Functional imaging of the brain in patients with liver cirrhosis

Altered brain iron deposition in patients with minimal hepatic encephalopathy: an MRI quantitative susceptibility mapping study

AIM

To explore the use of quantitative susceptibility mapping (QSM) in assessing changes in brain iron deposits and their association with cognitive function in patients with minimal hepatic encephalopathy (MHE).

MATERIALS AND METHODS

The study cohort comprised 27 cases with hepatitis B-associated cirrhosis with MHE (MHE group), 25 with hepatitis B-associated cirrhosis without MHE (NMHE group), and 25 healthy controls (HC group). Iron deposits in the bilateral frontal white matter, caudate nucleus (CN), putamen, globus pallidus, thalamus, red nucleus, substantia nigra (SN), hippocampus, and dentate nucleus were measured by QSM. The associations between iron deposition with the time taken to complete number connection tests A (NCT-A) and the score on digital-symbol test (DST) were analysed.

RESULTS

Susceptibility values differed significantly in the bilateral CN, left thalamus, right SN, and left hippocampus in the MHE group compared with the other groups and were positively associated with the times taken to complete the NCT-A in the bilateral CN, left thalamus, and right SN and negatively associated with DST scores in the bilateral CN, left TH, and left HP.

CONCLUSION

Reduced cognitive function in MHE patients was significantly associated with abnormally increased iron deposition in certain brain areas. The quantification of brain iron deposition by QSM may thus be an objective and accurate means of evaluating MHE.

[WITHDRAWN] Serum biomarkers of liver fibrosis identify changes in striatal metabolite levels

The full text of this preprint has been withdrawn by the authors due to author disagreement with the posting of the preprint. Therefore, the authors do not wish this work to be cited as a reference. Questions should be directed to the corresponding author.

Changes in dynamic and static brain fluctuation distinguish minimal hepatic encephalopathy and cirrhosis patients and predict the severity of liver damage

Purpose

Minimal hepatic encephalopathy (MHE) is characterized by mild neuropsychological and neurophysiological alterations that are not detectable by routine clinical examination. Abnormal brain activity (in terms of the amplitude of low-frequency fluctuation (ALFF) has been observed in MHE patients. However, little is known concerning temporal dynamics of intrinsic brain activity. The present study aimed to investigate the abnormal dynamics of brain activity (dynamic ALFF; dALFF) and static measures [static ALFF; (sALFF)] in MHE patients and to strive for a reliable imaging neuromarkers for distinguishing MHE patients from cirrhosis patients. In addition, the present study also investigated whether intrinsic brain activity predicted the severity of liver damage.

Methods

Thirty-four cirrhosis patients with MHE, 28 cirrhosis patients without MHE, and 33 age-, sex-, and education-matched healthy controls (HCs) underwent resting-state magnetic resonance imaging (rs-fMRI). dALFF was estimated by combining the ALFF method with the sliding-window method, in which temporal variability was quantized over the whole-scan timepoints and then compared among the three groups. Additionally, dALFF, sALFF and both two features were utilized as classification features in a support vector machine (SVM) to distinguish MHE patients from cirrhosis patients. The severity of liver damage was reflected by the Child–Pugh score. dALFF, sALFF and both two features were used to predict Child–Pugh scores in MHE patients using a general linear model.

Results

Compared with HCs, MHE patients showed significantly increased dALFF in the left inferior occipital gyrus, right middle occipital gyrus, and right insula; increased dALFF was also observed in the right posterior lobe of the cerebellum (CPL) and right thalamus. Compared with HCs, noMHE patients exhibited decreased dALFF in the right precuneus. In contrast, compared with noMHE patients, MHE patients showed increased dALFF in the right precuneus, right superior frontal gyrus, and right superior occipital gyrus. Furthermore, the increased dALFF values in the left precuneus were positively associated with poor digit-symbol test (DST) scores (r = 0.356, p = 0.038); however, dALFF in the right inferior temporal gyrus (ITG) was negatively associated with the number connection test–A (NCT-A) scores (r = -0.784, p = 0.000). A significant positive correlation was found between dALFF in the left inferior occipital gyrus (IOG) and high blood ammonia levels (r = 0.424, p = 0.012). Notably, dALFF values yielded a higher classification accuracy than sALFF values in distinguishing MHE patients from cirrhosis patients. Importantly, the dALFF values predicted the Child–Pugh score (r = 0.140, p = 0.030), whereas sALFF values did not in the current dataset. Combining two features had high accuracy in classification in distinguishing MHE patients from cirrhotic patients and yielded prediction in the severity of liver damage.

Conclusion

These findings suggest that combining dALFF and sALFF features is a useful neuromarkers for distinguishing MHE patients from cirrhosis patients and highlights the important role of dALFF feature in predicting the severity of liver damage in MHE.

Serum biomarkers of liver fibrosis identify globus pallidus vulnerability

The CNS manifestation of chronic liver disease can include magnetic resonance (MR) signal hyperintensities in basal ganglia structures. Here, relations between liver (serum-derived fibrosis scores) and brain (regional T1-weighted signal intensities and volumes) integrity were evaluated in a sample of 457 individuals including those with alcohol use disorders (AUD), people living with human immunodeficiency virus (HIV), those comorbid for AUD and HIV, and healthy controls. Liver fibrosis was identified from cutoff scores as follows: aspartate aminotransferase to platelet ratio index (APRI) > 0.7 in 9.4% (n = 43) of the cohort; fibrosis score (FIB4) > 1.5 in 28.0% (n = 128) of the cohort; and non-alcoholic fatty liver disease fibrosis score (NFS) > -1.4 in 30.2% (n = 138) of the cohort. Presence of serum-derived liver fibrosis was associated with high signal intensities selective to basal ganglia (i.e., caudate, putamen, and pallidum) structures. High signal intensities in the pallidum, however, explained a significant portion of the variance in APRI (25.0%) and FIB4 (23.6%) cutoff scores. Further, among the regions evaluated, only the globus pallidus showed a correlation between greater signal intensity and smaller volume (r = -0.44, p <.0001). Finally, higher pallidal signal intensity correlated worse ataxia (eyes open ρ = -0.23, p =.0002; eyes closed ρ = -0.21, p =.0005). This study suggests that clinically relevant serum biomarkers of liver fibrosis such as the APRI may identify individuals vulnerable to globus pallidus pathology and contribute to problems with postural balance.

Development of a New Index to Distinguish Hepatic Encephalopathy through Automated Quantification of Globus Pallidal Signal Intensity Using MRI

Hyperintensities within the bilateral globus pallidus on T1-weighted magnetic resonance images were present in some liver cirrhosis patients with hepatic encephalopathy. The symptoms of covert hepatic encephalopathy are similar to those of mild dementia. We aimed to develop a new diagnostic index in which to distinguish hepatic encephalopathy from dementia. The globus pallidus signal hyperintensity was quantified using three-dimensional images. In addition, the new index value distribution was evaluated in a cohort of dementia patients. Signal intensity of globus pallidus significantly increased in liver cirrhosis patients with hepatic encephalopathy compared to those without hepatic encephalopathy (p < 0.05), healthy subjects (p < 0.05) or dementia patients (p < 0.001). Only 12.5% of liver cirrhosis patients without hepatic encephalopathy and 2% of dementia patients exceeded the new index cut-off value of 0.994, which predicts hepatic encephalopathy. One dementia patient in our evaluation had a history of liver cancer treatment and was assumed to have concomitant hepatic encephalopathy. The automatic assessment of signal intensity in globus pallidus is useful for distinguishing liver cirrhosis patients with hepatic encephalopathy from healthy subjects and liver cirrhosis patients without hepatic encephalopathy. Our image analyses exclude possible cases of hepatic encephalopathy from patients with neurocognitive impairment, including dementia.

Regional Spontaneous Neural Activity Alterations in Type 2 Diabetes Mellitus: A Meta-Analysis of Resting-State Functional MRI Studies

Objective: Resting-state functional magnetic resonance imaging (rs-fMRI) studies have revealed inconsistent regional spontaneous neural activity alterations in patients with type 2 diabetes mellitus (T2DM). The aim of our meta-analysis was to identify concordant regional spontaneous neural activity abnormalities in patients with T2DM. Methods: A systematic search was conducted to identify voxel-based rs-fMRI studies comparing T2DM patients with healthy controls. The permutation of subject images seed-based d mapping (SDM) was used to quantitatively estimate the regional spontaneous neural activity abnormalities in patients with T2DM. Metaregression was conducted to examine the associations between clinical characteristics and functional alterations. Results: A total of 16 studies with 19 datasets including 434 patients with T2DM and 391 healthy controls were included. Patients with T2DM showed hypoactivity in the right medial superior frontal gyrus, right superior temporal gyrus, and left lingual gyrus, whereas hyperactivity in the right cerebellum. Metaregression analysis identified negative correlation between regional activity in the medial superior frontal and anterior cingulate gyri and illness duration of patients with T2DM. Conclusion: The patterns of regional spontaneous neural activity alterations, characterized by hypoactivity in the medial pre-frontal cortex, visual cortex, and superior temporal gyrus, whereas hyperactivity in the cerebellum, might represent the underlying neuropathological mechanisms of T2DM.

Evaluation of [13N]ammonia positron emission tomography as a potential method for quantifying glutamine synthetase activity in the human brain

PURPOSE

The conversion of synaptic glutamate to glutamine in astrocytes by glutamine synthetase (GS) is critical to maintaining healthy brain activity and may be disrupted in several brain disorders. As the GS catalysed conversion of glutamate to glutamine requires ammonia, we evaluated whether [13N]ammonia positron emission tomography (PET) could reliability quantify GS activity in humans.

METHODS

In this test-retest study, eight healthy volunteers each received two dynamic [13N]ammonia PET scans on the morning and afternoon of the same day. Each [13N]ammonia scan was preceded by a [15O]water PET scan to account for effects of cerebral blood flow (CBF).

RESULTS

Concentrations of radioactive metabolites in arterial blood were available for both sessions in five of the eight subjects. Our results demonstrated that kinetic modelling was unable to reliably distinguish estimates of the kinetic rate constant k3 (related to GS activity) from K1 (related to [13N]ammonia brain uptake), and indicated a non-negligible back-flux of [13N] to blood (k2). Model selection favoured a reversible one-tissue compartmental model, and [13N]ammonia K1 correlated reliably (r2 = 0.72-0.92) with [15O]water CBF.

CONCLUSION

The [13N]ammonia PET method was unable to reliably estimate GS activity in the human brain but may provide an alternative index of CBF.

Is there a link between bacteriuria and a reversible encephalopathy in dogs and cats?

Bacteriuria has been associated with abnormal neurological status in humans, especially geriatric patients. In this report, we review 11 cases (seven dogs and four cats) that suggest an association between bacteriuria and abnormal neurological status in veterinary medicine. These cases showed diffuse forebrain signs with or without brainstem signs, but primary brain disease was excluded by MRI and cerebrospinal fluid analysis. Bacteriological culture of urine was positive in each animal and neurological deficits improved or resolved with initiation of antibiosis ± fluid therapy and levetiracetam. While further studies are needed to definitively confirm or refute the link between bacteriuria and a reversible encephalopathy, urine bacteriological culture should be considered in veterinary patients presented with acute onset forebrain neuro-anatomical localisation, even in the absence of clinical signs of lower urinary tract inflammation.

Laboratory Abnormalities of Hepatic Encephalopathy

Currently, there is no gold standard serologic or imaging modality to detect hepatic encephalopathy (HE). It is a clinical diagnosis gathered from the history and physical. Imaging is nonspecific; however, PET and MRI have shown areas of utility, but are not widely available, cost-efficient, or necessary for diagnosis. Electroencephalogram has shown promise as it can be used in conjunction with the Portal Systemic Hepatic Encephalopathy Score test to diagnose minimal HE. Further research on these techniques would need to be performed to identify strict criteria and cutoffs for diagnosing HE as well as associated sensitivities and specificities.

Update on the clinical management of Wilson's disease

Wilson's disease (WD), albeit relatively rare, is an important genetic metabolic disease because of highly effective therapies that can be lifesaving. It is a great imitator and requires a high index of suspicion for correct and timely diagnosis. Neurologic, psychiatric and hepatologic problems in WD are very nonspecific, and we discuss the most common clinical phenotypes. The diagnosis remains laboratory based, and here we review the most important challenges and pitfalls in laboratory evaluation of WD, including the emerging role of genetic testing in WD diagnosis. WD is a monogenic disorder but has very high allelic heterogeneity with >500 disease-causing mutations identified, and new insights into phenotype-genotype correlations are also reviewed. The gold standard of therapy is chelation of excessive copper, but many unmet needs exist because of possible clinical deterioration in treated patients and potential adverse effects associated with currently available chelating medications. We also review the most promising novel therapeutic approaches, including chelators targeting specific cell types, cell transplantation and gene therapy.

The brain following transjugular intrahepatic portosystemic shunt: the perspective from neuroimaging

Hepatic encephalopathy (HE) is a common complication after implantation of a transjugular intrahepatic portosystemic shunt (TIPS). Neuroimaging offers a variety of techniques for non-invasive evaluation of alterations in metabolism, as well as structural and functional changes of the brain in patients after TIPS implantation. In this article, we review the epidemiology and pathophysiology of post-TIPS HE. The potential of neuroimaging including positron emission tomography and multimodality magnetic resonance imaging to investigate the pathophysiology of post-TIPS HE is presented. We also give a perspective on the role of neuroimaging in this field.

New Methods of Testing and Brain Imaging in Hepatic Encephalopathy: A Review

The diagnosis of hepatic encephalopathy is predominantly clinical, and the tests available assist in the diagnosis only by excluding other causes. Covert hepatic encephalopathy, which is defined as abnormal performance on psychometric tests when standard neurologic examination is completely normal, has gained widespread attention in recent years due to its effect on quality of life. This review focuses on the tests available to aid in the diagnosis of this significant complication of liver disease, and discusses the complex pathophysiologic mechanisms identified through new imaging techniques and their significance toward development of new therapeutic targets for this condition.

Ammonia metabolism and hyperammonemic disorders

Human adults produce around 1000 mmol of ammonia daily. Some is reutilized in biosynthesis. The remainder is waste and neurotoxic. Eventually most is excreted in urine as urea, together with ammonia used as a buffer. In extrahepatic tissues, ammonia is incorporated into nontoxic glutamine and released into blood. Large amounts are metabolized by the kidneys and small intestine. In the intestine, this yields ammonia, which is sequestered in portal blood and transported to the liver for ureagenesis, and citrulline, which is converted to arginine by the kidneys. The amazing developments in NMR imaging and spectroscopy and molecular biology have confirmed concepts derived from early studies in animals and cell cultures. The processes involved are exquisitely tuned. When they are faulty, ammonia accumulates. Severe acute hyperammonemia causes a rapidly progressive, often fatal, encephalopathy with brain edema. Chronic milder hyperammonemia causes a neuropsychiatric illness. Survivors of severe neonatal hyperammonemia have structural brain damage. Proposed explanations for brain edema are an increase in astrocyte osmolality, generally attributed to glutamine accumulation, and cytotoxic oxidative/nitrosative damage. However, ammonia neurotoxicity is multifactorial, with disturbances also in neurotransmitters, energy production, anaplerosis, cerebral blood flow, potassium, and sodium. Around 90% of hyperammonemic patients have liver disease. Inherited defects are rare. They are being recognized increasingly in adults. Deficiencies of urea cycle enzymes, citrin, and pyruvate carboxylase demonstrate the roles of isolated pathways in ammonia metabolism. Phenylbutyrate is used routinely to treat inherited urea cycle disorders, and its use for hepatic encephalopathy is under investigation.

Changes in CNS cells in hyperammonemic portal hypertensive rats

Rats with pre-hepatic portal hypertension because of partial portal vein ligation develop minimal hepatic encephalopathy (MHE) with hyperammonemia, impaired blood-brain barrier, mild brain edema, and severe mitochondrial changes in the hippocampus. The aim of this study was to evaluate changes of different neural cells in the cerebral cortex and the hippocampus. Animals were divided into two groups, MHE and sham. Astrocytes were studied by immunostaining with glial fibrillary acidic protein and S100β protein; neurons were immunostained with neuronal nuclear marker, microtubule associated protein-2, and NF-200 and capillaries with Nestin. The hypoxia-inducible factor 1α (HIF-1α) and its downstream proteins, P-glycoprotein (P-gp) and erythropoietin receptor (Epo-R), were also evaluated. Astrocytes were increased in area and number only in the hippocampus, while S100β increased in both brain areas in MHE animals. Microtubule associated protein-2 and NF-200 immunoreactivities (-ir) were significantly reduced in both areas. Hippocampal Nestin-ir was increased in MHE animals. These cellular changes were similar to those described in ischemic conditions, thus HIF-1α, P-gp, and Epo-R were also evaluated. A high expression of HIF-1α in cortical neurons was observed in the MHE group. It is likely that this hypoxia-like state is triggered via ammonia occupying the binding domain of HIF-1α and thereby preventing its degradation and inducing its stabilization, leading to the over-expression of P-gp and the Epo-R.

Pathogenesis of hepatic encephalopathy

Hepatic encephalopathy can be a serious complication of acute liver failure and chronic liver diseases, predominantly liver cirrhosis. Hyperammonemia plays the most important role in the pathogenesis of hepatic encephalopathy. The brain-blood barrier disturbances, changes in neurotransmission, neuroinflammation, oxidative stress, GABA-ergic or benzodiazepine pathway abnormalities, manganese neurotoxicity, brain energetic disturbances, and brain blood flow abnormalities are considered to be involved in the development of hepatic encephalopathy. The influence of small intestine bacterial overgrowth (SIBO) on the induction of minimal hepatic encephalopathy is recently emphasized. The aim of this paper is to present the current views on the pathogenesis of hepatic encephalopathy.

Aberrant resting-state corticostriatal functional connectivity in cirrhotic patients with hyperintense globus pallidus on T1-weighted MR imaging

Neurobiological and neuroimaging studies have emphasized the structural and functional alterations in the striatum of cirrhotic patients, but alterations in the functional connections between the striatum and other brain regions have not yet been explored. Of note, manganese accumulation in the nervous system, frequently reflected by hyperintensity at the bilateral globus pallidus (GP) on T1-weighted imaging, has been considered a factor affecting the striatal and cortical functions in hepatic decompensation. We employed resting-state functional magnetic resonance imaging to analyze the temporal correlation between the striatum and the remaining brain regions using seed-based correlation analyses. The two-sample t-test was conducted to detect the differences in corticostriatal connectivity between 44 cirrhotic patients with hyperintensity at the bilateral GP and 20 healthy controls. Decreased connectivity of the caudate was detected in the anterior/middle cingulate gyrus, and increased connectivity of the caudate was found in the left motor cortex. A reduction in functional connectivity was found between the putamen and several regions, including the anterior cingulate gyrus, right insular lobe, inferior frontal gyrus, left parahippocampal gyrus, and anterior lobe of the right cerebellum; increased connectivity was detected between the putamen and right middle temporal gyrus. There were significant correlations between the corticostriatal connectivity and neuropsychological performances in the patient group, but not between the striatal connectivity and GP signal intensity. These alterations in the corticostriatal functional connectivity suggested the abnormalities in the intrinsic brain functional organiztion among the cirrhotic patients with manganese deposition, and may be associated with development of metabolic encephalopathy. The manganese deposition in nervous system, however, can not be an independent factor predicting the resting-state brain dysfunction in real time.

T2* MRI of minimal hepatic encephalopathy and cognitive correlates in vivo

PURPOSE

To evaluate regional brain iron deposition in minimal hepatic encephalopathy (MHE) patients using T2*-weighted gradient-echo imaging and to explore the relationship between T2* MR changes and cognitive performance.

MATERIALS AND METHODS

Forty hepatitis-B virus (HBV)-related cirrhotic patients and 22 age-, sex-, and education-matched healthy controls were included in this study. Of the patients, twenty eight patients were diagnosed with MHE. All subjects were administered Number Connection Test-A (NCT-A), Letter Digit Substitution Test (LDST), Rey-Osterrieth Complex Figure Test (RCFT), and the Mini-Mental State Examination (MMSE). T2*-weighted gradient-echo images were acquired using 3 Tesla MRI. Phase values (putative iron levels) in the frontal-basal ganglia-thalamocortical circuits were measured. Spearman correlation and multiple linear regression analysis were performed.

RESULTS

MHE patients exhibited significantly prolonged NCT-A time and decreased LDST, RCFT immediate and delayed recall scores. Significant decreases of phase values in the bilateral putamen were detected in MHE patients compared to without MHE patients and controls. Multiple linear regression analysis confirmed significant correlations between the phase values in the putamen and right frontal white matter and cognitive performances by MHE patients.

CONCLUSION

Decreased phase values in the frontal cortical-basal ganglial circuits independently contribute to cognitive impairments in MHE patients.

Brain perfusion single photon emission computed tomography abnormalities in patients with minimal hepatic encephalopathy

BACKGROUND

Minimal hepatic encephalopathy (MHE) is the mildest form of hepatic encephalopathy (HE). Minimal hepatic encephalopathy patients do not demonstrate clinically overt symptoms of HE but present with abnormal neuropsychological and/or neurophysiological tests indicative of cerebral dysfunction. This study was performed in such patients to identify regions of abnormal cerebral perfusion and to correlate regional cerebral blood flow (rCBF) changes with psychometric hepatic encephalopathy score (PHES), Child-Turcotte-Pugh's score (CTP), and model for end-stage liver disease (MELD) score. We also compared abnormal patterns of rCBF in cirrhotic patients of alcoholic etiology with non-alcoholic etiology.

METHODS

This prospective study was performed to evaluate rCBF in 50 cirrhotic patients and 13 controls using technetium-99m ethyl cysteinate dimer (Tc-99m ECD) brain single photon emission computed tomography. All the patients underwent a battery of psychometry tests, PHES. Minimal hepatic encephalopathy was diagnosed if PHES was ≤-5. The rCBF changes were evaluated using region of interest (ROI) based semi-quantitative method of region/cerebellum and region/cortex ratios in 16 regions of the brain.

RESULTS

Cirrhotic patients with MHE showed impaired perfusion in the superior prefrontal cortex and increased perfusion in the thalamus, brain-stem, medial temporal cortex, and the hippocampus when compared with the controls. Cerebral perfusion in superior prefrontal cortex correlated negatively with MELD score (r=-0.323, P=0.022). We found significant positive correlation between PHES score and rCBF values in the left superior prefrontal cortex (r=0.385, P=0.006). Cirrhotic patients with alcohol etiology showed significantly decreased rCBF in right inferior prefrontal cortex, right superior prefrontal cortex, and the anterior cingulate cortex while increased rCBF was noted in the right medial temporal cortex and hippocampus.

CONCLUSION

Our results suggest that alterations in cognition in cirrhotic patients with MHE may be associated with impaired abnormalities of rCBF.

Dynamic susceptibility contrast-enhanced first-pass perfusion MR imaging in patients with subclinical hepatic encephalopathy

BACKGROUND AND PURPOSE

Subclinical hepatic encephalopathy (SHE) is frequently reported on single-photon emission computed tomography (SPECT), but is rarely described with magnetic resonance (MR) techniques. This study aimed to investigate hemodynamic changes in brain basal ganglia in patients with SHE using dynamic susceptibility contrast (DSC)-enhanced MR perfusion imaging.

METHODS

Twelve patients with SHE and ten age- and education-matched volunteers agreed to undergo MR examination. The scanning protocol included conventional anatomical images and DSC-enhanced perfusion MR imaging. Using MGH perfusion software, parameter maps of cerebral blood flow (CBF), cerebral blood volume (CBV) and mean transit time (MTT) were created. The bilateral caudate nucleus head, globus pallidus, putamen and thalamus were selected as regions of interest (ROI), with ipsilateral white matter of the frontal lobe used as reference. Ratios (ROI versus ipsilateral white matter in frontal lobe) for CBF, CBV and MTT in the patient group were compared with those in the control group.

RESULTS

The CBF ratios for every ROI were higher in SHE patients versus the controls. Statistically, significant increases were detected in the left globus pallidus, putamen and thalamus (P<0.05). For MTT, the reduction in values reached statistical significance in the right head of the caudate nucleus and bilateral thalamus (P<0.05). CBV values were not significantly different compared with those of the control group (P>0.05).

CONCLUSION

Increased CBF and decreased MTT values in the basal ganglia and thalamus were identified in patients with SHE. The CBF increase was compatible with the idea that the CBF is redistributed from cortical areas to the basal ganglia structures.

Gut-brain chemokine changes in portal hypertensive rats

BACKGROUND

Hepatic encephalopathy is a syndrome whose physiopathology is poorly understood; therefore, current diagnostic tests are imperfect and modern therapy is nonspecific. Particularly, it has been suggested that inflammation plays an important role in the pathogenesis of portal hypertensive encephalopathy in the rat.

AIM

We have studied an experimental model of portal hypertension based on a triple partial portal vein ligation in the rat to verify this hypothesis.

METHODS

One month after portal hypertension we assayed in the splanchnic area (liver, small bowel and mesenteric lymph nodes) and in the central nervous system (hippocampus and cerebellum) fractalkine (CX3CL1) and stromal cell-derived factor alpha (SDF1-α) as well as their respective receptors (CX3CR1 and CXCR4) because of their key role in inflammatory processes.

RESULTS

The significant increase of fractalkine in mesenteric lymph nodes (P<0.05) and its receptor (CX3CR1) in the small bowel (P<0.05) and hippocampus (P<0.01), associated with the increased expression of SDF1-α in the hippocampus (P<0.01) and the cerebellum (P<0.01) suggest that prehepatic portal hypertension in the rat induces important alterations in the expression of chemokines in the gut-brain axis.

CONCLUSION

The present study revealed that portal hypertension is associated with splanchnic-brain inflammatory alterations mediated by chemokines.

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

BACKGROUND & AIMS

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

METHODS

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

RESULTS

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

CONCLUSIONS

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

Abnormal brain processing in hepatic encephalopathy: evidence of cerebral reorganization?

BACKGROUND AND AIM

Hepatic encephalopathy (HE) is a severe and frequent complication of liver cirrhosis characterized by abnormal cerebral function. Little is known about the underlying neural mechanisms in HE and human data are sparse. Electrophysiological methods such as evoked brain potentials after somatic stimuli can be combined with inverse modeling of the underlying brain activity. Thereby, information on neuronal dynamics and brain activity can be studied in vivo. The aim of this study was to investigate the sensory brain processing in patients with HE.

PATIENTS AND METHODS

Twelve patients with minimal or overt HE and 26 healthy volunteers were included in the study. Cerebral sensory processing was investigated as (i) an auditory reaction time task; (ii) visual and somatosensory evoked brain potentials, and (iii) reconstruction of the underlying brain activity.

RESULTS

Somatosensory evoked potentials were reproducible (all P>0.05), whereas flash evoked potentials were not reproducible (all P<0.05). Compared with healthy volunteers, the patient group had a prolonged reaction time index (P=0.03) along with increasing prolongation of latencies of median nerve evoked potentials (P<0.03). Reconstruction of the underlying brain sources showed a lateral shift in source localization of the P45 (P<0.001) and N60 components (P=0.02). A correlation between the psychometric hepatic encephalopathy score and the dipole shift corresponding to the N60 (P=0.003) component was seen.

CONCLUSION

HE patients have evidence of prolonged intracerebral nerve conduction, along with lateralization of brain activity following median nerve stimulation. This possibly represents cortical reorganization and may be important in our understanding of this condition.

Astrocyte glutamine synthetase: importance in hyperammonemic syndromes and potential target for therapy

Many theories have been advanced to explain the encephalopathy associated with chronic liver disease and with the less common acute form. A major factor contributing to hepatic encephalopathy is hyperammonemia resulting from portacaval shunting and/or liver damage. However, an increasing number of causes of hyperammonemic encephalopathy have been discovered that present with the same clinical and laboratory features found in acute liver failure, but without liver failure. Here, we critically review the physiology, pathology, and biochemistry of ammonia (i.e., NH3 plus NH4+) and show how these elements interact to constitute a syndrome that clinicians refer to as hyperammonemic encephalopathy (i.e., acute liver failure, fulminant hepatic failure, chronic liver disease). Included will be a brief history of the status of ammonia and the centrality of the astrocyte in brain nitrogen metabolism. Ammonia is normally detoxified in the liver and extrahepatic tissues by conversion to urea and glutamine, respectively. In the brain, glutamine synthesis is largely confined to astrocytes, and it is generally accepted that in hyperammonemia excess glutamine compromises astrocyte morphology and function. Mechanisms postulated to account for this toxicity will be examined with emphasis on the osmotic effects of excess glutamine (the osmotic gliopathy theory). Because hyperammonemia causes osmotic stress and encephalopathy in patients with normal or abnormal liver function alike, the term "hyperammonemic encephalopathy" can be broadly applied to encephalopathy resulting from liver disease and from various other diseases that produce hyperammonemia. Finally, the possibility that a brain glutamine synthetase inhibitor may be of therapeutic benefit, especially in the acute form of liver disease, is discussed.

Rat CCl(4)-induced cirrhosis plus total portal vein ligation: a new model for the study of hyperammonaemia and brain oedema

INTRODUCTION

Animal models used to study hyperammonaemic disorders related to chronic liver disease are unsatisfactory. These animals only develop hyperammonaemia and brain oedema when fed with diets supplemented with amonium acetate.

AIM

To develop a novel experimental model of hyperammonaemia and brain oedema in CCl(4)-induced cirrhosis in rats.

METHODS

Four groups were studied: rats with sham intervention (S), rats with total portal vein ligation (TPVL), cirrhotic rats (LC), and cirrhotic rats with TPVL (LC+TPVL). When ascites was diagnosed, oral glutamine challenge (OGC) test was performed. Blood, liver, lungs and brain samples were collected to quantify liver function parameters, plasmatic and cerebral ammonia, endotoxaemia, liver and brain histology, brain oedema and portosystemic shunting degree.

RESULTS

LC+TPVL rats showed a significant increase in portosystemic shunting when compared with LC group and a significant derangement in liver function when compared with TPVL group. These alterations resulted in a significant increase in plasmatic and brain ammonia concentrations and a higher plasmatic endotoxaemia as compared with others. Similarly, the area under OGC curve was significantly increased in LC+TPVL group as compared with the others, and correlates with portal shunting. Low-grade brain oedema was only observed in LC+TPVL group. All cirrhotic groups showed liver regeneration nodules and type-II Alzheimer astrocytes

CONCLUSION

LC+TPVL reproduce the main alterations - portosystemic shunting, plasmatic and cerebral hyperammonaemia and low-grade brain oedema - observed in cirrhotic patients with hepatic encephalopathy.

Neurocognitive-neurological complications of liver transplantation: a review

Neurological complications are common after liver transplantation (LT) and they are associated with a significant morbidity. Long-term effects of LT on cognitive and psychological outcomes are not clear. The objective of this study was to summarize the present knowledge on the neurological and cognitive complications of LT, resulting from a systematic review of the literature in the last 10 years. Several studies have investigated the incidence and the pathophysiology of neurological complications; in contrast, the knowledge of cognitive and psychological status after LT is poor. Currently, the effect of LT on mental performance is debated. Some studies have shown an improvement of cognitive function after OLTX and, at the same time, a persistence of different cognitive deficits. In addition, the quality of life (QoL) and the psychological status after LT seem to improve but LT recipients have significant deficiencies in most QoL domains. Consequently, future studies are necessary in order to investigate cognitive alterations and QoL in LT recipients.

Human (13)N-ammonia PET studies: the importance of measuring (13)N-ammonia metabolites in blood

Dynamic (13)N-ammonia PET is used to assess ammonia metabolism in brain, liver and muscle based on kinetic modeling of metabolic pathways, using arterial blood (13)N-ammonia as input function. Rosenspire et al. (1990) introduced a solid phase extraction procedure for fractionation of (13)N-content in blood into (13)N-ammonia, (13)N-urea, (13)N-glutamine and (13)N-glutamate. Due to a radioactive half-life for (13)N of 10 min, the procedure is not suitable for blood samples taken beyond 5-7 min after tracer injection. By modifying Rosenspire's method, we established a method enabling analysis of up to 10 blood samples in the course of 30 min. The modified procedure was validated by HPLC and by 30-min reproducibility studies in humans examined by duplicate (13)N-ammonia injections with a 60-min interval. Blood data from a (13)N-ammonia brain PET study (from Keiding et al. 2006) showed: (1) time courses of (13)N-ammonia fractions could be described adequately by double exponential functions; (2) metabolic conversion of (13)N-ammonia to (13)N-metabolites were in the order: healthy subjects > cirrhotic patients without HE > cirrhotic patients with HE; (3) kinetics of initial tracer distribution in tissue can be assessed by using total (13)N-concentration in blood as input function, whereas assessment of metabolic processes requires (13)N-ammonia measurements.

Hepatic encephalopathy: pathophysiology and emerging therapies

Hepatic encephalopathy is characterized by neuropsychiatric abnormalities in patients with liver failure. Severe hepatic encephalopathy is an indication for liver transplantation as it portends poor outcome. Treatment of hepatic encephalopathy involves correction of precipitating factors such as sepsis, gastrointestinal bleeding, medications, and electrolyte imbalance. Effective therapies include lactulose and antibiotics such as neomycin, metronidazole, and rifaximin.

Neuropsychological assessment of hepatic encephalopathy: ISHEN practice guidelines

Low-grade or minimal hepatic encephalopathy (MHE) is characterised by relatively mild neurocognitive impairments, and occurs in a substantial percentage of patients with liver disease. The presence of MHE is associated with a significant compromise of quality of life, is predictive of the onset of overt hepatic encephalopathy and is associated with a poorer prognosis for outcome. Early identification and treatment of MHE can improve quality of life and may prevent the onset of overt encephalopathy, but to date, there has been little agreement regarding the optimum method for detecting MHE. The International Society on Hepatic Encephalopathy and Nitrogen Metabolism convened a group of experts for the purpose of reviewing available data and making recommendations for a standardised approach for neuropsychological assessment of patients with liver disease who are at risk of MHE. Specific recommendations are presented, along with a proposed methodology for further refining these assessment procedures through prospective research.

Radiotracer imaging studies in hepatic encephalopathy: ISHEN practice guidelines

There is lack of consensus on radiotracer usage in hepatic encephalopathy (HE). We have focused our attention on three main areas: (i) radiotracer imaging in animal models of HE, (ii) methodological issues of radiotracer imaging in HE and (iii) radiotracer imaging studies on the pathophysiology and (new) therapies in HE. We suggest the following: 1. Positron emission tomography (PET) and single photon emission computed tomography lend themselves to the study of animal models of HE, but the models that are suitable depend on the specific research question. Magnetic resonance imaging (MRI) may be a useful alternative technique. 2. Owing to the cost of the technique, there is a need for multicentre human PET studies to overcome the problem of underpowered small studies being undertaken in individual research centres. There should be a unified PET protocol with central, anonymised data analysis in one centre, using validated methodology, on behalf of all participating centres. Such studies would be useful for the assessment of early intervention in patients with subtle neuropsychiatric symptoms, or for clarification of the effect of liver transplantation on HE. 3. While radiotracer imaging modalities remain useful research tools for the study of pathogenesis and for the assessment of treatment effects, there is no consensus on the use of imaging in routine clinical practice for diagnosis and prognosis. The most promising objective tools appear to be magnetic resonance spectroscopy (MRS) and volumetric MRI, which can be performed in multiple centres without the difficulties that radiotracer imaging entail.

Neuropsychiatric disorders related to interferon and interleukins treatment

Certain cytokines such as interferon-alpha and interleukin-2 are often used in the treatment certain cancers and chronic diseases such as melanoma, hepatitis C infection and multiple sclerosis. Several neuropsychiatric side effects such as depression, anxiety, psychosis, suicidal ideation, hypomanic mood and cognitive impairment were reported in those patients who received those medications. In certain patients with those neuropsychiatric side effects, the symptoms ceased when the medication was stopped. However, in some cases, the cognitive impairment persisted even for years after cessation of the medication. In animal studies, those cytokines could induce sickness behaviour, anxiety behaviour and social anhedonia. The increased in pro-inflammatory cytokines in certain neuropsychiatric disorders was widely reported. In addition, in animal studies, the treatment with interferon-alpha or interleukin-1 could induce depressive like behaviour. Recently, the role of certain pro-inflammatory cytokines that could enhance the activity of the enzyme, indoleamine 2-3, dioxygenase (IDO) which in turn would increase tryptophan degradation into kynurenine and decrease tryptophan availability of tryptophan in the brain to synthesize serotonin, a neurotransmitter which is necessary for the normal mood state became of interest in pathophysiology of psychiatric disorders. Furthermore, the imbalance in the further downward catabolic kynurenine pathway and their interactions with other neurotransmitters has been proposed to play an important role. The presence of such an imbalance in patients being treated with cytokines and in patients with psychiatric disorders and the possible consequence of those changes on the neuroprotective function in the brain are discussed in this review.

Basal and learning task-related brain oxidative metabolism in cirrhotic rats

Hepatic encephalopathy is a neurological complication observed in patients with liver disease. Subjects with hepatic encephalopathy can develop memory alterations. In order to investigate brain oxidative metabolism in an animal model of chronic cirrhosis and its modification after spatial working memory task, we determined the neural metabolic activity of several brain limbic system regions by cytochrome oxidase (COx) histochemistry and assessed the spatial working memory in the Morris water maze of rats with cirrhosis by administration of thioacetamide. This COx histochemistry was done in cirrhotic and control rats under basal conditions and after the spatial working memory task. The histochemical results showed differences in basal COx activity between control and cirrhotic rats in hippocampal and thalamic regions. In cirrhotic rats basal COx activity was increased in the CA1 and CA3 areas of the hippocampus and reduced in the anterodorsal and anteroventral thalamic nuclei. We found impaired spatial working memory in animals with cirrhosis. These animals showed absence of metabolic activation of the CA3 hippocampal subfield and the lateral mammillary nucleus and disturbance of COx activity in the medial mammillary nucleus and the anteroventral thalamus. These findings suggest that cirrhotic rats show spatial working memory deficits that could be related to the alteration of metabolic activity of neural regions thought to be involved in the processing of spatial memories.

MR imaging findings in hepatic encephalopathy

The term hepatic encephalopathy (HE) includes a spectrum of neuropsychiatric abnormalities occurring in patients with liver dysfunction. Most cases are associated with cirrhosis and portal hypertension or portal-systemic shunts, but the condition can also be seen in patients with acute liver failure and, rarely, with portal-systemic bypass and no associated intrinsic hepatocellular disease. Although HE is a clinical condition, several neuroimaging techniques, particularly MR imaging, may eventually be useful for the diagnosis because they can identify and measure the consequences of central nervous system (CNS) increase in substances that under normal circumstances, are efficiently metabolized by the liver. Classic MR imaging abnormalities include high signal intensity in the globus pallidum on T1-weighted images, likely a reflection of increased tissue concentrations of manganese, and an elevated glutamine/glutamate peak coupled with decreased myo-inositol and choline signals on proton MR spectroscopy, representing disturbances in cell-volume homeostasis secondary to brain hyperammonemia. Recent data have shown that white matter abnormalities, also related to increased CNS ammonia concentration, can also be detected with several MR imaging techniques such as magnetization transfer ratio measurements, fast fluid-attenuated inversion recovery sequences, and diffusion-weighted images. All these MR imaging abnormalities, which return to normal with restoration of liver function, probably reflect the presence of mild diffuse brain edema, which seems to play an essential role in the pathogenesis of HE. It is likely that MR imaging will be increasingly used to evaluate the mechanisms involved in the pathogenesis of HE and to assess the effects of therapeutic measures focused on correcting brain edema in these patients.

Perioperative neurological complications after liver transplantation are best predicted by pre-transplant hepatic encephalopathy

INTRODUCTION

Liver transplant (LT) recipients are at significant risk for the development of neurological complications, such as altered mental status and seizures, in the postoperative period. Identifying accurate predictors of these events may allow optimal selection and preparation of candidates, and minimize risk after transplantation.

METHODS

One hundred and one consecutive adult LT recipients were evaluated retrospectively for neurological morbidity occurring in the first 30 days postoperatively. These events were analyzed in relation to specific predictive variables including preoperative complications of liver failure, such as hepatic encephalopathy (HE).

RESULTS

Median age was 50 years, 63% were male and hepatitis C was the most common indication for LT (n = 36). Median Child-Pugh score was 9 with 45% being Class C. Over half (n = 52) had experienced clinical HE prior to LT, while one quarter (n = 26) were encephalopathic at the time of LT. Neurological complications occurred in 31 patients in the postoperative period, with encephalopathy occurring in 28 and seizures occurring in 4; drug toxicity was responsible for neurological morbidity in 12 patients (39%). Length of hospital stay was significantly prolonged (median 19 vs. 12 days, P = 0.005) and all mortality (n = 3) occurred in those with neurological complications. There was no association between etiology of liver failure and complications; logistic regression identified active preoperative HE as the strongest predictor of postoperative morbidity (OR 10.7 95% CI 3.8-29.9).

CONCLUSION

Neurological events, manifesting most often as encephalopathy, occurred in almost one-third of patients after LT. Those suffering from HE at the time of LT may be more vulnerable to the metabolic stresses of surgery and the neurotoxicity of the drugs used, and were at highest risk for such complications.

New findings on cerebral ammonia uptake in HE using functional (13)N-ammonia PET

PET is a functional imaging technique suitable for studies of brain ammonia metabolism. Dynamic (13)N-ammonia PET yields time-courses of radioactivity concentrations in brain (PET camera) and blood (samples). Ahl et al. (Hepatology 40:73-79, 2004) and Keiding et al. (Hepatology 43:42-50, 2006) analysed such data in patients with HE by a kinetic model accounting for transfer of (13)N-ammonia across the blood-brain barrier (BBB) and intracellular formation of (13)N-glutamine. Initial unidirectional (13)N-ammonia transfer across BBB was characterized by the permeability-surface area product PS(BBB) (ml blood min(-1) ml(-1) tissue). There was a tendency to lower PS(BBB) values in patients with cirrhosis and HE than in patients with cirrhosis without HE and healthy controls but the differences were not statistically significant. Keiding et al. (Hepatology 43:42-50, 2006) also calculated PS(met) (ml blood min(-1) ml(-1) tissue) as a measure of the combined transfer of (13)N-ammonia across BBB and subsequent intracellular metabolism of (13)N-ammonia; neither did this PS-value show significant difference between the groups of subjects. Net flux of ammonia from blood into intracellular metabolites was linearly correlated to arterial ammonia. In conclusion, basic brain ammonia kinetics was not changed significantly in patients with cirrhosis +/- HE compared to healthy controls. Blood ammonia seems to be the more important factor for increased brain ammonia uptake in HE.

Energy metabolism in brain cells: effects of elevated ammonia concentrations

Both neurons and astrocytes have high rates of glucose utilization and oxidative metabolism. Fully 20% of glucose consumption is used for astrocytic production of glutamate and glutamine, which during intense glutamatergic activity leads to an increase in glutamate content, but at steady state is compensated for by an equally intense oxidation of glutamate. The amounts of ammonia used for glutamine synthesis and liberated during glutamine hydrolysis are large, compared to the additional demand for glutamine synthesis in hyperammonemic animals and patients with hepatic encephalopathy. Nevertheless, elevated ammonia concentrations lead to an increased astrocytic glutamine production and an elevated content of glutamine combined with a decrease in glutamate content, probably mainly in a cytosolic pool needed for normal activity of the malate-asparate shuttle (MAS); another compartment generated by glutamine hydrolysis is increased. As a result of reduced MAS activity the pyruvate/lactate ratio is decreased in astrocytes but not in neurons and decarboxylation of pyruvate to form acetyl coenzyme A is reduced. Elevated ammonia concentrations also inhibit decarboxylation of alpha-ketoglutarate in the TCA cycle. This effect occurs in both neurons and astrocytes, is unrelated to MAS activity and seen after chronic treatment with ammonia even in the absence of elevated ammonia concentrations.

Endotoxemia produces coma and brain swelling in bile duct ligated rats

This study explores the hypothesis that the inflammatory response induced by administration of lipopolysaccharide (LPS) exacerbates brain edema in cirrhotic rats; and if so whether this is associated with altered brain metabolism of ammonia or anatomical disturbance of the blood-brain barrier. Adult Sprague-Dawley rats 4 weeks after bile duct ligation (BDL)/Sham-operation, or naïve rats fed a hyperammonemic diet (HD), were injected with LPS (0.5 mg/kg, intraperitoneally) or saline, and killed 3 hours later. LPS administration increased brain water in HD, BDL, and sham-operated groups significantly (P < 0.05), but this was associated with progression to pre-coma stages only in BDL rats. LPS induced cytotoxic brain swelling and maintained anatomical integrity of the blood-brain barrier. Plasma/brain ammonia levels were higher in HD and BDL rats than in sham-operated controls and did not change with LPS administration. Brain glutamine/myoinositol ratio was increased in the HD group but reduced in the BDL animals. There was a background pro-inflammatory cytokine response in the brains of cirrhotic rats, and plasma/brain tumor necrosis factor alpha (TNF-alpha) and IL-6 significantly increased in LPS-treated animals. Plasma nitrite/nitrate levels increased significantly in LPS groups compared with non-LPS controls; however, frontal cortex nitrotyrosine levels only increased in the BDL + LPS rats (P < 0.005 versus BDL controls).

CONCLUSION

Injection of LPS into cirrhotic rats induces pre-coma and exacerbates cytotoxic edema because of the synergistic effect of hyperammonemia and the induced inflammatory response. Although the exact mechanism of how hyperammonemia and LPS facilitate cytotoxic edema and pre-coma in cirrhosis is not clear, our data support an important role for the nitrosation of brain proteins.

Review article: bacterial flora and pathogenesis in hepatic encephalopathy

The aetiology of hepatic encephalopathy has not been conclusively established, but it is widely agreed that ammonia derived primarily from enteric bacterial flora plays a central role. Recent research on the pathogenesis of hepatic encephalopathy reinforces previous findings, supporting an integral role of bacteria-derived ammonia and reveals other potential mechanisms by which bacterial flora and pathogens may be pathophysiologically important. This review discusses this research and considers its implications for the therapeutic management of hepatic encephalopathy. Besides producing ammonia, the enteric flora generates other neurotoxic products, such as phenols and mercaptans, that may potentiate the effects of ammonia. Bacteria may also constitute a primary source of the benzodiazepine-like compounds implicated in neuropsychiatric symptoms in patients with liver disease. New evidence suggests that acute bacterial infections, long recognized as important precipitants of hepatic encephalopathy, may mediate clinical worsening through effects on systemic inflammatory responses. Considered together, these data suggest wide-ranging pathophysiological contributions of bacteria to hepatic encephalopathy and underline the potential for an integral role of antibiotics and other bactericidal agents in its management.

Management of hepatic encephalopathy in patients with cirrhosis

The term hepatic encephalopathy encompasses a spectrum of neuropsychiatric abnormalities seen in patients with liver dysfunction. Distinct syndromes are identified in acute liver failure and cirrhosis. Rapid deterioration in consciousness level and increased intracranial pressure that may result in brain herniation and death are a feature of acute liver failure whereas manifestations of hepatic encephalopathy in cirrhosis include psychomotor dysfunction, impaired memory, increased reaction time, sensory abnormalities, poor concentration and in severe forms, coma. In patients with acute-on-chronic liver failure the pathophysiology remains undefined. Ammonia has been considered central to its pathogenesis. In the brain, the astrocyte is the main site for ammonia detoxification, during the conversion of glutamate to glutamine. An increased ammonia level raises the amount of glutamine within astrocytes, causing an osmotic imbalance resulting in cell swelling and ultimately brain oedema. Recent studies suggest that inflammation and it modulators may play a synergistic role with ammonia in the pathogenesis of hepatic encephalopathy. Therapy of hepatic encephalopathy is directed primarily at reducing ammonia generation and increasing its detoxification. The currently accepted regimens to treat hepatic encephalopathy such as lactulose and protein restricted diets need further clinical trials and therefore placebo controlled clinical trials in hepatic encephalopathy are justified. In liver failure, ammonia metabolism involves multiple organs and therefore ammonia reduction will require simultaneous targeting of these organs. The present review describes the pathophysiological basis of hepatic encephalopathy and evaluates the available therapies.