Erythema nodosum (also called EN) is a type of skin inflammation that is located in a certain portion of the fatty layer of skin. It results in reddish, painful, tender lumps most commonly located in the front of the legs. The tender lumps, or nodules, of erythema nodosum range in size from one to five centimeters. Inflammation in the fatty layer of skin causes the nodular swelling.

Erythema nodosum can be self-limited and resolved on its own in three to six weeks. Upon resolution, it may leave only a temporary bruised appearance or leave a chronic indentation in the skin where the fatty layer was injured.

Erythema nodosum may occur as an isolated condition or in association with other conditions.

Conditions that are associated with erythema nodosum include:

• medications (sulfa-related drugs, birth control pills, and estrogens),
• strep throat,
• Cat scratch disease,
• fungal diseases,
• infectious mononucleosis,
• sarcoidosis,
• Behçet's disease,
• inflammatory bowel diseases (Crohn's disease and ulcerative colitis), and
• normal pregnancy.

Characteristic features of erythema nodosum include barely raised, tender, reddish nodules, most commonly below the knees in the front of the legs. They are typically painful and can slowly come and go.

There are several scenarios for the outcome of erythema nodosum. Typically, these nodular areas are tender and inflamed off and on for weeks. They usually then resolve spontaneously, each one of the little areas of inflammation shrinking down and then becoming flat rather than raised and inflamed. They leave a bruised appearance. Then, they resolve completely on their own.

Other lesions can sometimes pop up elsewhere. This may occur for periods of weeks to months, and then they eventually disappear. However, chronic erythema nodosum that may last for years is another pattern.

Chronic erythema nodosum, with intermittent recurrences, can occur with or without an underlying disease present.

The nodules of erythema nodosum are sometimes confused with insect bites. Sometimes the inflammation of erythema nodosum is more widespread and can mimic skin infection (cellulitis).

Usually, erythema nodosum is a straightforward, simple diagnosis for a doctor to make by examining a patient and noting the typical firm area of raised tenderness that is red along with areas that have had lesions resolved, which might show a bruised-like appearance. It does not typically require other investigative tests.

Sometimes doctors perform a biopsy for confirmation. For example, if a patient presents with an isolated, nodule and a doctor is unable to make a diagnosis based on its appearance. The biopsy of the deeper layers of tissue of the skin can prove that it is erythema nodosum.

Doctors manage erythema nodosum by identifying and treating any underlying condition present. Simultaneously, healthcare professionals direct treatment toward inflamed skin from the erythema nodosum.

Treatments for erythema nodosum include anti-inflammatory drugs and corticosteroids by mouth or local injection. Physicians sometimes administer colchicine to reduce inflammation. Treatment must be customized for the particular patient and the conditions present, such as the existence of any associated diseases. It is important to note that erythema nodosum, while annoying and often painful, does not threaten internal organs and the long-term outlook is generally very good.