Erythema nodosum is an inflammatory disorder involving the deep dermis layer of skin and subcutaneous fat septa that underlies the skin. It is characterized by eruptions of recurrent or persistent multiple painful, red nodules under the skin that leave a bruised appearance when healing and do not scar.
The lower legs are most affected, but sores can appear anywhere there is subcutaneous fat.
Q: How do the nodules develop in erythema nodosum?
A: The edges of nodules are poorly defined, and the nodules vary from 2-6 cm.
During the first week of eruption, nodules become tense, hard, and painful. During the second week, they change color from bright red to bluish or livid and may become soft, but do not ulcerate. As absorption progresses, the color gradually fades to a yellowish hue, resembling a bruise. This disappears in 1 or 2 weeks as the overlying skin sloughs off and is replaced.1 
The eruptive phase of erythema nodosum begins with flulike symptoms of fever and generalized aching followed by a painful rash within 1-2 days. Aching legs and swelling ankles may occur and precede the eruption or appear during the eruptive phase and may persist for weeks.2 
Currently, the most common cause of erythema nodosum is streptococcal infection in children and streptococcal infection and sarcoidosis in adults.3  Most sores in infection-induced erythema nodosum heal within 7 weeks, but active disease may last up to 18 weeks.
In contrast, 30% of idiopathic erythema nodosum cases may last more than 6 months. Idiopathic means that the cause is not known.
What Is Erythema Nodosum In Celiac Disease and/or Gluten Sensitivity?
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