Congenital arrector pili hamartoma. A case report and review of the spectrum of Becker's melanosis and pilar smooth-muscle hamartoma.

Congenital pigmented arrector pili hamartomas are unique malformations of epidermis and pilar apparatus usually appearing as localized, lightly pigmented, hairy plaques. Characteristic microscopic features include smooth-muscle proliferation similar to irregularly disposed arrectores pilorum, and slight elongation of epidermal rete with hypermelanosis of the basal unit. An otherwise normally developed child who had this hamartoma at birth is described in an attempt to clarify the relationship between pilar smooth-muscle hamartomas and Becker's melanosis. We propose that these two entities belong at different poles of the same developmental spectrum of hamartomatous change.