A 32-year-old man from rural Maharashtra, diagnosed with lepromatous leprosy 6 months ago, presents with acute onset of painful nodules on his extensor surfaces, fever (38.5°C), and painful swelling of both knees and ankles. He has been compliant with multidrug therapy (MDT) for the past 4 months. On examination, tender erythematous nodules are noted on the forearms and shins. Laboratory investigations show elevated ESR (78 mm/hr) and CRP 12 mg/dL. Skin smear microscopy shows a decrease in bacillary load compared to baseline. What is the most likely diagnosis?

Explanation

## Clinical Diagnosis: Erythema Nodosum Leprosum (ENL) ### Key Clinical Features Present **High-Yield:** ENL (Type 3 lepra reaction) classically presents with: - Painful nodules on extensor surfaces (shins, forearms) - Systemic symptoms: fever, malaise - Polyarthralgia/arthritis (knees, ankles) - Occurs in lepromatous (LL) and borderline lepromatous (BL) leprosy - Timing: typically 6 months to several years after starting MDT (this patient at 4 months is within the window) **Key Point:** ENL is an **immune complex-mediated reaction (Type III hypersensitivity)**, NOT a delayed-type hypersensitivity. It occurs when the bacillary load is *decreasing* (hence the paradox of worsening symptoms with treatment success). ### Diagnostic Criteria Met | Feature | This Case | ENL Typical | | --- | --- | --- | | Leprosy type | Lepromatous | LL/BL | | Timing | 4 months on MDT | Months to years after starting treatment | | Nodules | Painful, erythematous | Yes, painful | | Systemic signs | Fever, arthritis | Yes, polyarthritis common | | Bacillary load | Decreasing | Decreasing (immune clearance) | | ESR/CRP | Elevated | Elevated (acute phase reactants) | **Clinical Pearl:** The *decrease* in bacillary load on smear microscopy is the key differentiator — this indicates successful treatment triggering an immune response, not treatment failure. ### Pathophysiology ```mermaid flowchart TD A[Lepromatous leprosy with high bacillary load]:::outcome --> B[Start MDT]:::action B --> C[Bacillary load decreases]:::outcome C --> D[Immune complex formation<br/>Type III hypersensitivity]:::outcome D --> E[ENL: painful nodules,<br/>fever, arthritis, systemic inflammation]:::urgent F[Antigen-antibody complexes<br/>deposit in skin, joints, eyes] -.-> E ``` **Mnemonic:** **ENLIST** = **E**rythema **N**odosum **L**eprosy **I**mmune **S**ystem **T**riggered (by successful treatment) ### Management Approach - Continue MDT (do NOT stop treatment) - NSAIDs for mild cases - Systemic corticosteroids (prednisolone 0.5–1 mg/kg/day) for moderate–severe ENL - Thalidomide (50–100 mg/day) for severe or recurrent ENL (highly effective) - Treat complications: iritis, neuritis, orchitis [cite:Park 26e Ch 15]