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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.
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Acute Hepatocellular Necrosis
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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 medications19,112,143 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
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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
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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
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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.
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Steatosis Associated with Mitochondrial Dysfunction.
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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
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Venoocclusive Disease
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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
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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
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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.
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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
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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
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Hepatic Injury Associated with Plants and Herbs
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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