## Understanding Erythema Nodosum in Vasculitis ### Pathophysiology of Erythema Nodosum **Key Point:** Erythema nodosum is a **septal panniculitis** (inflammation of subcutaneous fat septa) caused by **immune complex deposition**, NOT direct bacterial invasion of dermal vessels. ### Correct Statement Analysis | Feature | Mechanism | Vasculitis Type | |---------|-----------|------------------| | TB-associated EN | Immune complex (Type III hypersensitivity) in dermal septa | Septal panniculitis | | Sarcoidosis EN | Immune complex deposition; Löfgren = EN + hilar LAD + arthritis | Löfgren syndrome | | Streptococcal EN | Immune complex deposition (NOT direct invasion) | Septal panniculitis | | IBD-associated EN | Immune complex deposition in septa | Extraintestinal manifestation | ### Why Direct Bacterial Invasion is WRONG **High-Yield:** Erythema nodosum is fundamentally a **hypersensitivity reaction** mediated by circulating immune complexes that deposit in the septa of subcutaneous fat. The inflammatory response is triggered by the **antigen** (TB bacillus, streptococcal antigen, sarcoid antigen), but the lesion itself does NOT contain viable organisms or direct bacterial invasion of vessels. **Clinical Pearl:** Histology of erythema nodosum shows: - Septal fibrosis and inflammation - Granulomatous inflammation in septa (not vasculitis proper) - NO vasculitis of dermal vessels - NO organisms on culture or special stains ### Löfgren Syndrome Reminder **Mnemonic:** **LAE** = **L**öfgren + **A**rthritis + **E**rythema nodosum (+ hilar lymphadenopathy in sarcoidosis) ### Common Causes of Erythema Nodosum 1. **Infections:** TB, streptococcal pharyngitis, fungal (coccidioidomycosis, histoplasmosis) 2. **Systemic diseases:** Sarcoidosis, IBD (Crohn's > UC), SLE 3. **Drugs:** Oral contraceptives, sulfonamides, penicillins 4. **Malignancy:** Lymphoma, lung cancer [cite:Robbins 10e Ch 11]
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