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Patient Story
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A 9-year-old boy presented to the office with a 2-day history of fever and sore throat. At the time of presentation, he and his mother noted some painful bumps on his lower legs, and denied trauma (Figure 152-1). No history of recent cough or change in bowel habits was reported. The patient had no chronic medical problems, took no medications and had no known drug allergies. On examination, his oropharynx revealed tonsillar erythema and exudates. Bilateral lower extremities were spotted with multiple slightly-raised, tender, erythematous nodules that varied in size from 2 to 6 cm. Rapid strep test was positive and he was diagnosed clinically with erythema nodosum (EN) secondary to group A β-hemolytic Streptococcus. He was treated with penicillin and NSAIDs. He experienced complete resolution of the EN within 6 weeks.

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FIGURE 152-1

Erythema nodosum on the leg of a boy secondary to group A β-hemolytic Streptococcus. (Used with permission from Cleveland Clinic Children’s Hospital Photo Files.)

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Introduction
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EN is a common inflammatory panniculitis characterized by ill-defined, erythematous patches with underlying tender, subcutaneous nodules. It is a reactive process caused by chronic inflammatory states, infections, medications, malignancies, and unknown factors.

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Synonyms
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Lofgren syndrome (with hilar adenopathy).

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Epidemiology
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  • Erythema nodosum occurs in approximately 1 to 5 per 100,000 persons.1 It is the most frequent type of septal panniculitis (inflammation of the septa of fat lobules in the subcutaneous tissue).2

  • In the childhood form, the sexes are equally represented. In adults EN tends to occur more often in women, with a male-to-female ratio of 1:4.5.3

  • In 1 study, an overall incidence of 54 million people worldwide was cited in patients older than 14 years of age.4

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Etiology and Pathophysiology
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  • Most EN is idiopathic (Figures 152-2). Although the exact percentage is unknown, 1 study estimated that 55 percent of EN is idiopathic.5 This may be influenced by the fact that EN may precede the underlying illness. The distribution of etiologic causes may be seasonal.6 Identifiable causes can be infectious, reactive, pharmacologic, or neoplastic.

  • Histologic examination is most useful in defining EN. Defining characteristics of EN are a septal panniculitis without presence of vasculitis. That this pattern develops in certain areas of skin may be linked to local variations in temperature and efficient blood drainage.

  • Septal panniculitis begins with polymorphonuclear cells infiltrating the septa of fat lobules in the subcutaneous tissue. It is thought that this is in response to existing immune complex deposition in these areas.7  This inflammatory change consists of edema and hemorrhage which is responsible for the nodularity, warmth, and erythema.

  • The infiltrate progresses from predominantly polymorphonuclear cells, to lymphocytes, and then histiocytes where ...

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