Author: Assoc Prof Patrick Emanuel, Dermatopathologist, Auckland, New Zealand. January 2015.
Erythema nodosum is the most common form of panniculitis. It typically presents as tender, erythematous, subcutaneous nodules and plaques, arising in crops most commonly on the pretibial areas bilaterally.
Histology of erythema nodosum
Erythema nodosum histologically represents the prototype of a septal panniculitis. The classic histopathologic presentation of erythema nodosum is that of a septal panniculitis with a mixed cellular infiltrate of lymphocytes, histiocytes, giant cells, and occasional eosinophils and a characteristic absence of vasculitis (figures 1-3).
Miescher's radial granulomata that consist of small nodules composed of spindle to oval-shaped histiocytes arranged around minute slits are thought to be a relatively specific finding for erythema nodosum (figure 4).
Early lesions may be difficult to diagnose on histopathologic features alone. Sections may shows oedematous septa containing a prominent mixed inflammatoryinfiltrate, and minimal fibrosis. As the lesions evolve, there is neutrophilicinfiltration followed by chronicinfiltrates, granulomas and septal fibrosis.
Small vessel vasculitis (or venulitis) may be seen in early lesions.