Erythema nodosum, an idiopathic form of panniculitis, is seen in association with a wide range of inflammatory and infectious diseases. Pregnancy and use of oral con-traceptives are two of the most common associations. Erythema nodosum is believed to occur as a secondary phenomenon in response to the underlying disease state. The condition typically resolves spontaneously, but in some cases it is difficult to treat. Erythema nodosum affects the anterior part of the lower legs almost exclusively.
Clinical Findings: Erythema nodosum is most commonly seen in young adult women. There is no racial predilection. The skin findings in erythema nodosum have an insidious onset. Small, tender regions begin within the dermis and develop into firm, tender dermal nodules, with the anterior lower legs almost always involved. The rash typically affects both lower legs in synchronicity. The lesions can be multifocal or solitary in nature. Most patients have multiple areas of involvement, with varying sizes of the lesions. Involvement of other areas of the body has been reported but is exceedingly uncommon. In these dermal nodules, there is a slight red or purplish discoloration to the overlying normal-appearing epidermis. If ulcerations are present, one should consider another diagnosis, and a biopsy is war-ranted. Although almost all cases can be diagnosed on clinical grounds, skin biopsies are required for cases that are atypical in location or have unusual features such as ulcerations, surface change, palpable purpura, or other features inconsistent with classic erythema nodosum.
The diagnosis of erythema nodosum should lead to a search for a possible underlying association. One of the most frequent causes is use of oral contraceptive pills. If the rash is thought to be related to the use of oral contraceptives, they should be discontinued, after which the lesions of erythema nodosum typically resolve. Pregnancy is another major cause of erythema nodosum. The lesions may be difficult to treat during pregnancy, but they will spontaneously resolve after delivery. Erythema nodosum may also be seen in association with sarcoid. Löfgren’s syndrome is the combi- nation of fever, erythema nodosum, and bilateral hilar adenopathy that occurs as an acute form of sarcoid. In patients with no known reason for erythema nodosum, a standard chest radiograph should be considered to evaluate for sarcoid or the possibility of an underlying fungal or atypical infection. Valley fever (coccidioidomycosis), which is caused by the fungus Coccidioides immitis, has been linked with the development of erythema nodosum. Patients presenting with erythema nodosum who have lived in or traveled to an endemic area should be evaluated for this fungal infection. Streptococcal infection and tuberculosis are two other infections that should be considered. Erythema nodosum has also been reported to occur in the inflammatory bowel diseases and in Hodgkin’s lymphoma.
Histology: Erythema nodosum is a primary septal panniculitis. The inflammation is isolated primarily to the fibrous septa that are present within the subcutaneous tissue. The fibrous septa are responsible for providing a framework for the adipose tissue. No vasculitis is seen, and its presence should make one reconsider the diagnosis. The overlying dermis has a superficial and deep perivascular lymphocytic infiltrate. A characteristic finding is that of Miescher’s radial granulomas, which represent multiple histiocytes surrounding a central cleft. Multinucleate giant cells are also present within the septal infiltrate.
Pathogenesis: The etiology of erythema nodosum is unknown, but it is thought to be a hypersensitivity reaction pattern to multiple unique stimuli. It is theorized that the antigenic stimulus causes the formation of antibody-antigen complexes that localize to the septal region of the adipose tissue.
Treatment: Treatment is primarily symptomatic. One must pursue the possibility of an underlying disorder. Erythema nodosum induced by medications or pregnancy resolves spontaneously once the medication is withdrawn or after delivery. Those cases associated with an underlying infection, malignancy, or inflammatory bowel disease may be longer lasting and may show a waxing and waning course. Topical corticosteroids, compression stockings, elevation, and nonsteroidal antiinflammatory agents are first-line therapies. Severe cases can be treated with a short course of prednisone. Supersaturated potassium iodide and colchicine ave also been reported to be used successfully.