Case 15 -

Histiocytoid Sweet's Syndrome Noreen Walsh, MD

Sweet's syndrome (SS), also known as acute febrile neutrophilic dermatosis, was first described in 1964 by Robert Douglas Sweet. The condition is characterized clinically by an eruption of painful erythematous pseudovesicular plaques and/or nodules, accompanied by fever, leukocytosis and neutrophilia. At the light microscopic level lesional skin shows prominent edema of the papillary dermis beneath which is a dense diffuse infiltrate of polymorphonuclear leukocytes. Particles of nuclear dust are often present but vasculitic changes are absent. The epidermis is generally spared. SS can be idiopathic or associated with various inflammatory conditions such as viral infection, with malignancies of hematological or solid organ type, or with ingestion of various medications. Case 15 - Slide 1 Click to view with ImageScope Click to view with a Web-Based Viewer In 2005 a group of European dermatopathologists described an unusual and previously under-recognized histopathological variant of SS and titled this `histiocytoid Sweet syndrome.' The authors established that in some patients with early lesions of SS the dermal infiltrates were composed mainly of mononuclear cells resembling small histiocytes. These cells tended to have elongated, twisted or kidney-shaped vesicular nuclei with small amounts of eosinophilic cytoplasm. They stained positively for histiocytic markers, such as CD68, but, of significance, they also exhibited myeloperoxidase. The latter indicated that they were in fact immature myeloid cells. This variant of SS needs to be distinguished from cutaneous involvement by myeloid leukemia and from inflammatory skin diseases characterized by interstitial infiltrates of true histiocytes, such as interstitial granuloma annulare. The authors postulated that, in some cases of SS, for reasons that are currently unknown, immature myeloid cells can be released from the bone marrow and constitute the dermal infiltrates, in contrast to mature polymorphonuclear leukocytes which usually dominate the picture. References Histiocytoid Sweet Syndrome. Requena L, Kutzner H, Palmedo G et al. Arch Dermatol 2005;141:834-842.