Background: Smooth-muscle hamartoma is an uncommon, usually congenital cutaneous hyperplasia of the arrectores pilorum muscles. When it is acquired, it may be confused with Becker's nevus with a prominent smooth-muscle component. Both entities can be considered polar forms of a spectrum of dermal smooth muscle hyperplasia. We report three cases of acquired smooth-muscle hamartoma in adult patients.
Case reports: Three healthy women in their fifties presented with adult-onset asymptomatic, multiple small papules grouped on circumscribed areas of the trunk. Results of biopsy specimens showed an excess of haphazardly oriented smooth-muscle bundles in the mid to lower dermis with an unremarkable overlying epidermis. Immunohistochemical studies showed two components in this proliferation: a smooth muscle cell population expressing vimentin and smooth muscle actin, and another population of CD34 positive dendritic cells, interspersed among the former.
Discussion: Proliferation of CD34+ dermal dendritic cells has been recently described as an additional component in the proliferation of smooth muscle cells, hair follicles and nerve fascicles that constitute congenital smooth muscle hamartomas. The results of immunohistochemical studies in our three cases extend this observation to the acquired variant of cutaneous smooth muscle hamartoma, which can be included in the expanding spectrum of diseases, recently characterized by the presence of a dermal proliferation of CD34 + cells.