Abstract

Subcutaneous Sweet syndrome (SS) is an uncommon acute neutrophilic dermatosis involving the subcutaneous tissue as a lobular or sometimes septal panniculitis. There is clinical and histopathologic overlap with acute erythema nodosum (EN). Histiocytoid SS is a subtype of SS in which immature myeloid cells are the predominant cell type. We present a case of histiocytoid subcutaneous SS with features of EN. A 52-year-old female presented with a painful progressing rash on her arms. On examination there were indurated firm warm-to-touch subcutaneous nodules and plaques over bilateral dorsal hands and fingers, bilateral dorsal forearms and proximal arms, one tender nodule of right frontal scalp, and few scattered nodules on her legs. A punch biopsy was performed with a differential diagnosis that included erythema nodosom, infectious panniculitis, and erythema induratum. The biopsy showed septal widening and a partially lobular infiltrate of cells with histiocytoid morphology and leukocytoclasis admixed with neutrophils, lymphocytes, histiocytes, and occasional multinucleated giant cells. The histiocytoid cells had morphologic features of immature myeloid cells, and these aggregated around slit-like spaces, reminiscent of Miescher granulomas. The myeloid-appearing mononuclear cells were positive for MPO. CD68 stained histiocytes and mononuclear cells within these aggregates; a subpopulation of the myeloid cells were negative. The MPO positive population was essentially CD163 negative. The immunophenotype, myeloid features, and anatomic locations favored histiocytoid SS over EN. The lesions responded to corticosteroids. Hematologic malignancy may occur in this setting, and the workup is ongoing.

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