A 32-year-old man from Tamil Nadu with borderline lepromatous leprosy (BL) presents with acute onset fever, painful nodules on the extensor surfaces, and tender subcutaneous nodules. He had completed 4 months of multidrug therapy (MDT). On examination, there are tender erythematous nodules on the shins and forearms, and he has mild hepatosplenomegaly. His lepromin test is strongly positive. What is the most likely diagnosis?

Explanation

## Diagnosis: Erythema Nodosum Leprosum (ENL) ### Clinical Features Pointing to ENL **Key Point:** ENL (Lepra Reaction Type 2) is an immune complex-mediated reaction that occurs in lepromatous (LL) and borderline lepromatous (BL) leprosy patients, typically during or shortly after MDT initiation. **Clinical Pearl:** The constellation of fever, tender subcutaneous nodules (erythema nodosum pattern), and systemic involvement (hepatosplenomegaly) in a BL patient on MDT is pathognomonic for ENL. ### Distinguishing Features of ENL vs Type 1 Reaction | Feature | ENL (Type 2) | Type 1 Reaction | |---------|--------------|------------------| | **Leprosy type** | LL, BL (high bacillary load) | BT, BL, BB (intermediate) | | **Timing** | During/after MDT (months to years) | At diagnosis or during therapy | | **Lesion character** | Tender nodules, erythema nodosum pattern | Plaques, infiltrates (non-tender) | | **Systemic signs** | Fever, lymphadenitis, iritis, neuritis | Localized to skin/nerves | | **Lepromin test** | Strongly positive | Positive | | **Mechanism** | Type III (immune complex) | Type IV (cell-mediated) | | **Histology** | Neutrophilic infiltrate, vasculitis | Epithelioid granulomas | **High-Yield:** ENL occurs almost exclusively in LL and BL cases with high bacillary loads; it is rare in BT/TT leprosy. The presence of fever + tender nodules + systemic involvement makes ENL the diagnosis. ### Pathophysiology ENL is triggered by antigen–antibody complex deposition in skin and organs, leading to: 1. Activation of complement cascade 2. Neutrophilic infiltration and vasculitis 3. Release of inflammatory mediators (TNF-α, IL-6) 4. Clinical manifestations: nodules, fever, systemic inflammation **Mnemonic:** ENL = **E**rythema **N**odosum **L**eprosum = Type 2 reaction = **I**mmune complex = **I**nflammatory nodules + systemic signs ### Management Implications - **First-line:** Corticosteroids (prednisolone 0.5–1 mg/kg/day, taper over weeks) - **Severe/refractory:** Thalidomide (100–300 mg/day) — highly effective, but teratogenic - **Continue MDT** during ENL treatment **Clinical Pearl:** The strongly positive lepromin test in this patient indicates good cell-mediated immunity, which is consistent with ENL (not seen in pure LL with negative lepromin).