Roger F. Butterworth

Pathophysiology of hepatic encephalopathy: a new look at ammonia.

Results of neuropathologic, spectroscopic, and neurochemical studies continue to confirm a major role for ammonia in the pathogenesis of the central nervous system complications of both acute and chronic liver failure. Damage to astrocytes characterized by cell swelling (acute liver failure) or Alzheimer Type II astrocytosis (chronic liver failure) can be readily reproduced by acute or chronic exposure of these cells in vitro to pathophysiologically relevant concentrations of ammonia. Furthermore, exposure of the brain or cultured astrocytes to ammonia results in similar alterations in expression of genes coding for key astrocytic proteins. Such proteins include the structural glial fibrillary acidic protein, glutamate transporters, and peripheral-type (mitochondrial) benzodiazepine receptors. Brain–blood ammonia concentration ratios (normally of the order of 2) are increased up to fourfold in liver failure and arterial blood ammonia concentrations are good predictors of cerebral herniation in patients with acute liver failure. Studies using 1H magnetic resonance spectroscopy in patients with chronic liver failure reveal a positive correlation between the severity of neuropsychiatric symptoms and brain concentrations of the brain ammonia-detoxification product glutamine. Increased intracellular glutamine may be a contributory cause of brain edema in hyperammonemia. Positron emission tomography studies using 13HN3 provide evidence of increased blood–brain ammonia transfer and brain ammonia utilization rates in patients with chronic liver failure. In addition to the use of nonabsorbable disaccharides and antibiotics to reduce gut ammonia production, new approaches to the treatment of hepatic encephalopathy by lowering of brain ammonia include the use of L-ornithine–L-aspartate and mild hypothermia.

Manganese deposition in basal ganglia structures results from both portal-systemic shunting and liver dysfunction

BACKGROUND & AIMS Manganese (Mn) deposition could be responsible for the T(1)-weighted magnetic resonance signal hyperintensities observed in cirrhotic patients. These experiments were designed to assess the regional specificity of the Mn increases as well as their relationship to portal-systemic shunting or hepatobiliary dysfunction. METHODS Mn concentrations were measured in (1) brain samples from basal ganglia structures (pallidum, putamen, caudate nucleus) and cerebral cortical structures (frontal, occipital cortex) obtained at autopsy from 12 cirrhotic patients who died in hepatic coma and from 12 matched controls; and from (2) brain samples (caudate/putamen, globus pallidus, frontal cortex) from groups (n = 8) of rats either with end-to-side portacaval anastomosis, with biliary cirrhosis, or with fulminant hepatic failure as well as from sham-operated and normal rats. RESULTS Mn content was significantly increased in frontal cortex (by 38%), occipital cortex (by 55%), pallidum (by 186%), putamen (by 66%), and caudate (by 54%) of cirrhotic patients compared with controls. Brain Mn content did not correlate with patient age, etiology of cirrhosis, or history of chronic hepatic encephalopathy. In cirrhotic and portacaval-shunted rats, Mn content was increased in pallidum (by 27% and 57%, respectively) and in caudate/putamen (by 57% and 67%, respectively) compared with control groups. Mn concentration in pallidum was significantly higher in portacaval-shunted rats than in cirrhotic rats. No significant changes in brain Mn concentrations were observed in rats with acute liver failure. CONCLUSIONS These findings suggest that brain Mn deposition results both from portal-systemic shunting and from liver dysfunction.

Amino Acid Changes in Autopsied Brain Tissue from Cirrhotic Patients with Hepatic Encephalopathy

Abstract: Brain tissue was obtained at autopsy from nine cirrhotic patients dying in hepatic coma and from an equal number of controls, free from neurological, psychiatric, or hepatic diseases, matched for age and time interval from death to freezing of dissected brain samples. Glutamine, glutamate, aspartame, and γ‐aminobutyric acid (GABA) levels were measured in homogenates of cerebral cortex (prefrontal and frontal), caudate nuclei, hypothalamus, cerebellum (cortex and vermis), and medulla oblongata as their O‐phthalaldehyde derivatives by HPLC using fluorescence detection. Glutamine concentrations were found to be elevated two‐ to fourfold in all brain structures, the largest increases being observed in prefrontal cortex and medulla oblongata. Glutamate levels were selectively decreased in prefrontal cortex (by 20%), caudate nuclei (by 27%), and cerebellar vermis (by 17%) from cirrhotic patients. On the other hand, GABA content of autopsied brain tissue from these patients was found to be within normal limits in all brain structures. It is suggested that such region‐selective reductions of glutamate may reflect loss of the amino acid from the releasable (neurotransmitter) pool. These findings may be of significance in the pathogenesis of hepatic encephalopathy resulting from chronic liver disease. Key Words: Hepatic encephalopathy—Hyperammonemia— Cerebral amino acids—Glutamine—Glutamate—γ‐Aminobutyric acid. Lavoie J. et al. Amino acid changes in autopsied brain tissue from cirrhotic patients with hepatic encephalopathy.

Decreased glutamate transporter (GLT-1) expression in frontal cortex of rats with acute liver failure

It has been suggested that reduced astrocytic uptake of neuronally released glutamate contributes to the pathogenesis of hepatic encephalopathy in acute liver failure. In order to further address this issue, the recently cloned and sequenced astrocytic glutamate transporter GLT-1 was studied in brain preparations from rats with ischemic liver failure induced by portacaval anastomosis followed 24 h later by hepatic artery ligation and from appropriate sham-operated controls. GLT-1 expression was studied using reverse transcriptase-polymerase chain reaction (RT-PCR). Expression of GLT-1 transcript was significantly decreased in frontal cortex at coma stages of acute liver failure. Western blotting using a polyclonal antibody to GLT-1 revealed a concomitant decrease in expression of transporter protein in the brains of rats with acute liver failure. Reduced capacity of astrocytes to reuptake neuronally released glutamate, resulting from a GLT-1 transporter deficit and the consequently compromised neuron-astrocytic trafficking of glutamate could contribute to the pathogenesis of hepatic encephalopathy and brain edema, two major complications of acute liver failure.

Review article: the design of clinical trials in hepatic encephalopathy - an International Society for Hepatic Encephalopathy and Nitrogen Metabolism (ISHEN) consensus statement

Aliment Pharmacol Ther 2011; 33: 739–747

Complications of cirrhosis III. Hepatic encephalopathy

Hepatic encephalopathy (HE) is a major neuropsychiatric complication of cirrhosis. HE develops slowly in cirrhotic patients, starting with altered sleep patterns and eventually progressing through asterixis to stupor and coma. Precipitating factors are common and include an oral protein load, gastrointestinal bleeding and the use of sedatives. HE is common following transjugular intrahepatic portosystemic stent shunts (TIPS). Neuropathologically, HE in cirrhotic patients is characterized by astrocytic (rather than neuronal) changes known as Alzheimer type II astrocytosis and in altered expression of key astrocytic proteins. Magnetic resonance imaging in cirrhotic patients reveals bilateral signal hyperintensities particularly in globus pallidus on T1-weighted imaging, a phenomenon which may result from manganese deposition. Proton (1H) magnetic resonance spectroscopy shows increases in the glutamine resonance in brain, a finding which confirms previous biochemical studies and results no doubt from increased brain ammonia removal (glutamine synthesis). Additional evidence for increased brain ammonia uptake and removal in cirrhotic patients is provided by studies using positron emission tomography and 13NH3. Recent molecular biological studies demonstrate increased expression of genes coding for neurotransmitter-related proteins in chronic liver failure. Such genes include monoamine oxidase (MAO-A isoform), the peripheral-type benzodiazepine receptor and nitric oxide synthase (nNOS isoform). Activation of these systems has the potential to lead to alterations of monoamine and amino acid neurotransmitter function as well as modified cerebral perfusion in chronic liver failure. Prevention and treatment of HE in cirrhotic patients continues to rely on ammonia-lowering strategies which include assessment of dietary protein intake and the use of lactulose, neomycin, sodium benzoate and L-ornithine-aspartate. The benzodiazepine receptor antagonist flumazenil may be effective in certain cases. A more widespread use of central nervous system-acting drugs awaits a more complete understanding of the precise neurotransmitter systems involved in the pathogenesis of HE in chronic liver failure.

Experimental models of hepatic encephalopathy: ISHEN guidelines.

Objectives of the International Society for Hepatic Encephalopathy and Nitrogen Metabolism Commission were to identify well‐characterized animal models of hepatic encephalopathy (HE) and to highlight areas of animal modelling of the disorder that are in need of development. Features essential to HE modelling were identified. The best‐characterized animal models of HE in acute liver failure, the so‐called Type A HE, were found to be the hepatic devascularized rat and the rat with thioacetamide‐induced toxic liver injury. In case of chronic liver failure, surgical models in the rat involving end‐to‐side portacaval anastomosis or bile duct ligation were considered to best model minimal/mild (Type B) HE. Unfortunately, at this time, there are no satisfactory animal models of Type C HE resulting from end‐stage alcoholic liver disease or viral hepatitis, the most common aetiologies encountered in patients. The commission highlighted the urgent need for such models and of improved models of HE in chronic liver failure in general as well as a need for models of post‐transplant neuropsychiatric disorders. Studies of HE pathophysiology at the cellular and molecular level continue to benefit from in vitro and or ex vivo models involving brain slices or exposure of cultured cells (principally cultured astrocytes) to toxins such as ammonia, manganese and pro‐inflammatory cytokines. More attention could be paid in the future to in vitro models involving the neurovascular unit, microglia and neuronal co‐cultures in relation to HE pathogenesis.

Effect of ammonium ions on synaptic transmission in the mammalian central nervous system.

It is not surprising that a compound with such unique properties as NH3/NH4+, should have a large variety of biochemical and neurological effects and to find itself implicated in many pathological conditions. Its undissociated (NH3) or dissociated (NH4+) forms, having different physicochemical properties, enter neurons and other cells through differing pathways. These two forms then change internal pH in opposite directions, and initiate a variety of regulatory processes that attempt to overcome these pH changes. In addition, ammonia has a central role in normal intermediary metabolism, and when present in excess, it can disturb reversible reactions in which it participates. The challenge in interpreting these various observations lies in the difficulty in assigning to them a role in the generation of symptoms seen in experimental and clinical hyperammonemias. In this review we have attempted to summarize information available on the effects of ammonium ions on synaptic transmission, a central process in nervous system function. Evidence has been presented to show that ammonium ions, in pathologically relevant concentrations, interfere with glutamatergic excitatory transmission, not by decreasing the release of glutamate, but by preventing its action on post-synaptic AMPA receptors. Furthermore, NH4+ depolarizes neurons to a variable degree, without consistently changing membrane resistance, probably by reducing [K+]i. A decrease in EK+ may also be responsible for decreasing the effectiveness of the outward chloride pump, thus explaining the well known inhibitory effect of NH4+ on the hyperpolarizing IPSP. There is a consensus of opinion that chronic hyperammonemia increases 5HT turnover and this may be responsible for altered sleep patterns seen in hepatic encephalopathy. There does not seem to be a consistent effect on catecholaminergic transmission in hyperammonemias. However, chronic hyperammonemia causes pathological changes in perineuronal astrocytes, which may lead to a reduced uptake of released glutamate and a decreased detoxification of ammonia by the brain. Chronic moderate increase in extracellular glutamate results in a down-regulation of NMDA receptors, while the decreased detoxification of ammonia makes the central nervous system more vulnerable to a sudden hyperammonemia, due, for instance, to an increased dietary intake of proteins or to gastrointestinal bleeding in patients with liver disease. Clearly, data summarized in this review represent only the beginning in the elucidation of the mechanism of ammonia neurotoxicity. It should help, we hope, to direct future investigations towards some of the questions that need to be answered.

Thiamine-dependent enzyme changes in temporal cortex of patients with Alzheimer's disease.

Activites of thiamine-dependent enzymes [pyruvate dehydrogenase (PDHC), α-ketoglutarate dehydrogenase (αKGDH), and transketolase (TK)] were measured in autopsied samples of temporal cortex from six patients with Alzheimers disease and from eight age-matched control subjects who were free from neurological or psychiatric diseases. Times from death to freezing of dissected material at −70°C were matched. Significant decreases in PDHC (decreased by 70%;P<0.01), αKGDH (decreased by 70%; p<0.01), and TK (decreased by 52%;P<0.01) were observed in brain tissue from patients with Alzheimers disease. In contrast, activities of glutamate dehydrogenase were within normal limits. These findings suggest a possible role for alterations of brain thiamine metabolism or utilization in Alzheimers disease

The nutritional management of hepatic encephalopathy in patients with cirrhosis: International society for hepatic encephalopathy and nitrogen metabolism consensus

Nitrogen metabolism plays a major role in the development of hepatic encephalopathy (HE) in patients with cirrhosis. Modulation of this relationship is key to the management of HE, but is not the only nutritional issue that needs to be addressed. The assessment of nutritional status in patients with cirrhosis is problematic. In addition, there are significant sex‐related differences in body composition and in the characteristics of tissue loss, which limit the usefulness of techniques based on measures of muscle mass and function in women. Techniques that combine subjective and objective variables provide reasonably accurate information and are recommended. Energy and nitrogen requirements in patients with HE are unlikely to differ substantially from those recommended in patients with cirrhosis per se viz. 35‐45 kcal/g and 1.2‐1.5g/kg protein daily. Small meals evenly distributed throughout the day and a late‐night snack of complex carbohydrates will help minimize protein utilization. Compliance is, however, likely to be a problem. Diets rich in vegetables and dairy protein may be beneficial and are therefore recommended, but tolerance varies considerably in relation to the nature of the staple diet. Branched chain amino acid supplements may be of value in the occasional patient intolerant of dietary protein. Increasing dietary fiber may be of value, but the utility of probiotics is, as yet, unclear. Short‐term multivitamin supplementation should be considered in patients admitted with decompensated cirrhosis. Hyponatremia may worsen HE; it should be prevented as far as possible and should always be corrected slowly. Conclusion: Effective management of these patients requires an integrated multidimensional approach. However, further research is needed to fill the gaps in the current evidence base to optimize the nutritional management of patients with cirrhosis and HE. (Hepatology 2013)