## Hepatitis D Superinfection — Clinical Severity and Outcomes ### Definition of Superinfection **Key Point:** Superinfection occurs when HDV infects a patient who already has chronic HBV infection. This is distinct from coinfection (simultaneous acute HBV and HDV). ### Why HDV Superinfection is Most Severe | Scenario | Outcome | Severity | | --- | --- | --- | | **Acute HBV + acute HDV (coinfection)** | 5–10% fulminant hepatitis | Moderate–High | | **Chronic HBV + acute HDV (superinfection)** | 15–20% fulminant hepatitis | **Highest** | | **Chronic HBV alone** | Chronic hepatitis, slow progression | Low–Moderate | | **Chronic HBV + HAV superinfection** | Severe acute hepatitis, rarely fulminant | Moderate | | **Chronic HBV + HCV coinfection** | Accelerated cirrhosis | Moderate–High | **High-Yield:** HDV superinfection in chronic HBV carriers has the **highest risk of fulminant hepatic failure** (15–20%) and the fastest progression to cirrhosis (within 5–10 years). ### Pathophysiology of Increased Severity 1. Existing chronic HBV → already-damaged hepatic parenchyma and impaired regenerative capacity 2. Acute HDV replication → massive cytolytic injury on top of chronic inflammation 3. Immune response → vigorous CD8+ T-cell response against HDV-infected hepatocytes 4. Result: Acute-on-chronic liver failure with high mortality (up to 30% if fulminant) **Clinical Pearl:** In a patient with known chronic HBV presenting with acute hepatitis exacerbation, **always test for anti-HDV and HDV RNA** to diagnose superinfection early. ### Why Other Options Are Wrong - **HAV superinfection:** Causes acute hepatitis but rarely fulminant (< 1% fulminant rate even in chronic HBV) - **HCV superinfection:** Accelerates cirrhosis but does not typically cause acute fulminant hepatitis - **HEV superinfection:** Rare in developed countries; causes acute hepatitis but not typically fulminant in HBV carriers [cite:Harrison 21e Ch 297]
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