Erythema nodosum: clinicopathologic correlations and their use in differential diagnosis.Yonsei Med J. 2007 Aug 31;48(4):601-8.

PURPOSE: Typically, a diagnosis of erythema nodosum (EN) is based on clinical features. However, other diseases manifest with inflammatory nodules of the lower limbs in addition to EN, such as the EN-like lesions of Behcet's disease (BD). The purpose of this retrospective study was to investigate the frequency of histologically proven EN among diseases diagnosed clinically as EN, to determine underlying causes of EN, and to compare clinical and histologic features between EN and other diseases. PATIENTS AND METHODS: We selected 99 patients diagnosed clinically with EN and performed skin biopsies. All pathologic slides were evaluated and diagnosed; and after histologic diagnoses were made we reviewed the patients' medical records. RESULTS: Among the 99 patients diagnosed clinically with EN, 47 were biopsy-verified EN. The EN-like lesions of BD and nodular vasculitis were both in the primary differential diagnosis of EN. No definite difference in clinical features exists among these three diseases. Histologically, EN demonstrated septal panniculitis in the majority of patients. Lobular panniculitis was frequently observed in NV, and mixed or mostly lobular panniculitis was observed in the EN-like lesion. Vasculitis was rarely observed in EN; however lymphocytic vasculitis was observed frequently in EN-like lesions and neutrophilic vasculitis was observed in NV. The frequency of granulomatous inflammation was highest in NV. Some cases of patients with typical BD demonstrated classic EN lesions. CONCLUSION: It was extremely difficult to clinically differentiate EN from EN-like lesions or NV. We feel skin biopsy is mandatory for the diagnosis of lower extremity erythematous nodular lesions.

Erythema nodosum.Clin Dermatol. 2007 May-Jun;25(3):288-94.

Erythema nodosum (EN) is the most common form of panniculitis. It is characterized by the presence of rounded or oval, slightly raised, nonulcerative painful red nodules in the skin and subcutaneous fatty tissue, 1 to 6 cm in diameter, sometimes coalescing. They tend to be symmetrical in distribution and are usually located bilaterally on the lower extremities, particularly on the anterior tibial surface, although they may also involve the ankles, the lower parts of the thighs, and the forearms.