This woman presented with erythematous and painful nodules on her shins, bilaterally. Erythema nodosum (EN) was suspected clinically and confirmed histologically. EN is a septal panniculitis, and when thickened septae are present without lobular involvement, the diagnosis is straightforward. Early lesions may be more suppurative and a mixed septal/lobular picture may also be present. Another useful finding is the presence of Miescher radial granulomas, as in this case. The etiology of EN is unknown, but due to its association with many inflammatory and infectious conditions, a hypersensitivity reaction is favored. Treatment of the underlying condition often results in resolution of the cutaneous lesions.
Within the thickened septae, there are collections of histiocytes. Several Miescher radial granulomas are present, with histiocytes aggregating aroung slit-like spaces. There is background fibrosis. A significant lobular infiltrate is not appreciated.