Toxic and Drug-Induced Hepatitis¶
Chapter 351 | Part 12: Endocrinology
KEY CLINICAL POINTS¶
- Drug-induced liver injury (DILI) is classified as direct toxic (predictable, dose-dependent) or idiosyncratic (unpredictable, immune-mediated).
- Acetaminophen hepatotoxicity is the most common cause of acute liver failure in the West, with N-acetylcysteine (NAC) as the primary antidote.
- The R value (ALT/ALP ratio) distinguishes hepatocellular (R >5.0) from cholestatic (R <2.0) injury.
- HLA haplotypes and genetic polymorphisms play a role in drug metabolism and susceptibility to idiosyncratic hepatotoxicity.
- Hy’s Law: Jaundice during drug trials correlates with 10:1 risk of acute liver failure.
1. DEFINITION & OVERVIEW¶
Toxic and drug-induced hepatitis encompasses liver injury from xenobiotics, including medications, herbs, and environmental toxins. DILI is the most common cause of acute liver failure in the US, with acetaminophen being the leading agent. Two major mechanisms: direct toxic (predictable, dose-dependent) and idiosyncratic (immune-mediated, unpredictable).
Table 351-1: Features of Toxic and Drug-Induced Hepatic Injury¶
| FEATURES | DIRECT TOXIC EFFECT | IDIOSYNCRATIC | OTHER |
|---|---|---|---|
| Predictable and dose-related toxicity | + | + | + |
| Latent period | Short | Short | Delayed onset |
| Arthralgia, fever, rash, eosinophilia | 0 | 0 | 0 |
| Liver morphology | Necrosis, fatty infiltration | Centrilobular necrosis | Mixed hepatocellular/cholestatic |
1.1 Drug Metabolism¶
Drugs undergo phase I (oxidation/methylation by CYP450) and phase II (glucuronidation/sulfation) metabolism. Toxic metabolites (e.g., NAPQI from acetaminophen) or glutathione depletion can cause injury. Phase I reactions are more commonly associated with hepatotoxicity.
1.2 Liver Injury Mechanisms¶
Direct toxicity: Covalent binding of drugs/metabolites to proteins, lipid peroxidation. Idiosyncratic: Immune-mediated (cytotoxic T-cells, cytokines), genetic factors (HLA haplotypes), or metabolic idiosyncrasies (e.g., sulfamethoxazole toxicity).
2. EPIDEMIOLOGY¶
DILI accounts for 10–20% of acute liver failure cases in the US. Acetaminophen overdose is the leading cause, with ~56,000 US hospitalizations annually. Idiosyncratic reactions occur in 1:10^3–10^5 patients. Risk factors include chronic alcohol use, concurrent medications (e.g., alcohol + acetaminophen), and genetic predisposition (HLA-B*35:01, HLA-DR4).
2.1 Demographics¶
Acetaminophen toxicity is more common in women (due to higher body weight-adjusted doses). Idiosyncratic reactions are more frequent in older adults and those with chronic liver disease.
3. ETIOLOGY & PATHOPHYSIOLOGY¶
Direct toxicity: Covalent binding of drugs/metabolites to proteins (e.g., acetaminophen → NAPQI). Idiosyncratic: Immune-mediated (cytotoxic T-cells, cytokines), genetic factors (HLA haplotypes), or metabolic idiosyncrasies (e.g., sulfamethoxazole toxicity).
Table 351-2: Hepatic Morphology from Common Drugs¶
| PRINCIPAL MORPHOLOGIC CHANGE | CLASS OF AGENT | EXAMPLE |
|---|---|---|
| Cholestasis | Anabolic steroid | Methyl testosterone |
| Hepatitis | Antiandrogen | Flutamide |
| Toxic (necrosis) | Analgesic | Acetaminophen |
| Granulomas | Antibiotic | Sulfonamides |
3.1 Drug Metabolism¶
Phase I reactions (CYP450) generate toxic metabolites; phase II conjugation (glutathione, sulfation) detoxifies. Glutathione depletion (e.g., alcohol + acetaminophen) enhances toxicity.
3.2 Immune Mechanisms¶
Cytotoxic T-cells, cytokines (TNF- α , IFN- γ ), and HLA haplotypes (e.g., HLA-B*35:01) mediate idiosyncratic injury. Autoimmune features (e.g., anti-LKM1/2 antibodies) may occur with nitrofurantoin, minocycline, etc.
4. CLINICAL FEATURES¶
Symptoms: Nausea, vomiting, fatigue, jaundice. Signs: Elevated LFTs (ALT/AST), cholestasis (ALP), coagulopathy. Acute hepatitis (centrilobular necrosis) vs. cholestasis (bile duct injury).
4.1 Acute vs. Chronic Injury¶
Acute: Rapid onset (hours–days), severe LFTs, jaundice. Chronic: Prolonged elevation of LFTs, fibrosis, cirrhosis (e.g., with methotrexate, amiodarone).
5. DIFFERENTIAL DIAGNOSIS¶
Viral hepatitis, autoimmune hepatitis, alcoholic liver disease, metabolic disorders (Wilson’s disease), drug-induced cholestasis (e.g., cholestasis from amoxicillin-clavulanate).
5.1 Key Differentiators¶
DILI: No prior liver disease, drug history. Viral hepatitis: Positive serology. Autoimmune: ANA, anti-LKM1/2. Alcoholic: History of alcohol use.
6. INVESTIGATIONS & DIAGNOSIS¶
Liver function tests (ALT/AST, ALP, bilirubin), coagulation (INR), drug levels (acetaminophen), imaging (ultrasound for cholestasis), and liver biopsy. Hy’s Law: Jaundice + elevated LFTs = high risk of liver failure.
6.1 Diagnostic Criteria¶
R value (ALT/ALP) >5.0: hepatocellular; <2.0: cholestatic. Hy’s Law: Jaundice + LFTs >2× ULN = 10:1 risk of acute liver failure.
7. MANAGEMENT & TREATMENT¶
Discontinuation of suspect drug. Supportive care: NAC for acetaminophen, corticosteroids for autoimmune features. Severe cases: Liver transplantation. Monitoring: Aminotransferase levels, coagulation, renal function.
7.1 Acetaminophen Overdose¶
NAC (140 mg/kg IV loading, 70 mg/kg every 4h for 15–20 doses). Early administration (<12h) is critical. Avoid alcohol + acetaminophen.
7.2 Idiosyncratic Reactions¶
Corticosteroids for autoimmune features. Discontinue drug. Monitor for progression to cirrhosis (e.g., with methotrexate).
8. PROGNOSIS & COMPLICATIONS¶
Acute DILI: Spontaneous recovery in 30–40% with severe injury. Mortality: 10% for acute liver failure. Chronic injury: Fibrosis, cirrhosis, hepatocellular carcinoma (e.g., with amiodarone).
8.1 Complications¶
Hepatic encephalopathy, coagulopathy, renal failure, secondary infections. Long-term: Cirrhosis, portal hypertension, hepatocellular carcinoma.
9. SPECIAL CONSIDERATIONS¶
Pregnancy: Acetaminophen is safe; avoid hepatotoxic herbs. Pediatrics: Acetaminophen overdose is common; use weight-based dosing. Elderly: Higher risk of drug interactions and toxicity (e.g., alcohol + acetaminophen).
9.1 Genetic Predisposition¶
HLA-B35:01 (acetaminophen), HLA-DR4 (nitrofurantoin), HLA-B1502 (valproate).
10. KEY POINTS & CLINICAL PEARLS¶
- Acetaminophen toxicity is the leading cause of acute liver failure.
- NAC is the antidote for acetaminophen overdose.
- Hy’s Law: Jaundice + LFTs >2× ULN = 10:1 risk of liver failure.
- R value (ALT/ALP) distinguishes hepatocellular vs. cholestatic injury.
- HLA haplotypes (e.g., HLA-B*35:01) increase susceptibility to drug-induced liver injury.