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Brain Edema in Chronic Hepatic Encephalopathy

  • Cristina Cudalbu
    Correspondence
    Address for correspondence: Cristina Cudalbu, Centre d'Imagerie Biomedicale (CIBM), Ecole Polytechnique Fédérale de Lausanne (EPFL), EPFL-CIBM, Office F3 628, Station 6, CH-1015 Lausanne, Switzerland.
    Affiliations
    Centre d'Imagerie Biomedicale (CIBM), Ecole Polytechnique Fédérale de Lausanne (EPFL), Lausanne, Switzerland
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  • Simon D. Taylor-Robinson
    Affiliations
    Division of Integrative Systems Medicine and Digestive Disease, Department of Surgery and Cancer, St Mary's Hospital Campus, Imperial College London, London, United Kingdom
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Open AccessPublished:March 26, 2019DOI:https://doi.org/10.1016/j.jceh.2019.02.003
      Brain edema is a common feature associated with hepatic encephalopathy (HE). In patients with acute HE, brain edema has been shown to play a crucial role in the associated neurological deterioration. In chronic HE, advanced magnetic resonance imaging (MRI) techniques have demonstrated that low-grade brain edema appears also to be an important pathological feature. This review explores the different methods used to measure brain edema ex vivo and in vivo in animal models and in humans with chronic HE. In addition, an in-depth description of the main studies performed to date is provided. The role of brain edema in the neurological alterations linked to HE and whether HE and brain edema are the manifestations of the same pathophysiological mechanism or two different cerebral manifestations of brain dysfunction in liver disease are still under debate. In vivo MRI/magnetic resonance spectroscopy studies have allowed insight into the development of brain edema in chronic HE. However, additional in vivo longitudinal and multiparametric/multimodal studies are required (in humans and animal models) to elucidate the relationship between liver function, brain metabolic changes, cellular changes, cell swelling, and neurological manifestations in chronic HE.

      Keywords

      Abbreviations:

      ADC (apparent diffusion coefficient), ALF (acute liver failure), AQP (aquaporins), BBB (blood-brain barrier), BDL (bile duct ligation), CNS (central nervous system), Cr (creatine), CSF (cerebrospinal fluid), DTI (diffusion tensor imaging), DWI (diffusion-weighted imaging), FLAIR (fluid-attenuated inversion recovery), Gln (glutamine), Glx (sum of glutamine and glutamate), GM (gray matter), HE (hepatic encephalopathy), 1H MRS (proton magnetic resonance spectroscopy), Ins (inositol), Lac (lactate), LPS (lipopolysaccharide), MD (mean diffusivity), mIns (myo-inositol), MRI (magnetic resonance imaging), MRS (magnetic resonance spectroscopy), MT (magnetization transfer), MTR (MT ratio), NMR (nuclear magnetic resonance), PCA (portocaval anastomosis), tCr (total creatine), tCho (total choline), TE (echo time), WM (white matter)
      Brain edema is defined as an excessive accumulation of fluid (chiefly water) in the intracellular or extracellular spaces of the brain, which occurs on the background of an osmotic gradient. The pathological process is a complex phenomenon to measure and characterize, because it can be the result or effect of a certain disease or cerebral injury, but can also cause pathology or aggravate an existing disease process. The measurement of brain edema can be used to aid diagnosis and/or to measure targeted treatment effects. It is now well accepted that brain edema is a common feature associated with hepatic encephalopathy (HE).
      Net fluid entry to the brain from the vascular compartment (vasogenic edema) increases the brain volume, raises intracranial pressure, and potentially leads to fatal brainstem compression in the most severe, acute form.
      • Bémeur C.
      • Cudalbu C.
      • Dam G.
      • Thrane A.S.
      • Cooper A.J.L.
      • Rose C.F.
      Brain edema: a valid endpoint for measuring hepatic encephalopathy?.
      Vasogenic edema mainly occurs because of a breakdown of the tight endothelial junctions that make up the blood-brain barrier (BBB),
      • Bosoi C.R.
      • Rose C.F.
      Brain edema in acute liver failure and chronic liver disease: similarities and differences.
      while a disruption in cellular metabolism impairs functioning of the sodium and potassium pump in the glial cell membrane and causes accumulation of osmotically active molecules, leading to cellular retention of sodium and water and consequently to cytotoxic edema.
      • Bosoi C.R.
      • Rose C.F.
      Brain edema in acute liver failure and chronic liver disease: similarities and differences.
      • Norenberg M.D.
      • Rao K.V.
      • Jayakumar A.R.
      Mechanisms of ammonia-induced astrocyte swelling.
      • Unterberg A.W.
      • Stover J.
      • Kress B.
      • Kiening K.L.
      Edema and brain trauma.
      Although cytotoxic edema refers to intracellular swelling (an isolated fluid shift from the interstitial to the intracellular, cytosolic compartment with no net fluid entry to the brain), it can also occur following an increase in permeability (not physical breakdown) of the BBB. It is not unreasonable to assume that this pathological process is accompanied by some degree of net brain edema.
      • Bémeur C.
      • Cudalbu C.
      • Dam G.
      • Thrane A.S.
      • Cooper A.J.L.
      • Rose C.F.
      Brain edema: a valid endpoint for measuring hepatic encephalopathy?.
      • Bosoi C.R.
      • Rose C.F.
      Brain edema in acute liver failure and chronic liver disease: similarities and differences.
      This astrocytic swelling, accompanied by a shift of fluid from the interstitial/intravascular compartment to the intracellular (astrocytic) compartment, can lead to detrimental effects. The molecular mechanisms leading to astrocyte swelling are not yet fully understood and are believed to be linked with osmo-sensitive or stretch-sensitive intracellular signaling cascades, involving [Ca2+]i transients, aquaporins (AQPs) and volume-regulated anion channels.
      • Thrane A.S.
      • Rangroo Thrane V.
      • Nedergaard M.
      Drowning stars: reassessing the role of astrocytes in brain edema.
      • Rama Rao K.V.
      • Norenberg M.D.
      Aquaporin-4 in hepatic encephalopathy.
      • Jayakumar A.R.
      • Rama Rao K.V.
      • Murthy C.R.K.
      • Norenberg M.D.
      Glutamine in the mechanism of ammonia-induced astrocyte swelling.
      Astrocytes have a strategic perivascular location and high water permeability, and therefore their membrane is believed to be the main source of water entry in the brain.
      • Bémeur C.
      • Cudalbu C.
      • Dam G.
      • Thrane A.S.
      • Cooper A.J.L.
      • Rose C.F.
      Brain edema: a valid endpoint for measuring hepatic encephalopathy?.
      Moreover, water transport is the primary function of the main AQPs (plasma membrane water-transporting proteins) in the central nervous system (CNS). AQP-4 is expressed in astrocytic feet, lining the microcapillary endothelial cells of the BBB, and it is involved in water movement, cell volume regulation, cell migration, and neuroexcitation.
      • Rama Rao K.V.
      • Norenberg M.D.
      Aquaporin-4 in hepatic encephalopathy.
      • Papadopoulos M.C.
      • Verkman A.S.
      Aquaporin water channels in the nervous system.
      Accordingly, increased expression of AQP-4 has been shown to correlate with the development of brain edema in several diseases.
      • Bémeur C.
      • Cudalbu C.
      • Dam G.
      • Thrane A.S.
      • Cooper A.J.L.
      • Rose C.F.
      Brain edema: a valid endpoint for measuring hepatic encephalopathy?.
      • Rama Rao K.V.
      • Norenberg M.D.
      Aquaporin-4 in hepatic encephalopathy.
      Pathologically speaking, HE is characterized by astrocyte swelling, leading to brain edema. In acute HE (encephalopathy associated with acute liver failure [ALF]
      • Blei A.T.
      • Ferenci P.
      • Lockwood A.
      • Mullen K.
      • Tarter R.
      • Weissenborn K.
      Hepatic encephalopathy - definition, nomenclature, diagnosis, and quantification: final report of the working party at the 11th world congresses of gastroenterology, Vienna, 1998.
      ), brain edema occurs in the majority of patients to some degree and contributes to increased intracranial pressure, which can lead to brainstem herniation in the most severe cases.
      • Scott T.R.
      • Kronsten V.T.
      • Hughes R.D.
      • Shawcross D.L.
      Pathophysiology of cerebral oedema in acute liver failure.
      • Rama Rao K.V.
      • Jayakumar A.R.
      • Norenberg M.D.
      Brain edema in acute liver failure: mechanisms and concepts.
      • Donovan J.P.
      • Schafer Jr., D.F.
      • S B.W.
      • Sorrell M.F.
      Cerebral oedema and increased intracranial pressure in chronic liver disease.
      • Chavarria L.
      • Alonso J.
      • Rovira A.
      • Córdoba J.
      Neuroimaging in acute liver failure.
      In chronic HE (encephalopathy associated with cirrhosis and portal hypertension/or portal-systemic shunts
      • Blei A.T.
      • Ferenci P.
      • Lockwood A.
      • Mullen K.
      • Tarter R.
      • Weissenborn K.
      Hepatic encephalopathy - definition, nomenclature, diagnosis, and quantification: final report of the working party at the 11th world congresses of gastroenterology, Vienna, 1998.
      ), magnetic resonance imaging (MRI) techniques have demonstrated that low-grade brain edema appears also to be an important pathological feature, even though intracranial hypertension is rarely observed
      • Bosoi C.R.
      • Rose C.F.
      Brain edema in acute liver failure and chronic liver disease: similarities and differences.
      • McPhail M.J.W.
      • Taylor-Robinson S.D.
      The role of magnetic resonance imaging and spectroscopy in hepatic encephalopathy.
      • Chavarria L.
      • Cordoba J.
      Magnetic resonance imaging and spectroscopy in hepatic encephalopathy.
      • Rovira A.
      • Alonso J.
      • Cordoba J.
      MR imaging findings in hepatic encephalopathy.
      • Grover V.P.
      • Dresner M.A.
      • Forton D.M.
      • et al.
      Current and future applications of magnetic resonance imaging and spectroscopy of the brain in hepatic encephalopathy.
      • Mcphail M.J.W.
      • Thomas H.C.
      • Taylor-robinson S.D.
      Magnetic resonance studies of the brain in liver disease.
      • Chavarria L.
      • Cordoba J.
      Magnetic resonance of the brain in chronic and acute liver failure.
      (for more details please see Table 1, Table 2, Table 3). Edema in acute HE is believed to be mainly cytotoxic,
      • Scott T.R.
      • Kronsten V.T.
      • Hughes R.D.
      • Shawcross D.L.
      Pathophysiology of cerebral oedema in acute liver failure.
      • Rama Rao K.V.
      • Jayakumar A.R.
      • Norenberg M.D.
      Brain edema in acute liver failure: mechanisms and concepts.
      whereas in chronic HE, low-grade edema is also associated with Alzheimer type II changes as a morphological counterpart of astrocyte swelling.
      • Norenberg M.D.
      A light and electron microscopic study of experimental portal-systemic (ammonia) encephalopathy. Progression and reversal of the disorder.
      It is important to emphasize that labeling a particular case of edema as “vasogenic” or “cytotoxic” cannot be rigidly applied, since it is unusual for only one of the two mechanisms to exist in isolation.
      • Klatzo I.
      Pathophysiological aspects of brain edema.
      Overall, one type of edema will gradually lead to the development of the other type. This is also the case in HE where the two types of edema might coexist.
      • Bosoi C.R.
      • Rose C.F.
      Brain edema in acute liver failure and chronic liver disease: similarities and differences.
      • Chavarria L.
      • Alonso J.
      • García-Martínez R.
      • et al.
      Biexponential analysis of diffusion-tensor imaging of the brain in patients with cirrhosis before and after liver transplantation.
      Nevertheless, knowledge of the relative contribution of these two mechanisms in the various phases of edema development might be useful in understanding the dynamics of brain edema and theoretically, in designing useful means of clinical management.
      Table 1Summary of the Main Results Published to Date Using Different Ex Vivo Techniques in Chronic HE Animal Models.
      Animal modelSubjects (n)MethodBrain regionType of measurementFindingsCommentsRef
      EdemaType of edema Cell typeOther
      BDL rats











      Sham rats
      8







      9



      8-10
      Gravimetry, 3 weeks post-BDL





      GFAP staining



      HPLC – osmolytes



      Behavior studies
      CC, 2mm2







      FC, PC
      Ex-vivo, end point







      Ex-vivo, end point



      Ex-vivo, end point
      Direct, absolute assessment of water content



      Indirect indication
      N/A







      Direct evidence, astrocytes
      No change in water content = 79.73±0.12%







      No changes in GFAP staining in BDL rats



      Minor and non-significant changes in brain Gln and Ins
      No change in plasma and brain ammonia (122±70 μmol/L in plasma and 0.29±0.18μmol/g in brain of BDL)

      Mild impairment of motor coordination and a spontaneous motor activity in BDL rats



      LPS: ↑brain water content and Alzheimer type II astrocytes
      • Jover R.
      • Rodrigo R.
      • Felipo V.
      • et al.
      Brain edema and inflammatory activation in bile duct ligated rats with diet-induced hyperammonemia: a model of hepatic encephalopathy in cirrhosis.
      BDL rats

      Sham rats

      7

      6



      Gravimetry, 4 weeks post-BDL





      Ex-vivo 1H MRS, no information on quantification



      Electron microscopy

      Assessment of level of consciousness
      FC, CC – 2mm2Ex-vivo, end point







      Ex-vivo, end point







      Ex-vivo, end point
      Direct, absolute assessment of water content

      N/A















      Direct evidence- cytotoxic edema, astrocytes
      No change in water content = 79.9±0.27%





      ↓ Gln, NAA







      Partially collapsed microvessel

      Intact BBB

      ↑ plasma (168±14μmol/L) and brain (1.0±0.36μmol/g) ammonia

      No neurological modifications in BDL rats

      Among the very few reports showing a ↓Gln





      Minimal water accumulation in astrocytic, perivascular tissue

      LPS injection ↑brain water content and lead to a deterioration of tin the conscious level
      • Wright G.
      • Davies N.A.
      • Shawcross D.L.
      • et al.
      Endotoxemia produces coma and brain swelling in bile duct ligated rats.
      BDL rats

      Sham rats
      6

      6
      Gravimetry, 6 weeks post-BDL





      Locomotor activity
      FC, 2mm3Ex-vivo, end pointDirect, absolute assessment of water contentN/A

      ↑water content = 79.46±0.28% (BDL) vs 78.35±0.17% (sham)



      Allopurinol treatment decreased arterial ROS and brain edema but did not improve liver function nor fully restored locomotor activity-edema is not the only cause of HE
      ↑ arterial (119.7±15.2μM) and CSF (128.4±36.7μM) ammonia

      HA does not induce OS independently nor brain edema

      In combination systemic OS and HA stimulate an ↑water content

      Systemic OS is a result of primary liver injury

      • Bosoi C.R.
      • Yang X.
      • Huynh J.
      • et al.
      Systemic oxidative stress is implicated in the pathogenesis of brain edema in rats with chronic liver failure.
      BDL rats



      Sham rats
      7



      6
      Gravimetry, 6 weeks post-BDLFC, 1mm3Ex-vivo, end pointDirect, absolute assessment of water contentN/A

      -no significant change in water content between BDL and sham ratsExact water content difficult to assess from the graph = 81.5-82.5% (BDL)



      LPS injection ↑brain water content
      • Chavarria L.
      • Oria M.
      • Romero-Gimenez J.
      • Alonso J.
      • Lope-Piedrafita S.
      • Cordoba J.
      Brain magnetic resonance in experimental acute-on-chronic liver failure.
      BDL rats

      Sham rats
      No indication on number of rats was foundGravimetry, 6 weeks post-BDL





      Ex vivo 1H MRS, no information on quantification

      Ex vivo fluorescence
      FCEx-vivo, end point







      Ex-vivo, end point
      Direct, absolute assessment of water contentN/A↑water content







      ↑Gln, Glu, Tau

      ↓Ins

      sum of osmolytes

      ↑brain Lac, ↑CSF ammonia

      AST-120 and DCA treatments ↓ brain edema, Lac but not brain Gln

      Only AST-120 ↓ CSF ammonia
      Exact water content was difficult to assess from the graph = 78-79% (BDL)

      Correlations: No correlation between CSF ammonia and brain Gln

      Correlation between CSF ammonia and brain Lac

      ↑brain Lac and not Gln is a key factor in pathogenesis of brain edema together with impaired compensatory osmoregulatory mechanisms
      • Bosoi C.R.
      • Zwingmann C.
      • Marin H.
      • et al.
      Increased brain lactate is central to the development of brain edema in rats with chronic liver disease.
      BDL rats



      Sham rats
      6 groups (6/group)

      3 groups (6/group)



      Dry weight technique, 4 weeks post-BDL









      Assessment of level of consciousness
      50 mm2

      wet FC
      Ex-vivo, end point

      Direct, absolute assessment of water contentN/A

      No change in water content in BDL rats

      ↑water content in shams +HD and shams+LPS

      ↑water content in BDL+HD and BDL+HD+LPS

      ↓ water content after administration of OP and OP + infliximab

      ↑arterial and brain ammonia in HD and BDL rats; and ↓ after OP (±infliximab)

      ↓arterial ammonia with OP may prevent LPS induced worsening of HE and brain edema.



      Exact values of water content and ammonia were difficult to assess from the graphs
      • Wright G.
      • Vairappan B.
      • Stadlbauer V.
      • Mookerjee R.P.
      • Davies N.A.
      • Jalan R.
      Reduction in hyperammonaemia by ornithine phenylacetate prevents lipopolysaccharide-induced brain edema and coma in cirrhotic rats.
      BDL rats







      Sham rats
      9 groups (6-8/group)



      2 groups (7/group)

      Dry weight technique, 4 weeks post-BDL





      Ex vivo 1H MRS, no information on quantification
      50 mm2

      wet FC (GM)
      Ex-vivo, end pointDirect, absolute assessment of water contentN/A

      ↑plasma ammonia in BDL rats (67±6 to 186±20 μmol/L)

      ↑water content in BDL rats

      No change in brain Gln in BDL rats

      ↓ brain mIns in BDL rats

      OP treatment: ↓brain water content and plasma ammonia, no change in brain Gln or mIns,
      Exact values of water content were difficult to assess from the graphs (∼76% in Shams and ∼78% in BDL)
      • Davies N.A.
      • Wright G.
      • Ytrebø L.M.
      • et al.
      L-ornithine and phenylacetate synergistically produce sustained reduction in ammonia and brain water in cirrhotic rats.
      Abbreviations: Frontal cortex (FC), Cerebral cortex (CC), parietal cortex (PC), gray matter (GM), oxidative stress (OS), reactive oxygen species (ROS), blood brain barrier (BBB), hepatic encephalopathy (HE), cerebrospinal fluid (CSF), lactate (Lac), glutamine (Gln), taurine (Tau), inositol (Ins), myo-inositol (mIns), glutamate (Glu), lipopolysaccharide (LPS), hyperammonemia (HA), glial fibrillary acidic protein (GFAP), bile duct ligation (BDL), ornithine phenylacetate (OP), oral ammonia absorbent engineered activated carbon microspheres (AST-120), dichloroacetate (DCA), proton magnetic resonance spectroscopy (1H MRS), high protein/ammoniagenic diet (HD). Authors personal comments are in italics in the comments row.
      Table 2Summary of the Main Results Published to Date Now Using Different In Vivo MRI/MRS Techniques in Chronic HE Animal Models.
      Animal modelSubjects (n)Magnetic Field (B0)MethodBrain regionType of measurementFindingsCommentsRef
      EdemaType of edema Cell typeOther
      BDL rats



      Sham rats
      8



      6
      7T1H MRS, PRESS, TE=12ms



      7 metabolites quantified using LCModel, absolute quantification using water as internal reference

      DTI, 20 directions and 4 b-values (0-1000 s/mm2)
      6.5x6.5x6.5mm3 - No brain region specific







      VC, SC, MC, Hip, Tha, HypoT, Str, NC

      In vivo

      Longitudinal @ 4, 5, 6 weeks post-BDL
      Indirect indication









      In LPS – indication of intra and extra cellular edema supported by no changes in ADC
      N/A



      ↑Gln

      ↓Glu, tCho, tCr, NAA and Ins

      No change in Lac



      -No difference in ADC values between BDL and sham operated rats and neither in water content using gravimetry (Table 1)
      Statistical changes are between-group over the entire time course with LPS injections as last time point and not by individual time points



      LPS injection ↑water content in brain (gravimetry-Table 1)
      • Chavarria L.
      • Oria M.
      • Romero-Gimenez J.
      • Alonso J.
      • Lope-Piedrafita S.
      • Cordoba J.
      Brain magnetic resonance in experimental acute-on-chronic liver failure.
      BDL rats79.4T1H MRS, SPECIAL, TE=2.8ms

      18 metabolites quantified using LCModel, absolute quantification using water as internal reference

      Changes post-BDL always compared to those before BDL (week 0)
      4x7.5x6.5mm3 - No brain region specificIn vivo – longitudinal @ 0, 4, 8 weeks post-BDLIndirect indication

      N/A

      ↑Gln and plasma NH4+ post-BDL

      ↓Ins, tCho @ 8 weeks post-BDL

      ↓Glu, Asp @ 8 weeks post-BDL

      Sum of main brain organic osmolytes @ 8 weeks post-BDL
      Positive correlation between brain Gln and plasma NH4+

      Brain Gln showed stronger correlations than plasma NH4+ with the rest of metabolites
      • Rackayova V.
      • Braissant O.
      • McLin V.A.
      • Berset C.
      • Lanz B.
      • Cudalbu C.
      1H and 31P magnetic resonance spectroscopy in a rat model of chronic hepatic encephalopathy: in vivo longitudinal measurements of brain energy metabolism.
      Abbreviations: visual cortex (VC), sensorimotor cortex (SC), motor cortex (MC), hippocampus (Hip), thalamus (Tha), hypothalamus (HypoT), striatum (Str), nucleus accumbens (NC), lactate (Lac), glutamine (Gln), taurine (Tau), inositol (Ins), glutamate (Glu), total choline (tCho), total creatine (tCr), N-Acetylaspartate (NAA), aspartate (Asp), lipopolysaccharide (LPS), bile duct ligation (BDL), diffusion tensor imaging (DTI), proton magnetic resonance spectroscopy (1H MRS), apparent diffusion coefficient (ADC), SPin ECho, full Intensity Acquired Localized (SPECIAL), point resolved spectroscopy (PRESS), echo time (TE). Authors personal comments are in italics in the comments row.
      Table 3Summary of the Main Results Published to Date Using In Vivo MRI/MRS Techniques in Chronic HE Patients.
      HE typeSubjects (n)Magnetic Field (B0)MethodBrain regionType of measurementFindingsCommentsRef
      Edema measurementType of edema Cell typeOther
      Liver cirrhosis of different origins

      HE I+HE II =overt HE
      13-HE-0

      12-MHE

      10-HE I

      3-HE II

      1.5TFast absolute measurement of cerebral water content,

      TAPIR – T1 measure

      QUTE – quantitative T2 image

      Psychometric testing
      Pu, CR, OWM, FWM, OC, FC, Tha, GP, CN, AL, PLIn vivo - Single point

      Direct, absolute assessment of water content (%)N/A

      -↑0.4% water in HE-0, ↑0.8% in MHE, ↑2.1% in overt HE – WM (FWM, OWM)

      -No significant water content changes in GM, however 1.9%↑ in GP for overt HE
      Correlation between CFF and WM water content
      • Shah N.J.
      • Neeb H.
      • Kircheis G.
      • Engels P.
      • Häussinger D.
      • Zilles K.
      Quantitative cerebral water content mapping in hepatic encephalopathy.
      Mild chronic HE



      Controls
      3



      7
      1.5T1H MRS, STEAM, TE=30ms, quantification of 5 metabolites using the scanner data analysis package and ratios to tCrMidparietal cortex, WM+GM, 12.5-27cm3In vivo - Single point

      N/AN/A-trend of ↑Gln and ↓Cho and Ins

      -no statistics due to small number of patients
      • Kreis R.
      • Farrow N.
      • Ross B.D.
      Localized 1H NMR spectroscopy in patients with chronic hepatic encephalopathy. Analysis of changes in cerebral glutamine, choline and inositols.
      Liver cirrhosis of different origins



      Controls
      5-no HE

      10-mHE

      11-overt HE

      14
      1.5TT1 weighted images

      2D CSI, TE=130ms quantification of 3 metabolites using ratios to Cr

      Psychometric and EEG testing


      BG, temporal and occipital cortex
      In vivo - Single point

      N/AN/A- ↑Glx/Cr and ↓tCho/Cr in patients

      - no change in NAA/Cr

      - stronger ↑Glx/Cr in BG

      - stronger ↓tCho/Cr in occipital cortex
      - patients with no HE – normal spectra

      - patients with overt HE – abnormal spectra
      • Taylor-Robinson S.D.
      • Sargentoni J.
      • Marcus C.D.
      • Morgan M.Y.
      • Bryant D.J.
      Regional variations in cerebral proton spectroscopy in patients with chronic hepatic encephalopathy.
      Liver cirrhosis of different origins

      4-no HE

      7-mHE

      15-overt HE
      1TT1 weighted SE images

      T1 weighted MT images

      BGIn vivo - Single point

      N/AN/AHyperintensity of GP in 17 patients, and a difference between noHE vs mHE vs overt HE

      Hyperintensity of Pu in 5 patients
      Relationship between T1 contrast in GP and blood ammonia
      • Taylor-Robinson S.D.
      • Oatridge A.
      • Hajnal J.V.
      • Burroughs A.K.
      • McIntyre N.
      • deSouza N.M.
      MR imaging of the basal ganglia in chronic liver disease: correlation of T1-weighted and magnetisation transfer contrast measurements with liver dysfunction and neuropsychiatric status.
      Liver cirrhosis of different origins







      Controls
      24-no HE

      4-mHE

      4-HE I

      6-HE II

      1-HE IV

      20
      2TRoutine T1 and T2 weighted images

      1H MRS, PRESS, TE=30ms, quantification of 4 metabolites using a Marquardt curve-fitting algorithm and ratios to Cr

      Neuropsychological tests




      PWM, OGM (2.5cm)3
      In vivo - Single point

      Indirect indication based on ↓mIns/Cr and ↑Gln/Crassumption

      -Astrocytes swelling
      Asymptomatic (no HE) patients GM:

      -↓mIns/Cr

      Subclinical (mHE), overt HE(HE I-IV) GM:

      -↓mIns/Cr, ↑Gln/Cr

      -↑NAA/Cr only in over HE

      Asymptomatic and subclinical HE WM:

      -↓mIns/Cr

      Overt HE (HE I-IV) WM:

      -↓mIns/Cr, ↑Gln/Cr, ↓tCho/Cr
      Correlation between Gln in GM and plasma ammonium (r=0.62)

      No MRS differences between no HE and mHE

      MRS differences between mHE and overt HE

      ↑Gln and ↓mIns with HE grade
      • Laubenberger J.
      • Haussinger D.
      • Bayer S.
      • Gufler H.
      • Hennig J.
      • Langer M.
      Proton magnetic resonance spectroscopy of the brain in symptomatic and asymptomatic patients with liver cirrhosis.
      Liver cirrhosis of different origins





      Controls

      8-HE 0

      7-HE I

      2-HE II



      13
      1.5T1H MRS, STEAM, TE=30ms, quantification of 4 metabolites using peak integration and ratios to Cr



      Neuropsychological tests
      PWM, 18mlIn vivo and longitudinal: 30-60 days after LT or 2weeks after a low protein dietN/A

      N/A

      -↓mIns/Cr and tCho/Cr in HE

      - no change in Glx/Cr

      - no MRS changes observed with diet

      - no MRS changes 30-60 days after LT
      Correlations: mins/Cr and ammonia with the neuropsychological data
      • Huda A.
      • Guze B.H.
      • Thomas M.A.
      • et al.
      Clinical correlation of neuropsychological tests with 1H magnetic resonance spectroscopy in hepatic encephalopathy.
      Liver cirrhosis of different origins6-mHE

      3-overt HE
      1TCoregistered 3D T1 weighted images

      Semiautomated contour and thresholding program



      Neuropsychological tests, EEG
      whole brain and ventriclesIn vivo , longitudinal: 6weeks after lactulose (n=7), before and 24h after TIPSSIndirect indication of low-grade brain swellingN/A

      No structural abnormalities on T1 weighted images

      Change in brain and ventricular size after treatment: ↓brain, ↑ventricles and improved psychometric testing (n=3); ↑brain, ↓ventricles and worsen psychometric testing (n=2)

      Blood ammonia (66-98 μmol/L - mHE; 85-130 μmol/L- overt HE)

      No correlations between MRI, HE and liver function
      • Patel N.
      • White S.
      • Dhanjal N.S.
      • Oatridge A.
      • Taylor-Robinson S.D.
      Changes in brain size in hepatic encephalopathy: a coregistered MRI study.
      Liver cirrhosis of different origins







      Controls

      MHE

      24-MHE

      5-no HE

      5-HE I

      5-HE II



      18

      10
      1.5TDTI, single shot EPI dual SE sequence, b-value of 1000 s/mm2, 10 directions, MD and FA measured













      Neuropsychological tests
      CC, RIC, LIC, CN, Pu, FWM, OWMIn vivo









      Longitudinal: 3weeks after lactulose in 10 MHE and 10 controls
      Indirect indication

      ↑MD suggestive of ↑interstitial brain water
      AssumptionNo HE - ↑MD in CN

      MHE - ↑MD in CC, RIC, LIC, CN

      HE - ↑MD in CC, RIC, LIC, CN, Pu, FWM, OWM

      -no changes in FA





      - ↓MD in MHE after lactulose treatment and no change in FA
      MD ↑ from no HE to gr 2 HE- suggestive of increased water with HE grades

      Correlations between NP and MD in CC, RIC.



      Correlations between NP and MD in CC.

      Extracellular migration of macromolecules during the cellular osmoregulatory response may result in ↑ acculmulation of extracellular fluid
      • Kale R.A.
      • Gupta R.K.
      • Saraswat V.A.
      • et al.
      Demonstration of interstitial cerebral edema with diffusion tensor MR imaging in type C hepatic encephalopathy.
      Viral liver cirrhosis







      Controls
      7 –no HE

      6-HE I

      1-HE II



      12
      1.5TDWI, b-values:0, 300, 600,900 s/mm2CN, Pu, GP, OWM, FWM, PWM, ThaIn vivo - Single point

      Indirect indication of cytotoxic brain edemaAssumption

      ↑ADC in all brain regions except Tha

      Patient with HE II showed the highest ADC values

      No differences in ADC between no-HE and HE I

      Ammonia and related Gln accumulation might contribute to changes in water motility and content
      Correlation between venous ammonia and ADC values in deep gray and WM regions, except CN

      An increase in cell volume reduces the influence of restriction effects on intracellular diffusion pathways leading to ↑ADC
      • Lodi R.
      • Tonon C.
      • Stracciari A.
      • et al.
      Diffusion MRI shows increased water apparent diffusion coefficient in the brains of cirrhotics.
      Liver cirrhosis of different origins9-HE 0

      6-mHE

      6-HE I
      1.5TT1 weighted images

      1H MRS, STEAM, TE=18ms, quantification of 5 metabolites using peak integration and ratios to Cr

      13N –ammonia and FDG PET

      Psychometric examination


      BG, PWM, FGM, 8cm3
      In vivo - Single point

      N/AN/AMRS changes significant if patients divided into Child classes but not in HE classes

      -↓mIns/Cr in all 3 brain regions from Child A to C

      -↓tCho/Cr in BG, GM from Child A to C

      -↑Glx/Cr in BG, WM from Child A to C

      -↑NAA/Cr in WM from Child A to C

      No controls

      Correlations:

      -psychometric HE score with Glx/Cr in BG

      -venous plasma ammonia with MRS in WM

      -cerebral glucose utilization with mIns/Cr
      • Weissenborn K.
      • Ahl B.
      • Fischer-Wasels D.
      • et al.
      Correlations between magnetic resonance spectroscopy alterations and cerebral ammonia and glucose metabolism in cirrhotic patients with and without hepatic encephalopathy.
      Liver cirrhosis of different origins

      27

      1.5TT2 weighted, FSE

      Fast FLAIR images



      Neurologic assessment


      WM
      In vivo, longitudinal: before and after LTIndirect indication of brain edemaN/A-focal lesions were identified on the T2 weighted images before LT compatible with small-vessel brain disease in 19 patients

      - after LT (6-14 months)– average of 21.7% decrease of Wm lesion volumes
      No association between WM lesion, age, cause of cirrhosis, Child-Pugh score or laboratory findings

      Correlation: WM lesions and percent improvement in overall cognitive function
      • Rovira A.
      • Mínguez B.
      • Aymerich F.X.
      • et al.
      Decreased white matter lesion volume and improved cognitive function after liver transplantation.
      Cirrhotic patients with HE3No detailFLAIR imagesWMIn vivo, longitudinalIndirect indication of brain edemaN/A-supratentorial focal and diffuse WM lesions compatible with small-vessel brain disease which reduced with improvement of HE- these changes were associated with brain edema and support the participation of BBB in the pathogenesis of brain edema in HE
      • Mínguez B.
      • Rovira A.
      • Alonso J.
      • Córdoba J.
      Decrease in the volume of white matter lesions with improvement of hepatic encephalopathy.
      Liver cirrhosis of different origins



      Controls
      20-no HE

      10-mHE



      24
      1.5TDWI, single shot EPI sequence









      Neuropsychological tests
      Pu, GP, Tha, posterior cingulate GM, FWM, PWMIn vivo - Single point

      Indirect indication of minimal cellular edema-↑ ADC in mHE in WM compared to no HE

      -no difference in noHE compared to controls for ADC values

      Correlations: ADC in WM with venous ammonia; ADC in WM and neuropsychological tests

      minimal cellular edema with an increase of membrane permeability and increased intracellular diffusivity, as well as changes in the viscosity of the cytoplasm
      • Sugimoto R.
      • Iwasa M.
      • Maeda M.
      • et al.
      Value of the apparent diffusion coefficient for quantification of low-grade hepatic encephalopathy.
      Liver cirrhosis of different origins





      Controls
      33-mHE







      30
      1.5TProton density, T2 weighted images

      T1 weighted images, MPRAGE sequence

      1H MRS, 2D L-COS, TE=30ms, quantification of 13 metabolites using Felix NMR software and ratios to Cr

      Neuropsychological tests




      GP



      Occipital and prefrontal lobe, 27cm3
      In vivo - Single point

      N/AN/A-GP signal intensity

      -↑Glx/Cr in both brain regions

      -↓mICh/Cr, mIns/Cr and Ch_d/Cr in both brain regions
      Correlations between NP tests and MRS ratios

      mICh – most discriminant variable
      • Singhal A.
      • Nagarajan R.
      • Hinkin C.H.
      • et al.
      Two-dimensional MR spectroscopy of minimal hepatic encephalopathy and neuropsychological correlates in vivo.
      Liver cirrhosis and overt HE



      Controls
      41





      16
      1.5TT2 weighted, FSE

      T1 weighted, SE

      DTI, single shot EPI sequence, 6 noncollinear directions, 11b-values (0-7500s/mm2), mono and bi-exponential fitting

      Neuropsychological tests




      PWM, corticospinal tract
      In vivo, longitudinal: before and 1 year after LT (n=24)Indirect indication of increased brain water content based on ↑MDassumption interstitial

      edema
      -↑MD for fast diffusion in PWM which returned to normal after LT

      -↓FA that increased after LT

      -↑MD for fast and slow diffusion in corticospinal tract, only fast MD returned to normal after LT

      -↓ fast FA in corticospinal tract with a persistent decrease after LT
      - edema is reversible after LT but some microstructural changes might persist along the corticospinal tract as suggested by evolution of FA

      - extracellular edema - PWM

      - mixed edema -corticospinal tract

      No association between DTI parameters and neuropsychological tests
      • Chavarria L.
      • Alonso J.
      • García-Martínez R.
      • et al.
      Biexponential analysis of diffusion-tensor imaging of the brain in patients with cirrhosis before and after liver transplantation.
      Viral cirrhosis

      Controls
      28

      28
      3T3D FLAIR sequence

      Brain volume, vertex based shape analysis – FIRST/FSL software

      Total intracranial volume – Gaser’s VBM5 toolbox with SPM5

      Neuropsychological tests


      DGM (NC, Amy, CN, Hip, GP, Pu, Tha)
      In vivo, single pointN/AN/A volume in CN and Pu

      - a smaller volume was proportional to the severity of the disease

      -shape alteration in Pu, CN and GP
      Correlations: decreased DGM volume with poorer cognitive results
      • Lin W.C.
      • Chou K.H.
      • Chen C.L.
      • et al.
      Significant volume reduction and shape abnormalities of the basal ganglia in cases of chronic liver cirrhosis.
      Multiparametric studies / Multimodal studies
      Non-alcoholic cirrhosis





      Controls
      24 (16 with mHE)





      8
      1.5TT2 weighted, FSE

      T1 weighted, IR SE

      MT, 2D GE



      1H MRS, STEAM, TE=20ms, quantification of 5 metabolites using AMARES and ratios to tCr

      Neuropsychological tests




      PWM; FWM



      Parietal WM, 8cm3
      In vivo, single pointIndirect indication of low grade intracellular swelling (↑ water content) based on ↓MTRassumption

      No changes in T2 weighted images

      T1 signal intensity in BG and GP index

      ↓MTR in PWM and FWM

      ↑Glx/Cr in mHE only in PWM

      ↓mIns/Cr and Cho/Cr in all patients in PWM

      No changes in NAA/Cr

      Correlations: MTR with Glx/Cr; MTR with GP index
      • Rovira a
      • Grivé E.
      • Pedraza S.
      • Alonso J.
      Magnetization transfer ratio values and proton MR spectroscopy of normal-appearing cerebral white matter in patients with liver cirrhosis.
      Nalc cirrhosis without overt HE (70% mHE)

      After LT

      Controls
      24





      11

      10
      1.5TT2 weighted, FSE

      T1 weighted, IR SE

      MT, 2D GE



      1H MRS, STEAM, TE=20ms, quantification of 5 metabolites using AMARES and ratios to Cr



      Neuropsychological tests




      PWM; FWM



      Parieto-occipital WM, 8cm3
      In vivo

      Longitudinal: before and after LT at 1 month and 1 year

      Indirect indication of low grade edema (↑ water content) based on ↓MTRN/ANo changes in T2 weighted images

      T1 signal intensity in BG

      ↓MTR in PWM and FWM

      ↑Glx/Cr in mHE only

      ↓mIns/Cr and Cho/Cr in all patients

      No changes in NAA/Cr

      After LT: improvement in MTR; normalization of 1H MRS findings with a lower normalization for mIns/Cr; slower normalization of T1 hyperintensity in GP; neuropsychological impairment showed a rapid improvement
      Correlations between MTR and Glx/Cr and plasma osmolarity

      Glx/Cr and mIns/Cr correlated with liver and neuropsychological function

      No correlation between MTR and neuropsychological function

      Low grade edema and mHE are associated with ↑Gln –manifestations of metabolism of ammonia
      • Córdoba J.
      • Alonso J.
      • Rovira A.
      • et al.
      The development of low-grade cerebral edema in cirrhosis is supported by the evolution of 1H-magnetic resonance abnormalities after liver transplantation.
      PBC stage I-II

      PBS stage III-IV



      Controls
      14

      4



      11
      1.5TSE proton density image

      MT



      1H MRS, PRESS, TE=135ms, quantification of 3 metabolites using the scanner software (Philips)


      GP, CN, Pu, Tha, FWM

      8cm3, in BG and WM
      In vivo - Single point

      N/AN/A

      ↓MTR in GP

      No changes in 1H MRS
      Correlations between MTR and fatigue and MTR and blood manganese

      MTR changes are not a consequence of HE but rather of altered manganese homeostasis
      • Forton D.M.
      • Patel N.
      • Prince M.
      • et al.
      Fatigue and primary biliary cirrhosis: association of globus pallidus magnetisation transfer ratio measurements with fatigue severity and blood manganese levels.
      Liver cirrhosis

      Alcoholics

      Nonalcoholics





      Controls


      26

      16





      18
      1.5T1H MRS, STEAM, TE=20ms,

      5 metabolites quantified using LCModel and ratios to Cr



      MT, 2D GE images







      DWI, single shot SE EPI, b-values: 0-500-1000 s/mm2, 3 directions

      Neuropsychologic examination
      Left OWM and BG, 8cm3





      Tha, pons, OWM, GP, Pu, CN



      Tha, pons, OWM
      In vivo-single pointIndirect indication of ↑water content based on ↓MTRN/A

      Nalc group in BG: ↓mIns/Cr, Cho/Cr and ↑Glx/Cr

      Nalc group in OWM: ↓mIns/Cr and ↑Glx/Cr, NAA/Cr

      Alc group in BG: ↓mIns/Cr, Cho/Cr and ↑Glx/Cr

      Alc group in OWM: ↓mIns/Cr and ↑Glx/Cr

      MRS changes were significant for overt HE and similar in GM and WM

      ↓ MTR in both groups

      No change in ADC only a small trend of with increasing HE
      Correlations in Nalc: mIns/Cr and Glx/Cr with HE in both regions and MTR with HE

      Other correlations are presented

      No correlations in Alc group

      MR differences between Alc and Nalc –possible microstructural lesions due to chronic alcohol abuse
      • Miese F.
      • Kircheis G.
      • Wittsack H.J.
      • et al.
      1H-MR spectroscopy, magnetization transfer, and diffusion-weighted imaging in alcoholic and nonalcoholic patients with cirrhosis with hepatic encephalopathy.
      Liver cirrhosis of different causes and overt HE



      Liver cirrhosis without overt HE





      Controls
      24-overt HE







      9







      9
      1.5TDWI, b-values: 0-500-1000 s/mm2



      MT, 3D GE images







      1H MRS, TE=31ms, no sequence mentioned,

      5 metabolites quantified using AMARES and ratios to Cr
      GP, Pu, Tha, Hip, CR, PGM, PWM









      2x2x2cm3, PWM
      In vivo Longitudinal:24h after diagnosis and 5 days after resolution of HE episodeIndirect indication of ↑water content/low grade edema based on ↓MTR and ↑Glx/Cr, ↓Ins/Crassumption

      -No change in mean ADC between HE and non-HE patients



      ↓MTR in non-HE

      ↓↓MTR in HE in GP and PGM



      Glx/Cr –median =1.8 controls, 2.4 non-HE and 4.4 in HE.

      Ins/Cr – similar between HE and non-HE but lower than controls

      5 days after no change in MTR, Glx/Cr, Ins/Cr but a ↓ADC in PGM
      Correlation between MTR and Glx/Cr in WM in HE patients



      ↓ADC 5 days after – water flux from extracellular to intracellular compartment

      Brain regional difference – WM stronger water increase

      Small number of patients
      • Poveda M.J.
      • Bernabeu Á.
      • Concepción L.
      • et al.
      Brain edema dynamics in patients with overt hepatic encephalopathy. A magnetic resonance imaging study.
      Liver cirrhosis no evidence of overt HE24





      1.5TProton density and T2 weighted FSE

      T1 weighted SE imaging - Brain volume – SIENAX from FSL



      1H MRS, PRESS, TE=30ms, metabolites quantified using LCModel and ratios to Cr



      Neuropsychological assessment (n=52)










      Parieto-occipital WM, 8cm3
      In vivo, single point: 6 to 12 months post LTN/AN/A

      Improvement in neuropsychological tests after LT except for 7 patients

      Brain smaller volume showed poorer function on motor tests

      Bain metabolites were in normal range
      MRI and MRS data only after LT



      HE has an effect on cognitive function after LT, likely because it results in neuronal and brain volume loss
      • Garcia-Martinez R.
      • Rovira A.
      • Alonso J.
      • et al.
      Hepatic encephalopathy is associated with posttransplant cognitive function and brain volume.
      Stable liver cirrhosis of different causes (no-HE+mHE)

      133T3D T1 weighted, T2 weighted and FLAIR

      DTI, EPI, 2 b values:0-1000s/mm2, 6 directions



      1H MRS, PRESS, TE=36ms, 6 metabolites quantified using QUEST/jMRUI and water as internal reference

      Psychometric tests: PHES, CDRS




      WM





      Frontal WM, 8cm3
      In vivo

      Longitudinal at 0, 140 and 170 min after ingestion of amino acid capsules
      Indirect indication of in changes in brain water compartmentalization based on ↑trADCN/A

      No change in the CDRS after challenge

      ↑trADC (9%) after the challenge

      ↓Ins after challenge, no change in Gln, Glu, NAA, Cr, Cho





      No change in brain volume. Ammonia can directly drive changes in water distribution.

      No vasogenic mechanisms
      • Mcphail M.J.W.
      • Dhanjal N.S.
      • Grover V.P.
      • Taylor-robinson S.D.
      • Street S.W.
      Letters to the Editor Ammonia and cerebral water . Importance of structural analysis of the brain in hepatic encephalopathy Reply to: ‘“Ammonia and cerebral water . Importance of structural analysis of the brain in hepatic encephalopathy”’ C reactive.
      No controls

      Correlations: changes in trADC vs blood ammonia, changes in blood ammonia vs brain Gln, changes in trADC and brain Ins



      Glial swelling and redistribution of extra-intracellular water during HA – likely mechanisms of edema in HE
      • Mcphail M.J.W.
      • Dhanjal N.S.
      • Grover V.P.
      • Taylor-robinson S.D.
      • Street S.W.
      Letters to the Editor Ammonia and cerebral water . Importance of structural analysis of the brain in hepatic encephalopathy Reply to: ‘“Ammonia and cerebral water . Importance of structural analysis of the brain in hepatic encephalopathy”’ C reactive.
      • Mardini H.
      • Smith F.E.
      • Record C.O.
      • Blamire A.M.
      Magnetic resonance quantification of water and metabolites in the brain of cirrhotics following induced hyperammonaemia.
      Liver cirrhosis of different causes



      Controls

      6-HE II

      10-HE III

      2-HE IV

      8

      3TProton density and T2 weighted FSE and fast FLAIR

      T1 weighted imaging

      DWI, single shot EPI, 4 b values:0-3000s/mm2



      1H MRS, PRESS, TE=30ms, 5 metabolites quantified using LCModel and ratios to Cr



      HE patients: lactulose and rifaximin-severity grades were lower for the MRI






      PWM, corticospinal tract



      WM-parieto-occipital region, 8cm3
      In vivo –first 5 days after hospitalization

      Longitudinal – 6 weeks later (n=14)

      Indirect indication of extracellular edema based on ↑ADC which returned to normal after 6 weeksassumption↑ADC in patients vs controls

      ↑Gln/Cr in HE patients vs controls (2.4±0.78 vs 0.22±0.08)

      ↓Ins/Cr and Cho/Cr

      No change for Glu/Cr and NAA/Cr



      ↓ADC, ↓Gln/Cr and ↑Ins/Cr after 6 weeks in patients recovering after HE

      ADC in PWM similar to controls but ↑ in corticospinal tract 6 weeks after
      Correlations: Gln/Cr with HE grades, Gln/Cr and blood ammonia

      ↑ADC in patients with dehydration, ↓Ins/Cr in patients with hyponatremia



      Brain edema does not seem to be directly responsible for the neurological manifestation
      • Chavarria L.
      • Alonso J.
      • García-Martínez R.
      • et al.
      Brain magnetic resonance spectroscopy in episodic hepatic encephalopathy.
      Well-compensated liver cirrhosis of different causes and previous mHE





      Controls
      22











      21
      3TVolumetric imaging – 3D T1weighted sequence, SIENA – FSL software FSL

      fMRI, visuomotor task

      1H MRS, PRESS, TE=36ms, 4 metabolites quantified using ratios to Cr

      Psychometric testing: CDRS, PHES








      8cm3, left BG
      In vivo







      Longitudinal: 4weeks after LOLA
      N/AN/A

      No change in brain volume

      No change in activation after visual task before and after LOLA

      Greater activation in motor task after LOLA



      No Change in Glx/Cr, Cho/Cr, Ins/Cr, NAA/Cr pre and post-LOLA
      Improvements in CDRS and PHES after LOLA



      Correlations between the fMRI and psychometric tests



      • McPhail M.J.W.
      • Leech R.
      • Grover V.P.B.
      • et al.
      Modulation of neural activation following treatment of hepatic encephalopathy.
      Liver cirrhosis with mHE203TDTI, single shot SE EPI, b=1000s/mm2, 60 directions, FA, MD –FSL tool



      1H MRS, PROBE, TE=35ms, 4 metabolites quantified using LCModel and ratios to Cr

      fMRI, 2 tasks: N-back and inhibitory control tests

      Cognitive testing
      12 ROI – e.g. FWM, pWM, CC, IC, EC, cingulum

      ACC; pGM, rpWM, 8cm3
      In vivo

      Longitudinal: before and 8 weeks after rifaximin treatment
      N/A↑FA, no change in MD, imply cytotoxic edema correction

      No changes in MD

      Small ↑FA in 5 ROIs after rifaximin





      No metabolite changes before and after rifaximin



      Higher activation in some brain areas after rifaximin
      Improvement in cognitive tests after rifaximin

      Improvement in WM integrity after rifaximin



      No control or placebo group
      • Ahluwalia V.
      • Wade J.B.
      • Heuman D.M.
      • et al.
      Enhancement of functional connectivity, working memory and inhibitory control on multi-modal brain MR imaging with Rifaximin in Cirrhosis: implications for the gut-liver-brain axis.
      Liver cirrhosis with mHE or HE I





      Controls
      30







      16
      3T1H MRS, MEGA-PRESS, TE=68ms, 4 metabolites quantified using LCModel and ratios to Cr



      Fast absolute measurement of cerebral water content
      • Shah N.J.
      • Neeb H.
      • Kircheis G.
      • Engels P.
      • Häussinger D.
      • Zilles K.
      Quantitative cerebral water content mapping in hepatic encephalopathy.




      Psychometric tests
      Occipital lobe, sensory and motor cortex–“hand knob”, 27cm3 eachIn vivo - single point











      Direct, absolute assessment of water content (%)
      N/A

      ↑Gln/Cr in mHE and HE 1 in both voxels

      ↓Ins/Cr in mHE and HE 1 in both voxels compared to controls

      ↑GSx/Cr in mHE and HE 1

      ↓GABA/Cr in mHE and HE1 in occipital lobe

      No change in water content

      MEGA-PRESS sequence was optimized for GABA and not glutathione.
      Correlations: Gln/Cr with blood ammonia and CFF; Ins/Cr with ammonia and CFF, ↑GSx/Cr with ammonia

      Several other correlations are mentioned

      Edema is only marginally responsible for symptoms of covert HE
      • Oeltzschner G.
      • Butz M.
      • Wickrath F.
      • Wittsack H.J.
      • Schnitzler A.
      Covert hepatic encephalopathy: elevated total glutathione and absence of brain water content changes.
      Liver cirrhosis

      Alc (n=46)



      Nalc (n=102)





      No Controls


      19-no HE

      27-HE

      48-no HE

      44-HE

      1.5T

      Two sites
      T1 weighted images (MPRAGE) -VBM using FSL-VBM

      DTI, single shot SE EPI, b=1000s/mm2, 30 directions, FA, MD, CS –FSL tool



      1H MRS, PRESS, TE=35ms, 4 metabolites quantified using LCModel and ratios to Cr





      13 ROI – e.g. FWM, pWM, CC, IC, cingulum



      ACC; pGM, rpWM, 8cm3
      In vivo

      Longitudinal: 1 year after
      Indirect indication of interstitial edema based on ↑MD and CSassumption

      GM density reduced in Alc vs Nalc

      Alc vs Nalc: ↓FA, ↑MD, ↑CS in all ROI

      HE status affects Nalc (FA and CS)

      Alc vs Nalc: ↑Glx, ↓Ins (rpWM, ACC), ↓Ins (pGM)

      no HE: ↑Glx, ↓Ins

      HE: no difference

      In Nalc HE: ↑Glx in all 3 regions
      No changes in brain metabolites 1 year later













      • Ahluwalia V.
      • Wade J.B.
      • Moeller F.G.
      • et al.
      The etiology of cirrhosis is a strong determinant of brain reserve: a multimodal magnetic resonance imaging study.
      Liver cirrhosis



      Controls
      7-no HE

      7-mHE

      6
      3TT2 weighted, FLAIR and T1 weighted images (MPRAGE/SPGR sequence)

      DWI*

      MT*



      Neuropsychological tests

      Blood ammonia and cytokines






      FWM, PWM, IC, BG
      In vivo Longitudinal: 8 weeks after lactulose and rifaximin treatmentIndirect indication of low-grade brain edema in mHE based on ↓MTRN/A

      Diffuse atrophy–47.9% of patients

      Hyperintensity in BG-60.8% of patients

      No DWI results

      ↓ MTR in mHE in FWM, PWM, IC and BG compared to controls

      ↓ MTR in mHE compared to non HE – PWM, IC, BG

      ↑MTR after treatment except for BG in mHE

      No change in MTR in no HE after treatment
      Correlations:

      -IL-6 with MTR in PWM and IC

      -ammonia with MTR in PWM

      -NP with MTR in PWM, IC

      -no correlations after treatment



      ↑ammonia in mHE and noHE with mHE>no HE

      ↑IL-1 and IL-6 in mHE
      • Rai R.
      • Ahuja C.K.
      • Agrawal S.
      • et al.
      Reversal of low-grade cerebral edema after lactulose/rifaximin therapy in patients with cirrhosis and minimal hepatic encephalopathy.
      Cirrhotic patients of different causes





      Controls
      26







      19
      3TVolumetric imaging – 3D T1weighted sequence,

      T2 weighted sequence



      DTI, single-shot EPI sequence, 32 directions, b=1000s/mm2, ADC and FA measured, DTI Studio software

      MT, 2D GE, ImageJ software



      Psychometric testing








      Genu, body and splenium of CC, ACR, PCR

      FWM, Pu, GP, Tha, CN

      In vivo-single pointIndirect indication based on ↓MTR and ↑ADCAssumption

      No change in total brain volume



      ↑ADC in genus and body of CC

      No difference in FA











      ↓MTR in GP (5.8%), FWM (4%), CN, Pu,



      8 patients had mHE
      Trend of ↓MTR in mHE compared with other patients in FWM in GP

      Trend of ↓MTR in patients with alcohol-related disease



      ↓MTR and ↑ADC might demonstrate cytoplasmic changes of astrocytes

      Changes in astrocytes membrane permeability /redistribution of macromolecules
      • Grover V.P.B.
      • Crossey M.M.E.
      • Fitzpatrick J.A.
      • et al.
      Quantitative magnetic resonance imaging in patients with cirrhosis: a cross-sectional study.
      Well-compensated liver cirrhosis of different causes







      Controls
      22











      22
      3TVolumetric imaging – 3D T1weighted sequence, FMRIB software (FSL)

      T2 weighted sequence

      DTI, single-shot EPI sequence, 15 directions, b=1000s/mm2, ADC and FA measured, DTI Studio software

      MT, 2D GE, ImageJ software



      1H MRS, PRESS, TE=36ms, 5 metabolites quantified using AMARES and ratios to Cr

      Psychometric testing
      FWM, Pu, GP, Tha, CN





      Genu, body and splenium of CC







      15x15x15mm3, left BG
      In vivo







      Longitudinal: 4weeks after LOLA
      N/AN/A

      No change in total brain volume







      No change in ADC or FA nor in their relation to neuropsychiatric status

      ↓MTR in GP, Tha in patients with cirrhosis

      ↓MTR in FWM only in mHE

      No change in metabolite ratios



      7 patients out of 22 had mHE
      Psychometric performance was improved in 4 mHE patients after LOLA.

      No other changes were found after LOLA
      • Grover V.P.B.
      • McPhail M.J.W.
      • Wylezinska-Arridge M.
      • et al.
      A longitudinal study of patients with cirrhosis treated with L-ornithine L-aspartate, examined with magnetization transfer, diffusion-weighted imaging and magnetic resonance spectroscopy.
      There are many studies implicating brain edema in the pathogenesis of HE; in patients with acute HE, brain edema has been shown to play a crucial role in the associated neurological deterioration.
      • Chavarria L.
      • Alonso J.
      • Rovira A.
      • Córdoba J.
      Neuroimaging in acute liver failure.
      Patients who have cirrhosis with chronic HE may present with some degree of brain edema,
      • McPhail M.J.W.
      • Taylor-Robinson S.D.
      The role of magnetic resonance imaging and spectroscopy in hepatic encephalopathy.
      • Chavarria L.
      • Cordoba J.
      Magnetic resonance imaging and spectroscopy in hepatic encephalopathy.
      • Rovira A.
      • Alonso J.
      • Cordoba J.
      MR imaging findings in hepatic encephalopathy.
      • Grover V.P.
      • Dresner M.A.
      • Forton D.M.
      • et al.
      Current and future applications of magnetic resonance imaging and spectroscopy of the brain in hepatic encephalopathy.
      • Mcphail M.J.W.
      • Thomas H.C.
      • Taylor-robinson S.D.
      Magnetic resonance studies of the brain in liver disease.
      • Chavarria L.
      • Cordoba J.
      Magnetic resonance of the brain in chronic and acute liver failure.
      but it is not known if this is a universal finding. In addition, the correlations/associations between brain edema and neurological damage in chronic HE are not yet clearly established, with some studies showing a correlation and others not (for more details please see Table 1, Table 2, Table 3). This leads to the controversial question as to whether brain edema can be considered a valid endpoint in the evaluation of HE.
      • Bémeur C.
      • Cudalbu C.
      • Dam G.
      • Thrane A.S.
      • Cooper A.J.L.
      • Rose C.F.
      Brain edema: a valid endpoint for measuring hepatic encephalopathy?.
      • Chavarria L.
      • Alonso J.
      • García-Martínez R.
      • et al.
      Brain magnetic resonance spectroscopy in episodic hepatic encephalopathy.
      By extension, in rats with bile duct ligation (BDL), a type-C model of HE, brain edema, and HE are present.
      • Bosoi C.R.
      • Yang X.
      • Huynh J.
      • et al.
      Systemic oxidative stress is implicated in the pathogenesis of brain edema in rats with chronic liver failure.
      Other studies suggest that brain edema is not implicated in the pathogenesis of HE; in BDL rats, brain edema was also shown to be absent
      • Chavarria L.
      • Oria M.
      • Romero-Gimenez J.
      • Alonso J.
      • Lope-Piedrafita S.
      • Cordoba J.
      Brain magnetic resonance in experimental acute-on-chronic liver failure.
      with no modifications in their neurological status 4–6 weeks after surgery,
      • Wright G.
      • Davies N.A.
      • Shawcross D.L.
      • et al.
      Endotoxemia produces coma and brain swelling in bile duct ligated rats.
      • Oria M.
      • Chatauret N.
      • Chavarria L.
      • et al.
      Motor-evoked potentials in awake rats are a valid method of assessing hepatic encephalopathy and of studying its pathogenesis.
       while lipopolysaccharide (LPS) injection was shown to increase water content
      • Wright G.
      • Davies N.A.
      • Shawcross D.L.
      • et al.
      Endotoxemia produces coma and brain swelling in bile duct ligated rats.
      and alter the level of consciousness in these rats
      • Wright G.
      • Davies N.A.
      • Shawcross D.L.
      • et al.
      Endotoxemia produces coma and brain swelling in bile duct ligated rats.
      (for more details please see Table 1, Table 2). Moreover, in rats with portocaval anastomosis (PCA), a type-B model of HE (encephalopathy associated with portal-systemic bypass and no intrinsic hepatocellular disease
      • Blei A.T.
      • Ferenci P.
      • Lockwood A.
      • Mullen K.
      • Tarter R.
      • Weissenborn K.
      Hepatic encephalopathy - definition, nomenclature, diagnosis, and quantification: final report of the working party at the 11th world congresses of gastroenterology, Vienna, 1998.
      ), brain edema is not present.
      • Bosoi C.R.
      • Yang X.
      • Huynh J.
      • et al.
      Systemic oxidative stress is implicated in the pathogenesis of brain edema in rats with chronic liver failure.
      • Cauli O.
      • Llansola M.
      • Agustí A.
      • et al.
      Cerebral oedema is not responsible for motor or cognitive deficits in rats with hepatic encephalopathy.
      Finally, in rats with ALF, it was shown that motor tract function did not improve following attenuation of brain edema with the hypertonic solution, mannitol,
      • Oria M.
      • Chatauret N.
      • Chavarria L.
      • et al.
      Motor-evoked potentials in awake rats are a valid method of assessing hepatic encephalopathy and of studying its pathogenesis.
      while an acute injection of ammonia to PCA rats led to severe alterations of motor tract function, without the development of brain edema.
      • Oria M.
      • Chatauret N.
      • Chavarria L.
      • et al.
      Motor-evoked potentials in awake rats are a valid method of assessing hepatic encephalopathy and of studying its pathogenesis.
      It has been suggested that these discrepancies might be model specific (HE type A vs B vs C), since cerebral edema differs in terms of the temporality of the disease.
      • Bémeur C.
      • Cudalbu C.
      • Dam G.
      • Thrane A.S.
      • Cooper A.J.L.
      • Rose C.F.
      Brain edema: a valid endpoint for measuring hepatic encephalopathy?.
      • Bosoi C.R.
      • Rose C.F.
      Brain edema in acute liver failure and chronic liver disease: similarities and differences.
      • Kale R.A.
      • Gupta R.K.
      • Saraswat V.A.
      • et al.
      Demonstration of interstitial cerebral edema with diffusion tensor MR imaging in type C hepatic encephalopathy.
      In chronic HE, there is sufficient time for effective compensation and stabilization of the osmolyte shift to counteract the osmotic imbalance induced by the astrocytic accumulation of glutamine. In acute HE, the natural history of the syndrome is rapid and does not allow the system to compensate for metabolic changes.
      • Kale R.A.
      • Gupta R.K.
      • Saraswat V.A.
      • et al.
      Demonstration of interstitial cerebral edema with diffusion tensor MR imaging in type C hepatic encephalopathy.
      Moreover, in advanced chronic HE, there might be little room for activating additional volume-regulatory mechanisms against future challenges of cell volume (such as infection or neuroinflammation), which might explain the kinetics of HE occurrence and the episodic or persistent appearance of clinically overt cerebral edema in end-stage liver disease.
      • Häussinger D.
      Low grade cerebral edema and the pathogenesis of hepatic encephalopathy in cirrhosis.
      Nevertheless, all these assumptions remain to be determined.
      • Bémeur C.
      • Cudalbu C.
      • Dam G.
      • Thrane A.S.
      • Cooper A.J.L.
      • Rose C.F.
      Brain edema: a valid endpoint for measuring hepatic encephalopathy?.
      Moreover, these results raise the question as to the role of brain edema in the neurological alterations linked to HE and whether HE and brain edema are the manifestations of the same pathophysiological mechanism or of two different cerebral manifestations of brain dysfunction in liver disease. It has been also postulated that brain edema may be a predisposing factor in the development of HE or a terminal complication.
      • Bémeur C.
      • Cudalbu C.
      • Dam G.
      • Thrane A.S.
      • Cooper A.J.L.
      • Rose C.F.
      Brain edema: a valid endpoint for measuring hepatic encephalopathy?.
      • Bosoi C.R.
      • Rose C.F.
      Brain edema in acute liver failure and chronic liver disease: similarities and differences.

      Methods to measure brain edema ex vivo and in vivo in animal models and humans with chronic HE

      Several methods have been used to measure brain water content and consequently brain edema either ex-vivo or in vivo. Some of these methods will be briefly described below, and a summary of the main results published to date are listed in Table 1, Table 2, Table 3.
      Ex vivo measurements of water content using dissected tissue from sacrificed animals (no studies on human HE patients) are performed using the dry/wet weight technique or the specific gravity method.
      • Bémeur C.
      • Cudalbu C.
      • Dam G.
      • Thrane A.S.
      • Cooper A.J.L.
      • Rose C.F.
      Brain edema: a valid endpoint for measuring hepatic encephalopathy?.
      • Marmarou A.
      • Poll W.
      • Shulman K.
      • Bhagavan H.
      A simple gravimetric technique for measurement of brain edema.
      • Hayazaki K.
      • Matsuoka Y.
      Variation in Equation Coefficients in the Gravimetric Method to Determine Brain Water Content.
      The advantage of these two techniques is that both of them allow a direct/absolute estimation of the water amount in the brain. However, these techniques do not provide any information on the type of edema and they are endpoint measurements. Therefore, no longitudinal measurements on the same animal are possible. Table 1 presents a summary of the results published to date on type C HE animal models, while more details on these two techniques can be found in the published literature.
      • Bémeur C.
      • Cudalbu C.
      • Dam G.
      • Thrane A.S.
      • Cooper A.J.L.
      • Rose C.F.
      Brain edema: a valid endpoint for measuring hepatic encephalopathy?.
      • Bosoi C.R.
      • Rose C.F.
      Brain edema in acute liver failure and chronic liver disease: similarities and differences.
      The gravimetry technique appears to be most widely used and to have some advantages, such as a better specificity, together with the possibility of being able to use a smaller quantity of samples.
      • Bémeur C.
      • Cudalbu C.
      • Dam G.
      • Thrane A.S.
      • Cooper A.J.L.
      • Rose C.F.
      Brain edema: a valid endpoint for measuring hepatic encephalopathy?.
      • Bosoi C.R.
      • Rose C.F.
      Brain edema in acute liver failure and chronic liver disease: similarities and differences.
      However, at the time of writing, there are only a few published studies using these techniques, and the results appear to be controversial. At 3 or 4 weeks post-BDL, no increase in water content was measured in BDL rats using the gravimetry technique, while an increase in water content at 4 weeks post-BDL was measured using the dry-wet technique (from ∼76% in sham operated rats to ∼78% in BDL rats). At 6 weeks only, one group measured an increase in brain water content using the gravimetry technique (from 78.35 ± 0.17% in sham-operated rats to 79.46 ± 0.28% in BDL rats), while others did not observe this (please see Table 1 for more details).
      In vivo measurements of water content use several MRI or magnetic resonance spectroscopy (MRS) techniques, which have the main advantage of being non-invasive and thus allowing studies on the same individual longitudinally. The phenomenon of nuclear magnetic resonance (NMR) is based on the interaction of magnetic moments of nuclei of different atoms within the main (static) magnetic field (B0, usually expressed in Tesla). The magnetic moment of nuclei is associated with a nuclear spin (a form of angular momentum) characterized by a value called a spin number. The nucleus is defined by its number of protons and neutrons and its total nuclear spin. Nuclei with an odd number of protons or neutrons possess a non-zero spin and magnetic moment. Some of these nuclei have a spin number of ½ (e.g. 1H, 31P, 13C, and 15N), which is favorable for applications of magnetic resonance.
      • Mlynárik V.
      Introduction to nuclear magnetic resonance.
      MRI is mainly focused on imaging the hydrogen nucleus (1H) of water, since water is present in high concentrations in biological tissues, and 1H is the most sensitive nucleus in terms of high natural abundance (>99.9%) and intrinsic sensitivity (high gyromagnetic ratio), leading to a high signal-to-noise ratio. MRI techniques are presently available to detect subtle functional or structural changes in the human brain. The only MRI method allowing a direct in vivo water content measurement is brain water mapping, and this technique appears to be able to detect changes of approximately 1% in total brain water content, but it lacks specificity in relationship to the etiology of the water accumulation.
      • Shah N.J.
      • Neeb H.
      • Kircheis G.
      • Engels P.
      • Häussinger D.
      • Zilles K.
      Quantitative cerebral water content mapping in hepatic encephalopathy.
      Indirect or relative information regarding the content of water in the brain can be obtained using magnetization transfer (MT), diffusion-weighted or diffusion-tensor imaging (DWI or DTI), fast fluid-attenuated inversion recovery (FLAIR) MRI methodologies and MRS. All these techniques can provide some evidence of increased water content in HE, but they lack specificity in drawing conclusions about absolute water content changes, in addition to elucidating the origin of these perturbations in the brain. Therefore, these changes provide insight and pointers toward pathological mechanisms but are mainly interpretable simply as imaging manifestations of brain edema.
      • Bémeur C.
      • Cudalbu C.
      • Dam G.
      • Thrane A.S.
      • Cooper A.J.L.
      • Rose C.F.
      Brain edema: a valid endpoint for measuring hepatic encephalopathy?.
      • McPhail M.J.W.
      • Taylor-Robinson S.D.
      The role of magnetic resonance imaging and spectroscopy in hepatic encephalopathy.
      • Chavarria L.
      • Cordoba J.
      Magnetic resonance imaging and spectroscopy in hepatic encephalopathy.
      • Rovira A.
      • Alonso J.
      • Cordoba J.
      MR imaging findings in hepatic encephalopathy.
      • Grover V.P.
      • Dresner M.A.
      • Forton D.M.
      • et al.
      Current and future applications of magnetic resonance imaging and spectroscopy of the brain in hepatic encephalopathy.
      • Mcphail M.J.W.
      • Thomas H.C.
      • Taylor-robinson S.D.
      Magnetic resonance studies of the brain in liver disease.
      • Córdoba J.
      • Sanpedro F.
      • Alonso J.
      • Rovira A.
      1H magnetic resonance in the study of hepatic encephalopathy in humans.

      Volumetric MRI in Chronic HE

      MRI-based brain volumetry has been used in chronic HE to identify volume changes in a quantitative manner (total brain volume and/or specific brain regions) from T1-weighted structural MR images (Table 3). These volumetric methods are mainly based on brain segmentation (separation into non-brain and brain tissue, with the latter being sub-segmented into gray matter [GM], white matter [WM], and cerebrospinal fluid [CSF]).
      • Giorgio A.
      • De Stefano N.
      Clinical use of brain volumetry.
      As the position of the patient and, possibly, the shape and size of the brain are likely to have changed between examinations, co-registration is needed in longitudinal assessments, and this involves several MRI head images as a starting point. Advanced software packages can align or register brain images and delineate or segment tissue boundaries between CSF, cerebral WM, and GM.
      • Klauschen F.
      • Goldman A.
      • Barra V.
      • Meyer-Lindenberg A.
      • Lundervold A.
      Evaluation of automated brain MR image segmentation and volumetry methods.
      The final images can then be used for volumetry or morphometry measures.
      • Giorgio A.
      • De Stefano N.
      Clinical use of brain volumetry.
      • Mietchen D.
      • Gaser C.
      Computational morphometry for detecting changes in brain structure due to development, aging, learning, disease and evolution.
      Qualitative visual assessment of cerebral edema on MRI is usually only possible in ALF.
      • Fridman V.
      • Galetta S.L.
      • Pruitt A.A.
      • Levine J.M.
      MRI findings associated with acute liver failure.
      In minimal chronic HE, quantitative assessment of small percentage volume changes is only possible with advanced brain mapping software packages, where the conflicting effects of alcohol or age-related atrophy are assessed alongside the resultant changes in brain size due to HE. Several software packages are available for performing brain segmentation and volumetry/morphometry (including FSL software library, 3D slicer, SIENA, and SIENAX).
      • Giorgio A.
      • De Stefano N.
      Clinical use of brain volumetry.
      • Smith S.M.
      • Jenkinson M.
      • Woolrich M.W.
      • et al.
      Advances in functional and structural MR image analysis and implementation as FSL.
      • Smith S.M.
      Fast robust automated brain extraction.
      • Jenkinson M.
      • Bannister P.
      • Brady M.
      • Smith S.
      Improved optimization for the robust and accurate linear registration and motion correction of brain images.
      • Reuter M.
      • Schmansky N.J.
      • Rosas H.D.
      • Fischl B.
      Within-subject template estimation for unbiased longitudinal image analysis.
      • Fedorov A.
      • Beichel R.
      • Kalpathy-Cramer J.
      • et al.
      3D slicer as an image computing platform for the quantitative imaging network.
      More details on the methodology behind brain volumetry in the context of HE can be found in the published literature.
      • McPhail M.J.W.
      • Taylor-Robinson S.D.
      The role of magnetic resonance imaging and spectroscopy in hepatic encephalopathy.
      • Chavarria L.
      • Cordoba J.
      Magnetic resonance imaging and spectroscopy in hepatic encephalopathy.
      • Grover V.P.
      • Dresner M.A.
      • Forton D.M.
      • et al.
      Current and future applications of magnetic resonance imaging and spectroscopy of the brain in hepatic encephalopathy.
      • Mcphail M.J.W.
      • Thomas H.C.
      • Taylor-robinson S.D.
      Magnetic resonance studies of the brain in liver disease.
      • Tognarelli J.M.
      • Dawood M.
      • Shariff M.I.F.
      • et al.
      Magnetic resonance spectroscopy: principles and techniques: lessons for clinicians.
      The main volumetric MRI results obtained in chronic HE are summarized in Table 3. Some studies have shown a decrease in brain volume in HE
      • Lin W.C.
      • Chou K.H.
      • Chen C.L.
      • et al.
      Significant volume reduction and shape abnormalities of the basal ganglia in cases of chronic liver cirrhosis.
      • Rai R.
      • Ahuja C.K.
      • Agrawal S.
      • et al.
      Reversal of low-grade cerebral edema after lactulose/rifaximin therapy in patients with cirrhosis and minimal hepatic encephalopathy.
      mainly in GM while others have not.
      • Grover V.P.B.
      • McPhail M.J.W.
      • Wylezinska-Arridge M.
      • et al.
      A longitudinal study of patients with cirrhosis treated with L-ornithine L-aspartate, examined with magnetization transfer, diffusion-weighted imaging and magnetic resonance spectroscopy.
      • Grover V.P.B.
      • Crossey M.M.E.
      • Fitzpatrick J.A.
      • et al.
      Quantitative magnetic resonance imaging in patients with cirrhosis: a cross-sectional study.
      • Mardini H.
      • Smith F.E.
      • Record C.O.
      • Blamire A.M.
      Magnetic resonance quantification of water and metabolites in the brain of cirrhotics following induced hyperammonaemia.
      • McPhail M.J.W.
      • Leech R.
      • Grover V.P.B.
      • et al.
      Modulation of neural activation following treatment of hepatic encephalopathy.
      In addition, a relationship between brain volume and HE was sometimes observed.
      • Lin W.C.
      • Chou K.H.
      • Chen C.L.
      • et al.
      Significant volume reduction and shape abnormalities of the basal ganglia in cases of chronic liver cirrhosis.
      • Garcia-Martinez R.
      • Rovira A.
      • Alonso J.
      • et al.
      Hepatic encephalopathy is associated with posttransplant cognitive function and brain volume.
      It is important to underline that functionally well-compensated patients with cirrhosis showed no brain volume changes. There are a few reasons that could explain these discrepancies: the small number of studies performed to date and the small percentage volume changes associated with chronic HE, where the usage of higher magnetic fields might be more illuminating. The changes in brain volume measured in chronic HE were mainly associated with brain atrophy,
      • Chavarria L.
      • Cordoba J.
      Magnetic resonance imaging and spectroscopy in hepatic encephalopathy.
      but these findings require validation by other groups and additional studies using different multiparametric MRI techniques.

      Magnetization Transfer Imaging in Chronic HE

      MT was developed as a technique for manipulating tissue contrast for better image visualization on MRI,
      • Hajnal J.V.
      • Baudouin C.J.
      • Oatridge a
      • Young I.R.
      • Bydder G.M.
      Design and implementation of magnetization transfer pulse sequences for clinical use.
      • Wolff S.D.
      • Balaban R.S.
      Magnetization transfer contrast (MTC) and tissue water proton relaxation in vivo.
      also allowing an indirect measurement of bound and free water compartments in the brain. MT can be affected by variations in a variety of factors, including heavy metal concentration, membrane fluidity, and total water content.
      • Grover V.P.B.
      • McPhail M.J.W.
      • Wylezinska-Arridge M.
      • et al.
      A longitudinal study of patients with cirrhosis treated with L-ornithine L-aspartate, examined with magnetization transfer, diffusion-weighted imaging and magnetic resonance spectroscopy.
      • Grover V.P.B.
      • Crossey M.M.E.
      • Fitzpatrick J.A.
      • et al.
      Quantitative magnetic resonance imaging in patients with cirrhosis: a cross-sectional study.
      • Rovira a
      • Grivé E.
      • Pedraza S.
      • Alonso J.
      Magnetization transfer ratio values and proton MR spectroscopy of normal-appearing cerebral white matter in patients with liver cirrhosis.
      Of note, MT pulse sequences allow measurement of MT ratios (MTRs), which represent a quantitative tissue characteristic, reflecting the behavior of normally MR-invisible protons, bound to intracellular macromolecules. MTR measurement can detect alterations in brain water content that may not otherwise be seen using standard MR techniques. From a technical perspective, magnetization can be transferred between bound and free water pools bi-directionally through direct interaction between spins, transfer of nuclei, or through direct chemical means. Under normal circumstances, MT is the same in both directions, but MT pulse sequences can be designed to saturate the magnetization in the bound pool, leaving the free pool mostly unaffected. Such saturation of the bound pool causes a substantial reduction in the amount of the magnetization. Consequently, there is little transfer of the magnetization back to the free pool, with the MR longitudinal relaxation time reduced as a consequence.
      In chronic HE, MTR values have shown an overall trend toward decrease and appear to be one of the most consistent MRI findings as shown by the majority of the studies presented in Table 3. The decrease in MTR values has been demonstrated to be present in several brain regions and has been reported to be small in magnitude (around 10%).
      • Rovira A.
      • Alonso J.
      • Cordoba J.
      MR imaging findings in hepatic encephalopathy.
      Therefore, the main interpretation of this decrease includes the presence of low-grade astrocytic/cerebral edema which might also be linked to alterations in membrane permeability and cytoplasmic structure and to subsequent shifts in the distribution of macromolecules and intracellular water, with subtle alterations in intracellular and extracellular edema.
      • Rovira A.
      • Alonso J.
      • Cordoba J.
      MR imaging findings in hepatic encephalopathy.
      • Grover V.P.B.
      • McPhail M.J.W.
      • Wylezinska-Arridge M.
      • et al.
      A longitudinal study of patients with cirrhosis treated with L-ornithine L-aspartate, examined with magnetization transfer, diffusion-weighted imaging and magnetic resonance spectroscopy.
      • Grover V.P.B.
      • Crossey M.M.E.
      • Fitzpatrick J.A.
      • et al.
      Quantitative magnetic resonance imaging in patients with cirrhosis: a cross-sectional study.
      • Rovira a
      • Grivé E.
      • Pedraza S.
      • Alonso J.
      Magnetization transfer ratio values and proton MR spectroscopy of normal-appearing cerebral white matter in patients with liver cirrhosis.
      • Córdoba J.
      • Alonso J.
      • Rovira A.
      • et al.
      The development of low-grade cerebral edema in cirrhosis is supported by the evolution of 1H-magnetic resonance abnormalities after liver transplantation.
      Several other hypothesis have also been put forward. These are linked to damage to myelin or to axonal membrane and deposition of paramagnetic substances.
      • Grover V.P.B.
      • Crossey M.M.E.
      • Fitzpatrick J.A.
      • et al.
      Quantitative magnetic resonance imaging in patients with cirrhosis: a cross-sectional study.
      In addition, some interesting correlations were reported by some studies between MTR values and MRS findings, the globus pallidus index, blood ammonia levels, and serum manganese concentrations, while the correlations with the neuropsychological tests are controversial (Table 3). Additional multiparametric MRI and multimodal studies would be useful to establish a clear link between MTR values and their brain regional dependence, HE severity, MRS-measurable metabolites, and other important findings in chronic HE.

      Diffusion-Weighted/Diffusion-Tensor Imaging in Chronic HE

      DWI/DTI is a MR technique allowing quantification of water molecule movement.
      • Alexander A.L.
      • Lee J.E.
      • Lazar M.
      • Field A.S.
      Diffusion tensor imaging of the brain.
      • Chenevert T.L.
      • Brunberg J.A.
      • Pipe J.G.
      Anisotropic diffusion in human white matter: demonstration with MR techniques in vivo.
      • Le Bihan D.
      Diffusion and Perfusion Magnetic Resonance Imaging: Applications to Fonctional MRI.
      • Le Bihan D.
      The “wet mind”: water and functional neuroimaging.
      Water molecule diffusion follows the principles of Brownian motion. Unconstrained, water molecule movement is random and equal in all directions. This random movement is described as “isotropic”. However, motion of water molecules in structured environments is restricted due to physical surroundings and is described as being “anisotropic” (unequal in all directions). In the brain, the microstructure within GM and WM restricts water molecule movement. On average, water molecules tend to move parallel to WM tracts, as opposed to perpendicular to them.
      • Chenevert T.L.
      • Brunberg J.A.
      • Pipe J.G.
      Anisotropic diffusion in human white matter: demonstration with MR techniques in vivo.
      The molecules' motion in the x, y, and z planes and the correlation between these directions is described by a mathematical construct known as the diffusion tensor.
      • Basser P.J.
      • Mattiello J.
      • LeBihan D.
      MR diffusion tensor spectroscopy and imaging.
      In mathematics, a tensor defines the properties of a three-dimensional ellipsoid, the diffusion tensor describing the magnitude, the degree of anisotropy, and the orientation of diffusion anisotropy. For the diffusion tensor to be determined, diffusion data in a minimum of six non-collinear directions are required. This process is known as DTI. This technique collects detailed information allowing insight into the microstructure found within an area of interest within the brain, whose characteristic features are on the same length scale as the micrometer scale displacement of water molecules. These features may be used to map and characterize the three-dimensional diffusion of water as a function of spatial location. Factors calculated include the mean diffusivity (MD), degree of anisotropy, and direction of the diffusivities.
      • Basser P.J.
      • Mattiello J.
      • LeBihan D.
      MR diffusion tensor spectroscopy and imaging.
      MD is a measure of water diffusivity, dependent upon the surrounding chemical environment and the presence of obstacles to movement at a cellular and subcellular level. In parallel, using differently-weighted DWI images, a measure of diffusion can also be calculated. The different images can be mapped to create an apparent diffusion coefficient (ADC) image.
      • Mori S.
      • Barker P.
      Diffusion magnetic resonance imaging: its principle and applications.
      In chronic HE, where less obvious water shifts may be occurring, there is, nevertheless, a mild increase in ADC in patients with cirrhosis, even when HE may not be clinically overt, as in minimal HE.
      • Lodi R.
      • Tonon C.
      • Stracciari A.
      • et al.
      Diffusion MRI shows increased water apparent diffusion coefficient in the brains of cirrhotics.
      Even though the majority of previously published studies observed an increase in ADC (or MD), the overall interpretation of the diffusion data is difficult and sometimes controversial (Table 3). It is important to note that some studies were unable to report any change in ADC (or MD) values. The overall agreement appears to be linked to an increase in water content. However, some authors tend to believe that this increase in ADC is related to an increase in extracellular water content, others to astrocytes swelling while some believe that it reflects minimal cellular edema with an increase of membrane permeability and increased intracellular diffusivity, as well as changes in the viscosity of the cytoplasm.
      • Sugimoto R.
      • Iwasa M.
      • Maeda M.
      • et al.
      Value of the apparent diffusion coefficient for quantification of low-grade hepatic encephalopathy.
      The very basic interpretation of a two compartment model with intracellular or cytotoxic edema (linked to a decrease in ADC) and extracellular or vasogenic edema (linked to an increase in ADC) is not straightforward and is simplistic in its interpretation. As previously mentioned, it is rare for one of the two mechanisms to exist in isolation, and sometimes cytotoxic and vasogenic edema might coexist. DWI/DTI remains an indirect probe, because extracting quantitative metrics, characterizing the underlying tissue microstructure requires modeling of the diffusion signal. The limited specificity of DTI metrics and the need for biophysical modeling of the tissue to achieve specificity is discussed in the published literature.
      • Jelescu I.O.
      • Budde M.D.
      Design and validation of diffusion MRI models of white matter.

      Proton Magnetic Resonance Spectroscopy in Chronic HE

      In vivo localized proton magnetic resonance spectroscopy (1H MRS) is complementary to MRI and is a powerful technique to investigate brain metabolism of rodents and humans non-invasively and in a longitudinal manner.
      • Öz G.
      • Alger J.R.
      • Barker P.B.
      • et al.
      Clinical proton MR spectroscopy in central nervous system disorders.
      • Cudalbu C.
      • Cooper A.J.L.
      Editorial for the special issue on introduction to in vivo Magnetic Resonance Spectroscopy (MRS): a method to non-invasively study metabolism.
      It provides a spectrum as a readout, consisting of peaks at different resonant frequencies. In single voxel MRS, spectra are acquired from a well-defined volume, positioned in a specific brain region, using a combination of band-selective radiofrequency pulses and magnetic field gradients.
      • Mlynárik V.
      Introduction to nuclear magnetic resonance.
      • Lei H.
      • Xin L.
      • Gruetter R.
      • Mlynárik V.
      Localized single-voxel magnetic resonance spectroscopy, water suppression, and novel approaches for ultrashort echo-time measurements.
      1H MRS is one of the most sensitive techniques, and nearly all brain metabolites contain hydrogen nuclei. An important number of biologically relevant metabolites can be observed and quantified in vivo within minutes. This technique can detect low molecular weight metabolites at concentrations as low as 0.5 mM.
      Reliable quantification of the concentration of known metabolites and the extension of the number of quantifiable metabolites represent the main goal of in vivo 1H MRS.
      • Cudalbu C.
      • Mlynarik V.
      • Gruetter R.
      Handling macromolecule signals in the quantification of the neurochemical profile.
      • Cudalbu C.
      In vivo studies of brain metabolism in animal models of Hepatic Encephalopathy using 1H Magnetic Resonance Spectroscopy.
      • Lanz B.
      • Rackayova V.
      • Braissant O.
      • Cudalbu C.
      MRS studies of neuroenergetics and glutamate/glutamine exchange in rats: extensions to hyperammonemic models.
      • Xin L.
      • Tkáč I.
      A practical guide to in vivo proton magnetic resonance spectroscopy at high magnetic fields.
      • McKay J.
      • Tkáč I.
      Quantitative in vivo neurochemical profiling in humans: where are we now?.
      Accurate and precise quantification of brain metabolites is challenging and depends on hardware performance, pulse sequence design and adjustment, data processing, and quantification strategies. The choice of data processing software is very important, since many algorithms depend on user input, which might lead to inaccuracies. Moreover, published recommendations encourage the usage of quantification algorithms where metabolite concentrations are determined by fitting the measured in vivo 1H MRS spectrum to a linear combination of spectra of individual metabolites (the metabolite basis set).
      • Öz G.
      • Alger J.R.
      • Barker P.B.
      • et al.
      Clinical proton MR spectroscopy in central nervous system disorders.
      In clinical settings, metabolite concentration ratios are often used (mainly ratios to total creatine [tCr]); however, absolute metabolite concentrations are more valuable especially when tCr might change.
      1H MRS was among the first techniques which provided indications of the presence of low-grade cerebral edema in chronic HE by reporting changes in brain organic osmolytes (an increase in glutamine [Gln] concentration, together with a decrease in myo-inositol [mIns] that partially compensates for increased intracellular osmotic pressure).
      • Häussinger D.
      Low grade cerebral edema and the pathogenesis of hepatic encephalopathy in cirrhosis.
      • Häussinger D.
      • Kircheis G.
      • Fischer R.
      • Schliess F.
      • vom Dahl S.
      Hepatic encephalopathy in chronic liver disease: a clinical manifestation of astrocyte swelling and low-grade cerebral edema?.
      The glial localization of these osmolytes suggests a disturbance of astrocyte volume homeostasis.
      • Häussinger D.
      Low grade cerebral edema and the pathogenesis of hepatic encephalopathy in cirrhosis.
      • Häussinger D.
      • Kircheis G.
      • Fischer R.
      • Schliess F.
      • vom Dahl S.
      Hepatic encephalopathy in chronic liver disease: a clinical manifestation of astrocyte swelling and low-grade cerebral edema?.
      • Brusilow S.W.
      • Koehler R.C.
      • Traystman R.J.
      • Cooper A.J.L.
      Astrocyte glutamine synthetase: importance in hyperammonemic syndromes and potential target for therapy.
      However, the information provided by 1H MRS is an indirect evidence of astrocyte swelling.
      A detailed description of the main findings using 1H MRS in chronic HE in human patients can be found in Table 3. In clinical settings, the MRS acquisitions were performed at magnetic fields of 1.5T-3T and echo times (TEs) ≥20 ms, leading to the quantification of few metabolites (e.g. the sum of glutamine and glutamate [Glx], tCr [sometimes also simply called creatine {Cr}], total choline [tCho] and myo-inositol or inositol [mIns or Ins]). It is interesting to note that the stronger changes in brain metabolites (Glx/Cr, mIns/Cr, and tCho/Cr) were observed in overt HE, while in minimal HE, the decrease in mIns/Cr was observed more often than an increase in Glx/Cr. Finally, in functionally well-compensated liver cirrhosis, no significant changes were measured. This raises the question as to whether few metabolite changes occur in well-compensated liver disease patients, or if these changes are very small, and thus they are not detected at lower magnetic fields. Therefore, nowadays the availability of high magnetic fields (≥7T), together with MRS acquisitions at shorter TEs (<10–20 ms) might offer opportunities to better quantify and understand brain metabolites changes in chronic HE. Using this methodology, both in animal models and humans, about 19 brain metabolites can be quantified in the brain: glutamate, Gln, aspartate, γ-aminobutyrate, and glycine (neurotransmitters and associated metabolites); glucose, lactate (Lac), Cr, phosphocreatine, and alanine (markers of energy metabolism); taurine and mIns (markers of osmoregulation); phosphocholine, glycerophosphocholine, phosphoethanolamine, N-acetylaspartate, and N-acetylaspartylglutamate (markers of myelination/cell proliferation); and ascorbate and glutathione (antioxidants).
      • Öz G.
      • Alger J.R.
      • Barker P.B.
      • et al.
      Clinical proton MR spectroscopy in central nervous system disorders.
      • Cudalbu C.
      • Mlynarik V.
      • Gruetter R.
      Handling macromolecule signals in the quantification of the neurochemical profile.
      • Cudalbu C.
      In vivo studies of brain metabolism in animal models of Hepatic Encephalopathy using 1H Magnetic Resonance Spectroscopy.
      • Xin L.
      • Tkáč I.
      A practical guide to in vivo proton magnetic resonance spectroscopy at high magnetic fields.
      • Tkáć I.
      • Gruetter R.
      Methodology of 1 H NMR spectroscopy of the human brain at very high magnetic fields.
      • Lanz B.
      • Rackayova V.
      • Braissant O.
      • Cudalbu C.
      MRS studies of neuroenergetics and glutamate/glutamine exchange in rats: extensions to hyperammonemic models.
      Table 3 also presents some interesting correlations between MRS changes and other MRI or blood parameters. In addition, some brain regional differences were observed in brain metabolites, but this observation requires further validation.
      To date, brain water mapping
      • Shah N.J.
      • Neeb H.
      • Kircheis G.
      • Engels P.
      • Häussinger D.
      • Zilles K.
      Quantitative cerebral water content mapping in hepatic encephalopathy.
      is the direct method for absolute quantification of water content in vivo in humans. In animal models, a multimodal approach is desired combining in vivo and longitudinal measurements with an ex vivo technique assessing the absolute brain water content. This combination provides additional information on the temporal resolution of the onset of brain edema by monitoring the progression of the syndrome longitudinally. None of these techniques provides information on the type of the edema or which cell is involved. Therefore, using parallel electron microscopy or a similar technique would be very useful in animal models.

      Brain edema and HE treatments

      Drug therapy for HE largely focuses on removal of bacterial-derived toxins and manipulating gut flora levels, but underlying precipitating factors, such as gastrointestinal hemorrhage, infections, electrolyte disturbance, renal insufficiency, the use of psychoactive drugs, and the presence of constipation and the advent of ALF must be investigated and treated accordingly.
      • Blei A.T.
      • Cordoba J.
      Hepatic encephalopathy.
      Published studies suggest that probiotics, non-absorbable disaccharides (lactulose and lactitol), and non-absorbable antibiotics (such as rifaximin) can be useful in treating HE and may have an effect on brain water content.
      • Prakash R.K.
      • Kanna S.
      • Mullen K.D.
      Evolving concepts: the negative effect of minimal hepatic encephalopathy and role for prophylaxis in patients with cirrhosis.
      • Dhiman R.K.
      Gut microbiota and hepatic encephalopathy.
      • Rai R.
      • Saraswat V.A.
      • Dhiman R.K.
      Gut microbiota: its role in hepatic encephalopathy.
      • Dhiman R.K.
      • Rana B.
      • Agrawal S.
      • et al.
      Probiotic VSL#3 reduces liver disease severity and hospitalization in patients with cirrhosis: a randomized, controlled trial.
      • Al Sibae M.R.
      • McGuire B.M.
      Current trends in the treatment of hepatic encephalopathy.
      • Solga S.F.
      Probiotics can treat hepatic encephalopathy.
      • Morgan M.Y.
      • Blei A.
      • Grüngreiff K.
      • et al.
      The treatment of hepatic encephalopathy.
      The MRI/MRS results of some studies using different treatment strategies are detailed in Table 3.
      Non-absorbable disaccharides include lactulose and lactitol, which are well-known for their laxative effects; they also reduce the colonic pH and decrease gut mucosal uptake of glutamine.
      • Leeuwen PAM Van
      • Berlo CLH Van
      • Soeters P.B.
      New mode of action for lactulose.
      This reduces synthesis and absorption of ammonia. There has been one study demonstrating a small reduction in brain volume in patients with chronic HE on lactulose
      • Patel N.
      • White S.
      • Dhanjal N.S.
      • Oatridge A.
      • Taylor-Robinson S.D.
      Changes in brain size in hepatic encephalopathy: a coregistered MRI study.
      using a co-registration technique while another study observed a reduction in MD using the same treatment.
      • Kale R.A.
      • Gupta R.K.
      • Saraswat V.A.
      • et al.
      Demonstration of interstitial cerebral edema with diffusion tensor MR imaging in type C hepatic encephalopathy.
      Changes in T2 FLAIR WM lesions and ventricular volumes have been studied in chronic HE patients
      • Mínguez B.
      • Rovira A.
      • Alonso J.
      • Córdoba J.
      Decrease in the volume of white matter lesions with improvement of hepatic encephalopathy.
      and following liver transplantation.
      • Rovira A.
      • Mínguez B.
      • Aymerich F.X.
      • et al.
      Decreased white matter lesion volume and improved cognitive function after liver transplantation.
      Moreover, an improvement in MTR and MD was also observed after liver transplantation,
      • Chavarria L.
      • Alonso J.
      • García-Martínez R.
      • et al.
      Biexponential analysis of diffusion-tensor imaging of the brain in patients with cirrhosis before and after liver transplantation.
      • Córdoba J.
      • Alonso J.
      • Rovira A.
      • et al.
      The development of low-grade cerebral edema in cirrhosis is supported by the evolution of 1H-magnetic resonance abnormalities after liver transplantation.
      while normal MRS spectra were also acquired after liver transplantation.
      • Garcia-Martinez R.
      • Rovira A.
      • Alonso J.
      • et al.
      Hepatic encephalopathy is associated with posttransplant cognitive function and brain volume.
      Rifaximin is a minimally absorbed oral antibiotic with few adverse effects, no reported drug-drug interactions, and a low risk of inducing bacterial resistance.
      • Vilstrup H.
      • Amodio P.
      • Bajaj J.
      • et al.
      Hepatic encephalopathy in chronic liver disease: 2014 practice guideline by the American association for the study of liver diseases and the european association for the study of the liver.
      A multicenter trial published in 2010 found that HE remission was prolonged in rifaximin-treated patients, the drug exhibiting a protective effect, and reducing hospitalization rates.
      • Bass N.M.
      • Mullen K.D.
      • Sanyal A.
      • et al.
      Rifaximin Treatment in Hepatic Encephalopathy.
      Ahluwalia et al. demonstrated a reduction in fractional anisotropy (but not in MD), along with significant improvement in cognition, including working memory, after rifaximin treatment in a group of 16 minimal HE patients, indicating an effect on brain water content.
      • Ahluwalia V.
      • Wade J.B.
      • Heuman D.M.
      • et al.
      Enhancement of functional connectivity, working memory and inhibitory control on multi-modal brain MR imaging with Rifaximin in Cirrhosis: implications for the gut-liver-brain axis.

      Overall pathogenic mechanisms

      In the brain, glutamine synthesis is largely confined to astrocytes.
      • Norenberg M.D.
      Distribution of glutamine synthetase in the rat central nervous system.
      In case of liver disease or shunting, brain ammonium accumulation increases astrocytic Gln, raising intracellular osmotic pressure and leading to astrocyte swelling and brain edema.
      • Bémeur C.
      • Cudalbu C.
      • Dam G.
      • Thrane A.S.
      • Cooper A.J.L.
      • Rose C.F.
      Brain edema: a valid endpoint for measuring hepatic encephalopathy?.
      • Häussinger D.
      Low grade cerebral edema and the pathogenesis of hepatic encephalopathy in cirrhosis.
      • Cudalbu C.
      In vivo studies of brain metabolism in animal models of Hepatic Encephalopathy using 1H Magnetic Resonance Spectroscopy.
      • Brusilow S.W.
      • Koehler R.C.
      • Traystman R.J.
      • Cooper A.J.L.
      Astrocyte glutamine synthetase: importance in hyperammonemic syndromes and potential target for therapy.
      • Bosoi C.R.
      • Zwingmann C.
      • Marin H.
      • et al.
      Increased brain lactate is central to the development of brain edema in rats with chronic liver disease.
      • Rackayova V.
      • Braissant O.
      • McLin V.A.
      • Berset C.
      • Lanz B.
      • Cudalbu C.
      1H and 31P magnetic resonance spectroscopy in a rat model of chronic hepatic encephalopathy: in vivo longitudinal measurements of brain energy metabolism.
      • Braissant O.
      • McLin V.A.
      • Cudalbu C.
      Ammonia toxicity to the brain.
      • Haussinger D.
      • Kircheis G.
      • Fischer R.
      • Schliess F.
      • vom Dahl S.
      Hepatic encephalopathy in chronic liver disease: a clinical manifestation of astrocyte swelling and low-grade cerebral edema?.
      It is generally accepted that in hyperammonemia, excess glutamine compromises astrocyte function and morphology
      • Brusilow S.W.
      • Koehler R.C.
      • Traystman R.J.
      • Cooper A.J.L.
      Astrocyte glutamine synthetase: importance in hyperammonemic syndromes and potential target for therapy.
      and thus participates in the development of HE. Although the relationship between cause and effect, leading to HE, and the related spectrum of neurological symptoms remains unclear, ammonium and glutamine appear to be a common thread in the complex and multifactorial model of HE pathogenesis, since both precipitate a cascade of metabolic events that will ultimately result in the neurological disturbance. Ammonium triggers not only the increase in glutamine which will consequently perturb astrocyte metabolism and increase the intracellular osmotic pressure but also a series of signaling events: oxidative stress, activation of transcription factors, signaling kinases, mitochondrial permeability transition, and alterations in the neuronal processes growth.
      • Norenberg M.D.
      • Rao K.V.
      • Jayakumar A.R.
      Mechanisms of ammonia-induced astrocyte swelling.
      • Häussinger D.
      Low grade cerebral edema and the pathogenesis of hepatic encephalopathy in cirrhosis.
      • Braissant O.
      • McLin V.A.
      • Cudalbu C.
      Ammonia toxicity to the brain.
      • Görg B.
      • Schliess F.
      • Häussinger D.
      Osmotic and oxidative/nitrosative stress in ammonia toxicity and hepatic encephalopathy.
      • Butterworth R.F.
      Neuronal cell death in hepatic encephalopathy.
      • Norenberg M.D.
      • Rama Rao K.V.
      • Jayakumar A.R.
      Signaling factors in the mechanism of ammonia neurotoxicity.
      • Felipo V.
      • Butterworth R.F.
      Neurobiology of ammonia.
      • Liere V.
      • Sandhu G.
      • DeMorrow S.
      Recent advances in hepatic encephalopathy.
      • Rama Rao K.V.
      • Norenberg M.D.
      Glutamine in the pathogenesis of hepatic encephalopathy: the Trojan horse hypothesis revisited.
      • Braissant O.
      Current concepts in the pathogenesis of urea cycle disorders.
      • Cagnon L.
      • Braissant O.
      Hyperammonemia-induced toxicity for the developing central nervous system.
      • Bemeur C.
      • Desjardins P.
      • Butterworth R.F.
      Evidence for oxidative/nitrosative stress in the pathogenesis of hepatic encephalopathy.
      • Bosoi C.R.
      • Rose C.F.
      Oxidative stress: a systemic factor implicated in the pathogenesis of hepatic encephalopathy.
      • Lemberg A.
      • Fernández M.A.
      Hepatic encephalopathy, ammonia, glutamate, glutamine and oxidative stress.
      Moreover, increased astrocytic Gln can lead to the opening of the mitochondrial permeability transition pore
      • Albrecht J.
      • Zielińska M.
      • Norenberg M.D.
      Glutamine as a mediator of ammonia neurotoxicity: a critical appraisal.
      • Rama Rao K.V.
      • Jayakumar A.R.
      • Norenberg M.D.
      Induction of the mitochondrial permeability transition in cultured astrocytes by glutamine.
      and interfere with glutamatergic neurotransmission.
      • Ott P.
      • Vilstrup H.
      Cerebral effects of ammonia in liver disease: current hypotheses.
      More details about Gln-related hypotheses, related evidences, and controversies can be found in study by Brusilow et al.
      • Brusilow S.W.
      • Koehler R.C.
      • Traystman R.J.
      • Cooper A.J.L.
      Astrocyte glutamine synthetase: importance in hyperammonemic syndromes and potential target for therapy.
      In addition, other pathogenic mechanisms are also involved in HE: inflammation, alterations in neurotransmission, cerebral energy disturbances, Lac accumulation, and probably others more.
      • Aldridge D.R.
      • Tranah E.J.
      • Shawcross D.L.
      Pathogenesis of hepatic encephalopathy: role of ammonia and systemic inflammation.
      • Coltart I.
      • Tranah T.H.
      • Shawcross D.L.
      Inflammation and hepatic encephalopathy.
      • Rama Rao K.V.
      • Norenberg M.D.
      Brain energy metabolism and mitochondrial dysfunction in acute and chronic hepatic encephalopathy.
      • Rao K.V.R.
      • Norenberg M.D.
      Cerebral energy metabolism in hepatic encephalopathy and hyperammonemia.
      • Butterworth R.F.
      Effects of hyperammonaemia on brain function.
      • Rackayova V.
      • Braissant O.
      • McLin V.A.
      • Berset C.
      • Lanz B.
      • Cudalbu C.
      H and P magnetic resonance spectroscopy in a rat model of chronic hepatic encephalopathy: in vivo longitudinal measurements of brain energy metabolism.
      • Bak L.K.
      • Schousboe A.
      • Waagepetersen H.S.
      Brain energy and ammonia metabolism.
      • Zwingmann C.
      Nuclear magnetic resonance studies of energy metabolism and glutamine shunt in hepatic encephalopathy and hyperammonemia.
      • DeMorrow S.
      Bile acids in hepatic encephalopathy.
      • Shawcross D.L.
      • Wright G.
      • Olde Damink S.W.M.
      • Jalan R.
      Role of ammonia and inflammation in minimal hepatic encephalopathy.
      • Butterworth R.F.
      The concept of “the inflamed brain” in acute liver failure: mechanisms and new therapeutic opportunities.
      • Wright G.
      • Swain M.
      • Annane D.
      • et al.
      Neuroinflammation in liver disease: sessional talks from ISHEN.
      • Sergeeva O.A.
      GABAergic transmission in hepatic encephalopathy.
      • Jones E.A.
      Ammonia, the GABA neurotransmitter system, and hepatic encephalopathy.
      • Albrecht J.
      • Sidoryk-Węgrzynowicz M.
      • Zielińska M.
      • Aschner M.
      Roles of glutamine in neurotransmission.
      • Butterworth R.F.
      Neurotransmitter dysfunction in hepatic encephalopathy: new approaches and new findings.
      • Bosoi C.R.
      • Rose C.F.
      Elevated cerebral lactate: implications in the pathogenesis of hepatic encephalopathy.
      Even though astrocyte swelling and consequently brain edema are believed to act as a mediator in the neurological manifestations in HE, their pathophysiological role remains elusive. In the past years, several hypotheses have been elaborated regarding the relationship between brain metabolism changes, cellular changes, and cell swelling/edema in HE. The authors of the “osmotic gliopathy theory”
      • Brusilow S.W.
      • Koehler R.C.
      • Traystman R.J.
      • Cooper A.J.L.
      Astrocyte glutamine synthetase: importance in hyperammonemic syndromes and potential target for therapy.
      suggested that there is an initial pronounced osmotic stress in the astrocytes due to increased glutamine synthesis. With time, there is a gradual compensation as reflected by decreased organic osmolytes, and this compensation is accompanied by increased water in the extracellular space. However, this compensation cannot be complete since there is evidence that astrocyte swelling occurs, which may be more pronounced in the more severe disease. The Trojan horse hypothesis
      • Rama Rao K.V.
      • Norenberg M.D.
      Glutamine in the pathogenesis of hepatic encephalopathy: the Trojan horse hypothesis revisited.
      • Albrecht J.
      • Norenberg M.D.
      Glutamine: a Trojan horse in ammonia neurotoxicity.
      is another mechanism by which glutamine is considered to contribute to the pathogenesis of HE. It postulates that glutamine is transported into mitochondria, where it undergoes hydrolysis thus yielding high levels of ammonia and finally resulting in deleterious effects (e.g. induction of the mitochondrial permeability transition and oxidative/nitrative stress leading to astrocyte dysfunction and cell swelling). More details about this theory and related controversies can be found in study by Brusilow et al.
      • Brusilow S.W.
      • Koehler R.C.
      • Traystman R.J.
      • Cooper A.J.L.
      Astrocyte glutamine synthetase: importance in hyperammonemic syndromes and potential target for therapy.
      The transporter hypothesis postulates that increased Gln synthesis coupled with a partial suppression of SNAT3- and SNAT5-mediated efflux of Gln from astrocytes results in an accumulation of Gln in the astrocytic compartment leading to osmotic stress.
      • Desjardins P.
      • Du T.
      • Jiang W.
      • Peng L.
      • Butterworth R.F.
      Pathogenesis of hepatic encephalopathy and brain edema in acute liver failure: role of glutamine redefined.
      It is believed that small increases in astrocytes water content may have an important impact on astrocyte morphology, function, and gene expression despite the absence of clinically overt increases of intracranial pressure in chronic HE.
      • Häussinger D.
      • Kircheis G.
      • Fischer R.
      • Schliess F.
      • vom Dahl S.
      Hepatic encephalopathy in chronic liver disease: a clinical manifestation of astrocyte swelling and low-grade cerebral edema?.
      For example, prolonged osmotic and/or metabolic stress has been shown to cause production of reactive oxygen species, mitochondrial permeability transition, and inflammatory signals, which have physiological and pathophysiological consequences.
      • Bémeur C.
      • Cudalbu C.
      • Dam G.
      • Thrane A.S.
      • Cooper A.J.L.
      • Rose C.F.
      Brain edema: a valid endpoint for measuring hepatic encephalopathy?.
      Altered astrocyte function eventually leads to deranged neuroglial communication and neurotransmitter system imbalance, which will impact synaptic plasticity and oscillatory cerebral networks, thus enabling a pathological environment characterizing HE.
      • Häussinger D.
      Low grade cerebral edema and the pathogenesis of hepatic encephalopathy in cirrhosis.

      Conclusion

      Although some of the discussed studies established a link between brain edema and alterations in cognitive function, the role of brain edema as a neuropathological feature/consequence or cause of HE remains controversial. It was speculated that different degrees of astrocyte swelling or brain edema might have different effects on cerebral function.
      • Bosoi C.R.
      • Rose C.F.
      Brain edema in acute liver failure and chronic liver disease: similarities and differences.
      In addition, brain edema might act synergistically with other pathogenic factors or only be a predisposing or precipitating factor in the development of HE. The in vivo MRI/MRS studies were very helpful in the process of evaluating brain edema in chronic HE and in improving our understanding of the pathophysiological alterations in HE. As can be seen form Table 1, Table 2, Table 3, there is an overall tendency in using multimodal (more than two MRI/MRS techniques) and multiparametric (MRS/MRS studies combined with neurological tests, biochemical analysis) approaches. However, additional in vivo, longitudinal, and multiparametric/multimodal studies are required (in humans and animal models) to elucidate the relationship between liver function, brain metabolism changes, cellular changes, cell swelling/edema, and neurological manifestations in chronic HE. The brain regional difference in chronic HE also remains an open question.

      Conflicts of interest

      The authors have none to declare.

      Acknowledgments

      Financial support was provided by the SNSF project no 310030_173222/1 and by the CIBM ( UNIL , UNIGE , HUG , CHUV , EPFL , as well as the Leenaards and Jeantet Foundations ). SDTR is grateful to the United Kingdom NIHR Biomedical Facility at Imperial College London for infrastructure support.

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