Erythema nodosum (EN) (commonly known as cicada bites) is an inflammatory condition characterised 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 3–6 weeks. It is common in young people between 12–20 years of age.
Erythema nodosum lesion in a person with tuberculosis.
A single lesion of erythema nodosum
The first sign of erythema nodosum is often flu-like symptoms. It may involve a hyperthermia, weakness and arthralgia.
It is characterized by the appearance of dermo-hypodermic rounded nodules which may be bright red or purplish, bilaterally symmetrical, located on the extensor surface of the legs. They are movable relative to the deep plane and skin. They generally disappear in 1–6 weeks without sequelae.
Erythema nodosum is characterised by red nodules that are tender and warm. Nodules occur under the skin, classically in front of the shins. Nodules may occur anywhere there is fat under the skin, including the thighs, arms, trunk, face, and neck.
Erythema nodosum may occur concurrently with fever, malaise, and joint pain and inflammation. Nodules vary from 1–10 cm in diameter, and may coalesce to form large areas of hardened skin.
As the nodules age, they become bluish purple, brownish, yellowish, and finally green, similar to the color changes that occur in a resolving bruise. The nodules usually subside over a period of 2–6 weeks without ulceration or scarring.
Erythema contusiforme, an erythema nodosum lesion with subcutaneous haemorrhage 
Chronic erythema nodosum, more likely to be unilateral and migratory.
Erythema nodosum migrans, (also known as subacute nodular migratory panniculitis of Vilanova and Piñol), a form of erythema nodosum with lesions that spread centrifugally with central clearing, and a form of chronic erythema nodosum.) is a form of septal panniculitis that is much less common than acute erythema nodosum.:488
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.
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.
Erythema nodosum is diagnosed clinically. A biopsy can be taken and examined microscopically to confirm an uncertain diagnosis.:148 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.
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.:148
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 bedrest, 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.