Keeping cool in acute liver failure: rationale for the use of mild hypothermia

Acute liver failure

Acute liver failure (ALF) is a life-threatening disorder characterised by rapid deterioration of liver function, coagulopathy, and hepatic encephalopathy in the absence of pre-existing liver disease. The cause of ALF varies across the world. Common causes of ALF in adults include drug toxicity, hepatotropic and non-hepatotropic viruses, herbal and dietary supplements, antituberculosis drugs, and autoimmune hepatitis. The cause of liver failure affects the management and prognosis, and therefore extensive investigation for cause is strongly suggested. Sepsis with multiorgan failure and cerebral oedema remain the leading causes of death in patients with ALF and early identification and appropriate management can alter the course of ALF. Liver transplantation is the best current therapy, although the role of artificial liver support systems, particularly therapeutic plasma exchange, can be useful for patients with ALF, especially in non-transplant centres. In this Seminar, we discuss the cause, prognostic models, and management of ALF.

Current concepts in acute liver failure

Acute liver failure (ALF) is a severe condition secondary to a myriad of causes associated with poor outcomes. The prompt diagnosis and identification of the aetiology allow the administration of specific treatments plus supportive strategies and to define the overall prognosis, the probability of developing complications and the need for liver transplantation. Pivotal issues are adequate monitoring and the institution of prophylactic strategies to reduce the risk of complications, such as progressive liver failure, cerebral oedema, renal failure, coagulopathies or infections. In this article, we review the main aspects of ALF, including the definition, diagnosis and complications. Also, we describe the standard-of-care strategies and recent advances in the treatment of ALF. Finally, we include our experience of care patients with ALF.

Subnormothermic Perfusion in the Isolated Rat Liver Preserves the Antioxidant Glutathione and Enhances the Function of the Ubiquitin Proteasome System

The reduction of oxidative stress is suggested to be one of the main mechanisms to explain the benefits of subnormothermic perfusion against ischemic liver damage. In this study we investigated the early cellular mechanisms induced in isolated rat livers after 15 min perfusion at temperatures ranging from normothermia (37°C) to subnormothermia (26°C and 22°C). Subnormothermic perfusion was found to maintain hepatic viability. Perfusion at 22°C raised reduced glutathione levels and the activity of glutathione reductase; however, lipid and protein oxidation still occurred as determined by malondialdehyde, 4-hydroxynonenal-protein adducts, and advanced oxidation protein products. In livers perfused at 22°C the lysosomal and ubiquitin proteasome system (UPS) were both activated. The 26S chymotrypsin-like (β5) proteasome activity was significantly increased in the 26°C (46%) and 22°C (42%) groups. The increased proteasome activity may be due to increased Rpt6 Ser120 phosphorylation, which is known to enhance 26S proteasome activity. Together, our results indicate that the early events produced by subnormothermic perfusion in the liver can induce oxidative stress concomitantly with antioxidant glutathione preservation and enhanced function of the lysosomal and UPS systems. Thus, a brief hypothermia could trigger antioxidant mechanisms and may be functioning as a preconditioning stimulus.

Finasteride improves motor, EEG, and cellular changes in rat brain in thioacetamide-induced hepatic encephalopathy

Neurosteroids are involved in the pathogenesis of hepatic encephalopathy (HE). This study evaluated the effects of finasteride, inhibitor of neurosteroid synthesis, on motor, EEG, and cellular changes in rat brain in thioacetamide-induced HE. Male Wistar rats were divided into the following groups: 1) control; 2) thioacetamide-treated group, TAA (300 mg·kg(-1)·day(-1)); 3) finasteride-treated group, FIN (50 mg·kg(-1)·day(-1)); and 4) group treated with FIN and TAA (FIN + TAA). Daily doses of TAA and FIN were administered in three subsequent days intraperitoneally, and in the FIN + TAA group FIN was administered 2 h before every dose of TAA. Motor and reflex activity was determined at 0, 2, 4, 6, and 24 h, whereas EEG activity was registered about 24 h after treatment. The expressions of neuronal (NeuN), astrocytic [glial fibrilary acidic protein (GFAP)], microglial (Iba1), and oligodendrocyte (myelin oligodendrocyte glycoprotein) marker were determined 24 h after treatment. While TAA decreased all tests, FIN pretreatment (FIN + TAA) significantly improved equilibrium, placement test, auditory startle, head shake reflex, motor activity, and exploratory behavior vs. the TAA group. Vital reflexes (withdrawal, grasping, righting and corneal reflex) together with mean EEG voltage were significantly higher (P < 0.01) in the FIN + TAA vs. the TAA group. Hippocampal NeuN expression was significantly lower in TAA vs. control (P < 0.05). Cortical Iba1 expression was significantly higher in experimental groups vs. control (P < 0.05), whereas hippocampal GFAP expression was increased in TAA and decreased in the FIN + TAA group vs. control (P < 0.05). Finasteride improves motor and EEG changes in TAA-induced HE and completely prevents the development of hepatic coma.

Preventing cerebral oedema in acute liver failure: the case for quadruple-H therapy

Severe cerebral oedema is a life-threatening complication of acute liver failure. Hyperammonaemia and cerebral hyperaemia are major contributing factors. A multimodal approach, which incorporates hyperventilation, haemodiafiltration, hypernatraemia and hypothermia (quadruple-H therapy), may prevent or attenuate severe cerebral oedema. This approach is readily administered by critical care clinicians and is likely to be more effective than the use of single therapies. Targeting of PaCO2 in the mild hyperventilation range, as seen in acute liver failure patients before intubation, aims to minimise hyperaemic cerebral oedema. Haemodiafiltration aims to achieve the rapid control of elevated blood ammonia concentrations by its removal and to reduce production via the lowering of core temperature. The administration of concentrated saline increases serum tonicity and further reduces cerebral swelling. In addition, the pathologically increased cerebral blood-flow is further attenuated by therapeutic hypothermia. The combination of all four treatments in a multimodal approach may be a safe and effective means of attenuating or treating the cerebral oedema of acute liver failure and preventing death from neurological complications.

Fulminant viral hepatitis

Acute liver failure (ALF) is a condition wherein the previously healthy liver rapidly deteriorates, resulting in jaundice, encephalopathy, and coagulopathy. There are approximately 2000 cases per year of ALF in the United States. Viral causes (fulminant viral hepatitis [FVH]) are the predominant cause of ALF in developing countries. Given the ease of spread of viral hepatitis and the high morbidity and mortality associated with ALF, a systematic approach to the diagnosis and treatment of FVH is required. In this review, the authors describe the viral causes of ALF and review the intensive care unit management of patients with FVH.

Hypothermia predicts hepatic failure after extensive hepatectomy in mice

AIM

To investigate the effect of hypothermia on the function of the liver remnant (LR) after extended hepatectomy.

METHODS

We performed a 75% partial hepatectomy (PH) in male C57BL/6J mice. Body temperature was measured with a rectal probe. The study mice were prospectively grouped as hypothermic (HT) or normothermic (NT) if their body temperature was < 34 °C vs ≥ 34 °C, respectively. Blood and liver samples were obtained at 24 and 48 h after 75% PH. Various factors during and after 75% PH were compared at each time point and the most important factor for a good outcome after 75% PH was determined.

RESULTS

At 24 and 48 h after 75% PH, LR weight was decreased in HT mice compared with that in NT mice and the assay results in the HT mice were consistent with liver failure. NT mice had normal liver regeneration. Each intra- and post-operative factor which showed statistical significance in univariate analysis was evaluated by multivariate analysis. The most important factor for a good outcome after 75% PH was body temperature at both 24 and 48 h after surgery.

CONCLUSION

Hypothermia after an extensive hepatectomy predicts impending liver failure and may be a useful clinical marker for early detection of liver failure after extended hepatectomy.

Management and prognosis of acute liver failure in children

Although the etiologies of pediatric acute liver failure (ALF) are diverse, ultimate pathophysiologic pathways and management challenges for these disorders, usually lethal in the pre-transplant era, are similar. This review considers particularly the mechanisms of, and monitoring for, intracranial hypertension and coagulopathy; summarizes detailed advice for management of the ALF-associated failures of multiple body systems; and reviews the variety of prognostic scores available to guide management and assist in choosing the patients most apt to benefit from liver transplantation and the optimal timing for such transplantation.

Therapeutic hypothermia for acute neurological injuries

Therapeutic hypothermia (TH) is the intentional reduction of core body temperature to 32°C to 35°C, and is increasingly applied by intensivists for a variety of acute neurological injuries to achieve neuroprotection and reduction of elevated intracranial pressure. TH improves outcomes in comatose patients after a cardiac arrest with a shockable rhythm, but other off-label applications exist and are likely to increase in the future. This comprehensive review summarizes the physiology and cellular mechanism of action of TH, as well as different means of TH induction and maintenance with potential side effects. Indications of TH are critically reviewed by disease entity, as reported in the most recent literature, and evidence-based recommendations are provided.

A simple dummy liver assist device prolongs anhepatic survival in a porcine model of total hepatectomy by slight hypothermia

BACKGROUND

Advances in intensive care support such as therapeutic hypothermia or new liver assist devices have been the mainstay of treatment attempting to bridge the gap from acute liver failure to liver transplantation, but the efficacy of the available devices in reducing mortality has been questioned. To address this issue, the present animal study was aimed to analyze the pure clinical effects of a simple extracorporeal dummy device in an anhepatic porcine model of acute liver failure.

METHODS

Total hepatectomy was performed in ten female pigs followed by standardized intensive care support until death. Five animals (dummy group, n = 5) underwent additional cyclic connection to an extracorporeal dummy device which consisted of a plasma separation unit. The separated undetoxified plasma was completely returned to the pigs circulation without any plasma substitution or exchange in contrast to animals receiving intensive care support alone (control group, n = 5). All physiological parameters such as vital and ventilation parameters were monitored electronically; laboratory values and endotoxin levels were measured every 8 hours.

RESULTS

Survival of the dummy device group was 74 ± 6 hours in contrast to 53 ± 5 hours of the control group which was statistically significant (p < 0.05). Body temperature 24 hours after hepatectomy was significantly lower (36.5 ± 0.5°C vs. 38.2 ± 0.7°C) in the dummy device group. Significant lower values were measured for blood lactate (1.9 ± 0.2 vs. 2.5 ± 0.5 mM/L) from 16 hours, creatinine (1.5 ± 0.2 vs. 2.0 ± 0.3 mg/dL) from 40 hours and ammonia (273 ± 122 vs. 1345 ± 700 μg/dL) from 48 hours after hepatectomy until death. A significant rise of endotoxin levels indicated the onset of sepsis at time of death in 60% (3/5) of the dummy device group animals surviving beyond 60 hours from hepatectomy.

CONCLUSIONS

Episodes of slight hypothermia induced by cyclic connection to the extracorporeal dummy device produced a significant survival benefit of more than 20 hours through organ protection and hemodynamic stabilisation. Animal studies which focus on a survival benefit generated by liver assist devices should especially address the aspect of slight transient hypothermia by extracorporeal cooling.

Contemporary management of traumatic intracranial hypertension: is there a role for therapeutic hypothermia?

OBJECTIVE

Intracranial hypertension (ICH) remains the single most difficult therapeutic challenge for the acute management of severe traumatic brain injury (TBI). We reviewed the published trials of therapeutic moderate hypothermia to determine its effect on ICH and compared its efficacy to other commonly used therapies for ICH.

METHODS

A PubMed database search was done using various combinations of the search terms "brain injury," "therapeutic hypothermia," "intracranial hypertension," "barbiturates," "mannitol," "hypertonic saline," "hyperventilation," "decompressive craniectomy," and "CSF drainage."

RESULTS

We identified 11 prospective randomized clinical TBI trials comparing hypothermia vs. normothermia treatment for which intracranial pressure (ICP) data was provided, and 6 prospective cohort studies that provided ICP data before and during hypothermia treatment. In addition, we identified 37 clinical TBI studies of lumbar CSF drainage, mannitol, hyperventilation, barbiturates, hypertonic saline, and decompressive craniectomy that provided pre- and posttreatment ICP data. Hypothermia was at least as effective as the traditional therapies for ICH (hyperventilation, mannitol, and barbiturates), but was less effective than hypertonic saline, lumbar CSF drainage, and decompressive craniectomy. Ultimately, however, therapeutic hypothermia does appear to have a favorable risk/benefit profile.

CONCLUSION

Therapeutic moderate hypothermia is as effective, or more effective, than most other treatments for ICH. If used for 2-3 days or less there is no evidence that it causes clinically significant adverse events. The lack of consistent evidence that hypothermia improves long-term neurologic outcome should not preclude consideration of its use for the primary treatment of ICH since no other ICP therapy is held to this standard.

Acute liver failure

Acute liver failure is a rare disorder with high mortality and resource cost. In the developing world, viral causes predominate, with hepatitis E infection recognised as a common cause in many countries. In the USA and much of western Europe, the incidence of virally induced disease has declined substantially in the past few years, with most cases now arising from drug-induced liver injury, often from paracetamol. However, a large proportion of cases are of unknown origin. Acute liver failure can be associated with rapidly progressive multiorgan failure and devastating complications; however, outcomes have been improved by use of emergency liver transplantation. An evidence base for practice is emerging for supportive care, and a better understanding of the pathophysiology of the disorder, especially in relation to hepatic encephalopathy, will probably soon lead to further improvements in survival rates.

Antioxidant and anti-inflammatory effects of mild hypothermia in the attenuation of liver injury due to azoxymethane toxicity in the mouse

Previous studies have demonstrated protective effects of mild hypothermia following acetaminophen (APAP)-induced acute liver failure (ALF). However, effects of this treatment in ALF due to other toxins have not yet been fully investigated. In the present study, the effects of mild hypothermia in relation to liver pathology, hepatic and cerebral glutathione, plasma ammonia concentrations, progression of encephalopathy, cerebral edema, and plasma proinflammatory cytokines were assessed in mice with ALF resulting from azoxymethane (AOM) hepatotoxicity, a well characterized model of toxic liver injury. Male C57BL/6 mice were treated with AOM (100 microg/g; i.p.) or saline and sacrificed at coma stages of encephalopathy in parallel with AOM mice maintained mildly hypothermic (35 degrees C). AOM treatment led to hepatic damage, significant increase in plasma transaminase activity, decreased hepatic glutathione levels, and brain GSH/GSSG ratios as well as selective increases in expression of plasma proinflammatory cytokines. Mild hypothermia resulted in reduced hepatic damage, improvement in neurological function, normalization of glutathione levels, and selective attenuation in expression of circulating proinflammatory cytokines. These findings demonstrate that the beneficial effects of mild hypothermia in experimental AOM-induced ALF involve both antioxidant and anti-inflammatory mechanisms.

Treatment of brain edema in acute liver failure

OPINION STATEMENT

Cerebral edema is very common in patients with acute liver failure and encephalopathy. In severe cases, it produces brain tissue shift and potentially fatal herniation. Brain swelling in acute liver failure is produced by a combination of cytotoxic (cellular) and vasogenic edema. Accumulation of ammonia and glutamine leads to disturbances in the regulation of cerebral osmolytes, increased free radical production and calcium-mediated mitochondrial injury, and alterations in glucose metabolism (inducing high levels of brain lactate), resulting in astrocyte swelling. Activation of inflammatory cytokines can cause increased blood-brain barrier permeability leading to vasogenic edema, although the relative contribution of vasogenic edema is probably minor compared with cellular swelling. Cerebral blood flow is disturbed and generally increased in patients with acute liver failure; persistent vasodilatation and loss of autoregulation may generate hyperemia, and the consequent augmentation in cerebral blood volume may exacerbate brain edema.Adequate management of intracranial hypertension demands continuous monitoring of intracranial pressure and cerebral perfusion pressure. Coagulation status should be assessed and bleeding diathesis should be treated prior to insertion of the intracranial pressure monitor. Standard treatment measures such as hyperventilation and osmotic agents (e.g., mannitol, hypertonic saline) remain useful first-line interventions. Although hypertonic saline may be preferred in patients with coexistent hyponatremia, the rate of correction of hyponatremia must be gradual to avoid the risk of osmotic demyelination. Barbiturate coma and intravenous indomethacin are available options in refractory cases. The most promising novel therapeutic alternative is the induction of moderate hypothermia (aiming for a core temperature of 32-34°C). However, the safety and efficacy of therapeutic hypothermia for brain swelling caused by liver failure still needs to be proven in randomized, controlled clinical trials. Management of intracranial pressure in patients with acute liver failure should be guided by well-defined treatment protocols.

Current and future role of therapeutic hypothermia

Therapeutic moderate hypothermia has been advocated for use in traumatic brain injury, stroke, cardiac arrest-induced encephalopathy, neonatal hypoxic-ischemic encephalopathy, hepatic encephalopathy, and spinal cord injury, and as an adjunct to aneurysm surgery. In this review, we address the trials that have been performed for each of these indications, and review the strength of the evidence to support treatment with mild/moderate hypothermia. We review the data to support an optimal target temperature for each indication, as well as the duration of the cooling, and the rate at which cooling is induced and rewarming instituted. Evidence is strongest for prehospital cardiac arrest and neonatal hypoxic-ischemic encephalopathy. For traumatic brain injury, a recent meta-analysis suggests that cooling may increase the likelihood of a good outcome, but does not change mortality rates. For many of the other indications, such as stroke and spinal cord injury, trials are ongoing, but the data are insufficient to recommend routine use of hypothermia at this time.

ICU management of acute liver failure

Survival of patients presenting with acute liver failure (ALF) has improved because of earlier disease recognition, better understanding of pathophysiology of various insults leading to ALF, and advances in supportive measures including a team approach, better ICU care, and liver transplantation. This article focuses on patient management and evaluation that takes place in the ICU for patients who have acute liver injury. An organized team approach to decision making about critical care delivered during this period of time is important for achieving a good patient outcome.

Mild hypothermia does not affect liver regeneration after partial hepatectomy in mice

BACKGROUND

The use of mild hypothermia has been suggested to be therapeutically useful in treating acute liver failure. It is not known if hypothermia influences liver regeneration.

AIM

To assess the effect of hypothermia on liver regeneration in mice.

METHODS

After partial (70%) hepatectomy (PHx), C57BL6/J mice were randomly assigned to either a hypothermic group or a normothermic group. Controlled mild hypothermia was maintained for up to 3 h after surgery. In addition, assessment of liver mass restitution was examined by studying the induction of key cell cycle proteins (cyclin A, D1 and E) and hepatocyte proliferation [assessment of proliferating cell nuclear antigen (PCNA) protein expression] by Western blotting and DNA synthesis by measuring 5-bromo-2-deoxyuridine (BrdU) incorporation by immunohistochemical techniques 45 h after PHx.

RESULTS

Partial hepatectomy induced a vigorous proliferative response in the remnant livers of both groups of mice (normothermic and hypothermic groups), as evidenced by the induction of key cyclins, PCNA and incorporation of BrdU after PHx. The liver/body weight ratio and both cyclin and PCNA protein expression as well as BrdU incorporation did not differ between the regenerating livers of hypothermic and normothermic groups.

CONCLUSION

Mild hypothermia does not influence liver regeneration in mice.

Current approach to intraoperative monitoring in liver transplantation

PURPOSE OF REVIEW

Although liver transplantation has become a standardized treatment and the only established definite therapy for end-stage liver disease it remains a unique clinical procedure. Increased understanding of the specific pathophysiological changes in end-stage liver disease and the transplantation procedure have led to the adaptation of concepts including overall monitoring of the patient and assessment of specific organ function.

RECENT FINDINGS

Major emphasis is placed on adequate monitoring during perioperative care of liver transplantation patients in order to ensure optimal hemodynamic and respiratory performance. The immediate assessment of metabolism and graft function will also serve to guide therapy according to the individual patient's needs.

SUMMARY

The evolution of monitoring during standardized liver transplantation, as well as currently recommended novel devices and concepts, are described and discussed.

Iron chelation attenuates intracranial pressure and improves survival in a swine model of acute liver failure

Oxidative mechanisms have been implicated in the pathogenesis of brain edema in acute liver failure (ALF). The aim of this study was to test the hypothesis that inhibition of iron-catalyzed oxidative reactions through iron chelation using deferoxamine could attenuate brain edema in a swine model of ischemic ALF. Following ALF induction (end-to-side portacaval anastomosis and ligation of the hepatoduodenal ligament), 14 animals were randomized to a study group that received an intravenous infusion of 150 mg/kg deferoxamine (group DF; n = 7) or a control group (group C; n = 7). Six sham-operated animals were also assigned to a deferoxamine-treated group (n = 3) or a control group (n = 3). Hemodynamic, neurological, and hematological parameters were monitored postoperatively. All sham animals maintained normal hemodynamics and intracranial pressure. At 18 hours, group DF animals had higher mean arterial pressure (mean +/- standard deviation: 98.0 +/- 15.9 versus 69.9 +/- 15.8 mmHg, P < 0.004), lower intracranial pressure (18.1 +/- 8.6 versus 32.7 +/- 13.4 mmHg, P < 0.032), and higher cerebral perfusion pressure (76.4 +/- 16.4 versus 37.1 +/- 25.6 mmHg, P < 0.006) in comparison with group C. Similar differences were recorded up to the 24th postoperative hour, leading to a significant difference in animal survival (88% in group DF versus 17% in group C, P < 0.001). Furthermore, group DF exhibited an attenuated increase of serum malondialdehyde from the baseline (16% versus 74%, P < 0.05) and lower brain malondialdehyde concentrations (3.7 +/- 1.3 versus 5.7 +/- 2.0 microM/mg of protein, P < 0.05) in comparison with controls. In conclusion, deferoxamine delayed the development of intracranial hypertension and improved survival in pigs with ischemic ALF.

Acute liver failure: Summary of a workshop

Acute liver failure (ALF) is a rare but challenging clinical syndrome with multiple causes; a specific etiology cannot be identified in 15% of adult and 50% of pediatric cases. The course of ALF is variable and the mortality rate is high. Liver transplantation is the only therapy of proven benefit, but the rapidity of progression and the variable course of ALF limit its use. Currently in the United States, spontaneous survival occurs in approximately 45%, liver transplantation in 25%, and death without transplantation in 30% of adults with ALF. Higher rates of spontaneous recovery (56%) and transplantation (31%) with lower rates of death (13%) occur in children. The outcome of ALF varies by etiology, favorable prognoses being found with acetaminophen overdose, hepatitis A, and ischemia (approximately 60% spontaneous survival), and poor prognoses with drug-induced ALF, hepatitis B, and indeterminate cases (approximately 25% spontaneous survival). Excellent intensive care is critical in management of patients with ALF. Nonspecific therapies are of unproven benefit. Future possible therapeutic approaches include N-acetylcysteine, hypothermia, liver assist devices, and hepatocyte transplantation. Advances in stem cell research may allow provision of cells for bioartificial liver support. ALF presents many challenging opportunities in both clinical and basic research.

Mechanisms of brain edema in acute liver failure and impact of novel therapeutic interventions

Continued elucidation of the mechanisms of brain edema in acute liver failure (ALF) has established ammonia and the astrocyte as major players in its pathogenesis. The metabolism of ammonia to glutamine appears to be a requisite, and is followed by an osmotic disturbance in the brain, mitochondrial dysfunction with oxidative/nitrosative stress, and alterations of brain glucose metabolism. Cerebral blood flow (CBF) is also altered in ALF and strongly influence the development of brain edema and intracranial hypertension. Additional factors such as systemic inflammation, alterations of the brain extracellular concentration of amino acids and neurotransmitters, and others have been identified and may contribute to the cerebral alterations of ALF. Such pathophysiologic insights are reflected in the various clinical trials of novel therapeutic interventions using ammonia-lowering agents, N-acetylcysteine, hypertonic saline, indomethacin, high-volume plasmapheresis, bio-artificial liver assist devices, albumin dialysis and mild hypothermia.

Arterial ammonia and clinical risk factors for encephalopathy and intracranial hypertension in acute liver failure

High circulating ammonia concentrations are common in patients with acute liver failure (ALF) and are associated with hepatic encephalopathy (HE) and intracranial hypertension (ICH). Other risk factors are poorly characterized. We evaluated the relation of the admission arterial ammonia concentration and other clinical variables with the development of HE and ICH. Arterial ammonia was measured on admission to the intensive care unit in 257 patients; 165 had ALF and severe HE, and there were 3 control groups: acute hepatic dysfunction without severe HE (n = 50), chronic liver disease (n = 33), and elective surgery (n = 9). Variables associated with ICH and HE were investigated with regression analysis. Ammonia was higher in ALF patients than controls. An independent risk factor for the development of severe HE and ICH, a level greater than 100 mumol/L predicted the onset of severe HE with 70% accuracy. The model for end-stage liver disease (MELD) score was also independently predictive of HE, and its combination with ammonia increased specificity and accuracy. ICH developed in 55% of ALF patients with a level greater than 200 mumol/L, although this threshold failed to identify most cases. After admission, ammonia levels remained high in those developing ICH and fell in those who did not. Youth, a requirement for vasopressors, and renal replacement therapy were additional independent risk factors.

CONCLUSION

Ammonia is an independent risk factor for the development of both HE and ICH. Additional MELD scoring improved the prediction of HE. Factors other than ammonia also appear important in the pathogenesis of ICH. Ammonia measurements could form part of risk stratification for HE and ICH, identifying patients for ammonia-lowering therapies and invasive monitoring.

Neurological implications of urea cycle disorders

The urea cycle disorders constitute a group of rare congenital disorders caused by a deficiency of the enzymes or transport proteins required to remove ammonia from the body. Via a series of biochemical steps, nitrogen, the waste product of protein metabolism, is removed from the blood and converted into urea. A consequence of these disorders is hyperammonaemia, resulting in central nervous system dysfunction with mental status changes, brain oedema, seizures, coma, and potentially death. Both acute and chronic hyperammonaemia result in alterations of neurotransmitter systems. In acute hyperammonaemia, activation of the NMDA receptor leads to excitotoxic cell death, changes in energy metabolism and alterations in protein expression of the astrocyte that affect volume regulation and contribute to oedema. Neuropathological evaluation demonstrates alterations in the astrocyte morphology. Imaging studies, in particular (1)H MRS, can reveal markers of impaired metabolism such as elevations of glutamine and reduction of myoinositol. In contrast, chronic hyperammonaemia leads to adaptive responses in the NMDA receptor and impairments in the glutamate-nitric oxide-cGMP pathway, leading to alterations in cognition and learning. Therapy of acute hyperammonaemia has relied on ammonia-lowering agents but in recent years there has been considerable interest in neuroprotective strategies. Recent studies have suggested restoration of learning abilities by pharmacological manipulation of brain cGMP with phosphodiesterase inhibitors. Thus, both strategies are intriguing areas for potential investigation in human urea cycle disorders.