Erythema nodosum is a type of skin inflammation that is located in a part of the fatty layer of skin. Erythema nodosum results in reddish, painful, tender lumps most commonly located in the front of the legs below the knees. The tender lumps, or nodules, of erythema nodosum range in size from a dime to a quarter. They may be inflamed off and on for a period of weeks, then shrink and become flat, leaving a bruised appearance.
Erythema nodosum can go away on its own in three to six weeks. After it's gone, it may leave only a temporary bruised appearance or a chronic indentation in the skin where the fatty layer has been injured. Chronic erythema nodosum is a condition in which lesions pop up elsewhere, for a period of weeks to months. However, chronic erythema nodosum that may last for years is another pattern. Chronic erythema nodosum, with occasional recurrences, can occur with or without an underlying disease present.
Causes:
Erythema nodosum may occur with or without another medical condition. Conditions that are linked with erythema nodosum include medications (sulfa-related drugs, birth control pills, estrogens), strep throat, Cat scratch disease, fungal diseases, infectious mononucleosis, sarcoidosis, Behcet's disease, inflammatory bowel diseases (Crohn's disease and ulcerative colitis), and normal pregnancy.
Courtesy: WebMD