Blue nevus with smooth muscle hyperplasia: A rare hamartoma.

A limited number of distinct histopathological variants of blue nevus exist, and hamartomatous proliferations involving melanocytic components, dendritic, or otherwise, have also been described. Blue nevus/smooth muscle hamartomas represent a rarely described entity. In this paper, we add two examples of this unusual hamartoma to the existing literature. These additional blue nevus/smooth muscle hamartomas occurred on the left mid-upper back of a 50-year-old woman and the central upper back of a 54-year-old man. Both lesions were clinically atypical pigmented lesions. Histopathologic review of both specimens revealed proliferations of predominantly spindled and pigmented dermal melanocytes with associated smooth muscle hyperplasia, compatible with blue nevus/smooth muscle hamartoma. Both specimens were accompanied by subtle changes suggesting follicular induction, a phenomenon previously described as occurring in a minority of specimens. A brief re-examination of recently diagnosed blue nevus at our institution did not reveal any additional cases in which a subtle smooth muscle component had been missed, suggesting this type of hamartoma is, indeed, exceedingly rare.