Panniculitis.

The classification of inflammatory disorders of the subcutaneous tissue has mystified dermatologists for decades. Overlapping clinical and histologic features, and a lack of specific treatments have added to the confusion. This article initially classifies the various panniculititides by their primary histopathologic pattern: (1) septal panniculitis without vasculitis, (2) septal panniculitis with vasculitis, (3) lobular panniculitis without vasculitis, and (4) lobular panniculitis with vasculitis. Subsequently, we describe the key clinical findings in the most important forms of panniculitis. We begin with the most common form of panniculitis, erythema nodosum. Indeed, in many patients suspected of having panniculitis, a worthwhile question to consider initially might be, "Is this, or is this not, erythema nodosum?" before engaging in an elaborate (and expensive) exercise in differential diagnosis.