23: Hepatic Principles

The liver plays an essential role in metabolic homeostasis. Hepatic functions include the synthesis, storage, and breakdown of glycogen. In addition, the liver is important in the metabolism of lipids; the synthesis of albumin, clotting factors, and other important proteins; the synthesis of the bile acids necessary for absorption of lipids and lipid soluble vitamins; and the metabolism of cholesterol.^64,159 Hepatocytes facilitate the excretion of metals, most importantly iron, copper, zinc, manganese, mercury, and aluminum, as well as the detoxification of products of metabolism, such as bilirubin and ammonia.^32,74 Generalized disruption of these important functions results in manifestations of liver failure: hyperbilirubinemia, coagulopathy, hypoalbuminemia, hyperammonemia, and hypoglycemia.^86,89,137 Disturbances of more specific functions result in accumulation of lipids, metals, and ­bilirubin, and the development of lipid-soluble vitamin deficiencies.^64,159

The liver is also the primary site of biotransformation and detoxification of xenobiotics. Its interposition between the gut and systemic circulation makes it the first-pass recipient of xenobiotics absorbed from the gastrointestinal tract. The liver receives blood from the systemic ­circulation and participates in the detoxification and elimination of xenobiotics that reach the bloodstream through other routes, such as inhalation or cutaneous absorption.^142,159,160

Many xenobiotics are lipophilic and inert and require chemical ­modification followed by conjugation to make them sufficiently water-soluble to be eliminated. The liver contains the highest concentration of enzymes involved in phase I oxidation-reduction reactions, the first stage of detoxification for many lipophilic xenobiotics. Conjugation of the reactive products of phase I biotransformation with molecules such as glucuronide facilitates excretion (Chap. 13). Although many xenobiotics that are detoxified in the liver are subsequently excreted in the urine, the biliary tract provides a second essential route for the elimination of detoxified xenobiotics and products of metabolism.^32,56,160 Although phase I activation of xenobiotics is usually followed by phase II conjugation that results in detoxification, it can also lead to the production of xenobiotics with increased toxicity, which may cause injury to hepatocytes at the site of their synthesis.^142,160

Two pathologic concepts are used to describe the appearance and function of the liver: a structural one represented by the hepatic lobule and a functional one represented by the acinus. The basic structural unit of the liver as characterized by light microscopy is the hepatic lobule, a hexagon with the central hepatic vein at the center and the portal triads at the angles. The portal triad consists of the portal vein, the common bile duct, and the hepatic artery. Cords of hepatocytes are oriented radially around the central hepatic vein, forming sinusoids. In contrast, the acinus, or “metabolic lobule” is the functional unit of the liver. Located between two central hepatic veins, it is bisected by terminal branches of the hepatic artery and portal vein that extend from the bases of the acini toward hepatic venules at the apices. The acinus is subdivided into three metabolically distinct zones. Zone 1 lies near the portal triad, zone 3 lies near the central hepatic vein, and zone 2 is ­intermediate.^64,159Figure 23–1 illustrates the relationship of the structural and functional concepts of the liver.

Approximately 75% of the blood supply to the liver is derived from the portal vein, which drains the alimentary tract, spleen, and pancreas. This blood is enriched with nutrients and other absorbed xenobiotics and is poor in oxygen. The remainder of the hepatic blood flow comes from the hepatic artery, which delivers well-oxygenated blood from the systemic circulation. Blood from the hepatic artery and portal vein mixes in the sinusoids, coming in close contact with cords of hepatocytes before it exits through small fenestrations in the wall of the vein.¹⁷⁵ Oxygen content diminishes several fold as blood flows from the portal area to the central hepatic vein.⁶⁴

There are six types of cells in the liver. Hepatocytes and bile duct epithelia make up the parenchyma. Cells found in the vicinity of the sinusoids include endothelial cells, fixed macrophages (Kupffer cells), hepatic stellate cells (so-called Ito cells), and a large population of lymphocytes that roam the sinusoids. The sinusoidal lining formed by endothelial cells is thin and fenestrated, allowing transfer of fluid, chylomicrons, and proteins across the space of Disse, an extrasinusoidal space filled with microvilli.¹⁷⁵ Kupffer cells remove particles and cell debris that include bacteria and endotoxin from the portal circulation. They also clear many biologically active substances from the systemic circulation.¹⁶ When immunologically activated by xenobiotics, Kupffer cells contribute to the generation of oxygen free radicals^141,178 and may also participate in the production of autoimmune injury to hepatocytes, including activation of hepatic stellate cells.^36,73,141 Hepatic stellate cells are primary sites for the storage of fat and vitamin A.^48,58 In a quiescent state they spread out between the sinusoidal endothelium and hepatic parenchymal cells. Filled with microtubules and microfilaments, they project cytoplasmic extensions that contact several cell types.^48,58 Activated stellate cells produce collagen, proteoglycans, and adhesive glycoproteins, which are crucial to the development of hepatic fibrosis.^10,46 The liver lymphocyte population is enriched with natural killer (NK) or pit cells, which play a key role in host defense by actively lysing tumor cells and cells infected by viruses^23,47 (Fig. 23–2). Evidence from animal models suggests that NK cells selectively kill activated stellate cells, inhibiting the development of liver fibrosis.^46,47

Bile acids, organic anions, bilirubin, phospholipids, xenobiotics, and other molecules excreted in bile are actively transported across the hepatocyte plasma membrane into the bile canaliculi at sites that have specificity­ for acids, bases, and neutral xenobiotics.¹²¹ Tight junctions separate the contents of the bile canaliculi from the sinusoids and hepatocytes, maintaining a rigid and functionally necessary compartmentalization. Bile acids use three active transport systems: a sodium-dependent bile salt transporter in the sinusoidal membrane, an adenosine triphosphate (ATP)-dependent bile salt carrier in the canalicular membrane, and a canalicular membrane transport site driven by the membrane voltage potential.^79,121 Xenobiotics bound to glucuronide are substrates for the bile acid transport systems and are actively secreted into bile. Xenobiotics with molecular weights greater than 500 Da are also preferentially secreted into bile. Like the transport and concentration of constituents from the sinusoids and hepatocytes, the flow of bile through the canaliculi is also an active process facilitated by ATP-dependent contractions of actin filaments that encircle the canaliculi.^79,121,171

The enterohepatic circulation of bile acids and some vitamins plays a crucial role in their conservation. Unfortunately, this physiologically important process impedes the fecal elimination of some xenobiotics by reabsorbing and returning them back into the systemic circulation, prolonging their apparent half-lives and toxicity. Xenobiotics that have low molecular weights and are not ionized at intestinal pH, such as methylmercury, phencyclidine, and nortriptyline, are most likely to be reabsorbed.^32,140

Due to its location at the end of the portal system and its substantial complement of biotransformation enzymes, the liver is especially vulnerable to toxic injury. The pathological spectrum of liver injury includes combinations of hepatocellular necrosis with focal or generalized lysis of hepatocytes and elevations of aspartate aminotransferase (AST) and alanine aminotransferase (ALT); hepatitis associated with inflammatory cellular infiltrates and varied elevation of hepatocellular enzymes; cholestasis with pruritus, jaundice, and insignificant elevations of hepatocellular enzymes; steatosis caused by intracellular deposits of fat; apoptosis, the formation of shrunken, nonfunctioning, eosinophilic bodies; and fibrosis.¹⁵⁹

Hepatocellular necrosis that occurs near the portal vein is called periportal, or zone 1 necrosis. The term centrilobular or zone 3 necrosis refers to injury that surrounds the central hepatic vein. Figure 23–3 shows centrilobular necrosis caused by exposure to bromobenzene. Metabolic characteristics of the zones of the acinus have important relevance to the anatomic distribution of toxic liver injury. Because of its location in the periportal area, zone 1 has a two-fold higher oxygen content than zone 3. Hepatic injury that results from the metabolic production of oxygen free radicals predominates in zone 1. Allyl alcohol, an industrial chemical that is metabolized to a highly reactive aldehyde, is associated with oxygen dependent lipid peroxidation injury to hepatocytes in zone 1.⁶ The tendency for centrilobular or zone 3 accumulation of fat in patients with alcoholic steatosis is attributed to the effect of relative hypoxia in the central vein area on the oxidation potential of the hepatocyte.^9,95 The availability of substrates for detoxification and the localization of enzymes involved in biotransformation also affect the site of injury. Zone 1 has a higher concentration of glutathione, whereas zone 3 has a greater capacity for glucuronidation and sulfation.¹⁶³ Zone 3 has higher concentrations of alcohol dehydrogenase, which may lead to increased production of its toxic metabolite acetaldehyde at centrilobular sites.^31,95,101 Zone 3 also has high concentrations of cytochrome oxidase CYP2E1, which converts many xenobiotics including acetaminophen (APAP), nitrosamines, benzene, and carbon tetrachloride (CCl₄) to reactive intermediates that may cause centrilobular injury. Although CCl₄ can be metabolized to a highly reactive oxygen free radical in zone 1, it primarily injures zone 3 for the following reasons:^18,36,96 CCl₄ is metabolized by CYP2E1 in zone 3 to a trichloromethyl free radical (·CCl₃) that can form covalent bonds with cellular proteins, cause lipid peroxidation, or spontaneously react with oxygen to form the more highly reactive trichloromethyl peroxy radical (CCl₃OO·).^18,36,96 Higher oxygen tension in zone 1 fosters the formation of CCl₃OO·, which is rapidly detoxified by glutathione. Since the less reactive ·CCl₃ that predominates in zone 3 is not readily detoxified by glutathione, zone 3 incurs the greater amount of injury. Hyperbaric oxygen increases the oxygen tension throughout the liver and decreases liver injury caused by CCl₄, possibly by increasing the formation of CCl₃OO· in zone 3, which is then efficiently detoxified by glutathione.^18,160 The observed effects of isoniazid (an inhibitor of the enzyme CYP2E1) and chronic ethanol intake (an inducer of the CYP2E1 gene) on injury in cell cultures from periportal and centrilobular areas exposed to CCl₄ support the association of CCl₄ injury with the localization of CYP2E1 activity. Acute exposure to isoniazid significantly decreases the injury associated with exposure of zone 3 cells to CCl₄, whereas chronic treatment with ethanol significantly enhances injury.⁹⁶

Xenobiotics that produce liver damage in all humans in a predictable and dose-dependent manner are called intrinsic hepatotoxins. They include APAP, CCl₄, and yellow phosphorous. Those that cause liver damage in a small number of individuals and whose effect is not apparently predictable or dose dependent are called idiosyncratic hepatotoxins.^50,88

Prediction of Drug-Induced Liver Injury

Drug-induced liver injury (DILI) refers to the production of liver injury by pharmaceuticals. It is the most common apparent cause of acute liver failure in the United States.^20,134 DILI causes 33% of terminations of clinical trials of new drugs and accounts for a majority of post-marketing withdrawals of marketed drugs.¹⁶⁹ Recent studies of DILI question the lack of dose dependency of some apparently idiosyncratic hepatotoxins.^{1,20,50, 84} Analysis of a Swedish data base of patients with DILI demonstrated dose dependency, as more reported cases were associated with higher doses of drugs metabolized in the liver.⁸⁴

Prediction of DILI in any individual patient based on clinical trials is difficult because the incidence is rare.^102,169 DILI depends largely on factors that affect host susceptibility.^5,156 These include age, gender, diet, underlying diseases, and concurrent exposure to other xenobiotics.²⁰ Genetic factors are increasingly elucidated. Many enzymes involved in biotransformation show genetic polymorphism. Inherited variations in CYP enzymes affect susceptibility to DILI.^{55,62,87, 97} Approximately 8% of Caucasians are deficient in CYP2D6, which is responsible for the metabolism of a number of pharmaceuticals, including debrisoquine (an antihypertensive first identified as the substrate of this enzyme), several antidepressants and antidysrhythmics, some opioids, and phenformin.^87,150 Perhexiline, an antianginal marketed in Europe in the 1980s, caused severe liver injury and peripheral neuropathy in persons with an inability to metabolize debrisoquine.¹⁵⁰ The congenital disorder that results in Gilbert syndrome is characterized by impaired glucuronyltransferase. Patients demonstrate decreased glucuronidation and increased bioactivation of APAP during chronic therapeutic dosing, suggesting an increased risk of hepatic injury following ingestion of APAP.²⁹ Gastrointestinal toxicity due to irinotecan, an antineoplastic used in the treatment of colon cancer, is associated with deficiency of glucuronyltransferase.¹¹¹

Studies that compare the genomes of patients who develop hyper­sensitivity reactions to various drugs with those who do not show significant associations with human leukocyte antigen (HLA) alleles.^{5,27,53,99,156,164, 169} Hypersensitivity reactions to abacavir are significantly associated with HLA-B*5701, such that the Food and Drug Administration now recommends HLA screening prior to the initiation of abacivir.¹⁰⁴ HLA-B*5701 is also associated with cholestatic DILI in patients taking flucloxacillin, an antibiotic marketed in Europe.²⁷ Severe hypersensitivity reactions to carbamazepine are associated with the HLA allele HLA-A*3101.¹⁰⁶ Specific HLA phenotypes are implicated in the development of cholestatic liver injury in patients exposed to amoxicillin-clavulanate.^5,99,156 DILI in the form of elevation of the ALT occurred in 15% of patients exposed to the first-generation direct thrombin inhibitor ximelagatran, with some developing clinically significant hepatitis. This was associated with two specific HLA phenotypes DRB1’07 and DQA1*02.^69,72

Effects of Xenobiotics on Enzyme Function

Some xenobiotic combinations increase the possibility of hepatotoxic reactions because one xenobiotic alters the metabolism of the other, leading to the production of toxic metabolites. This is the case with combinations of rifampin and isoniazid,^65,143 amoxicillin and clavulanic acid,^85,128 and trimethoprim and sulfamethoxazole.² Changes in the activities of biotransformation enzymes that result in increased formation of hepatotoxic metabolites suggest increased susceptibility to hepatic injury. Chronic administration of isoniazid induces CYP2E1 activity.^143,179 During chronic ethanol exposure, proliferation of the smooth endoplasmic reticulum in the centrilobular areas is associated with increased activity of CYP2E1.^77,96,98 In one rat study, the administration of an average of 9.2 g/kg of ethanol in increasing doses for 4 weeks resulted in a significant increase in the extent of injury to cultured zone 3 hepatocytes when exposed to CCl₄. This was associated with a higher expression of CYP2E1 in the livers of ethanol-treated rats.⁹⁶ Bromobenzene is a xenobiotic whose metabolism and hepatotoxicity are similar to that of APAP. When administered to rats chronically exposed to ethanol, the onset of hepatotoxicity occurs more rapidly in study animals, with only a small increase in the extent of hepatic necrosis. The dose of bromobenzene required for hepatic injury to occur is not altered by pretreatment with ethanol.⁵⁹ Chronic administration of phenobarbital to rats results in a significant increase in the hepatotoxic effects of bromobenzene.¹³² In humans, hepatic toxicity caused by solvents such as CCl₄, dimethylformamide, and bromobenzene may be exacerbated by chronic exposure to ethanol.^{96,105,129, 130} Whether ethanol increases the clinical risk of APAP hepatotoxicity in humans has been debated.^{80,125,145,149, 182}

Availability of Substrates

The availability of substrates for detoxification may significantly affect both the likelihood and localization of hepatic injury. The metabolism of APAP illustrates the effect of glutathione concentration on the delicate balance between detoxification and the production of injurious metabolites. In healthy adults taking therapeutic amounts of APAP, approximately 90% of hepatic metabolism results in formation of the glucuronide or sulfate metabolites.¹⁶⁰ Most of the remainder undergoes oxidative metabolism to the toxic electrophile N-acetyl-p-benzoquinone imine (NAPQI) and is rapidly detoxified by conjugation with glutathione.^21,64 Glutathione may be depleted during the course of metabolism of APAP by ­otherwise normal livers, or it may be decreased by inadequate nutrition or liver disease.^87,88 Excessive amounts of APAP result in increased synthesis of NAPQI, which, in the absence of glutathione, reacts avidly with hepatocellular macromolecules. The cellular concentration of glutathione correlates inversely with the demonstrable covalent binding of NAPQI to liver cells.²¹ Centrilobular (zone 3) necrosis predominates in APAPinduced hepatic injury, likely related both to the centrilobular localization of CYP2E1 and to the relatively low glutathione concentrations in zone 3 compared to the periportal areas (zone 1).⁶⁴

Several pathologic mechanisms of injury that are associated with various hepatocyte “targets” are described.^88,159

Immune-Mediated Liver Injury

Immune-mediated liver injury is an idiosyncratic and host-dependent hypersensitivity response to exposure to xenobiotics.^11,20,97 Damage to the hepatocyte may be mediated by complement- or antibody-directed lysis, by specific cell-mediated cytotoxicity, or by an inflammatory response stimulated by immune complexes and complement.^{10,14,15,34,70,73, 110} The antibody or cell-mediated response may be precipitated by a covalently bound xenobiotic-cell protein adduct that acts as a hapten.⁵⁶ The resultant inflammatory response provokes secondary cytokine release that may promote further cell injury by neutrophils.⁵⁶ Subsequent apoptosis appears to be partly mediated by tumor necrosis factor (TNF).¹³¹ Immune-mediated toxic hepatitis is differentiated from liver injury caused by other autoimmune disorders by the absence of self-perpetuation; that is, there is a need for continuous exposure to the xenobiotic to perpetuate the injury.^11,97 It is also less likely to recur following withdrawal of immune suppression therapy.^11,97 Hypersensitivity reactions result in forms of liver injury that include hepatitis, cholestasis, and mixed disorders. Whether autoantibodies stimulated by the xenobiotic–protein adducts are in all cases the actual mediators of cell injury is not clear.^11,91 Adducts and associated autoantibodies are demonstrated for APAP,²¹ halothane,^14,34,154 dihydralazine,¹⁵ phenytoin,⁹¹ and ­germander.⁸³ In cases where the metabolite is highly unstable, an electrophilic attack may be directed against the CYP enzyme at the site of formation of the metabolite.^{14,15,95, 97} Autoantibodies against the CYP2E1 enzyme have been demonstrated in halothane liver injury (Chap. 68).^14,70 The most severe form of idiosyncratic halothane liver injury is manifest as fulminant hepatic necrosis associated with formation of adducts of its trifluoroacetyl chloride (TFA) metabolite with numerous hepatoproteins that include CYP2E1 and pyruvate dehydrogenase.^34,70,154 Autoantibodies specifically directed against CYP enzymes are also demonstrated for dihydralazine¹⁵ and phenytoin.⁹¹ A trifluoroacetyl protein adduct similar to that associated with halothane hepatitis was detected in workers who developed hepatic necrosis following exposure to hydrochlorofluorocarbons.⁶⁰ Early reports of lymphocyte sensitization in cases of xenobiotic-mediated liver toxicity suggested that cell-mediated immunity may play a role.¹⁶⁷ Cell-mediated autoimmune mechanisms are implicated in the idiosyncratic type of halothane hepatitis and are suspected in an increasing number of models of experimental xenobiotic-mediated liver injury.^35,167 Polymorphonucleocyte (PMN) activation and infiltration appear to be important factors in the production of cholangitis in a rat model of α-naphthyl-isothiocyanate (ANIT) liver injury. ANIT stimulates the release of cytotoxic lysosomal enzymes and oxygen free radicals by activated PMNs.¹¹⁰ Antibodies directed against circulating neutrophils decrease the extent of liver damage caused by ANIT.²⁵ NK T-cells are ubiquitous in the liver, and their possible role in the facilitation of cell-mediated autoimmune liver injury is being investigated.^23,24,52

Pharmaceuticals most commonly involved in autoimmune injury are nitrofurantoin and minocycline.¹¹ Drugs with hypersensitivity reactions that typically present with hepatitis include halothane,^14,70,154 trimethoprim-sulfamethoxazole,^2,117 anticonvulsants,⁹¹ and allopurinol.^3,153 Drugs associated with hypersensitivity that typically present with cholestatic signs include chlorpromazine, erythromycin, penicillins, rifampin, and sulfonamides.^79,121,167 Signs of injury typically begin 1 to 8 weeks following the initiation of the drug, although they may begin as late as 20 weeks for drugs such as isoniazid or dantrolene. The onset of signs of injury associated with the oxypenicillins may occur as late as 2 weeks after the drug is stopped.⁹⁷ In all cases, the onset is earlier when the patient is rechallenged with the drug. Antinuclear antibodies and smooth muscle antibodies are frequently present, as are eosinophilia, atypical lymphocytosis, fever, and rash. Their absence does not exclude an autoimmune mechanism of drug-associated liver injury.^11,76,97 Liver injury characterized by the formation of hepatic granulomas may also be a consequence of hypersensitivity reactions.¹⁰⁹

Xenobiotics That Target the Biliary Tract

Xenobiotics that undergo biliary excretion are most commonly associated with jaundice in patients with hepatoxicity.¹²¹ Xenobiotics induce cholestasis by targeting specific mechanisms of bile synthesis and flow or by damaging canalicular cells.^54,79 Elucidations of various bile transport proteins have led to better understanding of mechanisms of cholestasis in toxic liver injury.¹²¹ Cholestasis may occur with or without associated hepatitis. The development of jaundice following hepatic necrosis is a manifestation of general failure of liver function. More discrete mechanisms that result in intrahepatic cholestasis include (a) impairment of the integrity of tight membrane junctions that functionally isolate the canaliculus from the hepatocyte and sinusoids, (b) failure of transport of bile components across the hepatocytes, (c) blockade of specific membrane active transport sites, (d) decreased membrane fluidity resulting in altered transport, and (e) decreased canalicular contractility resulting in decreased bile flow.^{13,79,121, 146} Xenobiotics that specifically target bile canaliculi may lead to irreversible injury, the so-called “vanishing bile duct syndrome.”¹⁶¹ Estrogens cause intrahepatic cholestasis by altering the composition of the lipid membrane and inhibiting the rate of secretion of bile into the canaliculi.^79,146 Rifampin impedes the uptake of bilirubin into hepatocytes. Methyltestosterone and C-17 alkylated anabolic steroids impair the secretion of bilirubin into canaliculi.⁹⁴ Cyclosporine inhibits sodium-dependent uptake of bile salts across the sinusoidal membrane and blocks ATP-dependent bile salt transport across the canalicular membrane.¹³ Floxacillin causes cholestasis with minimal inflammation or evidence of hepatocellular injury.¹⁶⁷ Exposure of rats to ANIT causes a specific injury localized to the tight junctions that separate the hepatocyte from the canaliculi. This results in reflux of bile ­constituents into the sinusoidal space and increased access of sinusoidal molecules to the biliary tree.^25,79,110

Mitochondrial Injury

Direct mitochondrial injury impairs cellular respiration and is associated with fat accumulation, diminution of ATP production, and metabolic acidosis with elevated lactate.^82,89,122 This may involve attacks by xenobiotics on structural components of the mitochondria such as DNA, respiratory chain enzymes, and membranes. Nucleoside analogs that inhibit viral reverse transcriptase also inhibit mitochondrial DNA synthesis, leading to depletion of mitochondria.^{22,82,108, 144}

Other Targets

Stimulation of Kupffer cells enhances hepatic injury. When immunologically activated by xenobiotics, Kupffer cells contribute to the generation of oxygen free radicals^141,178 and may also participate in the production of autoimmune injury to hepatocytes, including activation of hepatic stellate cells.^36,73,141 Activated stellate cells produce collagen, proteoglycans, and adhesive glycoproteins, which are crucial to the development of hepatic fibrosis.^9,46 Hepatic venoocclusive disease is caused by xenobiotics that injure the endothelium of terminal hepatic venules, resulting in intimal thickening, edema, and nonthrombotic obstruction.^{81,107,113,174, 176}

The liver responds to injury in a limited number of ways.^{67,68,88,118, 159} Cells may swell (ballooning degeneration) and accumulate fat (steatosis) or biliary material. They may necrose and lyse or undergo the slower process of apoptosis, forming shrunken, nonfunctioning, eosinophilic bodies. Necrosis may be focal or bridging, linking the periportal or centrilobular areas; zonal or panacinar; or it may be massive.^59,86,108 An autoimmune type of injury is characterized by hepatitis with a prominent plasma cell infiltrate.^11,97,164 Injury to the bile ducts results in cholestasis.^79,121,167 Xenobiotics that target canalicular transport proteins may cause cholestasis in the absence of injury to hepatocytes.¹²¹ Direct mitochondrial injury impairs cellular respiration and is associated with fat accumulation, diminution of ATP production, and metabolic acidosis.^82,122 Injuries to the intima of postsinusoidal veins cause obstruction to venous flow.^81,176 The vascular effects of cocaine may cause ischemic liver injury.^87,165Table 23–1 lists characteristic morphologies of hepatic injury and associated xenobiotics.

Acute Hepatocellular Necrosis

Numerous xenobiotics are associated with hepatocellular necrosis (Table 23–1). APAP is a common cause, as are herbal remedies that ­contain hepatotoxins, whose risks are increasingly recognized.^37,61,116 Many halogenated hydrocarbons that include carbon tetrachloride,^18,96 bromobenzene,^59,132 hydrochlorofluorocarbons,⁶⁰ halothane,^15,70,154 and antituberculous medications^19,112,143 also produce hepatocellular necrosis. A study of more than 11,000 patients exposed to isoniazid during preventive treatment showed that the risk of hepatocellular necrosis was low, occurring in 0.1% of those starting treatment, and in 0.15% of those completing treatment.^19,120 Risk factors for the development of hepatotoxicity from ­isoniazid exposure are female sex, increasing age, coadministration with rifampin, and alcoholism¹⁹ (Chap. 58). The thiazolidinedione antidiabetics troglitazone and rosiglitazone are associated with acute hepatocellular necrosis.^44,51 Occupational exposure to solvents including dimethylformamide and CCl₄ causes dose-related hepatocellular necrosis.^129,130

Acute necrosis of a hepatocyte disrupts all aspects of its function. Because there is a great deal of functional reserve in the liver, hepatic function may be preserved despite the development of focal necrosis.¹⁵⁹ Extensive necrosis results in functional liver failure. The processes that lead to cell necrosis are not well known. Cell lysis is preceded by the formation of blebs in the lipid membrane and leakage of cytosolic enzymes, primarily aminotransferases and lactate dehydrogenase. Coalescence of blebs leads to rupture of the cellular membrane and acute irreversible cell death, with disintegration of the nucleus and termination of all cellular function. Prior to membrane rupture, this injury is reversible by membrane repair processes.¹¹⁰ The release of intracellular constituents attracts circulating ­leukocytes and results in an inflammatory response in the hepatic parenchyma. A proposed mechanism of rapid injury to the cell membrane is the initiation of a cascading lipid peroxidation reaction following attack by a free radical. The CYP2E1 enzyme has a significant potential to produce oxygen free radicals, as do activated PMNs and Kupffer cells.^{26,36,46,110,141, 178} Oxidant stress is an important cause of liver injury during the metabolism of ethanol by CYP2E1. This results in cell death and the stimulation of stellate cells, which promotes fibrosis.^31,46,159 In addition to peroxidation of membrane lipids, the oxidation of proteins, phospholipid fatty acyl side chains, and nucleosides appears to be widespread. Mitochondrial injury and resultant ATP depletion may also be associated with necrosis.^67,103 Xenobiotics known to cause mitochondrial injury include antiretroviral drugs that inhibit the replication of mitochondrial DNA,^22,28,108 tetracycline,¹⁴⁷ valproic acid,¹⁸⁰ hypoglycin, margosa oil, and cereulide.^103,144

Steatosis

Steatosis is the abnormal accumulation of fat in hepatocytes. It reflects abnormal cellular metabolism in conditions that include responses to xenobiotics. Cell injury depends on the severity of the underlying metabolic disturbance. Steatosis per se is normally well tolerated and reversible in many cases, although approximately one-third of patients with nonalcoholic steatosis may develop steatohepatitis.^41,82 Nonalcoholic steatosis associated with obesity, insulin resistance, and the metabolic syndrome may account for many cases of cryptogenic cirrhosis.⁴¹ The β-oxidation of fatty acids depends on a steady synthesis of cellular energy in the form of ATP and takes place in the mitochondria. Mechanisms of impaired β-oxidation of fatty acids include direct inhibition or sequestration of critical cofactors such as coenzyme-A and l-carnitine.^82,122

Intracellular fat accumulation may also occur as a result of any one or more of the following mechanisms: impaired synthesis of lipoproteins, increased mobilization of peripheral adipose stores, increased uptake of circulating lipids, increased triglyceride production, decreased binding of triglycerides to lipoprotein, and decreased release of very-low-density lipoproteins from the hepatocytes.^82,95 There are two light microscopic manifestations of steatosis: macrovesicular steatosis, in which the nucleus is displaced by large droplets of intracellular fat, and microvesicular steatosis, which is characterized by tiny cytoplasmic fat droplets that do not displace the nucleus. Xenobiotics associated with macrovesicular steatosis include ethanol and amiodarone. Ethanol increases the uptake of fatty acids into hepatocytes and decreases lipoprotein secretion. In addition, the increased ratio of the reduced form of nicotinamide adenine dinucleotide (NADH) to the oxidized form of nicotinamide adenine dinucleotide (NAD⁺), associated with hepatic metabolism of ethanol, decreases oxidation of fatty acids and promotes fatty acid synthesis.⁹⁵ An early pathologic lesion that occurs in alcoholic liver disease is reversible macrovesicular steatosis. Steatohepatitis is a more virulent form, characterized by ­ballooning degeneration of hepatocytes and apoptosis that may progress to cirrhosis.^92,177 Mallory bodies, eosinophilic cytoplasmic deposits of keratin filaments in degenerating hepatocytes, are also common microscopic findings in alcoholic liver disease.^{9,95,100, 101} Amiodarone hepatic toxicity resembles that of alcoholic hepatitis, with steatosis, Mallory bodies, and potential for progression to cirrhosis.⁸⁷ Lamellated intralysosomal phospholipid inclusion bodies are specific for amiodarone toxicity.¹³⁶Figure 23–4 shows macrovesicular steatosis with Mallory bodies caused by amiodarone.

Steatosis Associated with Mitochondrial Dysfunction.

Microvesicular steatosis is caused by severe impairment of β-oxidation of fatty acids. Valproic acid causes mild elevations of aminotransferases in approximately 11% of patients, usually during the first few months of therapy. The earliest pathologic lesion that signals progression of liver injury is microvesicular steatosis, which occurs in the absence of necrosis. An association between deficiency of carnitine, microsteatosis, and the development of hyperammonemia is observed in children treated with valproic acid.^12,126,166 A small percentage of patients progress to fulminant hepatic failure characterized by centrilobular necrosis.¹⁸⁰ The incidence of fatal hepatocellular injury is highest in children, approaching 1 in 800 children younger than 2 years of age.¹²⁶ Other mechanisms of impaired β-oxidation of fatty acids include processes that disrupt cellular ATP production, either directly by xenobiotics such as sodium azide or cyanide that inhibit electron transport in the respiratory chain, by xenobiotics that increase permeability of mitochondrial membranes and degrade the proton gradient that is critical to ATP synthesis, or by xenobiotics that uncouple oxidative phosphorylation.^93,144,147 Microvesicular steatosis is described in patients taking antiretrovirals such as zidovudine, zalcitabine, and didanosine.^28,155,158 In all cases, metabolic acidosis with elevated lactate is a prominent biochemical feature.^28,144 The nucleoside analog fialuridine caused severe hepatotoxicity during a study of its use in the treatment of chronic hepatitis B infection. Microscopic examinations of liver specimens in these cases showed marked accumulation of fat with minimal necrosis or structural injury. Severe acidosis and failure of hepatic synthetic function suggested failure of cellular energy production. Mitochondria examined under the electron microscope were demonstrably abnormal.¹⁰⁸Figure 23–5 demonstrates microvesicular steatosis in a patient with fialuridine hepatotoxicity. High doses of tetracycline produce microvesicular steatosis associated with moderate elevations of aminotransferases, markedly prolonged prothrombin time (PT), and progression to fulminant hepatic failure.¹⁴⁷ Microvesicular steatosis attributed to failure of mitochondrial energy production was reported in a fatal case of Bacillus cereus food poisoning, where high concentrations of the bacterial emetic toxin cereulide are found in the bile and liver. In this case, microvesicular steatosis is associated with extensive hepatocellular necrosis.¹⁰³ Other xenobiotics that cause mitochondrial failure are hypoglycin, the cause of Jamaican vomiting sickness, aflatoxin, and margosa oil.¹⁴⁴ Steatosis is also observed following exposure to the industrial solvent dimethylformamide. Liver biopsies in patients with acute illness show focal hepatocellular necrosis and microvesicular steatosis. More prolonged, less symptomatic exposures result in significant macrovesicular steatosis with mild aminotransferase elevations.^129,130

Venoocclusive Disease

Hepatic venoocclusive disease is caused by xenobiotics that injure the endothelium of terminal hepatic venules, resulting in intimal thickening, edema, and nonthrombotic obstruction. Central and sublobular hepatic veins may also become edematous and fibrosed. There is intense sinusoidal dilation in the centrilobular areas that is associated with liver cell atrophy and necrosis.¹⁷⁴ The gross pathologic appearance is that of a “nutmeg” liver.^81,176 Massive hepatic congestion and ascites ensue.^81,135 Hepatic venoocclusive disease is rapidly fatal in 15% to 20% of cases. It is associated with exposure to pyrrolizidine alkaloids found in many plant species including Symphytum (comfrey tea),^172,176Heliotrope, Senecio, and Crotalaria.⁸¹ It has occurred in epidemic proportions, in South Africa after the ingestion of flour contaminated with ragwort (Senecio), in Jamaica after the ingestion of “bush teas” (Crotalaria spp), and in India and Afghanistan when food was contaminated with Heliotropium lasiocarpine and Crotalaria.^17,113 A rapidly progressive form has been reported in bone marrow transplant patients ­following high-dose treatment with cyclophosphamide.¹⁰⁷

Chronic Hepatitis

A form of toxic hepatitis that clinically resembles autoimmune hepatitis occurs with the chronic administration of drugs such as methyldopa, allopurinol, nitrofurantoin, propylthiouracil, nafcillin, dantrolene, and diclofenac.^{3,7,11,71,89,133,148, 157} Many cases are associated with positive antinuclear antibody (ANA), smooth muscle antibody (SMA), and hyperglobulinemia. Jaundice is prominent and hepatocellular enzymes are elevated 5- to 60-fold. Liver biopsy commonly reveals intrahepatic cholestasis, as well as centrilobular inflammation.^11,97

Granulomatous hepatitis is characterized by infiltration of the hepatic parenchyma with caseating granulomata. At least 60 drugs are associated with this disorder. Fever and systemic symptoms are common, and 25% of patients have splenomegaly. Liver enzymes are mixed, reflecting variable degrees of cholestasis and hepatocellular injury. Eosinophilia occurs in 30% as an extrahepatic manifestation of drug hypersensitivity. Continued exposure may result in a more severe form of liver disease. Small vessel vasculitis, which may involve the skin, lungs, and kidney, is a disturbing sign associated with increased mortality.^88,109,181Table 23–1 lists a number of the xenobiotics that are implicated in this disorder.

Cirrhosis

Cirrhosis is caused by progressive fibrosis and scarring of the liver, which results in irreversible hepatic dysfunction and portal hypertension. This causes shunting of blood away from hepatocytes and subsequent hepatocellular dysfunction. Activated stellate cells produce collagen, proteoglycans, and adhesive glycoproteins, which are deposited in the space of Disse and are crucial to the development of hepatic fibrosis.^46,159 The development of fibrosis requires inflammation. Reactive oxygen species derived from lipid peroxidation, reduced NADPH, and apoptotic cells are important inflammatory stimuli that activate stellate cells.⁴⁶ In alcoholic cirrhosis, acetaldehyde and tumor necrosis factor provide a cytokine-mediated inflammatory stimulus.^9,95,159 Chronic ingestion of excessive amounts of vitamin A (25,000 units/day for 6 years or 100,000 units/day for 2.5 years) results in cirrhosis. An increase in the fat content of the sinusoidal stellate cells with increasing degrees of collagen formation are characteristic lesions that occur early in vitamin A toxicity (Chap. 47). Portal hypertension may be early and striking.⁴⁹ Like vitamin A, methyldopa and methotrexate also cause a slow progressive development of cirrhosis with few clinical symptoms.^90,173 Methotrexate-induced hepatic fibrosis is dose dependent. Risk factors include associated alcohol intake and preexisting liver disease. Reduced dosing has largely eliminated the risk of the development of cirrhosis in patients receiving methotrexate.^67,173

Hepatic Tumors

There is persuasive evidence that the use of oral contraceptive steroids increases the risk of hepatic adenomas.⁷⁵ Oral contraceptives also increase the overall risk of hepatocellular carcinoma; however, the number of cases associated with estrogen therapy is low.⁶³ Anabolic steroids are rarely associated with the development of both benign and malignant hepatic tumors.⁴⁰ Angiosarcoma is strongly associated with exposure to vinyl chloride, in addition to arsenic, thorium dioxide, and androgenic hormones.³⁸

Hepatic Injury Associated with Plants and Herbs

In addition to the venoocclusive disease associated with pyrrolizidine alkaloids described above, herbal remedies are increasingly recognized as a cause of acute hepatocellular injury. Numerous plants or plant products are known or suspected to cause hepatic injury (Chaps. 45 and 121).^{33,37,61,83, 116}

Clinical presentations of toxic liver injury range from indolent, often asymptomatic progression of impairment of hepatic function to rapid development of hepatic failure. Jaundice and pruritus are due to increased concentrations of bile acids and bilirubin in the blood. Failure of hepatocellular synthetic function results in bleeding due to coagulopathy and edema due to hypoalbuminemia. Encephalopathy may be due to hypoglycemia, impaired neurotransmission, or accumulation of toxic products of metabolism such as ammonia. Fever may occur with autoimmune-mediated liver injury. Impaired hepatic blood flow results in familiar manifestations of portal hypertension such as caput medusa, splenomegaly, ascites, and ­varices. Spider angiomata and gynecomastia also occur due to altered estrogen metabolism.¹⁵⁹

Acute Liver Failure

Acute liver failure is defined as a rapid onset of liver injury progressing over 2 to 3 weeks that results in altered mental status, vasodilation, kidney and pulmonary failure, frequent infection, and a poor outcome without transplantation.^{45,90,137, 159} Complications from acute liver failure include encephalopathy, cerebral edema, coagulopathy, kidney dysfunction, ­hypogly­-cemia, hypotension, acute respiratory distress syndrome, sepsis, and death. The encephalopathy associated with chronic liver disease should be considered separately from the rapidly progressive disorder of acute liver failure.^38,137 In some cases, a patient may progress from health to death in as little as 2 to 10 days.^{42,87,103,119,137}Table 23–2 emphasizes the clinical progression of encephalopathy as acute liver failure develops. The prognosis of acute liver failure is related to the time that passes between the onset of jaundice and the onset of encephalopathy. Perhaps surprisingly, a better prognosis is associated with shorter (2 to 4 weeks) jaundice-to-encephalopathy intervals.^137,138 Most cases are caused by xenobiotics or viral hepatitis. Acute liver failure is usually associated with extensive necrosis, although it may occur in the absence of demonstrable necrosis following exposures to ­xenobiotics that injure mitochondria.^103,108,158 Some xenobiotics that are associated with acute liver failure are shown in Table 23–1.

Hepatic Encephalopathy

Hepatic encephalopathy (HE) is a complex disorder that affects both cognitive and neuromuscular functions of the central nervous system.^8,138,139 Cognitive changes range from barely discernible impairment evident only on psychometric testing to confusion, stupor and coma. Neuromuscular signs include slowed motor responses, asterixis, myoclonus, and rigidity.^8,139 HE is potentially fully reversible, even in cases of deep coma.^43,139Table 23–2 lists the clinical stages of acute HE. Ammonia concentrations are elevated in 60% to 80% of patients with HE.¹³⁹ The demonstration that ammonia is not elevated in many cases suggests that there are other pathogenic etiologies and the etiology is multifactorial.¹³⁸ Disruption of central neurotransmitter regulation including dopamine receptor binding may contribute.¹⁷⁰ There is evidence that liver failure is associated with the accumulation of substances that stimulate central benzodiazepine receptors, leading to enhancement of γ-aminobutyric acid transmission.^4,139 Other toxins that include mercaptans, phenol, and manganese may also play a role.¹³⁹ Inflammation and infection is also proposed as an important precipitant.¹⁵² Ammonia is produced in the colon by bacterial breakdown of ingested proteins then transported to the liver via the portal circulation where it is detoxified to glutamine and urea by urea cycle enzymes.^8,123,138 Processes that raise central nervous system ammonia concentrations include infection, hypokalemia, alkalosis, increased muscle wasting, volume depletion, azotemia, and gastrointestinal bleeding.^{8,138,139, 152} Alkalosis and hypokalemia facilitate conversion of NH₄⁺ to NH₃, which moves more easily across the bowel wall and the blood–brain barrier. Acidification of the bowel by bacterial breakdown of lactulose creates a hostile environment for ammonia-producing bacteria and constructs a gradient that favors entrapment of ammonia in the bowel.^123,138 In a study of lactulose alone versus placebo, over a 14-month period 19.6% of patients on lactulose had recurrent HE, compared with 48.6% of patients on placebo.¹⁵¹ A nonabsorbed broad-spectrum antibiotic rifaximin has been shown to decrease the recurrence of HE in cirrhotic patients, presumably by decreasing the numbers of urease-containing colonic bacteria that produce ammonia. Recurrence rates at 6 months were 22.1% in the rifaximin groups versus 46.8% in the placebo group. In this study, 90% of the patients in each group were also on lactulose.⁸

Flumazenil has been proposed to treat HE. Although it is clear that benzodiazepines can make encephalopathy worse, studies of the use of flumazenil to reverse encephalopathy show conflicting results.^4,137 Although there is no clear evidence that all patients will benefit from flumazenil, some individuals may benefit for a short time. Certainly, the administration of benzodiazepines should be avoided.⁴

The history is critical in establishing the etiologic diagnosis of the patient with liver disease. A medication history should include careful investigation of nonprescription xenobiotics, especially APAP and the possible use of alternative or complementary therapies including herbal therapies. Nearly any chronically used medications should be suspect. An occupational history may indicate exposure to vinyl chloride (plastics industry), dimethylformamide (leather industry), or other industrial solvents. Table 23–3 lists occupational exposures that result in liver injury. Alcohol abuse is a common cause of acute hepatitis and the most common cause of cirrhosis in the USA.^9,95,101 A history of male homosexual contacts, health care occupation, or intravenous drug use indicates the possibility of hepatitis B and C. Recent travel to a developing country suggests the possibility of hepatitis A. In patients with significant pain, the possibility of cholelithiasis should be considered.¹⁵⁹

Biochemical Patterns of Liver Injury

There are two basic biochemical patterns associated with liver injury induced by xenobiotics. The hepatocellular pattern is characterized by elevation of liver aminotransferases due to the injury of hepatocytes by apoptosis or necrosis.¹¹⁸ The cholestatic pattern is characterized by elevation of the serum alkaline phosphatase concentration and bilirubin and usually results from injury or functional impairment of the bile ductules.⁵⁴ Processes associated with intrahepatic cholestasis in the absence of hepatitis may not lead to significant aminotransferase elevation.^117,128,181

Aminotransferases.

Laboratory tests are helpful, and certain patterns may be suggestive of specific etiologies (Table 23–4). Elevation of hepatocellular enzymes, especially the AST and ALT, indicates hepatocellular injury, and within a given clinical context, has useful diagnostic significance. Aminotransferases may be increased up to 500 times normal when hepatic necrosis is extensive, such as in severe acute viral or toxic hepatitis.^86,124 The degree of elevation does not always reflect the severity of injury because concentrations may decline as liver failure progresses. Only moderately elevated, or occasionally normal, aminotransferase concentrations occur in some patients with hepatic failure caused by mitochondrial failure, cirrhosis, or venoocclusive disease.^{49,81,108, 180} Aminotransferase concentrations may be normal or only slightly elevated in processes associated with intrahepatic cholestasis in the absence of hepatitis.^117,128,181 In alcoholic liver disease, in contrast to other forms of hepatitis, the AST concentration is typically two to three times greater than the ALT. Elevation of either of these enzymes greater than 300 IU/L is inconsistent with injury caused by ethanol.^100,124 During acute extrahepatic obstruction of the biliary tract, the AST or ALT may be as high as 1000 IU/L, indicating inflammation caused by reflux of bile acids into the biliary tree.¹²⁴ The measurement of γ-glutamyl transpeptidase (GGTP) is not very useful because it is present throughout the liver and its elevation is often nonspecific.¹²⁴

Alkaline Phosphatase.

In patients with cholestasis, bile acids stimulate the synthesis of alkaline phosphatase by hepatocytes and biliary epithelium in response to a number of pathologic processes in the liver. Elevations of the alkaline phosphatase as great as 10-fold may occur with infiltrative liver diseases but are most commonly associated with extrahepatic obstruction.¹²⁴ Although the alkaline phosphatase may be normal or elevated only minimally in hepatocellular injury, it is unusual for obstruction to occur without some elevation of the alkaline phosphatase. Elevations of alkaline phosphatase and GGTP parallel each other in diseases of the biliary tract.¹²⁴

Bilirubin.

Elevation of conjugated (direct) bilirubin implies impairment of secretion into bile, whereas elevation of unconjugated (indirect) bilirubin implies impairment of conjugation. Unconjugated hyperbilirubinemia also occurs during hemolysis and in rare disorders of hepatic conjugation such as Gilbert or Crigler-Najjar syndromes. Except in cases of pure unconjugated hyperbilirubinemia, the fractionation of bilirubin in the case of hepatobiliary disorders does not have any important diagnostic utility, and it will not distinguish patients with parenchymal disorders of the liver from intrinsic or extrinsic cholestasis. The presence of bilirubin in the urine implies elevation of conjugated (direct) bilirubin, which is water soluble and filtered by the glomerulus, obviating the need for laboratory fractionation.^124,159

Urobilinogen is produced by the bacterial metabolism of bilirubin in the bowel lumen. It is absorbed and excreted in the urine. Its presence in the urine indicates the normal excretion of bilirubin in bile, while its absence is associated with complete biliary obstruction. As a result of more modern methods of detection of complete obstruction of the biliary tract, this test is mainly of historical interest.

Serum Albumin.

Quantitatively, albumin is the most important ­protein that is made in the liver. With a half-life as great as 20 days, the albumin is usually normal in the previously healthy patient with acute liver injury. In the absence of disorders that affect albumin, such as nephrotic syndrome, protein-losing enteropathy, or starvation, a low serum albumin concentration is a useful marker for the severity of chronic liver disease.^124,159

Coagulation Factors.

Impairment of coagulation is a marker of the severity of hepatic dysfunction in both acute and chronic liver disease. ­Elevation of the PT in acute hepatitis is associated with a higher risk of hepatic failure.^57,87,108 Unlike the case with serum albumin, the onset of coagulopathy as a consequence of impaired synthesis of coagulation factors produced in the liver is rapid. Acute changes in coagulation reflect the concentration of factor VII, which has the shortest half-life.⁶⁶ The extrinsic coagulation pathway, as measured by the PT, is affected by reductions in ­factors VII, IX, and X. In addition to failure of hepatic synthesis, inadequate concentrations of factors II, VII, IX, and X may also result from ingestion of warfarin anticoagulants or malabsorption of vitamin K (Chap. 60).

Because different thromboplastin reagents give different PT values on the same sample, the international normalized ratio (INR) was developed to normalize PT measurements in patients treated with warfarin, allowing comparisons of therapeutic outcomes across different care settings and across the literature. The INR uses the International Sensitivity Index (ISI) that is derived from a cohort of patients on stable anticoagulant therapy. It normalizes the responsiveness of a given thromboplastin reagent in comparison to a WHO reference standard that is assigned a value of 1.0.⁷⁸ There is little controversy regarding the value of the INR in comparison with the PT ratio for measuring the extent of warfarin-induced anticoagulation. In patients with liver disease, use of the INR implies a normalized correlation that does not exist and is therefore potentially misleading. Because factor deficiencies in patients with liver disease are different from those in patients on warfarin, there is considerable controversy regarding which measurement is best for patients with liver disease.^30,66,78 Although comparison of factor concentrations in warfarin-treated patients with those with liver disease showed no difference in factor VII, there are significant differences in factors II, V, X, and fibrinogen. Comparison of the PT with INR in the evaluation of test results with three different thromboplastin reagents showed consistency among the control groups of warfarin-treated patients, but no consistency among PT or INR measurements using the same thromboplastin reagents in patients with liver disease.⁷⁸ Because of a failure to demonstrate an advantage, liver specialists have supported the continued use of the PT to describe the degree of liver injury, lacking the availability of a single reliable standard that would help predict operative risk.^30,78 The implication for toxicologists is that caution should be exercised in relying too heavily on published INR values that purportedly predict the severity of illness in patients with acute liver failure.

A thoughtful review of bleeding risk due to coagulopathy in chronic liver disease discussed the balance of procoagulation forces (elevated factor VIII concentrations, elevated von Willebrand factor, decreased protein C and antithrombin) and anticoagulation forces (decreased fibrinogen and other clotting factors, high tissue plasminogen activator, low platelet count). The balance is simply characterized as a seesaw effect between stimulation of thrombin generation by factor VIII and the suppression of thrombin generation by protein C. This fragile balance between procoagulation and anticoagulation may be tipped by events such as kidney failure or infection. It is not effectively characterized by the INR or the PT. The review proposed that the standard belief that patients with liver failure are “auto-anticoagulated” and not at risk of venous or arterial thrombosis is erroneous. The authors suggested that neither the PT nor the INR could predict bleeding risk in the cirrhotic patient and proposed further investigations to help identify bleeding risk and prevent or manage thrombotic events in cirrhotic patients.¹⁶²

Ammonia.

Severe generalized impairment of hepatic function leads to a rise in the serum ammonia concentration as a result of impairment of hepatic detoxification of ammonia produced during catabolism of proteins.^8,123,138 Urease-containing bacteria within the bowel, especially Klebsiella and Proteus species, are the most prominent producers of ammonia.¹³⁸ Some patients treated with valproic acid develop alterations in mental status associated with elevated ammonia concentrations in the absence of other laboratory indicators of hepatic injury.^37,127,168 Hyperammonemia in patients on valproic acid is also commonly asymptomatic.¹¹⁵ This has been attributed to selective impairment of urea cycle enzymes ornithine transcarbamylase or carbamyl phosphate synthetase by pentanoic acid metabolites (Chap. 48). As noted above, an association between deficiency of carnitine, microsteatosis, and the development of hyperammonemia in the setting of liver failure is observed in children treated with valproic acid. This is attributed to impairment of carnitine-dependent β-oxidation of sodium valproate and free fatty acids in the mitochondria.^12,126,166

Other Laboratory Tests

Serologic studies for the presence of markers of hepatitis A, B, and C should be obtained routinely in patients with hepatitis. In the patient with severe liver injury, hypoglycemia is a major concern because of impairment of glycogen storage and gluconeogenesis. Hyperglycemia also occurs as a result of the inability of the liver to handle a large glucose load. The arterial or venous blood gas commonly shows a respiratory alkalosis. Severe metabolic acidosis with elevated lactate occurs in patients with hepatic failure caused by mitochondrial injury. Measurements of serum lactate concentration may be useful in identifying the cause of acidosis in a patient with suspected toxic liver injury.^{103,108,144, 159}

Computed tomography and magnetic resonance imaging scans are useful tests for evaluation of parenchymal disease of the liver. An ultrasound examination reliably demonstrates dilation of the extrahepatic bile ducts. Liver biopsy may be helpful but is not specifically diagnostic of xenobiotic-induced hepatic injury.

In many patients, toxic liver injury resolves with simple withdrawal of the offending xenobiotic. In patients with severe injury, significant improvement in survival is associated with good supportive care in an intensive care environment.⁸⁷ Transplantation may be lifesaving for critically ill patients with acute liver failure. Early referral to a transplant center for patients with evidence of severe or rapidly progressive toxic injury is indicated.^45,102 For discussion of indications for the use of N-acetylcysteine and discussion of indications for transplantation (Antidotes in Depth: A3).

The primary role of the liver in the biotransformation of xenobiotics results in an increased risk of hepatotoxicity. Xenobiotic-induced liver injury can be dose-dependent and predictable or idiosyncratic and unpredictable.

• Predictability is evolving with the science of genome analysis. Injury that is apparently idiosyncratic is affected by host characteristics that include genetic makeup, concomitant or previous exposure to ­xenobiotics, and the underlying condition of the liver. The ability to ­predict this susceptibility is increasing.

• The pathological spectrum of liver injury includes combinations of hepatocellular necrosis, hepatitis, cholestasis, steatosis, apoptosis, and fibrosis.

• Mechanisms of hepatic injury include cell-mediated immune mechanisms that result in inflammation and fibrosis; antibody-mediated immune mechanisms that attack various hepatocyte targets including critical cellular enzymes; lipid peroxidation initiated by free radicals; and mitochondrial injury.

Acknowledgment

Charles Maltz, MD, PhD, and Todd Bania, MD, contributed to this chapter in previous editions.

References