S10C7P5-Erythema Induratum

S10C7P5-1: In this field, infiltrates of inflammatory cells are relatively confined to a lobule of adipose tissue that extends to the lower margin of the dermis. On some sections of this lesion, the infiltrates were also prominent along vessels of the reticular dermis. In the central lobule of fat near its left margin and extending close to the bottom of the field, the changes are less cellular (more acidophilic). In this area, the infiltrate has granulomatous qualities (the histiocytes in this area have more abundant cytoplasm than do the lymphocytes in the neighboring blue areas; nuclei are more widely spaced in the pink areas). There is a defect in this same area; the pale acidophilic zone outlining the defect is a zone of necrosis. In the absence of demonstrable organisms, a diagnosis of erythema induratum would be considered in the differential diagnosis; special stains and cultures would be required to rule out an infectious process. In this characterization, erythema induratum gives recognition to a lobular granulomatous panniculitis; it does not necessarily imply a relationship with an infectious process such as tuberculosis. Infectious processes must be ruled out.

S10C7P5-2: At higher magnification, the granulomatous area focally has lipogranulomatous qualities; this pattern, in which symmetrically rounded spaces are outlined by layers of epithelioid histiocytes, is a non-specific reaction of fat to injury. Liberated lipids form rounded pools and elicit a granulomatous response. Near the apex of the tear-drop shaped defect, there is a zone of acidophilic necrosis with fibrinoid qualities (red arrows); the lesion has necrotizing qualities. The pattern is that of a necrotizing, granulomatous panniculitis; special stains for organisms would be indicated.

S10C7P5-3: In this field, histiocytes focally are clustered in granulomatous patterns. Infiltrates of lymphoid cells partly surround the granulomas. The granulomas are not tightly rounded aggregates (they are not classic tubercle-like aggregates) (lobular granulomatous panniculitis).

S10C7P5-4: In this field, there is a zone of necrosis to the left and above, near the center of the field. Histiocytes form ill-defined palisades at the interface between viable and necrotic tissue; there are fragments of nuclear debris at this same interface. To the right of the area of necrosis, some of the epithelioid histiocytes form tubercle-like aggregates. If the work-up has eliminated an infectious process, the changes would be compatible with “erythema induratum.”

S10C7P5-5: Epithelioid histiocytes are clustered in a granulomatous pattern.

S10C7P5-6: Near the left margin of the field, a vessel extends from the top to the bottom of the field. Near its center in this field, a branch extends to the right (red arrows identify the vertically oriented vessel and the branch extending to the right). To the left of the vertically oriented vessel, there is a zone of fibrinoid necrosis (green arrows). The branch to the right, near its origin from the vertically oriented vessel, shows extravasated red blood cells in its wall. This is an area of fibrinoid necrosis in a granuloma; a vasculitis is intimately involved in the necrotizing process. An infectious process and “erythema induratum” should be considered in the differential diagnosis.

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