Halothane Hepatitis

Introduction

Halothane (introduced 1955) is a halogenated volatile anesthetic that can produce two distinct forms of hepatic injury. The severe, fulminant form is termed halothane hepatitis and was the primary reason halothane was withdrawn from adult anesthetic practice in most developed nations.

Types of Hepatic Injury

Type 1 - Mild / Subclinical

Occurs in up to 20% of adults after halothane exposure
Characterized by mild, transient elevation of ALT and AST
Self-limited and reversible
Mechanism: reductive metabolism of halothane by CYP2A6 produces a 2-chloro-1,1,1-trifluoroethyl free radical, combined with hepatic hypoxia due to disturbed oxygen supply-demand balance
No clinical consequences

Type 2 - Fulminant Hepatic Necrosis (Halothane Hepatitis)

Incidence: 1 in 5,000-35,000 adult administrations; rises to 1 in 1,000 after 2 or more exposures
Fatality rate: 50-75% (pre-liver transplant era: up to 80%)
Mechanism: predominantly immune-mediated (hypersensitivity)

Pathogenesis

Oxidative metabolism: ~20-33% of halothane is metabolized by hepatic CYP2E1 via oxidative pathways
This produces a highly reactive trifluoroacetyl chloride (TFA-Cl) intermediate
TFA-Cl covalently binds to hepatocyte surface proteins, forming trifluoroacetyl (TFA)-protein adducts (neoantigens)
In sensitized individuals, these neoantigens trigger:
- Formation of anti-TFA IgG antibodies
- Cytotoxic T-cell reaction (CD4+ T cells, NK cells)
- Eosinophil infiltration
This immune response leads to massive centrilobular (zone 3) hepatic necrosis

The reductive pathway (<1%) via CYP2A6 produces free radicals that contribute to the mild form of injury. Zimmerman's model proposes that immunologic enhancement of reductive-metabolite-induced zone 3 necrosis produces the full clinical picture.

Risk Factors

Factor	Detail
Repeated exposure	Up to 90% of cases; shorter interval = greater risk; within weeks/months is highest risk
Age > 40 years	60% of patients; rare in children (1 in 80,000-200,000)
Female sex	Two-thirds of cases are women; estrogen may increase γ-interferon levels
Obesity	Halothane stored in body fat, prolonging exposure
Enzyme inducers	Phenobarbital, alcohol, isoniazid induce CYP2E1, increasing toxic metabolite production
Genetic susceptibility	HLA-linked; possible familial predisposition

Clinical Features

Onset typically 3-14 days after last halothane exposure (shorter with repeat exposure):

Fever (often the earliest sign)
Anorexia, nausea, vomiting
Jaundice
Rash and peripheral eosinophilia (hallmarks of immune reaction)
Arthralgia
Progressive hepatic failure with coagulopathy

Laboratory findings: Elevated ALT, AST, bilirubin, and alkaline phosphatase; coagulopathy in severe cases

Pathology

Massive or submassive centrilobular (zone 3) hepatic necrosis in all autopsy specimens
Range from spotty necrosis (mild cases) to sharply demarcated zone 3 necrosis (severe)
Eosinophilic infiltrate (~20% of cases)
Granulomatous hepatitis (occasional)
Inflammatory response less severe than acute viral hepatitis

Adverse Prognostic Indicators

Age > 40 years
Obesity
Short interval to onset of jaundice
Serum bilirubin > 20 mg/dL
Coagulopathy

Management

Withdrawal of halothane - most important step; avoid re-exposure (a previous history of unexplained fever and jaundice after halothane is an absolute contraindication)
Supportive care: correction of coagulopathy, management of hepatic failure
Immunosuppressive agents have only rarely helped
Liver transplantation for fulminant hepatic failure - the only definitive treatment in severe cases
Symptoms can resolve within 5-14 days; full recovery takes up to 12 weeks

Comparison with Other Halogenated Agents

The degree of TFA metabolite production correlates directly with hepatotoxicity risk:

Agent	% Metabolized	Hepatitis Incidence
Halothane	20%	1 in 5,000-35,000
Enflurane	2%	1 in 800,000
Isoflurane	0.2%	Very rare
Desflurane	0.01%	Extremely rare

All produce TFA metabolites, but halothane's extensive metabolism explains why it is by far the most hepatotoxic.

Sources: Miller's Anesthesia 10e; Barash's Clinical Anesthesia 9e; Goodman & Gilman's Pharmacological Basis of Therapeutics; Sleisenger and Fordtran's Gastrointestinal and Liver Disease; Katzung's Basic and Clinical Pharmacology 16e

This is a shared conversation. Sign in to Orris to start your own chat.