Rhabdomyomatous mesenchymal hamartoma: an unusual dermal entity with a report of two cases and a review of the literature

Background: Rhabdomyomatous mesenchymal hamartoma (RMH) is a rare congenital lesion of the dermis and soft tissues consisting of a disordered and varied collection of mature adipose tissue, skeletal muscle, adnexal elements and nerve bundles. This entity exists under various names including striated muscle hamartoma, congenital midline hamartoma, and hamartoma of cutaneous adnexa and mesenchyme. Several published cases report the occurrence of RMH within the setting of other uncommon congenital abnormalities. Methods: We report the clinical and pathologic features of two cases of rhabdomyomatous mesenchymal hamartoma. Patient 1 is a 71-year-old man who presented for removal of a nodule located on his temple that had been traumatized during a recent haircut. Patient 2 is a 4-month-old infant with amnion rupture sequence and rare craniofacial abnormalities including facial clefts, microphthalmia, bilateral colobomas, and a mobile fingerlike projection above the left medial canthus. Results: Histological examination in both cases showed a deep dermal and subcutaneous fat collection of disorganized skeletal muscle fibers, adipose and neural tissue, and adnexal structures. Characteristically, the skeletal muscle approximated folliculosebaceous structures in a haphazard manner. Conclusions: RMH is a rare benign condition of the deep dermis and subcutaneous fat with only 22 cases existing in the English literature. To our knowledge, this is the first report of an elderly man presenting with RMH, presumptively present since birth. There was no evidence of cellular or malignant degeneration. While the etiology of RMH is unknown, possible explanations include aberrancy in the embryonic migration of mesodermally derived tissues or a genetic defect predisposing to the formation of hamartomas.