They include identifying the insult causing hepatic injury, providing organ systems support to optimize the patient's physical condition, anticipation and prevention of development of complications. Increasing insights into the pathophysiological mechanisms of ALF are contributing to better therapies. For instance, the evident role of cerebral hyperemia in the pathogenesis of increased ICP has led to a re-evaluation of established therapies such as hyperventilation, N-acetylcysteine, thiopentone sodium and propofol. The role of systemic inflammatory response in the pathogenesis of increased ICP has also gained importance supporting the concept that antibiotics given prophylactically reduce the risk of developing sepsis during the course of illness. Moderate hypothermia has also been established as a therapy able to reduce ICP in patients with uncontrolled intracranial hypertension and to prevent increases in ICP during orthopic liver transplantation. Ornithine phenylacetate, a new drug in the treatment of liver failure, and liver replacement therapies are still being investigated both experimentally and clinically. Despite many advances in the understanding of the pathophysiological basis and the management of intracranial hypertension in ALF, more clinical trials should be conducted to determine the best therapeutic management for this difficult clinical event.

Brain energy metabolism and mitochondrial dysfunction in acute and chronic hepatic encephalopathy

One proposed mechanism for acute and chronic hepatic encephalopathy (HE) is a disturbance in cerebral energy metabolism. It also reviews the current status of this mechanism in both acute and chronic HE, as well as in other hyperammonemic disorders. It also reviews abnormalities in glycolysis, lactate metabolism, citric acid cycle, and oxidative phosphorylation as well as associated energy impairment. Additionally, the role of mitochondrial permeability transition (mPT), a recently established factor in the pathogenesis of HE and hyperammonemia, is emphasized. Energy failure appears to be an important pathogenetic component of both acute and chronic HE and a potential target for therapy.

Alterations of blood brain barrier function in hyperammonemia: an overview

Ammonia is a neurotoxin involved in the pathogenesis of neurological conditions associated with hyperammonemia, including hepatic encephalopathy, a condition associated with acute—(ALF) or chronic liver failure. This article reviews evidence that apart from directly affecting the metabolism and function of the central nervous system cells, ammonia influences the passage of different molecules across the blood brain barrier (BBB). A brief description is provided of the tight junctions, which couple adjacent cerebral capillary endothelial cells to each other to form the barrier. Ammonia modulates the transcellular passage of low-to medium-size molecules, by affecting their carriers located at the BBB. Ammonia induces interrelated aberrations of the transport of the large neutral amino acids and aromatic amino acids (AAA), whose influx is augmented by exchange with glutamine produced in the course of ammonia detoxification, and maybe also modulated by the extracellularly acting gamma-glutamyl moiety transferring enzyme, gamma-glutamyl-transpeptidase. Impaired AAA transport affects neurotransmission by altering intracerebral synthesis of catecholamines (serotonin and dopamine), and producing “false neurotransmitters” (octopamine and phenylethylamine). Ammonia also modulates BBB transport of the cationic amino acids: the nitric oxide precursor, arginine, and ornithine, which is an ammonia trap, and affects the transport of energy metabolites glucose and creatine. Moreover, ammonia acting either directly or in synergy with liver injury-derived inflammatory cytokines also evokes subtle increases of the transcellular passage of molecules of different size (BBB “leakage”), which appears to be responsible for the vasogenic component of cerebral edema associated with ALF.

Neurological management of fulminant hepatic failure

Acute liver failure (ALF) is uncommon in the United States, but presents acutely and catastrophically, often with deadly consequences. Hepatic encephalopathy, cerebral edema, elevated intracranial pressure, and intracranial hemorrhage due to coagulopathy are common occurrences in patients with ALF. Appropriate management of multi-system organ failure and neurological complications are essential in bridging patients to transplant and ensuring satisfactory outcomes.

Interaction between cytokines and ammonia in the mitochondrial permeability transition in cultured astrocytes

Hepatic encephalopathy (HE) is the major neurological complication occurring in patients with acute and chronic liver failure. Elevated levels of blood and brain ammonia are characteristic of HE, and astrocytes are the primary target of ammonia toxicity. In addition to ammonia, recent studies suggest that inflammation and associated cytokines (CKs) also contribute to the pathogenesis of HE. It was previously established that ammonia induces the mitochondrial permeability transition (mPT) in cultured astrocytes. As CKs have been shown to cause mitochondrial dysfunction in other conditions, we examined whether CKs induce the mPT in cultured astrocytes. Cultures treated with tumor necrosis factor-α, interleukin-1β, interleukin-6, and interferon-γ, individually or in a mixture, resulted in the induction of the mPT in a time-dependent manner. Simultaneous treatment of cultures with a mixture of CKs and ammonia showed a marked additive effect on the mPT. As oxidative stress (OS) is known to induce the mPT, so we examined the effect of CKs and ammonia on hemeoxygenase-1 (HO-1) protein expression, a marker of OS. Such treatment displayed a synergistic effect in the upregulation of HO-1. Antioxidants significantly blocked the additive effects on the mPT by CKs and ammonia, suggesting that OS represents a major mechanism in the induction of the mPT. Treatment of cultures with minocycline, an antiinflammatory agent, which is known to inhibit OS, also diminished the additive effects on the mPT caused by CKs and ammonia. Induction of the mPT in astrocytes appears to represent a major pathogenetic factor in HE, in which CKs and ammonia are critically involved.

Hepatic encephalopathy: a central neuroinflammatory disorder?

Encephalopathy and brain edema are serious central nervous system complications of liver failure. Recent studies using molecular probes and antibodies to cell-specific marker proteins have demonstrated the activation of microglial cells in the brain during liver failure and confirmed a central neuroinflammatory response. In animal models of ischemic or toxic liver injury, microglial activation and concomitantly increased expression of genes coding for proinflammatory cytokines in the brain occur early in the progression of encephalopathy and brain edema. Moreover, the prevention of these complications with mild hypothermia or N-acetylcysteine (two treatments known to manifest both peripheral and central cytoprotective properties) averts central neuroinflammation due to liver failure. Recent studies using anti-inflammatory agents such as ibuprofen and indomethacin have shown promise for the treatment of mild encephalopathy in patients with cirrhosis, whereas treatment with minocycline, a potent inhibitor of microglial activation, attenuates the encephalopathy grade and prevents brain edema in experimental acute liver failure. The precise nature of the signaling mechanisms between the failing liver and central neuroinflammation has yet to be fully elucidated; mechanisms involving blood-brain cytokine transfer and receptor-mediated cytokine signal transduction as well as a role for liver-related toxic metabolites such as ammonia have been proposed. The prevention of central proinflammatory processes will undoubtedly herald a new chapter in the development of agents for the prevention and treatment of the central nervous system complications of liver failure.

Acute endotoxemia following transjugular intrahepatic stent-shunt insertion is associated with systemic and cerebral vasodilatation with increased whole body nitric oxide production in critically ill cirrhotic patients

BACKGROUND & AIMS

Transjugular intrahepatic stent-shunt (TIPSS) insertion, in patients with uncontrolled gastro-intestinal bleeding, often results in worsening of the systemic hemodynamics which can be associated with intracranial hypertension but the underlying mechanisms are unclear. This study explored the hypothesis that TIPSS insertion results in acute endotoxemia which is associated with increased nitric oxide production resulting in systemic and cerebral vasodilatation.

METHODS

Twelve patients with cirrhosis who were undergoing TIPSS for uncontrolled variceal bleeding were studied prior to and 1-h after TIPSS insertion. Changes in cardiac output (CO) and cerebral blood flow (CBF) were measured. NO production was measured using stable isotopes using l-[guanidino-(15)N(2)] arginine and l-[ureido-(13)C;5,5-(2)H(2)] citrulline infusion. The effect of pre- and post-TIPSS plasma on nitric oxide synthase (NOS) activity on human endothelial cell-line (HUVEC) was measured.

RESULTS

TIPSS insertion resulted in a significant increase in CO and CBF. Endotoxin and induced neutrophil oxidative burst increased significantly without any significant changes in cytokines. Whole body NO production increased significantly and this was associated with increased iNOS activity in the HUVEC lines. The change in NO production correlated with the changes in CO and CBF. Brain flux of ammonia increased without significant changes in arterial ammonia.

CONCLUSIONS

In conclusion, the insertion of TIPSS results in acute endotoxemia which is associated with increased nitric oxide production possibly through an iNOS dependent mechanism which may have important pathophysiological and therapeutic relevance to understanding the basis of circulatory failure in the critically ill cirrhotic patient.

Brain region-selective mechanisms contribute to the progression of cerebral alterations in acute liver failure in rats

BACKGROUND & AIMS

Patients with acute liver failure (ALF) often die of intracranial pressure (IP) and cerebral herniation. Main contributors to increased IP are ammonia, glutamine, edema, and blood flow. The sequence of events and underlying mechanisms, as well as the temporal pattern, regional distribution, and contribution of each parameter to the progression of neurologic deterioration and IP, are unclear. We studied rats with ALF to follow the progression of changes in ammonia, glutamine, grade and type (vasogenic or cytotoxic) of edema, blood-brain barrier permeability, cerebral blood flow, and IP. We assessed whether the changes in these parameters were similar between frontal cortex and cerebellum and evaluated the presence, type, and progression of edema in 12 brain areas.

METHODS

ALF was induced by injection of galactosamine. The grade and type of edema was assessed by measuring the apparent diffusion coefficient by magnetic resonance imaging. Cerebral blood flow was measured by magnetic resonance and blood-brain barrier permeability by Evans blue-albumin extravasation.

RESULTS

Increased IP arises from an early increase of blood-brain barrier permeability in certain areas (including cerebellum but not frontal cortex) followed by vasogenic edema. Ammonia and glutamine then increase progressively, leading to cytotoxic edema in many areas. Alterations in lactate and cerebral blood flow are later events that further increase IP.

CONCLUSIONS

Different mechanisms in specific regions of the brain contribute, with different temporal patterns, to the progression of cerebral alterations and IP in ALF.

Hepatic encephalopathy: from pathophysiology to therapeutic management

Hepatic encephalopathy is a complex and potentially reversible neuropsychiatric syndrome complicating acute or chronic liver disease. Clinical manifestations are multiple and varied, ranging from minimal neurological changes to coma. Ammonia is the main toxic substance involved in the pathogenesis of hepatic encephalopathy, although other mechanisms, such as modifications of the blood-brain barrier, disruptions in neurotransmission and abnormalities in GABAergic and benzodiazepine pathways may also play a role. The identification and treatment of precipitating factors is crucial in the management of patients with hepatic encephalopathy. Current treatments are based on reducing intestinal ammonia load by agents such as antibiotics or disaccharides, although their efficacy is yet to be clearly established.

Extracellular brain ammonia levels in association with arterial ammonia, intracranial pressure and the use of albumin dialysis devices in pigs with acute liver failure

In acute liver failure (ALF) hyperammonemia plays a mayor role in the pathogenesis of hepatic encephalopathy (HE) but does not always correlate with the severity of mental deterioration and intracranial pressure (ICP). The aim of our study was to evaluate the association with extracellular brain ammonia, ICP and the therapeutical impact of two albumin dialysis devices. ALF was induced by complete hepatectomy in 13 pigs. All pigs were monitored and treated under intensive care conditions until death. Arterial blood and cerebral microdialysis samples were collected and ICP data recorded. Additionally in 5 pigs, standard albumin dialysis and in 3 animals an albumin dialysis prototype was initiated as a tool. Arterial ammonia increased straight after hepatectomy, while extracellular brain ammonia remained on a moderate level 10 h post ALF initiation. After 16 h the brain ammonia reached arterial ammonia levels before plateauing at 1,200 microM, though the arterial ammonia continued to rise. The ICP correlated with the brain ammonia levels. No impact of the different dialysis therapies on neither blood nor brain ammonia levels was observed. In ALF the extracellular brain ammonia revealed a delayed increase compared to arterial ammonia. It correlated strongly with the ICP and could serve as a sensitive marker for HE development. Albumin dialysis did not affect blood or brain ammonia levels.