Panniculitis. Part I. Mostly septal panniculitis

The panniculitides represent a group of heterogeneous inflammatory diseases that involve the subcutaneous fat. The specific diagnosis of these disease s requires histopathologic study because different panniculitides usually s how the same clinical appearance, which consists of subcutaneous erythemato us nodules on the lower extremities. However, the histopathologic study of panniculitis is difficult because of an inadequate clinicopathologic correl ation, and the changing evolutionary nature of the lesions means that biops y specimens are often taken from late-stage lesions, which results in nonsp ecific histopathologic findings. In addition, large-scalpel incisional biop sies are required. However, we believe that by obtaining appropriate biopsy specimens and-with adequate clinicopathologic correlation, a specific diag nosis may be rendered fn most cases of panniculitis. It must be accepted th at all panniculitides are somewhat mixed because the inflammatory infiltrat e involves both the septa and lobules; however, in general the differential diagnosis between a mostly septal and a mostly lobular panniculitis is str aightforward at scanning magnification. Mostly septal panniculitides with v asculitis include leukocytoclastic vasculitis involving the small blood ves sels of the septa; superficial thrombophlebitis resulting from inflammation and subsequent thrombosis of large veins of the septa; and cutaneous polya rteritis nodosa, which is a vasculitis involving arteries and arterioles of the septa of subcutaneous fat with few or no systemic manifestations. Ofte n septal panniculitides with no vasculitis are the consequence of dermal in flammatory processes extending to the subcutaneous fat, such as necrobiosis lipoidica, scleroderma, subcutaneous granuloma annulare, rheumatoid nodule ; and necrobiotic xanthagranuloma. However, in other cases, the inflammator y process is primarily located in the fibrous septa of the subcutis with or without involvement of the overlying dermis. The most frequently seen sept al panniculitis is erythema nodosum, which, in fully developed lesions, is characterized histopathologically by Miescher's radial granulomas in the se pta.