## Diagnosis of Type 1 Lepra Reaction **Key Point:** Type 1 lepra reaction (reversal reaction) is a delayed-type hypersensitivity (Type IV) reaction that occurs in borderline forms of leprosy (BT, BL, LL). The diagnosis is clinical and histopathological, not microbiological. ### Why Skin Biopsy is the Gold Standard Histopathological examination of affected skin shows: - Epithelioid granulomas with caseation - Increased inflammatory infiltrate in dermis and subcutis - Destruction of skin appendages and nerves - Edema and vasculitis - Acid-fast bacilli (AFB) may be present but are NOT diagnostic of the reaction itself ### Comparison of Investigations | Investigation | Role in Type 1 Reaction | Diagnostic Value | |---|---|---| | **Skin Biopsy** | Confirms diagnosis; shows granulomatous inflammation | Gold standard | | Slit-skin smear | Assesses bacterial load; unchanged during reaction | Not diagnostic of reaction | | Lepromin test | Measures cell-mediated immunity; positive in BT/BL | Not useful during acute reaction | | NCS | Assesses nerve damage; non-specific | Supportive only | **High-Yield:** Type 1 reactions occur in patients with moderate cell-mediated immunity (borderline forms). They are immune-mediated, not due to increased bacillary load. Slit-skin smear remains unchanged or may even decrease during a reaction. **Clinical Pearl:** Type 1 reactions can occur before, during, or after MDT completion. They are reversible with corticosteroids and do not indicate treatment failure. ### Distinction from Type 2 Reaction (ENL) Type 2 reaction (Erythema Nodosum Leprosum) occurs in lepromatous and borderline lepromatous forms and is an immune complex-mediated reaction (Type III). It presents with painful nodules, neuritis, and systemic symptoms but occurs in patients with high bacillary loads. 
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