Mild hypothermia modifies ammonia-induced brain edema in rats after portacaval anastomosis

Therapeutic Hypothermia in Critically Ill Patients: A Systematic Review and Meta-Analysis of High Quality Randomized Trials

OBJECTIVES

To investigate the effect of the application of therapeutic hypothermia (32-35°C) on survival and major clinical endpoints in critically ill patients.

DATA SOURCES

We searched online database and clinical trial registries dated up to April 30, 2019, and references of relevant studies.

STUDY SELECTION

Low risk of bias randomized trials which compared hypothermia applied for at least 24 hours and conventional therapy in critically ill patients were included. We excluded trials investigating therapeutic hypothermia in indications already supported by international guidelines (adult cardiac arrest and hypoxic-ischemic encephalopathy of newborns) or intraoperative hypothermia.

DATA EXTRACTION

Titles and abstracts were reviewed independently by two authors. If the articles seemed eligible, full-text articles were reviewed, and data were abstracted using a structured template.

DATA SYNTHESIS

Our search retained 14 low risk of bias randomized trials (2,670 patients) performed in three different settings: traumatic brain injury, serious infections, and stroke. Therapeutic hypothermia was associated with an increase in mortality at longest follow-up available (432/1,375 [31%] vs 330/1,295 [25%]; risk ratio, 1.24; 95% CI, 1.10-1.39; p = 0.0004; I = 0%). Pooled results showed no difference of good neurologic outcome among survivors between the two treatment arms (493/1,142 [43%] vs 486/1,067 [46%]; risk ratio, 1.04; 95% CI, 0.97-1.12; p = 0.27; I = 1%). Arrhythmias were significantly increased among patients undergoing therapeutic hypothermia. We found no difference between groups in pneumonia, serious infections, any infection, hemorrhage, renal failure, deep vein thrombosis, and uncontrollable intracranial hypertension.

CONCLUSIONS

High-quality randomized evidence indicates that therapeutic hypothermia is associated with higher mortality and no difference in good neurologic outcome compared with normothermia in critically ill patients. Although there still might be a possibility that therapeutic hypothermia is beneficial in a specific setting, routine application of therapeutic hypothermia would better be avoided outside the settings indicated by international guidelines (adult cardiac arrest and hypoxic-ischemic encephalopathy of newborns).

Cerebral edema and liver disease: Classic perspectives and contemporary hypotheses on mechanism

Liver disease is a growing public health concern. Hepatic encephalopathy, the syndrome of brain dysfunction secondary to liver disease, is a frequent complication of both acute and chronic liver disease and cerebral edema (CE) is a key feature. While altered ammonia metabolism is a key contributor to hepatic encephalopathy and CE in liver disease, there is a growing appreciation that additional mechanisms contribute to CE. In this review we will begin by presenting three classic perspectives that form a foundation for a discussion of CE in liver disease: 1) CE is unique to acute liver failure, 2) CE in liver disease is only cytotoxic, and 3) CE in liver disease is primarily an osmotically mediated consequence of ammonia and glutamine metabolism. We will present each classic perspective along with more recent observations that call in to question that classic perspective. After highlighting these areas of debate, we will explore the leading contemporary mechanisms hypothesized to contribute to CE during liver disease.

Relationship between oxidative stress and brain swelling in goldfish (Carassius auratus) exposed to high environmental ammonia

Buildups of ammonia can cause potentially fatal brain swelling in mammals, but such swelling is reversible in the anoxia- and ammonia-tolerant goldfish (Carassius auratus). We investigated brain swelling and its possible relationship to oxidative stress in the brain and liver of goldfish acutely exposed to high external ammonia (HEA; 5 mmol/l NH₄Cl) at two different acclimation temperatures (14°C, 4°C). Exposure to HEA at 14°C for 72h resulted in increased internal ammonia and glutamine concentrations in the brain, and it caused cellular oxidative damage in the brain and liver. However, oxidative damage was most pronounced in brain, in which there was a twofold increase in thiobarbituric acid-reactive substances, a threefold increase in protein carbonylation, and a 20% increase in water volume (indicative of brain swelling). Increased activities of catalase, glutathione peroxidase, and glutathione reductase in the brain suggested that goldfish upregulate their antioxidant capacity to partially offset oxidative stress during hyperammonemia at 14°C. Notably, acclimation to colder (4°C) water completely attenuated the oxidative stress response to HEA in both tissues, and there was no change in brain water volume despite similar increases in internal ammonia. We suggest that ammonia-induced oxidative stress may be responsible for the swelling of goldfish brain during HEA, but further studies are needed to establish a mechanistic link between reactive oxygen species production and brain swelling. Nevertheless, a high capacity to withstand oxidative stress in response to variations in internal ammonia likely explains why goldfish are more resilient to this stressor than most other vertebrates.

A multicentre randomized controlled trial of moderate hypothermia to prevent intracranial hypertension in acute liver failure

BACKGROUND & AIMS

Animal models and human case series of acute liver failure (ALF) suggest moderate hypothermia (MH) to have protective effects against cerebral oedema (CO) development and intracranial hypertension (ICH). However, the optimum temperature for patient management is unknown. In a prospective randomized controlled trial we investigated if maintenance of MH prevented development of ICH in ALF patients at high risk of the complication.

METHODS

Patients with ALF, high-grade encephalopathy and intracranial pressure (ICP) monitoring in specialist intensive care units were randomized by sealed envelope to targeted temperature management (TTM) groups of 34°C (MH) or 36°C (control) for a period of 72h. Investigators were not blinded to group assignment. The primary outcome was a sustained elevation in ICP >25mmHg, with secondary outcomes the occurrence of predefined serious adverse effects, magnitude of ICP elevations and cerebral and all-cause hospital mortality (with or without transplantation).

RESULTS

Forty-six patients were randomized, of whom forty-three were studied. There was no significant difference between the TTM groups in the primary outcome during the study period (35% vs. 27%, p=0.56), for the MH (n=17) or control (n=26) groups respectively, relative risk 1.31 (95% CI 0.53-3.2). Groups had similar incidence of adverse events and overall mortality (41% vs. 46%, p=0.75).

CONCLUSIONS

In patients with ALF at high risk of ICH, MH at 33-34°C did not confer a benefit above management at 36°C in prevention of ICH or in overall survival. This study did not confirm advantage of its prophylactic use. (ISRCTN registration number 74268282; no funding.)

LAY SUMMARY

Studies in animals with acute liver failure (ALF) have suggested that cooling (hypothermia) could prevent or limit the development of brain swelling, a dangerous complication of the condition. There is limited data on its effects in humans. In a randomized controlled trial in severely ill patients with ALF we compared the effects of different temperatures and found no benefit on improving survival or preventing brain swelling by controlling temperature at 33-34°C against 36°C.

Mild hypothermia attenuate kidney injury in canines with oleic acid-induced acute respiratory distress syndrome

BACKGROUND

Hypothermia may attenuate ventilator induced-lung injury in acute respiratory distress syndrome (ARDS). However, the impact of hypothermia on extra-pulmonary organ injury in ARDS remains unclear. The purpose of this study was to investigate whether hypothermia affects extra-pulmonary organ injury in a canine ARDS model induced by oleic acid.

OBJECTIVES

Twelve anesthetized canines with oleic acid-induced ARDS were randomly divided (n=6 per group) into a hypothermia group (core temperature of 33±1°C, HT group) and a normothermia group (core temperature of 38±1°C, NT group) and treated for four hours. The liver, small intestine and kidney were assessed by evaluating biochemical parameters, plasma and tissue cytokine levels, and tissue histopathological injury scores.

RESULTS

The HT group showed a lower plateau pressure, lung elastance and pulmonary vascular resistance. Hypothermia was associated with lower oxygen consumption (138.4±55.0mlmin(-1)vs. 72.0±11.2mlmin(-1), P<0.05) and higher oxygen saturation of mixed venous blood (62.8%±8.0% vs. 77.5%±10.1%, P<0.05). Both groups had similar levels of tumour necrosis factor-α in the plasma and extra-pulmonary organ, however, plasma interleukin-10 (97.1±25.0pgml(-1)vs. 131.4±27.0pgml(-1), P<0.05) was higher in the HT group. Further, the animals in the HT group had a lower levels of plasma creatinine (54.6±19.1UL(-1)vs. 29.1±8.0UL(-1), P<0.05), and lower renal histopathological injury scores [4.0(3.5;7.0) vs. 1.5(0.8;3.0), P<0.05]. Hypothermia did not affect the histopathological injury of the liver and small intestine.

CONCLUSIONS

Short-term mild hypothermia can reduce lung elastance and pulmonary vascular resistance, increase the systemic anti-inflammatory response and attenuate kidney histopathological injury in a canine ARDS model induced by oleic acid.

Multifactorial Effects on Different Types of Brain Cells Contribute to Ammonia Toxicity

Effects of ammonia on astrocytes play a major role in hepatic encephalopathy, acute liver failure and other diseases caused by increased arterial ammonia concentrations (e.g., inborn errors of metabolism, drug or mushroom poisoning). There is a direct correlation between arterial ammonia concentration, brain ammonia level and disease severity. However, the pathophysiology of hyperammonemic diseases is disputed. One long recognized factor is that increased brain ammonia triggers its own detoxification by glutamine formation from glutamate. This is an astrocytic process due to the selective expression of the glutamine synthetase in astrocytes. A possible deleterious effect of the resulting increase in glutamine concentration has repeatedly been discussed and is supported by improvement of some pathologic effects by GS inhibition. However, this procedure also inhibits a large part of astrocytic energy metabolism and may prevent astrocytes from responding to pathogenic factors. A decrease of the already low glutamate concentration in astrocytes due to increased synthesis of glutamine inhibits the malate-aspartate shuttle and energy metabolism. A more recently described pathogenic factor is the resemblance between NH₄⁺ and K⁺ in their effects on the Na⁺,K⁺-ATPase and the Na⁺,K⁺, 2 Cl^- and water transporter NKCC1. Stimulation of the Na⁺,K⁺-ATPase driven NKCC1 in both astrocytes and endothelial cells is essential for the development of brain edema. Na⁺,K⁺-ATPase stimulation also activates production of endogenous ouabains. This leads to oxidative and nitrosative damage and sensitizes NKCC1. Administration of ouabain antagonists may accordingly have therapeutic potential in hyperammonemic diseases.

High environmental ammonia elicits differential oxidative stress and antioxidant responses in five different organs of a model estuarine teleost (Dicentrarchus labrax)

We investigated oxidative status and antioxidant profile in five tissues (brain, liver, gills, muscle and kidney) of European sea bass (Dicentrarchus labrax) when exposed to high environmental ammonia (HEA, 20 mg/L~1.18 mM as NH4HCO3) for 12 h, 2 days, 3.5 days, 7.5 days and 10 days. Results show that HEA triggered ammonia accumulation and induced oxidative stress in all tissues. Unlike other organs, hydrogen peroxide (H2O2) and malondialdehyde (MDA) accumulation in liver were restored to control levels. This recovery was associated with a concomitant augmentation in superoxide dismutase (SOD), catalase (CAT), components of glutathione redox cycle (glutathione peroxidase GPX, glutathione reductase, reduced glutathione), ascorbate peroxidase activity and reduced ascorbate content. On the contrary, in brain during prolonged exposure many of these anti-oxidant enzymes were either unaffected or inhibited, which resulted in persistent over-accumulation of H2O2 and MDA. Branchial and renal tissue both involved in osmo-regulation, revealed an entirely dissimilar compensatory response; the former rely mainly on the ascorbate dependent defensive system while the glutathione catalytic pathway was activated in the latter. In muscle, GPX activity first rose (3.5 days) followed by a subsequent drop, counterbalanced by simultaneous increment of CAT. HEA resulted in a relatively mild oxidative stress in the muscle and kidney, probably explaining the modest anti-oxidative responses. Our findings exemplify that oxidative stress as well as antioxidant potential are qualitatively diverse amongst different tissues, thereby demonstrating that for biomonitoring studies the screening of adaptive responses at organ level should be preferred over whole body response.

Reversible brain swelling in crucian carp (Carassius carassius) and goldfish (Carassius auratus) in response to high external ammonia and anoxia

Increased internal ammonia (hyperammonemia) and ischemic/anoxic insults are known to result in a cascade of deleterious events that can culminate in potentially fatal brain swelling in mammals. It is less clear, however, if the brains of fishes respond to ammonia in a similar manner. The present study demonstrated that the crucian carp (Carassius carassius) was not only able to endure high environmental ammonia exposure (HEA; 2 to 22 mmol L(-1)) but that they experienced 30% increases in brain water content at the highest ammonia concentrations. This swelling was accompanied by 4-fold increases in plasma total ammonia (TAmm) concentration, but both plasma TAmm and brain water content were restored to pre-exposure levels following depuration in ammonia-free water. The closely related, ammonia-tolerant goldfish (Carassius auratus) responded similarly to HEA (up to 3.6 mmol L(-1)), which was accompanied by 4-fold increases in brain glutamine. Subsequent administration of the glutamine synthetase inhibitor, methionine sulfoximine (MSO), reduced brain glutamine accumulation by 80% during HEA. However, MSO failed to prevent ammonia-induced increases in brain water content suggesting that glutamine may not be directly involved in initiating ammonia-induced brain swelling in fishes. Although the mechanisms of brain swelling are likely different, exposure to anoxia for 96 h caused similar, but lesser (10%) increases in brain water content in crucian carp. We conclude that brain swelling in some fishes may be a common response to increased internal ammonia or lower oxygen but further research is needed to deduce the underlying mechanisms behind such responses.

Therapeutic hypothermia in acute liver failure: a multicenter retrospective cohort analysis

The benefit of therapeutic hypothermia (TH) in acute liver failure (ALF) has not been previously demonstrated in a controlled fashion. This study sought to determine the impact of TH on 21-day survival and complications in ALF patients at high risk for cerebral edema. This was a retrospective cohort study of ALF patients in the US Acute Liver Failure Study Group with grade III or IV hepatic encephalopathy. TH (32°C-35°C) was used in 97 patients (8%); 1135 (92%) who were not cooled were controls. Intracranial pressure was monitored in 38 TH ALF patients (39.2% versus 22% of controls, P < 0.001). Rates of bleeding (12% for both) and bloodstream (17% versus 18%) and tracheal infections (21% versus 23%, P > 0.5 for all) were similar. Unadjusted 21-day overall (62% versus 60%) and transplant-free survival rates (45% versus 39%, P > 0.4 for both) were similar. Multivariate models were created for acetaminophen (APAP) patients (n = 582) and non-APAP patients (n = 613). For APAP patients, the Model for End-Stage Liver Disease [MELD; odds ratio (OR) = 0.91 per increment, 95% confidence interval (CI) = 0.89-0.94, P < 0.001] and vasopressors (OR = 0.16, 95% CI = 0.11-0.24, P < 0.001) were associated with decreased 21-day spontaneous survival. Survival was improved with TH in APAP patients who were <25 years old (age of 25 years: OR = 2.735, 95% CI = 1.001-7.467) but worsened for APAP patients who were 64 years old or older (age of 64 years: OR = 0.167, 95% CI = 0.028-0.999). For non-APAP patients, MELD (OR = 0.93 per increment, 95% CI = 0.91-0.95, P < 0.001) and vasopressors (OR = 0.60, 95% CI = 0.40-0.90, P = 0.01) were associated with worse outcomes, whereas TH had no impact (P = 0.93). In conclusion, TH in ALF was not associated with increased bleeding or infections. Although young APAP ALF patients may benefit, TH did not consistently affect 21-day survival. A prospective trial is required to clarify the utility of TH in ALF patients.

Cerebral effects of ammonia in liver disease: current hypotheses

Hyperammonemia is necessary for development of the cerebral complications to liver disease including hepatic encephalopathy and cerebral edema but the mechanisms are unclear. Ammonia is taken up by the brain in proportion to its arterial concentration. The flux into the brain is most likely by both diffusion of NH3 and mediated transport of NH4 (+) . Astrocytic detoxification of ammonia involves formation of glutamine at concentrations high enough to produce cellular edema, but compensatory mechanisms reduce this effect. Glutamine can be taken up by astrocytic mitochondria and initiate the mitochondrial permeability transition but the clinical relevance is uncertain. Elevated astrocytic glutamine interferes with neurotransmission. Thus, animal studies show enhanced glutamatergic neurotransmission via the NMDA receptor which may be related to the acute cerebral complications to liver failure, while impairment of the NMDA activated glutamate-NO-cGMP pathway could relate to the behavioural changes seen in hepatic encephalopathy. Elevated glutamine also increases GABA-ergic tone, an effect which is aggravated by mitochondrial production of neurosteroids; this may relate to decreased neurotransmission and precipitation of encephalopathy by GABA targeting drugs. Hyperammonemia may compromise cerebral energy metabolism as elevated cerebral lactate is generally reported. Hypoxia is unlikely since cerebral oxygen:glucose utilisation and lactate:pyruvate ratio are both normal in clinical studies. Ammonia inhibits α-ketoglutaratedehydrogenase in isolated mitochondria, but the clinical relevance is dubious due to the observed normal cerebral oxygen:glucose utilization. Recent studies suggest that ammonia stimulates glycolysis in excess of TCA cycle activity, a hypothesis that may warrant further testing, in being in accordance with the limited clinical observations.

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.

Zinc is released by cultured astrocytes as a gliotransmitter under hypoosmotic stress-loaded conditions and regulates microglial activity

AIM

Astrocytes contribute to the maintenance of brain homeostasis via the release of gliotransmitters such as ATP and glutamate. Here we examined whether zinc was released from astrocytes under stress-loaded conditions, and was involved in the regulation of microglial activity as a gliotransmitter.

MAIN METHODS

Hypoosmotic stress was loaded to astrocytes using balanced salt solution prepared to 214-314 mOsmol/L, and then intra- and extra-cellular zinc levels were assessed using Newport Green DCF diacetate (NG) and ICP-MS, respectively. Microglial activation by the astrocytic supernatant was assessed by their morphological changes and poly(ADP-ribose) (PAR) polymer accumulation.

KEY FINDINGS

Exposure of astrocytes to hypoosmotic buffer, increased the extracellular ATP level in osmolarity-dependent manners, indicating a load of hypoosmotic stress. In hypoosmotic stress-loaded astrocytes, there were apparent increases in the intra- and extra-cellular zinc levels. Incubation of microglia in the astrocytic conditioned medium transformed them into the activated "amoeboid" form and induced PAR formation. Administration of an extracellular zinc chelator, CaEDTA, to the astrocytic conditioned medium almost completely prevented the microglial activation. Treatment of astrocytes with an intracellular zinc chelator, TPEN, suppressed the hypoosmotic stress-increased intracellular, but not the extracellular, zinc level, and the increase in the intracellular zinc level was blocked partially by a nitric oxide synthase inhibitor, but not by CaEDTA, indicating that the mechanisms underlying the increases in the intra- and extra-cellular zinc levels might be different.

SIGNIFICANCE

These findings suggest that under hypoosmotic stress-loaded conditions, zinc is released from astrocytes and then plays a primary role in microglial activation as a gliotransmitter.

Glutamine in the pathogenesis of hepatic encephalopathy: the trojan horse hypothesis revisited

Hepatic encephalopathy (HE) is major neuropsychiatric disorder occurring in patients with severe liver disease and ammonia is generally considered to represent the major toxin responsible for this condition. Ammonia in brain is chiefly metabolized ("detoxified") to glutamine in astrocytes due to predominant localization of glutamine synthetase in these cells. While glutamine has long been considered innocuous, a deleterious role more recently has been attributed to this amino acid. This article reviews the mechanisms by which glutamine contributes to the pathogenesis of HE, how glutamine is transported into mitochondria and subsequently hydrolyzed leading to high levels of ammonia, the latter triggering oxidative and nitrative stress, the mitochondrial permeability transition and mitochondrial injury, a sequence of events we have collectively termed as the Trojan horse hypothesis of hepatic encephalopathy.

Acute liver failure: current practice and recent advances

ALF is an important cause of liver-related morbidity and mortality. Advances in the management of ICH and SIRS, and cardiorespiratory, metabolic, and renal support have improved the outlook of such patients. Early transfer to a liver transplant center is essential. Routine use of NAC is recommended for patients with early hepatic encephalopathy, irrespective of the etiology. The role of hypothermia remains to be determined. Liver transplantation plays a critical role, particularly for those with advanced encephalopathy. Several detoxification and BAL support systems have been developed to serve as a bridge to transplantation or to spontaneous recovery. However, such systems lack sufficient reliability and efficacy to be applied routinely in clinical practice. Hepatocyte and stem cell transplantation may provide valuable adjunctive therapy in the future.

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.

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

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

CONCLUSION

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

Role of astrocytes in brain function and disease

Astrocytes assume multiple roles in maintaining an optimally suited milieu for neuronal function. Select astrocytic functions include the maintenance of redox potential, the production of trophic factors, the regulation of neurotransmitter and ion concentrations, and the removal of toxins and debris from the cerebrospinal fluid (CSF). Impairments in these and other functions, as well as physiological reactions of astrocytes to injury, can trigger or exacerbate neuronal dysfunction. This review addresses select metabolic interactions between neurons and astrocytes and emphasizes the role of astrocytes in mediating and amplifying the progression of several neurodegenerative disorders, such as Parkinson's disease (PD), hepatic encephalopathy (HE), hyperammonemia (HA), Alzheimer's disease (AD), and ischemia.

Evidence for oxidative/nitrosative stress in the pathogenesis of hepatic encephalopathy

Hepatic encephalopathy (HE) is a serious complication of liver failure. HE manifests as a series of neuropsychiatric and neuromuscular symptoms including personality changes, sleep abnormalities, asterixis and muscle rigidity progressing through stupor to coma. The pathophysiologic basis of HE remains unclear. There is general agreement that ammonia plays a key role. In recent years, it has been suggested that oxidative/nitrosative stress constitutes part of the pathophysiologic cascade in HE. Direct evidence for oxidative/nitrosative stress in the pathogenesis of HE has been demonstrated in experimental animal models of acute or chronic liver failure. However, evidence from studies in HE patients is limited. This review summarizes this evidence for a role of oxidative/nitrosative stress in relation to ammonia toxicity and to the pathogenesis of HE.

Therapeutic hypothermia: implications for acute care practitioners

The use of therapeutic hypothermia (TH) in acute care medicine has evolved over the past 2 centuries, and its use over the past decade has increased in emergency departments, intensive care units, and operating rooms. Therapeutic hypothermia has several potential clinical applications based on its putative mechanisms of action. It appears to improve oxygen supply to ischemic areas of the brain and decreases intracranial pressure. Mild-to-moderate TH (33 degrees C +/- 1 degrees C) after resuscitation from cardiac arrest is neuroprotective, and also acts on the cardiovascular system with evidence of a decrease in heart rate and increase in systemic vascular resistance. Therapeutic hypothermia decreases cardiac output by 7% for each 1 degrees C decrease in core body temperature, but maintains the stroke volume and the mean arterial pressure. Despite a growing amount of data, this life-saving technique is underutilized in hospitals worldwide. The purpose of this comprehensive review is to show the evolution and the clinical use of TH as it pertains to acute care practitioners.

Brain edema in acute liver failure: inhibition by L-histidine

Brain edema and the associated increase in intracranial pressure are potentially lethal complications of acute liver failure (ALF). Astrocyte swelling (cytotoxic edema) represents a significant component of the brain edema in ALF, and elevated blood and brain ammonia levels have been strongly implicated in its formation. We earlier showed in cultured astrocytes that oxidative stress (OS) and the mitochondrial permeability transition (mPT) play major roles in the mechanism of ammonia-induced astrocyte swelling. Glutamine, a byproduct of ammonia metabolism, has also been shown to induce OS, the mPT, and astrocyte swelling. Such effects of glutamine were suggested to be mediated by its hydrolysis in mitochondria, potentially yielding high levels of ammonia in this organelle and leading to OS and the mPT. L-histidine, an inhibitor of mitochondrial glutamine transport, was recently shown to mitigate OS, mPT, and cell swelling in cultured astrocytes treated with ammonia. The present study examined whether L-histidine similarly abolishes OS, the mPT, and brain edema in a rat model of ALF. Treatment of rats with thioacetamide caused a significant degree of brain edema, which was associated with induction of OS and the mPT. These changes were completely abolished by L-histidine, supporting a key role of mitochondrial glutamine transport and hydrolysis in the mechanism of the brain edema associated with ALF.

Therapeutic hypothermia for acute liver failure

Cerebral edema is a potentially life-threatening complication of acute liver failure, the syndrome of abrupt loss of liver function in a patient with a previously healthy liver. Although the prevalence of cerebral edema appears to be decreasing, patients with rapidly progressive (hyperacute) liver failure, such as after acetaminophen overdose, remain at highest risk. In severe cases of cerebral edema, intracranial hypertension develops and leads to brain death after brainstem herniation or to anoxic brain injury and permanent neurologic impairment. Intracranial hypertension in patients with acute liver failure often can be temporarily controlled by manipulating body position, increasing the degree of sedation, and increasing blood osmolarity through pharmacologic means. However, these maneuvers often postpone, but do not eliminate, the risk of brainstem herniation unless orthotopic liver transplantation or spontaneous liver regeneration follows in short order. To buy time, the induction of therapeutic hypothermia (core temperature 32 degrees C-35 degrees C) has been shown to effectively bridge patients to transplant. Similar to the experience in patients with cerebral edema after other neurologic insults, hypothermia reduces cerebral edema and intracranial hypertension in patients with acute liver failure by decreasing splanchnic ammonia production, restoring normal regulation of cerebral hemodynamics, and lowering oxidative metabolism within the brain. Hypothermia may also ameliorate the degree of liver injury. Hypothermia has not been adequately studied for its safety and theoretically may increase the risk of infection, cardiac dysrhythmias, and bleeding, all complications independently associated with acute liver failure. Therefore, although an ample body of experimental and human data provides a rationale for the use of therapeutic hypothermia in patients with acute liver failure, multicenter, randomized, controlled clinical trials are needed to confirm that hypothermia secures brain viability and improves survival without causing harm.

An overview of animal models for investigating the pathogenesis and therapeutic strategies in acute hepatic failure

Acute hepatic failure (AHF) is a severe liver injury accompanied by hepatic encephalopathy which causes multiorgan failure with an extremely high mortality rate, even if intensive care is provided. Management of severe AHF continues to be one of the most challenging problems in clinical medicine. Liver transplantation has been shown to be the most effective therapy, but the procedure is limited by shortage of donor organs. Although a number of clinical trials testing different liver assist devices are under way, these systems alone have no significant effect on patient survival and are only regarded as a useful approach to bridge patients with AHF to liver transplantation. As a result, reproducible experimental animal models resembling the clinical conditions are still needed. The three main approaches used to create an animal model for AHF are: surgical procedures, toxic liver injury and infective procedures. Most common models are based on surgical techniques (total/partial hepatectomy, complete/transient devascularization) or the use of hepatotoxic drugs (acetaminophen, galactosamine, thioacetamide, and others), and very few satisfactory viral models are available. We have recently developed a viral model of AHF by means of the inoculation of rabbits with the virus of rabbit hemorrhagic disease. This model displays biochemical and histological characteristics, and clinical features that resemble those in human AHF. In the present article an overview is given of the most widely used animal models of AHF, and their main advantages and disadvantages are reviewed.

Therapeutic hypothermia for acute liver failure: toward a randomized, controlled trial in patients with advanced hepatic encephalopathy

Acute liver failure (ALF), the abrupt loss of liver function in a patient without previous liver disease, remains a highly mortal condition. Patients with ALF often succumb to their liver injury after the development of cerebral edema, resulting in intracranial hypertension and brain herniation. While the management of cerebral edema in ALF always includes the administration of osmotically active agents, osmotherapy often reduces intracranial pressure (ICP) insufficiently, such that herniation may be delayed but not prevented. Therapeutic hypothermia, the intentional reduction of body core temperature, has been increasingly used to treat cerebral edema in patients with traumatic and hypoxic brain injury. Data in animal models of ALF also suggest that hypothermia is effective in the prevention and treatment of cerebral edema, and case reports in humans have suggested that hypothermia is an effective bridge to orthotopic liver transplantation. A randomized, controlled trial comparing the management of ALF patients under normothermic and hypothermic conditions is a logical extension of these preliminary observations. Herein, we consider the many difficulties which will be encountered in the design of such a trial in patients with ALF at high risk of developing cerebral edema.

A 1H nuclear magnetic resonance-based metabonomic approach for grading hepatic encephalopathy and monitoring the effects of therapeutic hypothermia in rats

BACKGROUND

There are no good biomarkers for grading hepatic encephalopathy (HE) and monitoring the effectiveness of therapeutic measures.

METHODS

We applied (1)H nuclear magnetic resonance (NMR)-based metabonomics of brain samples obtained from acute liver failure rats sacrificed after ligation of the hepatic artery (at 6 h, precoma and coma stages), sham-operated controls and mild hypothermia (35 degrees C) for 6 or 15 h as a therapeutic measure.

RESULTS

Partial least square discriminant analysis established a classification model that scored the severity of encephalopathy. Animals treated with hypothermia did not develop manifestations of encephalopathy and were graded accordingly using the NMR-based metabonomic approach. Hypothermic animals showed lower levels of alanine and lactate as well as higher levels of N-acetylaspartate and myo-inositol compared with normothermic animals. The course of metabolic deterioration was more rapid in the brainstem than in the cortex.

CONCLUSION

Metabonomic analysis is capable of grading HE, detecting regional differences and monitoring the protective effects of hypothermia. This approach elucidates differences of brain energetic metabolism and compensatory osmotic response to explain the effects of hypothermia.

Simvastatin for rats with thioacetamide-induced liver failure and encephalopathy

BACKGROUND AND AIM

Nitric oxide (NO) inhibition aggravates hepatic damage and encephalopathy and increases mortality in rats with thioacetamide (TAA)-induced acute liver failure. Statins enhance NO synthase expression beyond their lipid-lowering capability, but the impact on encephalopathy remains unexplored. The aim of this study was to assess the effects of simvastatin on rats with TAA-induced acute liver damage and hepatic encephalopathy.

METHODS

Sprague-Dawley rats received TAA (350 mg/kg/day) or normal saline (NS) by intraperitoneal injection for 3 consecutive days. Two days before injections, each group was divided into three subgroups, taking (i) distilled water; (ii) simvastatin (20 mg/kg/day); or (iii) simvastatin plus N(G)-nitro-l-arginine methyl ester (L-NAME, 25 mg/kg/day) by oral gavage for 5 days. On the fifth day, severity of encephalopathy was assessed and plasma levels of alanine aminotransferase (ALT), aspartate aminotransferase (AST), total bilirubin and ammonia were measured.

RESULTS

The TAA subgroups showed higher ALT, AST, bilirubin and ammonia levels and lower motor activity counts as compared with the NS subgroups. Among the TAA-treated subgroups, rats with simvastatin treatment exerted higher motor activity counts and survival rate (P = 0.043), and a trend of lower ALT, AST, bilirubin and ammonia levels than those receiving saline. All rats that underwent simvastatin plus L-NAME treatment died during or after TAA injections.

CONCLUSIONS

Simvastatin improved encephalopathy and survival in TAA-administered rats. The beneficial effect was offset by L-NAME, suggesting the role of NO in liver damage and encephalopathy.

Therapeutic hypothermia: past, present, and future

Cardiac arrest causes devastating neurologic morbidity and mortality. The preservation of the brain function is the final goal of resuscitation. Therapeutic hypothermia (TH) has been considered as an effective method for reducing ischemic injury of the brain. The therapeutic use of hypothermia has been utilized for millennia, and over the last 50 years has been routinely employed in the operating room. TH gained recognition in the past 6 years as a neuroprotective agent in victims of cardiac arrest after two large, randomized, prospective clinical trials demonstrated its benefits in the postresuscitation setting. Extensive research has been done at the cellular and molecular levels and in animal models. There are a number of proposed applications of TH, including traumatic brain injury, acute encephalitis, stroke, neonatal hypoxemia, and near-drowning, among others. Several devices are being designed with the purpose of decreasing temperature at a fast and steady rate, and trying to avoid potential complications. This article reviews the historical development of TH, and its current indications, methods of induction, and potential future.

Cerebral blood flow autoregulation in experimental liver failure

Patients with acute liver failure (ALF) display impairment of cerebral blood flow (CBF) autoregulation, which may contribute to the development of fatal intracranial hypertension, but the pathophysiological mechanism remains unclear. In this study, we examined whether loss of liver mass causes impairment of CBF autoregulation. Four rat models were chosen, each representing different aspects of ALF: galactosamine (GlN) intoxication represented liver necrosis, 90% hepatectomy (PHx90) represented reduction in liver mass, portacaval anastomosis (PCA) represented shunting of blood/toxins into the systemic circulation thus mimicking intrahepatic shunting in ALF, PCA+NH(3) provided information about the additional effects of hyperammonemia Rats were intubated and sedated with pentobarbital. We measured CBF with laser Doppler, intracranial pressure (ICP) was measured in the fossa posterior and registered with a pressure transducer, brain water was measured using the wet-to-dry method, and cerebral glutamine/glutamate was measured enzymatically. The CBF autoregulatory index in both the GlN and PHx90 groups differed significantly from the control group. Conversely, CBF autoregulation was intact in the PCA and PCA+NH(3) groups despite high arterial ammonia, high cerebral glutamine concentration, and increased CBF and ICP. Increased water content of the brainstem or cerebellum was not associated with defective CBF autoregulation. In conclusion, impairment of CBF autoregulation is not caused by brain edema/high ICP. Nor does portacaval shunting or hyperammonemia impair autoregulation. Rather, massive liver necrosis and reduced liver mass are associated with loss of CBF autoregulation.

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.

Downregulation of the 18-kDa translocator protein: effects on the ammonia-induced mitochondrial permeability transition and cell swelling in cultured astrocytes

Hepatic encephalopathy (HE) is a major neurological complication in patients with severe liver disease. While the pathogenesis of HE is unclear, elevated blood and brain ammonia levels are believed to be major etiological factors, and astrocytes appear to be the primary target of its toxicity. A notable feature of ammonia neurotoxicity is an upregulation of the 18-kDa translocator protein (TSPO) (formerly referred to as the peripheral benzodiazepine receptor or PBR), which is found on the outer mitochondrial membrane. However, the precise significance of this upregulation is unclear. To examine its potential role in ammonia-induced astrocyte dysfunction, we downregulated the TSPO using antisense oligonucleotides, and examined whether such downregulation could alter two prominent features of ammonia gliotoxicity, namely, the mitochondrial permeability transition (MPT) and astrocyte swelling. Nontransfected cultures treated with NH4Cl (5 mM; 48 h) showed a significant increase in astrocyte cell volume (37.5%). In cultured astrocytes transfected with TSPO antisense oligonucleotides, such cell swelling was reduced to 17%, but this change was not significantly different from control cell volume. Similarly, nontransfected cultures treated with NH4Cl (5 mM; 24 h) exhibited a 40% decline in the cyclosporin A-sensitive mitochondrial inner membrane potential (DeltaPsi(m)) (P < 0.01) (a measure of the MPT). By contrast, cells transfected with TSPO antisense oligonucleotides did not display a significant loss of the DeltaPsi(m) following ammonia exposure. Our findings highlight the important role of the TSPO in the mechanism of ammonia neurotoxicity.

A surgical model of fulminant hepatic failure in rabbits

AIM

Animal models of fulminant hepatic failure (FHF) have been developed for characterization of disease progression and to evaluate the effectiveness of liver-assist devices, some by treatment with hepatotoxic drugs, viral hepatitis or surgical procedures. We have developed a model in the rabbit by combining resection of the three anterior lobes with ligation of the pedicle of the right lateral lobes, resulting in liver necrosis; the remnant quadrate lobes are left intact.

MATERIALS AND METHODS

Adult male New Zealand white rabbits (n=16) were used. Six animals were killed to measure the weight of the separate liver lobes. The others (n=10) underwent left neck central line placement to monitor continuous blood pressure and collect blood for laboratory analysis, and a burr hole on the right parietal bone to monitor the intracranial pressure (ICP). Blood laboratory analysis, clinical hepatic encephalopathy and ICP levels were measured in FHF animals (n=6). Animals (n=4) undergoing a sham operation served as controls.

RESULTS

All FHF animals died between 12 and 26 h after liver surgery from FHF characterized by a progressive increase in liver enzymes, ammonia, total bilirubin, coagulopathy, hepatic encephalopathy and intracranial hypertension. Histological features of the ischaemic lobes showed coagulative necrosis of hepatocytes with absence of nuclei and collapse of cell plates. Brain histology revealed hypoxic cell damage.

CONCLUSION

We have developed a simple, reproducible model of FHF in rabbits that has a number of features comparable with clinical FHF patients and is well suited for testing experimental bioartificial liver systems and investigating the pathogenesis of FHF.

The anaplerotic flux and ammonia detoxification in hepatic encephalopathy

Metabolic alterations in the brain underly many of the mechanisms leading to acute and chronic Hepatic Encephalopathy (HE). Controversy exists about the role of glutamine accumulation as a causal factor in HE. Glutamine formation contributes to detoxify ammonia, whereby anaplerotic mechanisms in the astrocytes have to be sufficient to replenish Krebs cycle intermediates. The application of ex vivo high-resolution nuclear magnetic resonance (NMR) spectroscopy permits direct measurements of metabolites and different metabolic pathways. Ex vivo (13)C-NMR studies in experimental animal models of acute and chronic HE have provided new insights. In an experimental rat model of ALF, (13)C isotopomer analysis of glucose metabolism showed that alterations of glucose flux through astrocytic pyruvate carboxylase might be linked to the pathogenesis of ALF as a limited anaplerotic flux in the brain, but not in the muscle, correlates with the development of brain edema. Moreover, (13)C-NMR data from a rat model of mild HE demonstrated relative differences in the pathway of glucose through pyruvate carboxylase in thalamus compared to frontal cortex, which might explain the vulnerability of this brain region compared to thalamus. These findings further support that glutamine accumulation might be not the primary cause of neurological symptoms in HE, and show that anaplerotic mechanisms could be essential for ammonia detoxification in HE.

New concepts in the mechanism of ammonia-induced astrocyte swelling

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

Nuclear magnetic resonance studies of energy metabolism and glutamine shunt in hepatic encephalopathy and hyperammonemia

Hepatic encephalopathy (HE) in both acute and chronic liver failure is more likely a reversible functional disease rather than an irreversible pathological lesion of brain cells. Metabolic alterations underlie many of the mechanisms leading to HE. This paper summarizes in vivo and ex vivo (1)H-, (13)C-, and (15)N-nuclear magnetic resonance (NMR) spectroscopy data on patients and experimental models of HE. In vivo NMR spectroscopy provides a unique opportunity to study metabolic changes noninvasively in the brain in vivo, and to quantify various metabolites in localized brain areas, and ex vivo NMR permits the high-resolution measurement of metabolites and the identification of different metabolic pathways. In vivo and ex vivo (1)H-NMR investigations consistently reveal severalfold increases in brain glutamine and concomitant decreases in myo-inositol, an important osmolyte in astrocytes. An osmotic disturbance in these cells has long been suggested to be responsible for astrocyte swelling and brain edema. However, ex vivo (13)C-NMR studies have challenged the convention that glutamine accumulation is the major cause of brain edema in acute HE. They rather indicate a limited anaplerotic flux and capacity of astrocytes to detoxify ammonia by glutamine synthesis and emphasize distortions of energy and neurotransmitter metabolism. However, recent (15)N-NMR investigations have demonstrated that glutamine fluxes between neurons and astrocytes are affected by ammonia. Further NMR studies may provide novel insights into the relationship between brain edema and/or astrocyte pathology and changes in inter- and intracellular glutamine homeostasis, which may secondarily alter brain energy metabolism.

Pathogenetic interplay between osmotic and oxidative stress: the hepatic encephalopathy paradigm

Hepatic encephalopathy (HE) defines a primary gliopathy associated with acute and chronic liver disease. Astrocyte swelling triggered by ammonia in synergism with different precipitating factors, including hyponatremia, tumor necrosis factor (TNF)-alpha, glutamate and ligands of the peripheral benzodiazepine receptor (PBR), is an early pathogenetic event in HE. On the other hand, reactive nitrogen and oxygen species (RNOS) including nitric oxide are considered to play a major role in HE. There is growing evidence that osmotic and oxidative stresses are closely interrelated. Astrocyte swelling produces RNOS and vice versa. Based on recent investigations, this review proposes a working model that integrates the pathogenetic action of osmotic and oxidative stresses in HE. Under participation of the N-methyl-D-aspartate (NMDA) receptor, Ca(2+), the PBR and organic osmolyte depletion, astrocyte swelling and RNOS production may constitute an autoamplificatory signaling loop that integrates at least some of the signals released by HE-precipitating factors.

Glutamine: a Trojan horse in ammonia neurotoxicity

Mechanisms involved in hepatic encephalopathy still remain to be defined. Nonetheless, it is well recognized that ammonia is a major factor in its pathogenesis, and that the astrocyte represents a major target of its CNS toxicity. In vivo and in vitro studies have shown that ammonia evokes oxidative/nitrosative stress, mitochondrial abnormalities (the mitochondrial permeability transition, MPT) and astrocyte swelling, a major component of the brain edema associated with fulminant hepatic failure. How ammonia brings about these changes in astrocytes is not well understood. It has long been accepted that the conversion of glutamate to glutamine, catalyzed by glutamine synthetase, a cytoplasmic enzyme largely localized to astrocytes in brain, represented the principal means of cerebral ammonia detoxification. Yet, the "benign" aspect of glutamine synthesis has been questioned. This article highlights evidence that, at elevated levels, glutamine is indeed a noxious agent. We also propose a mechanism by which glutamine executes its toxic effects in astrocytes, the "Trojan horse" hypothesis. Much of the newly synthesized glutamine is subsequently metabolized in mitochondria by phosphate-activated glutaminase, yielding glutamate and ammonia. In this manner, glutamine (the Trojan horse) is transported in excess from the cytoplasm to mitochondria serving as a carrier of ammonia. We propose that it is the glutamine-derived ammonia within mitochondria that interferes with mitochondrial function giving rise to excessive production of free radicals and induction of the MPT, two phenomena known to bring about astrocyte dysfunction, including cell swelling. Future therapeutic approaches might include controlling excessive transport of newly synthesized glutamine to mitochondria and its subsequent hydrolysis.

Relation of plasma thyroid-stimulating hormone levels to vascular lesions and atrophy of the brain in the elderly

CONTEXT

Recent studies suggest that thyroid disease is associated with cardiovascular and peripheral vascular disease. However, little is known about the underlying pathophysiology and the relation with cerebrovascular disease or brain atrophy in the elderly.

OBJECTIVE

To determine if plasma thyroid-stimulating hormone (TSH) levels are associated with vascular brain changes and cortical atrophy in the elderly.

DESIGN

Community based, cross-sectional study.

PARTICIPANTS AND SETTING

268 participants of the Memory and Morbidity in Augsburg Elderly Study, 65-83 years of age and without contraindications for magnetic resonance imaging (MRI) of the brain.

MAIN OUTCOME MEASURES

The presence of brain lesions and atrophy was determined using a standardized MRI protocol, an established rating scale and a single rater. Plasma TSH levels were assessed using standard laboratory methods. The association between plasma TSH levels and MRI findings was analyzed using logistic and linear regression models.

RESULTS

Higher TSH levels within the normal clinical range were significantly associated with severer cortical atrophy (p = 0.04) and a higher proportion of infarct-like vascular lesions (p = 0.005) in men. These associations were independent of potential confounders, including thyroid hormone therapy, in multivariable regression analysis. No association between plasma TSH levels and both MRI outcomes were observed in women. In addition, neither in men nor in women was an association between TSH levels and white-matter lesions found.

CONCLUSIONS

Increasing TSH levels even within the normal clinical range are associated with severer brain atrophy and infarct-like vascular lesions in elderly men.

Therapeutic hypothermia

Therapeutic hypothermia has been used for millennia, but in recent years was not in much clinical use due to an apparent high risk of complications. More recently, the benefits of induced therapeutic hypothermia have been rediscovered, mainly with the improvement in neurological outcome in out-of-hospital cardiac arrest victims. In addition, therapeutic hypothermia has been suggested to improve outcome in other neurological conditions such as traumatic brain injury, neonatal asphyxia, cerebrovascular accidents and intracranial hypertension. This article reviews the history of the discovery of therapeutic hypothermia, as well as the current therapeutic applications and ways to deliver this treatment. Cooling techniques and recovery processes, as well as potential complications are also reviewed. Clinicians caring for a wide variety of critically ill patients should be familiar with the use of therapeutic hypothermia.

Effect of ammonia on astrocytic glutamate uptake/release mechanisms

Hyperammonemic disorders such as acute liver failure (ALF) or urea cycle enzymopathies are associated with hyperexcitability, seizures, brain edema and increased extracellular brain glutamate. Mechanisms responsible for increased glutamate content in the extracellular space of the brain include decreased uptake by perineuronal astrocytes and/or increased release from neurons and/or astrocytes. Exposure of astrocytes to millimolar concentrations of ammonia results in cell swelling, loss of expression of the glutamate transporters excitatory amino acid transporter (EAAT-1) and EAAT-2 and increased release of glutamate. Three distinct mechanisms are theoretically possible to explain ammonia-induced glutamate release from astrocytes namely: release due to swelling; reversal of glutamate transporters and due to Ca2+-dependent vesicular release. Recent identification of vesicular docking and fusion proteins in astrocytes together with glutamate-release (due to intracellular alkanization and mobilization of intracellular Ca2+-stores) studies implies that vesicular release is a predominant mechanism responsible for ammonia-induced release of glutamate from astrocytes.

Glutamine in the mechanism of ammonia-induced astrocyte swelling

Brain edema and the subsequent increase in intracranial pressure are the major neurological complications in fulminant hepatic failure (FHF). Brain edema in FHF is predominantly "cytotoxic" due principally to astrocyte swelling. It is generally believed that ammonia plays a key role in this process, although the mechanism by which ammonia brings about such swelling is yet to be defined. It has been postulated that glutamine accumulation in astrocytes subsequent to ammonia detoxification results in increased osmotic forces leading to cell swelling. While the hypothesis is plausible and has gained support, it has never been critically tested. In this study, we examined whether a correlation exists between cellular glutamine levels and the degree of cell swelling in cultured astrocytes exposed to ammonia. Cultured astrocytes derived from rat brain cortices were exposed to ammonia (5 mM) for different time periods and cell swelling was measured. Cultures treated with ammonia for 1-3 days showed a progressive increase in astrocyte cell volume (59-127%). Parallel treatment of astrocyte cultures with ammonia showed a significant increase in cellular glutamine content (60-80%) only at 1-4 h, a time when swelling was absent, while glutamine levels were normal at 1-3 days, a time when peak cell swelling was observed. Thus no direct correlation between cell swelling and glutamine levels was detected. Additionally, acute increase in intracellular levels of glutamine by treatment with the glutaminase inhibitor 6-diazo-5-oxo-L-norleucine (DON) after ammonia exposure also did not result in swelling. On the contrary, DON treatment significantly blocked (66%) ammonia-induced astrocyte swelling at a later time point (24 h), suggesting that some process resulting from glutamine metabolism is responsible for astrocyte swelling. Additionally, ammonia-induced free radical production and induction of the mitochondrial permeability transition (MPT) were significantly blocked by treatment with DON, suggesting a key role of glutamine in the ammonia-induced free radical generation and the MPT. In summary, our findings indicate a lack of direct correlation between the extent of cell swelling and cellular levels of glutamine. While glutamine may not be acting as an osmolyte, we propose that glutamine-mediated oxidative stress and/or the MPT may be responsible for the astrocyte swelling by ammonia.

Persistent arterial hyperammonemia increases the concentration of glutamine and alanine in the brain and correlates with intracranial pressure in patients with fulminant hepatic failure

In this prospective study of patients with fulminant hepatic failure (FHF), we tested the hypothesis that arterial hyperammonemia results in cerebral accumulation of the osmotic active amino acids glutamine and alanine, processes that were expected to correlate with intracranial pressure (ICP). By using in vivo brain microdialysis technique together with ICP monitoring in 17 FHF patients (10 females/7 males; median age 49 (range 18 to 66) years), we found that arterial ammonia concentration correlated to brain content of glutamine (r=0.47; P<0.05) but not to alanine. A persisting high arterial ammonia concentration (above 200 micromol/L) characterized patients who developed high ICP (n=8) while patients who did not experience surges of increased ICP (n=9) had a decline in the ammonia level (P<0.05). Moreover, brain glutamine and alanine concentrations were higher at baseline and increased further in patients who developed intracranial hypertension compared with patients who experienced no surges of high ICP. Brain glutamine concentration increased 32% from baseline to 6536 (697 to 9712) micromol/L (P<0.05), and alanine 44% from baseline to 104 (81 to 381) micromol/L (P<0.05). Brain concentration of glutamine (r=0.59, P<0.05), but not alanine, correlated to ICP. Also arterial ammonia concentration correlated to ICP (r=0.73, P<0.01). To conclude, this study shows that persistence of arterial hyperammonemia is associated with profound changes in the cerebral concentration of glutamine and alanine. The elevation of brain glutamine concentration correlated to ICP in patients with FHF.

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

Encephalopathy, brain edema and intracranial hypertension are neurological complications responsible for substantial morbidity/mortality in patients with acute liver failure (ALF), where, aside from liver transplantation, there is currently a paucity of effective therapies. Mirroring its cerebro-protective effects in other clinical conditions, the induction of mild hypothermia may provide a potential therapeutic approach to the management of ALF. A solid mechanistic rationale for the use of mild hypothermia is provided by clinical and experimental studies showing its beneficial effects in relation to many of the key factors that determine the development of brain edema and intracranial hypertension in ALF, namely the delivery of ammonia to the brain, the disturbances of brain organic osmolytes and brain extracellular amino acids, cerebro-vascular haemodynamics, brain glucose metabolism, inflammation, subclinical seizure activity and alterations of gene expression. Initial uncontrolled clinical studies of mild hypothermia in patients with ALF suggest that it is an effective, feasible and safe approach. Randomized controlled clinical trials are now needed to adequately assess its efficacy, safety, clinical impact on global outcomes and to provide the guidelines for its use in ALF.

Protein tyrosine nitration in hyperammonemia and hepatic encephalopathy

Hepatic encephalopathy is seen as a clinical manifestation of a chronic low grade cerebral edema, which is thought to trigger disturbances of astrocyte function, glioneuronal communication, and finally HE symptoms. In cultured astrocytes, hypoosmotic swelling triggers a rapid oxidative stress response, which involves the action of NADPH oxidase isoenzymes, followed by tyrosine nitration of distinct astrocytic proteins. Oxidative stress and protein tyrosine nitration (PTN) are also observed in response to ammonia, inflammatory cytokines, such as TNF-alpha or interferons, and benzodiazepines with affinity to the peripheral benzodiazepine receptor (PBR). NMDA receptor activation was identified as upstream event in protein tyrosine nitration (PTN). Cerebral PTN is also found in vivo after administration of ammonia, benzodiazepines or lipopolysaccharide and in portocaval shunted rats. PTN predominantly affects astrocytes surrounding cerebral vessels with potential impact on blood-brain-barrier permeability. Among the tyrosine-nitrated proteins, glutamine synthetase, GAPDH, extracellular signal-regulated kinase and the PBR were identified. PTN of glutamine synthetase is associated with inactivation of the enzyme. Thus, factors known to trigger hepatic encephalopathy induce oxidative/nitrosative stress on astrocytes with protein modifications through PTN. The pathobiochemical relevance of astrocytic PTN for the development of HE symptoms remains to be established.