Erythema nodosum - a review of an uncommon panniculitis.

Panniculitis, inflammation of the subcutaneous fat, is a relatively uncommon condition that usually presents with inflammatory nodules or plaques. Erythema nodosum (EN) is clinically the most frequent form of panniculitis and is considered a reactive process that may be triggered by a wide variety of stimuli. Whilst up to 55% of EN is considered idiopathic, the most common causes include infections, drugs, systemic illnesses such as sarcoidosis and inflammatory bowel disease, pregnancy, and malignancy. EN typically presents in the teens and 20s, and is seen more commonly in females. It is often preceded by a non-specific prodrome of one to three weeks, which may include fever, malaise, and symptoms of an upper respiratory tract infection. Cutaneous lesions then follow, typically localized on the extensor aspect of the limbs. The lesions are painful rounded or oval, slightly raised, non-ulcerative red nodules. The exact pathogenesis of EN is not understood, although is thought to result from deposition of immune complexes in the venules of the septae in subcutaneous fat, causing a neutrophilic panniculitis. The classical histopathological picture is of a septal panniculitis without vasculitis. However, the pathological features vary with the chronology of the lesions. Even without specific therapy for a causative condition, EN typically resolves without treatment. Therefore, symptomatic support is adequate for the majority of patients.