A 28-year-old woman with borderline lepromatous leprosy, 3 months into MDT, presents with sudden swelling and erythema of existing skin lesions and nodules. She also reports acute neuritis affecting her ulnar and posterior tibial nerves with pain and tenderness. Histopathology shows epithelioid granulomas with activated macrophages. Bacillary index remains high. What is the most likely diagnosis?

Explanation

## Diagnosis: Type 1 Lepra Reaction (Reversal Reaction) **Key Point:** Type 1 lepra reaction is a delayed-type hypersensitivity (Type IV) reaction that occurs in immunologically unstable forms of leprosy: borderline tuberculoid (BT), borderline (BB), and borderline lepromatous (BL). It is characterized by acute inflammation of existing lesions and nerves. **Clinical Features of Type 1 Reaction:** - Sudden swelling, erythema, and inflammation of **existing lesions** (not new lesions) - Acute neuritis with nerve tenderness and thickening - Can occur before, during, or after MDT - No systemic symptoms (fever, constitutional symptoms are absent) - High bacillary index (unlike ENL, which occurs in high-bacillary-load patients) **Histopathology:** - Epithelioid granulomas with activated macrophages - Increased cellularity and inflammation within existing granulomas - NO immune complex deposition (distinguishes from ENL) - Langerhans cells and T-cell infiltration **High-Yield:** Type 1 reactions occur in **BT, BB, BL** (unstable forms), whereas ENL occurs in **LL and BL** (high bacillary load). The key distinguishing feature is histology: epithelioid granulomas (Type 1) vs. immune complexes (ENL). **Mnemonic: TYPE 1 vs. TYPE 2 LEPRA REACTIONS** - **Type 1 (Reversal):** Cell-mediated, **existing lesions**, epithelioid granulomas, unstable forms (BT, BB, BL), no fever - **Type 2 (ENL):** Immune complex, **new nodules**, neutrophilic infiltration, high-bacillary forms (LL, BL), fever present **Management:** - Continue MDT - Corticosteroids (prednisolone 0.5–1 mg/kg) for inflammation and neuritis - NSAIDs for mild cases - Physiotherapy for affected nerves **Clinical Pearl:** Acute neuritis in Type 1 reaction is a medical emergency because it can lead to permanent nerve damage and disability. Early aggressive anti-inflammatory therapy is crucial.