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Journal of Hepato/og,v 2000; 32 (suppl. 1): 171-180 Printed in Denmark All rights reserved Munksgaard Copenhagen Copyright 0 European Association for the Study of the Liver 2000 Journal of Hepatology ISSN 0169-5185 Complications of cirrhosis III. Hepatic encephalopathy Roger E Butterworth Neuroscience Research Unit, CHUM (H6pital Saint-Luc), University of Montreal, Montreal, Quebec, Canada Hepatic encephalopathy (HE) is a major neuropsychi- atric 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 in- clude 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. Mag- netic resonance imaging in cirrhotic patients reveals bilateral signal hyperintensities particularly in globus pallidus on T,-weighted imaging, a phenomenon which may result from manganese deposition. Proton (‘H) magnetic resonance spectroscopy shows in- creases in the glutamine resonance in brain, a finding which confirms previous biochemical studies and re- sults no doubt from increased brain ammonia removal (glutamine synthesis). Additional evidence for in- creased brain ammonia uptake and removal in cir- rhotic patients is provided by studies using positron emission tomography and 3NH3. Recent molecular biological studies demonstrate increased expression of H EPATIC ENCEPHALOPATHY (HE), or portal-systemic encephalopathy, is a major neuropsychiatric complication of cirrhosis. HE accompanies portal-sys- temic shunting of venous blood that arises either spon- taneously, due to portal hypertension, or surgically, following portacaval anastomosis surgery or trans- Correspondence: Roger E Butterworth, Neuroscience Re- search Unit, C.H.U.M./HGpital Saint-Luc, 1058 St-Denis Street, Montreal, Quebec H2X 3J4, Canada. Tel: 514 281 2444 ext. 5759. Fax: 514 281 2492/2107. e-mail: [email protected] genes coding for neurotransmitter-related proteins in chronic liver failure. Such genes include monoamine oxidase (MAO-A isoform), the peripheral-type benzo- diazepine 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 assess- ment of dietary protein intake and the use of lactul- ose, 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. Key words: Ammonia; Chronic liver failure; Cir- rhosis; Hepatic encephalopathy; Magnetic resonance imaging; Manganese; Monoamine oxidase; Nitric ox- ide synthase; Peripheral-type benzodiazepine recep- tors; Portal-systemic encephalopathy; Positron emis- sion tomography; Serotonin; TIPS. jugular intrahepatic portosystemic stent shunt (TIPS) aimed at relieving portal hypertension. Neurologically, HE in chronic liver failure develops slowly. The onset of neuropsychiatric symptoms is often insidious, starting with changes of personality and alter- ations in sleep patterns. Shortened attention span and lack of muscular coordination including asterixis (or “flapping tremor”) follow, progressing eventually through lethargy to stupor and coma. Motor signs in- cluding rigidity and ataxia are not uncommon, as are multiple episodes of HE, frequently precipitated by am- moniagenic conditions such as an oral protein load, gas- trointestinal bleeding or by use of a sedative. 171

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Page 1: Complications of cirrhosis III. Hepatic encephalopathy€¦ · III. Hepatic encephalopathy Roger E Butterworth Neuroscience Research Unit, CHUM (H6pital Saint-Luc), University of

Journal of Hepato/og,v 2000; 32 (suppl. 1): 171-180

Printed in Denmark All rights reserved Munksgaard Copenhagen

Copyright 0 European Association for the Study of the Liver 2000

Journal of Hepatology ISSN 0169-5185

Complications of cirrhosis III. Hepatic encephalopathy

Roger E Butterworth

Neuroscience Research Unit, CHUM (H6pital Saint-Luc), University of Montreal, Montreal, Quebec, Canada

Hepatic encephalopathy (HE) is a major neuropsychi- atric 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 in- clude 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. Mag- netic resonance imaging in cirrhotic patients reveals bilateral signal hyperintensities particularly in globus pallidus on T,-weighted imaging, a phenomenon which may result from manganese deposition. Proton (‘H) magnetic resonance spectroscopy shows in- creases in the glutamine resonance in brain, a finding which confirms previous biochemical studies and re- sults no doubt from increased brain ammonia removal (glutamine synthesis). Additional evidence for in- creased brain ammonia uptake and removal in cir- rhotic patients is provided by studies using positron emission tomography and ’ 3NH3. Recent molecular biological studies demonstrate increased expression of

H EPATIC ENCEPHALOPATHY (HE), or portal-systemic encephalopathy, is a major neuropsychiatric

complication of cirrhosis. HE accompanies portal-sys- temic shunting of venous blood that arises either spon- taneously, due to portal hypertension, or surgically, following portacaval anastomosis surgery or trans-

Correspondence: Roger E Butterworth, Neuroscience Re- search Unit, C.H.U.M./HGpital Saint-Luc, 1058 St-Denis Street, Montreal, Quebec H2X 3J4, Canada. Tel: 514 281 2444 ext. 5759. Fax: 514 281 2492/2107. e-mail: [email protected]

genes coding for neurotransmitter-related proteins in chronic liver failure. Such genes include monoamine oxidase (MAO-A isoform), the peripheral-type benzo- diazepine 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 assess- ment of dietary protein intake and the use of lactul- ose, 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.

Key words: Ammonia; Chronic liver failure; Cir- rhosis; Hepatic encephalopathy; Magnetic resonance imaging; Manganese; Monoamine oxidase; Nitric ox- ide synthase; Peripheral-type benzodiazepine recep- tors; Portal-systemic encephalopathy; Positron emis- sion tomography; Serotonin; TIPS.

jugular intrahepatic portosystemic stent shunt (TIPS) aimed at relieving portal hypertension.

Neurologically, HE in chronic liver failure develops slowly. The onset of neuropsychiatric symptoms is often insidious, starting with changes of personality and alter- ations in sleep patterns. Shortened attention span and lack of muscular coordination including asterixis (or “flapping tremor”) follow, progressing eventually through lethargy to stupor and coma. Motor signs in- cluding rigidity and ataxia are not uncommon, as are multiple episodes of HE, frequently precipitated by am- moniagenic conditions such as an oral protein load, gas- trointestinal bleeding or by use of a sedative.

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R. F. Butterworth

In “subclinical” hepatic encephalopathy, patients may have a normal neurological status on standard clinical evaluation but manifest neuropsychological deficits involving both cognitive and motor perform- ance skills. The prevalence of subclinical hepatic encephalopathy as defined by deficits in psychometric test scores in cirrhotic patients may be as high as 84% (1). Psychometric tests routinely used for the diagnosis and assessment of subclinical hepatic encephalopathy include the Number Connection Test, the Block Design Test, the Digit Symbol Test and Reaction Times to Sound or Light stimuli. Further details of these tests and their uses and possible pitfalls are described in a recent review article (1). Psychometric tests are more sensitive than electrophysiological measurements based upon stimulus-evoked potentials in patients with sub- clinical hepatic encephalopathy. However, electrophys- iological tests may be more specific, so that a combi- nation of the two approaches could be advantageous.

Subclinical hepatic encephalopathy impacts nega- tively on the quality of life in cirrhotic patients, par- ticularly with regard to their quality of sleep and rest (2).

Post-TIPS encephalopathy The TIPS procedure is a non-surgical intervention that is used for the treatment of portal hypertension and often results in an effective means of prevention of var- iceal bleeding and in the treatment of ascites. However, the TIPS procedure results in new or worsening epi- sodes of HE in a large percentage of cirrhotic patients as shown in Table 1 (3-l 1).

Post-TIPS encephalopathy is characterized by the occurrence of spontaneous episodes of HE, by the oc- currence of chronic HE in lo-20% of patients and by improvement in neurological status during follow-up (5). This improvement most likely relates to a progress- ive stenosis of the shunt over time as well as to meta-

bolic adaptation to the hemodynamic changes caused by TIPS (3).

Predictors of post-TIPS encephalopathy include (i) the presence of encephalopathy prior to the TIPS pro- cedure, (ii) a non-alcoholic etiology of cirrhosis, (iii) hypoalbuminemia and (iv) the age of the patient. The consistent observation of increased incidence of post- TIPS encephalopathy in older cirrhotic patients may relate to the increased sensitivity of brain to liverlgut- derived neurotoxins such as ammonia (12).

Neuropathology of HE in chronic liver failure HE in chronic liver failure is characterized neuropatho- logically by alterations of astrocyte morphology and function; no consistent histological changes to neurons have been described in the brain in chronic liver dis- ease. It has therefore been proposed that HE represents the first example of a primary “astrocytopathy” or “gliopathy” (13). The morphological changes that characterize HE are collectively known as Alzheimer Type II astrocytosis, a typical example of which is shown in Fig. 1.

Alzheimer Type II astrocytes exhibit large swollen nuclei with margination of the chromatin pattern, re- sulting in a large pale nucleus and prominent nu- cleolus. These astrocytic changes are seen throughout the brains of patients with end-stage chronic liver fail- ure (14) in both grey and white matter structures.

Astrocytes are the only cells in the mammalian brain to contain the necessary metabolic machinery (gluta- mine synthetase) for ammonia removal since brain is devoid of an effective urea cycle. It is therefore the as- trocyte that bears the brunt of ammonia removal in brain and the astrocyte that ultimately shows the most consistent morphological changes. Astrocytes in chronic liver failure also manifest alterations in ex- pression of many key proteins, as discussed in a sub- sequent section of this review.

TABLE 1

Incidence of hepatic encephalopathy (HE) after transjugular intrahepatic portosystemic stent shunt (TIPS)

Study (Reference) N Pugh Class (WI) New or Chronic

A B C worsened HE (“%) HE (‘%)

Somberg et al., 1995 (4) 77 16 55 29 23 5 Sanyal et al., 1994 (5) 30 20 33 47 33 3 Dagenais et al., 1994 (6) 45 2 51 47 25 18 Coldwell et al., 1995 (7) 96 25 40 35 29 I Rossle et al., 1994 (8) 100 27 52 22 16 7 Martin et al., 1993 (9) 45 22 58 20 43 20 Ochs et al., 1995 (10) 50 ~ 36 64 44 16 Jalan et al., 1995 (11) 68 25 4 31 13

Adapted from Pomier Layrargues, 1996 (ref. 3).

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Hepatic encephalopathy

Fig. 1. Alzheimer type II astrocytes (ALZ) in frontal cor- tex of a 37-yr-old alcoholic cirrhotic patient who died in hepatic coma. Note presence of neurons and normal astro- cytes (N) in the same jeld. (H+E staining, magkjication X40).

Non-invasive techniques for the study of HE Non-invasive techniques such as magnetic resonance imaging and spectroscopy as well as positron emission tomography are increasingly being used for the study of the central nervous system consequences of chronic liver failure (15).

Magnetic resonance imaging (MRI) The most consistent finding on MRI of cirrhotic pa- tients is bilateral, symmetrical hyperintensities in glo- bus pallidus on Ti-weighted imaging. A typical ex- ample of these signal hyperintensities is shown in Fig. 2.

The cause of these pallidal signal hyperintensities as well as their relationship to the extent of liver damage, the presence of portal-systemic shunting and to the clinical neuropsychiatric status of the patient have been the subject of several investigations. However, little (if any) consensus has emerged (1618). On the other hand, it is clear that the pallidal signal hyperin- tensities on MRI of patients with end-stage liver failure are reversible, since successful liver transplantation re- sults in their disappearance (19).

As to the cause of these signal hyperintensities, most evidence available at the present time points to a role of manganese deposition. Manganese is a neurotoxic metal whose elimination takes place via the hepatobili- ary route. Blood manganese concentrations are in- creased in cirrhotic patients who manifest pallidal sig- nal hyperintensities on MRI (18). Furthermore, similar

Fig. 2. Bilateral signal hyperintensities (open arrow) on T,- weighted magnetic resonance imaging in a 51-yr-old male cirrhotic patient.

pallidal signal hyperintensities have been described in a patient with Alagille’s Syndrome, a hereditary disorder characterized by cholestasis, intrahepatic bile duct paucity and increased blood manganese (20). Pallidal MRI signal hyperintensities have also been described in patients during total parenteral nutrition where it was again proposed that the cause was manganese ac- cumulation in the brain (21). Histopathological evalu- ation of pallidal tissue from cirrhotic patients who had manifested pallidal signal hyperintensities on MRI during life reveals Alzheimer Type II astrocytosis (16), suggesting that the astrocytic pathology characteristic of HE in chronic liver failure may be caused, at least in part, by manganese accumulation.

The most convincing evidence for manganese as the causal agent in the pallidal signal hyperintensities ob- served in cirrhotic patients comes from direct studies of manganese content of dissected pallidal tissue ob- tained at autopsy from cirrhotic patients (Table 2). In contrast to other metals, including copper and zinc, manganese concentrations were found to be increased up to 7-fold in pallidal samples from these patients (22,23). More recent studies in experimental animals with surgical portacaval shunts or experimental biliary cirrhosis revealed selective accumulation of manganese in pallidum (Table 2) (and to a lesser extent in other basal ganglia structures), suggesting that both portal- systemic shunting and impaired hepatobiliary elimin- ation may be implicated in brain manganese accumu- lation in chronic liver failure (24).

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R. F. Butterworth

TABLE 2

Brain manganese concentrations in dissected pallidal tissue from cir- rhotic patients who died in hepatic coma and from rats with experi- mental acute or chronic liver failure

Pallidal manganese concentrations

kg/g)

Patients Controls (n) Cirrhotics (n)

Rats

1.41 i-o.91 (8) 4.0-c 1.54** (8)

Controls (Sham-operated) (n) Portacaval-shunted (n) Bile-duct ligated (cirrhosis) (n) Hepdtic devdscularization (acute her

failure (n)

0.54-cO.02 (6) 1.05iO.O7** (6) 0.85t0.06* (6) 0.72ZO.07 (6)

Values represent mean%SE of determinations in dissected pallidal tissue. n=number of samples. Values significantly different from the appropriate control group indicated by *p<O.OS, **p<O.Ol (23,24). Data from ref. 24.

Magnetic Resonance Spectroscopy (MRS) ‘H (proton) MRS is currently being used as a research tool to measure brain metabolites in cirrhotic patients with HE (25,26). Studies using sufficiently high mag- netic field strengths reveal increases in resonances as- signed to the protons of the glutamine molecule (26). Increases of brain glutamine no doubt result from in- creased removal of ammonia via glutamine synthesis in the brain. In addition to increases in the glutamine resonances, reductions in resonances assigned to the protons of myoinositol have consistently been reported in the brains of cirrhotic patients using the ‘H-MRS procedure (27) and it has been suggested that these changes reflect disturbances in cell volume regulation in the brain in chronic liver failure. Studies using lower field strengths yield information in the form of meta- bolite ratios which, although possibly useful as a “fingerprint” for the diagnosis of HE, are of limited value in the study of pathophysiologic mechanisms.

“P (phosphorus) MRS yields seven major reson- ances, three of which have been assigned to the (x, /I and ‘J phosphate groups of the ATP molecule. Conse- quently, this technique has been used to study brain energy status in the brains of cirrhotic patients with varying degrees of severity of HE. To date, no convinc- ing evidence of alterations of ATP, phosphocreatine or ATPlphosphocreatine ratios has been observed in these patients (28). Furthermore, 3’P-MRS studies per- formed in pallidurn of cirrhotic patients who manifest MRI signal hyperintensities did not show any signifi- cant alterations of 31P resonances (29) suggesting that manganese deposition in the brains of these patients does not result in local cerebral energy failure.

174

Positron emission tomography (PET) Using the PET ligand “F-fluorodeoxyglucose, several reports have described alterations of local cerebral glu- cose utilization (LCGU) in the brains of cirrhotic pa- tients with mild-to-moderate HE. In one study, sig- nificant decreases in LCGU were observed in the an- terior cingulate gyrus, a brain region known to be im- plicated in the mediation of attention as well as in tar- get analysis and response formulation (30). These changes were accompanied by small but significant in- creases in LCGU in some other cortical and thalamic structures. This pattern of alterations of LCGU paral- lels the alterations in cerebral blood flow described in cirrhotic patients (3 1,32).

In a landmark study using PET and “NH3, Lock- wood et al. (33) were able to demonstrate an increase in the cerebral metabolic rate for ammonia (CMRJ (i.e. a measure of the rate at which ammonia is uptaken and metabolized by brain). Furthermore, this in- creased rate of ammonia utilization by brain was ac- companied by an increase in the permeability/surface area product (PS), a measure of blood-brain barrier permeability to ammonia (Fig. 3).

This apparently increased ease with which ammonia moves from blood to brain in cirrhotic patients offers a plausible explanation for the imperfect correlation between neurological status and blood ammonia con- centrations in these patients.

CMR, PS

Fig. 3. ‘-‘NH,-positron emission tomogruphic imuges of (I control subject compuretl to a cirrhotic putient Ic,ith mild HE. Note increased CMRA urd PS throughout the bruin of the patient. CMRA: Cerebral metubolic rute jbr um- moniu. PS: Permeuhilit~~ surf&e ureu product. Adapted from rejl 33.

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Hepatic encephalopathy

Neurotoxic substances generated in chronic liver failure Chronic liver failure and significant portal-systemic shunting result in the accumulation of neurotoxic sub- stances in brain. Two such substances are ammonia and manganese.

Ammonia Ammonia derived from colonic bacteria as well as from the deamination of glutamine in the small bowel is absorbed by passive diffusion and normally un- dergoes a high first-pass extraction by the liver (34). In chronic liver failure, hepatic urea synthesis declines and this, in addition to portal-systemic shunting, results in increased blood ammonia concentrations. Further- more, cirrhotic patients are hypersensitive to am- moniagenic conditions such as an oral protein load or gastrointestinal hemorrhage. An illustration of this hy- persensitivity is provided by a report of studies in the 1950’s in which attempts were made to treat ascites in cirrhotic patients with ion-exchange resins that ab- sorbed sodium but released ammonium ions (35). This treatment led to a significant reduction in ascitic vol- ume but precipitated severe encephalopathy in many of the patients treated.

If present in high concentrations, ammonia has the potential to adversely affect central nervous system (CNS) function by several mechanisms. Such mechan- isms include a direct effect of the ammonium ion [NH,+] on inhibitory and excitatory neurotransmis- sion (36) as well as inhibition of the tricarboxylic acid cycle enzyme ketoglutarate dehydrogenase (37) with potential impairment of brain energy metabolism. However, brain energy metabolism does not appear to be impaired in chronic liver failure until very late stages associated with isoelectric electroencephalography (EEG) traces (38). On the other hand, increases of cerebrospinal fluid lactate have been described both in cirrhotic patients with HE (39) and in experimental animals with chronic liver failure and ammonia-pre- cipitated encephalopathy (40), findings which are con- sistent with an inhibitory effect of ammonia on cer- ebral glucose oxidation.

Other effects of ammonia on cerebral function in- clude a stimulatory effect on L-arginine uptake by brain preparations resulting in increased production of nitric oxide (41) and inhibition of the capacity of astro- cytes to accumulate glutamate (42,43), a major excit- atory neurotransmitter.

Manganese As previously mentioned in this review, chronic liver failure results in increased blood and brain concen-

trations of manganese (18,22-24) and manganese de- position is the most plausible explanation for the pal- lidal signal hyperintensities on MRI in cirrhotic pa- tients. Manganese is neurotoxic, affecting both neur- onal and astrocytic integrity. In the case of astrocytes, exposure to manganese results in altered expression of several key astrocytic proteins (44,45) and Alzheimer Type II changes (16).

Other toxins in addition to ammonia and manga- nese are known to increase in the systemic circulation in chronic liver failure. Such toxins include mercaptans, phenols and short chain fatty acids (46). While there is no convincing evidence that these toxins alone cause cerebral dysfunction in chronic liver failure, they could combine with ammonia or manganese to act synergis- tically (47).

Increased expression of genes coding for neurotransmitter-related proteins in brain in chronic liver failure The lack of alterations in cerebral energy metabolism has resulted in a focus of attention on changes in brain neurotransmitter systems as the likely mediators of the neuropsychiatric manifestations of HE in chronic liver failure. Recent studies using molecular biological ap- proaches continue to confirm that, when liver fails, brain responds with significant alterations in gene ex- pression. In many cases these alterations involve genes which code for neurotransmitter-related proteins, many of which are essential for CNS function.

Monoamine oxidase (MAO-A isoform) Many of the symptoms of early HE in chronic liver failure such as altered personality, depression and in- verted sleep patterns are symptoms which have classic- ally been associated with alterations in biogenic amine function. RNA extracts of brain tissue obtained at au- topsy from cirrhotic patients who died in hepatic coma have been found to show increased expression of the neuronal isoform of the monoamine-metabolizing en- zyme MAO-A (48). This increase in MAO-A gene ex- pression was found to be associated with increased ac- tivities of the enzyme and increased densities of cata- lytic sites on the enzyme protein (48). Moreover, studies of the same brain extracts revealed increased concentrations of homovanillic and 5-hydroxyindolea- cetic acids (49) the final metabolites of dopamine and serotonin respectively. Increased concentrations of 5- hydroxyindoleacetic acid were also reported in cerebro- spinal fluid from patients (50) and experimental ani- mals (51) with chronic liver failure. On the basis of these findings, it has been suggested that altered mono- aminergic function may be responsible for the early

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R. F. Butterworth

neuropsychiatric symptoms of HE in chronic liver dis- ease (51,52).

The “‘peripheral-type” benzodiazepine receptor (PTBR) PTBR is a hetero-oligomeric protein complex located (like MAO-A) on the outer mitochondrial membrane of the astrocyte. Increased PTBR gene expression has been reported in brain extracts from portacaval-shunt- ed rats (53). This increased gene expression resulted in increased receptor sites in the brains and peripheral tissues of these animals as revealed by quantitative re- ceptor autoradiography and the highly selective PTBR ligand 3H-PK1 1195 (54,55). Increased ‘H-PK11195 binding sites were also reported in autopsied brain tissue from cirrhotic patients who died in hepatic coma (56).

There is evidence to suggest that the increased ex- pression of PTBRs in brain in chronic liver failure is the consequence of exposure to ammonia and/or manganese. Evidence in favor of this includes reports that: (i) increased densities of 3H-PK1 1195 binding sites are also observed in brain and peripheral tissues of mice with chronic hyperammonemia due to a con- genital defect in the urea cycle enzyme ornithine trans- carbamylase (57); (ii) exposure of rat cerebral cortical astrocytes in culture to ammonia results in increased densities of 3H-PK11195 binding sites (58); (iii) ex- posure to manganese also results in increased densities of ‘H-PKll195 binding sites (44).

The precise mechanism whereby increased express- ion or activation of PTBRs results in altered brain exci- tability (characteristic of HE) has not been established. However, the mitochondrial localization of this recep- tor complex has led to the speculation that it plays a role in the maintenance of astrocytic energy met- abolism and in the uptake of cholesterol by the mito- chondria of these cells. This latter action results in the synthesis of a newly-characterized group of com- pounds known collectively as neurosteroids, some of which have potent neuroinhibitory properties (13). Further studies of neurosteroids in chronic liver failure and their possible role in the pathogenesis of HE are clearly warranted.

Neuronal nitric oxide synthase (nNOS) Portacaval anastomosis in the rat results in increased gene expression of the constitutive (neuronal) isoform of nitric oxide synthase (nNOS) in brain (59). In- creased nNOS mRNA is accompanied by increased nNOS protein (59) and by increased nNOS enzyme ac- tivities (41). There is recent evidence to suggest that, in addition to an induction in nNOS gene expression,

increased nNOS activities may also result from a stimulatory effect of ammonia on L-arginine uptake by neuronal preparations (shown both in vitro and in vivo) (60). Increased production of nitric oxide as a consequence of increased nNOS activities could con- tribute to the alterations of cerebral perfusion observed in chronic liver disease (61).

Other neurotransmitter systems in HE The appearance of extrapyramidal symptoms (particu- larly rigidity) in cirrhotic patients with end-stage liver disease has prompted, by analogy with the well-estab- lished dopamine deficit in Parkinson’s Disease, evalu- ations of the dopamine system in relation to HE. Studies in autopsied brain tissue from cirrhotic patients (49) and from rats with portacaval shunts (51) reveal several-fold increases in concentration of the dopamine metabolite homovanillic acid, a finding which could result from in- creased activities of monoamine oxidase reported in the same material (48) (see above). In another study, densi- ties of the postsynaptic dopamine Dz receptor were sig- nificantly reduced in pallidum/putamen from cirrhotic patients (62), a finding that could have resulted from manganese deposition in the brains of these patients (22).

Cirrhotic patients are hypersensitive to morphine (63) and portacaval shunting in the rat results in in- creased pain sensitivity (64) a phenomenon known to involve the endogenous opioid system. Increased plasma levels of the endogenous opioid met-enkephalin have been recorded in patients with primary biliary cir- rhosis (65) and brain B-endorphin levels are increased in experimental chronic liver failure (66). D-endorphin is an endogenous opioid peptide that is involved in the positive reinforcing effects of drugs of abuse, including ethanol (67). It is of interest, therefore, that portacaval- shunted rats drink significantly more ethanol in a free- choice drinking paradigm (68,69). Autoradiographic studies in the brains of these animals reveal region- selective alterations of ~1 and 6 opioid receptor sites (70). Moreover, the increase in ethanol preference manifested by the rats after portacaval shunting is sig- nificantly attenuated following administration of the opioid receptor antagonist naloxone (69). Extrapol- ation of these findings to humans suggests the in- triguing possibility that significant liver disease and portal-systemic shunting could result in activation of the brain opioid system with the potential to lead to increased drinking.

Prevention and treatment of HE Strategies aimed at the prevention and treatment of HE in chronic liver failure are of two major types,

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Hepatic encephalopathy

namely, ammonia-lowering strategies and approaches aimed directly at the CNS (34,71).

Ammonia-lowering strategies Since HE is frequently precipitated by ammoniagenic situations such as an oral protein load or a gastrointes- tinal hemorrhage, various treatment modalities are aimed at the gut. Such strategies include reduction of the absorption of nitrogenous substances arising from bacterial action in the colon. Colonic cleansing reduces the luminal ammonia content and lowers blood am- monia content in cirrhotic patients (72).

Non-absorbable disaccharides such as lactulose are routinely used to decrease ammonia production in the gut. The action of the most popular substance in this class, lactulose, involves increased fecal nitrogen ex- cretion by facilitation of the incorporation of ammonia into bacteria as well as a cathartic effect (34). Lactu- lose administered orally reaches the cecum where it is metabolized by enteric bacteria, causing a fall in pH (73). The dose is adjusted to produce two or three soft bowel movements daily (34).

Antibiotics such as neomycin are also useful for low- ering blood ammonia, mainly by an effect on ammonia production by intestinal bacteria. However, neomycin therapy is associated with significant toxic side effects (34).

Restriction of dietary protein remains a cornerstone of therapy for HE in cirrhotic patients (34). However, long-term nitrogen restriction is potentially harmful and a positive nitrogen balance is necessary to promote liver regeneration as well as to increase the capacity of skeletal muscle to remove ammonia in the form of glutamine (74). Protein intake of l-2 g/kg per day may be required in order to maintain an adequate nitrogen balance (75).

An alternative strategy for the lowering of blood ammonia is the stimulation of ammonia fixation (71). Under normal physiological conditions, ammonia is removed by the formation of urea in periportal hepato- cytes and by glutamine synthesis in perivenous hepato- cytes, skeletal muscle and brain. In cirrhosis, both urea cycle enzymes and glutamine synthetase in liver are de- creased in activity. Strategies to stimulate residual urea cycle activities and/or glutamine synthesis have been tried over the last 20 yr. One of most successful agents to be used so far is L-ornithine-L-aspartate (OA). Ran- domized controlled clinical trials with OA demonstrate significant ammonia-lowering and concomitant im- provement in psychometric test scores in cirrhotic pa- tients with HE (76). Studies in experimental animals suggest that the metabolic basis for the beneficial effect of OA on blood ammonia in chronic liver failure re-

sides in its ability to stimulate residual hepatic urea cycle function and also to promote glutamine syn- thesis, particularly in skeletal muscle (77).

Benzoate is also effective in reducing blood am- monia both in patients with inherited urea cycle dis- orders and in cirrhotic patients (71). In a randomized controlled clinical trial with sodium benzoate versus lactulose, improvement in neuropsychiatric perform- ance was found to be comparable using both treat- ments (78).

Use of CNS-acting drugs In contrast to the multiple strategies used successfully to lower blood ammonia and improve neurological sta- tus in patients with chronic liver failure (above), drugs which act directly on neuronal excitability have not been widely applied in this patient group. The major reason for this is that the precise neurotransmitter changes responsible for HE in chronic liver failure are still being elucidated. Some attempts to treat HE in cirrhotic patients with benzodiazepine receptor anta- gonists and dopamine agonists have been attempted, but with limited success.

Several controlled clinical trials have been per- formed to assess the efficacy of the benzodiazepine re- ceptor antagonist flumazenil in cirrhotic patients with various degrees of severity of HE (71). Spectacular im- provements in neuropsychiatric status were recorded in a subset of patients receiving flumazenil (79,80). How- ever, enthusiasm for this approach has been tempered by the possible confounding effects of prior exposure to benzodiazepines and the seeming lack of correlation between clinical response and blood levels of sub- stances with benzodiazepine receptor agonist prop- erties in these patients (81).

Drugs known to increase dopaminergic neurotrans- mission such as L-DOPA and the dopamine receptor agonist bromocriptine have been used in clinical trials in cirrhotic patients with HE. Results were not encour- aging in terms of improvement of overall neuropsychi- atric status (82,83). However, it is possible that these agents could improve motor performance in these pa- tients (34).

It is anticipated that new therapeutic avenues will emerge as the precise abnormalities in the mono- aminergic, glutamatergic and opioidergic systems be- come more clearly defined in relation to the various cognitive, psychiatric and motor symptoms that char- acterize HE in chronic liver failure.

Acknowledgments Studies from the Author’s research unit were funded by The Medical Research Council of Canada. The author

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R. F. Buttrrwvrth

thanks Dominique D. Roy for her valuable assistance in the preparation of the manuscript.

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