## Diagnosis of Type 2 Leprosy Reaction (ENL) ### Histopathological Confirmation of ENL **Key Point:** Histopathology showing **immune complex deposition (IgG, IgM, C3) in dermal and subcutaneous vessels** is the gold standard for confirming Type 2 leprosy reaction (ENL). ### Pathology of ENL - ENL is an **immune complex-mediated (Type III hypersensitivity) reaction** - Occurs in **lepromatous (LL) and borderline lepromatous (BL) leprosy** - Typically develops **during or after MDT** (most common 6–12 months into treatment) - Histology shows: - Neutrophilic infiltration in dermis and subcutis - **Immune complex deposition** (IgG, IgM, C3) in vessel walls - Vasculitis and panniculitis ### Diagnostic Comparison Table | Investigation | Finding in ENL | Diagnostic Value | |---|---|---| | **Histopathology** | Immune complex deposition in vessels | **Gold standard for confirmation** | | Slit-skin smear/BI | No change from baseline | Does NOT diagnose ENL | | Lepromin test | Positive (TT/BT) or negative (LL) | Does NOT change; does NOT diagnose ENL | | Serum complement | May be low (consumed) | Supportive but NOT diagnostic | **Clinical Pearl:** ENL is a **post-treatment complication** that occurs in 50% of LL and 10% of BL patients. The diagnosis is **clinical + histopathological**. Histology is essential to rule out other causes of nodules (relapse, new infection, drug reaction). **High-Yield:** Do NOT confuse: - **Type 1 reaction (reversal reaction)** = delayed hypersensitivity, occurs in BT/BB/BL, shows epithelioid granulomas on histology - **Type 2 reaction (ENL)** = immune complex deposition, occurs in LL/BL, shows vasculitis and immune complex deposition on histology **Mnemonic: ENL = Erythema Nodosum Leprosum = Immune complex = Vasculitis on histology** [cite:Park 26e Ch 8; Robbins 10e Ch 8] 
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