Erythema nodosum (EN) is an inflammatory condition characterized by inflammation of the fat cells under the skin, resulting in tender red nodules or lumps that are usually seen on both shins. It can be caused by a variety of conditions, and typically resolves spontaneously within 30 days. It is common in young people aged 12–20 years. Signs and symptoms Pre-eruptive phase The first signs of erythema nodosum are often flu-like symptoms such as a fever, cough, malaise, and aching joints. Some people also experience stiffness or swelling in the joints and weight loss. Eruptive stage Erythema nodosum is characterised by nodules (rounded lumps) below the skin surface, usually on the shins. These subcutaneous nodules can appear anywhere on the body, but the most common sites are the shins, arms, thighs, and torso. Each nodule typically disappears after around two weeks, though new ones may continue to form for up to six or eight weeks. A new nodule usually appears red and is hot and firm to the touch. The redness starts to fade and it gradually becomes softer and smaller until it disappears. Each nodule usually heals completely without scarring over the course of about two weeks. Joint pain and inflammation sometimes continue for several weeks or months after the nodules appear. Less common variants of erythema nodosum include: Ulcerating forms, seen in Crohn's disease Erythema contusiforme, when a subcutaneous hemorrhage (bleeding under the skin) occurs with an erythema nodosum lesion, causing the lesion to look like a contusion (bruise) Erythema nodosum migrans (also known as subacute nodular migratory panniculitis), a rare form of chronic erythema nodosum characterized by asymmetrical nodules that are mildly tender and migrate over time. Causes EN is associated with a wide variety of conditions. Idiopathic About 30–50% of EN cases are idiopathic (of an unknown cause). Infection Infections associated with EN include: Streptococcal infection which, in children, is by far the most common precipitant Primary infection of tuberculosis Mycoplasma pneumoniae Histoplasma capsulatum Yersinia Lymphogranuloma venereum (LGV), caused by the bacteria Chlamydia trachomatis Epstein-Barr virus Coccidioides immitis (Valley fever) Cat scratch disease Autoimmune disorders Autoimmune disorders associated with EN include: Inflammatory bowel disease (IBD): about 15% of patients develop erythema nodosum. Behçet's disease Sarcoidosis Pregnancy Pregnancy may be associated with EN. Medications Medications associated with EN include: Omeprazole Sulfonamides Penicillins Bromides Hepatitis B vaccination Cancer Cancers associated with EN include: Non-Hodgkin's lymphoma (NHL) Carcinoid tumours Pancreatic