Lesions present as tender, nonulcerated, erythematous nodules located most commonly over the shins, and are sometimes associated with arthralgia and fever.
Lesions occur as a response to a variety of conditions, including infection, chronic inflammatory disease, autoimmune disease, malignancy, and pregnancy. They are also related to the ingestion of certain medications.
Histopathology demonstrates a septal panniculitis without vasculitis. Miescher radial granulomas are a diagnostic feature, but are not necessary for confirmation.
Treatment consists of supportive care, including bed rest and leg elevation, as well as nonsteroidal anti-inflammatory drugs (NSAIDs), potassium iodide, and intralesional or oral corticosteroids. Nodules may also resolve spontaneously. Healing occurs without atrophy or scarring.
Erythema nodosum is a common cutaneous hypersensitivity reaction consisting of erythematous, tender nodules most commonly located over the shins, but also reported over the thighs, upper extremities, calves, buttocks, and face. Fever and arthralgia may also be present. Lesions arise as single or clustered, tender patches, later evolving into painful nodules over the course of several days. Occasionally these nodules are associated with extravated blood, resulting in the appearance of purpura. These ecchymotic areas fade into variably pigmented patches before resolving. As new crops of nodules may form weeks after the initial presentation, lesions at varying stages of resolution may coexist. Microscopy of these lesions demonstrates a septal panniculitis without vasculitis. 
Director of Pediatric Dermatology
Department of Dermatology
Massachusetts General Hospital
DK declares that she has no competing interests.
Dr Daniela Kroshinsky would like to gratefully acknowledge Dr Om P. Sharma, a previous contributor to this monograph. Dr Kroshinsky would also like to thank Priyanka Vedak for her contribution to this monograph. OPS declares that he has no competing interests.
Professor of Internal Medicine
Clinical Professor of Dermatopathology
Northeastern Ohio Universities College of Medicine
Associate Clinical Professor of Dermatology
Case Western Reserve University School of Medicine
RTB declares that he has no competing interests.
King's College Hospital
EH declares that she has no competing interests.
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