cancer EN may also be due to excessive antibody production in lepromatous leprosy leading to deposition of immune complexes. There is an association with the HLA-B27 histocompatibility antigen, which is present in 65% of patients with erythema nodosum. A useful mnemonic for causes is SORE SHINS (Streptococci, OCP, Rickettsia, Eponymous (Behçet), Sulfonamides, Hansen's Disease (Leprosy), IBD, NHL, Sarcoidosis. Pathophysiology Erythema nodosum is probably a delayed hypersensitivity reaction to a variety of antigens. Although circulating immune complexes have been demonstrated in patients with inflammatory bowel disease, they have not been found in idiopathic or uncomplicated cases. Diagnosis Erythema nodosum is diagnosed clinically. A biopsy can be taken and examined microscopically to confirm an uncertain diagnosis. Microscopic examination usually reveals a neutrophilic infiltrate surrounding capillaries that results in septal thickening, with fibrotic changes in the fat around blood vessels. A characteristic microscopic finding is radial granulomas, well-defined nodular aggregates of histiocytes surrounding a stellate cleft. Additional evaluation should be performed to determine the underlying cause of erythema nodosum. This may include a full blood count, erythrocyte sedimentation rate (ESR), antistreptolysin-O (ASO) titer and throat culture, urinalysis, intradermal tuberculin test, and a chest x-ray. The ESR is typically high, the C-reactive protein elevated, and the blood showing an increase in white blood cells. The ESR is initially very high and falls as the nodules of erythema nodosum. The ASO titer is high in cases associated with a streptococcal throat infection. A chest X-ray should be performed to rule out pulmonary diseases, in particular sarcoidosis and Löfgren syndrome. Treatment Erythema nodosum is self-limiting and usually resolves itself within 3–6 weeks. A recurring form does exist, and in children, it is attributed to repeated infections with streptococcus. Treatment should focus on the underlying cause. Symptoms can be treated with bed rest, leg elevation, compressive bandages, wet dressings, and nonsteroidal anti-inflammatory agents (NSAIDs). NSAIDs are usually more effective at the onset of EN versus with chronic disease. Potassium iodide can be used for persistent lesions whose cause remains unknown. Corticosteroids and colchicine can be used in severe refractory cases. Thalidomide has been used successfully in the treatment of Erythema nodosum leprosum, and it was approved by the U.S. FDA for this use in July 1998. According to a 2009 meta-analysis, there is some evidence of benefit for both thalidomide and clofazimine in the treatment of erythema nodosum leprosum. Epidemiology Erythema nodosum is the most common form