Erythema nodosum usually resolves itself 3-6 weeks after an event, either internal or external to the body, that initiates a hypersensitivity reaction in subcutaneous fat . EN is frequently associated with fever, malaise, and joint pain and inflammation. It presents as tender red nodules on the shins that are smooth and shiny. The nodules may occur anywhere there is fat under the skin, including the thighs, arms, trunk, face, and neck . The nodules are 1-5 cm in diameter, and individual nodules 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.
Once EN is diagnosed, additional evaluation needs to be performed to determine the underlying cause. A complete blood count, erythrocyte sedimentation rate (ESR), antistreptolysin-O (ASO) titer, urinalysis, throat culture, intradermal tuberculin test, and chest x-ray is part of the initial examination.
The ESR is initially very high, and falls as the nodules fade. 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. Hilar lymphadenopathy may be due to tuberculosis, sarcoidosis, or Löfgren syndrome (a form of acute sarcoidosis with erythema nodosum , parotid swelling and bilateral hilar adenopathy, often accompanied by joint symptoms).
Potassium iodide can be used for persistent lesions whose cause remains unknown. Corticosteroids and colchicine can be used in severe refractory cases (Yurdakul et al, 2001).