Erythema Nodosum

Overview
  • Erythema Nodosum presents as deep tender red nodules on the legs and represents an inflammatory reaction of the subcutaneous fat in response to a distant focus of infection or inflammation.
Pathogenesis
  • Erythema Nodosum can be triggered by a wide variety of bacterial, viral, and fungal infections as well as inflammatory conditions, most classically Ulcerative Colitis. Drugs and malignancies have also been associated with the lesions. The precise immunological pathogenesis is not known although immune complexes are suspected.
Morphology
  • Erythema Nodosum presents as multiple, ill-defined, deep-seated, erythematous nodules bilaterally on the lower extremities, especially the pretibial surfaces. The nodules are classically very tender and over time evolve into a dusky violaceous color.
  • Histologically, lesions initially display neutrophilic inflammation and edema of the subcutaneous fat. Over time, the inflammatory infiltrate evolves to be predominantly lymphocytic with the presence of granulomas.
Clinical Consequences
  • Beyond their tenderness, lesions of erythema nodosum is not dangerous in and of themselves and usually fade over a period of weeks once the underlying trigger resolves.