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- Erythema nodosum (EN) is characterized by multiple symmetric, nonulcerative tender nodules on the extensor surface of the lowerextremities, typically in young adults.
- Peak incidence in 3rd decade
- More common in women (4:1)
- Nodules are round with poorly demarcated edges and vary in size from 1 to 10 cm.
- Skin lesions are initially red, become progressively ecchymotic appearing as they resolve over 3–6 wk.
- Lesions are caused by inflammation of the septa between SC fat nodules (septal panniculitis).
- Spontaneous regression of lesions within 3–6 wk
- Major disease variants include:
- EN migrans (usually mild unilateral disease with little or no systemic symptoms)
- Chronic EN (lesions spread via extension, and associated systemic symptoms tend to be milder)
- Immune-mediated response
- 30–50% of the time etiology is idiopathic
- Often a marker for underlying disease; specific etiologies include:
- Drug reactions:
- Oral contraceptives
- Infections including:
- Streptococcal pharyngitis
- Mycobacterium tuberculosis (TB)
- Atypical mycobacteria
- Systemic diseases:
- Inflammatory bowel disease
- Behçet disease
- Connective tissue disorders
- Malignancies such as lymphoma and leukemia
- Catscratch disease
- HIV infection
- Rarely can be caused by vaccines for hepatitis and TB (BCG)
Typically, EN begins 2–3 wk after onset of S. pharyngitis.
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