Erythema nodosum primarily affects people in their 20s and 30s but can occur at any age; women are more often affected.
Etiology of erythema nodosum is unknown, but an immunologic reaction is suspected because erythema nodosum is frequently accompanied by other disorders. The most common accompanying disorders are
Other possible triggers include
Up to one third of cases of erythema nodosum are idiopathic.
Erythema induratum, a similar disorder, manifests with lesions on the calves and classically affects patients with tuberculosis.
Erythema nodosum is a subset of panniculitis that manifests as erythematous, tender nodules or plaques, primarily in the pretibial region, often preceded or accompanied by fever, malaise, and arthralgia. Lesions may be detected more easily by palpation than inspection and can evolve into bruiselike areas over weeks.
Diagnosis of erythema nodosum is usually by clinical appearance and can be confirmed by excisional biopsy of a nodule when necessary. A diagnosis of erythema nodosum should prompt evaluation for causes. Evaluation might include biopsy, tuberculosis skin testing (PPD or anergy panel), antinuclear antibodies, complete blood count, chest x-ray, and serial antistreptolysin O titers or a pharyngeal culture. Erythrocyte sedimentation rate is often high.
Erythema nodosum almost always resolves spontaneously. Treatment includes bed rest, elevation, cool compresses, and nonsteroidal anti-inflammatory drugs. Potassium iodide 300 to 500 mg orally 3 times a day can be given to decrease inflammation. Systemic corticosteroids are effective but should be used only as a last resort because they can worsen an occult infection.
If an underlying disorder is identified, it should be treated.
The most common causes of erythema nodosum are streptococcal infections (particularly in children), sarcoidosis, and inflammatory bowel disease.
Diagnose erythema nodosum primarily by clinical appearance but, when necessary, excise a nodule for biopsy confirmation.
Treat erythema nodosum supportively and use nonsteroidal anti-inflammatory drugs or potassium iodide as needed until the disorder resolves spontaneously.