Rhabdomyomatous mesenchymal hamartoma presenting as a chin nodule in a 15-year-old male.

Rhabdomyomatous mesenchymal hamartoma (RMH), also known as congenital midline hamartoma and striated muscle hamartoma, is a rare congenital malformation presenting most commonly in midline sites of the head and neck region. Since its first description in 1986, 67 cases have been reported to date. We report a case of RMH presenting as a chin nodule in an otherwise healthy 15-year-old male. The patient presented with a dome-shaped subcutaneous lesion on his chin which had been present since birth, but had grown and was interfering with his ability to shave. He otherwise had no history of congenital anomalies or malformations. Histopathological examination of the excised lesion revealed a haphazard proliferation of striated muscle admixed with adipose tissue and adnexal structures within the dermis and subcutaneous tissue, consistent with a diagnosis of RMH. While the majority of reported cases are of newborns or children under 3 years of age, RMH may not come to clinical attention until later in life. This rare malformation should be included in the differential diagnosis of lesions containing dermal striated muscle and/or adipose tissue, to include nevus lipomatosus superficialis, fibrous hamartoma of infancy, neuromuscular choristoma, fetal rhabdomyoma, and rhabdomyosarcoma.