Idiopathic histiocytoid Sweet syndrome: a case report with clinical and histopathological considerations

Background Histiocytoid Sweet syndrome is characterized by a predominant neutrophilic dermal infiltrate. Usual clinical differential diagnosis includes erythema multiforme, drug eruption, and erythema nodosum. Histiocytoid Sweet syndrome is considered an uncommon histopathological variant of the disease. Methods We evaluated clinical, histopathological, and immunohistochemical findings of a case categorized as idiopathic histiocytoid Sweet syndrome in which clinical‐epidemiological data raised the possibilities of Sweet syndrome, leprosy, and drug reaction. Results Positive reaction to myeloperoxidase (MPO) in histiocytoid cells of the dermal infiltrate, response to oral corticosteroids, clinical and laboratory investigation, and absence of cutaneous lesions or clinical complaints within 1 year of follow‐up are consistent with the diagnosis of idiopathic histiocytoid Sweet syndrome. CD68 (PG‐M1) and CD15 positive cells were also present among dermal cells. Conclusions Epidemiological data are relevant while considering a clinical differential diagnosis of Sweet syndrome that can be further expanded, from a histopathological point of view, when dealing with the histiocytoid variant since neutrophils, macrophages, and immature myelomonocytic cells with histiocytoid morphology are present. The significance of the MPO positive mononuclear dermal cells are not completely established.