Rhabdomyomatous mesenchymal hamartoma associated with nasofrontal meningocele and dermoid cyst.

A 1-year-6-month-old girl presented with a subcutaneous tumor of the forehead, which had developed since birth. The preoperative examinations showed nasofrontal bone defect with meningocele and subcutaneous tumor with cyst. The patient underwent excision of the tumor and reconstruction of the bone defect. Histologically, the skin tumor exhibited disordered arrangement of striated muscle fibers among normal dermal components, and the cyst was lined by cornified epithelium with a few hair adnexa and contained lamellated keratin. These findings were consistent with rhabdomyomatous mesenchymal hamartoma (RMH) and dermoid cyst. This is an interesting case of RMH co-existing with nasofrontal meningocele and dermoid cyst in the same area. We suggest embryologic errors as a possible etiology, which is incomplete dysjunction of the neural ectoderm from the cutaneous ectoderm. Failure of insertion of mesoderm between the ectoderms caused the bone defect and the disordered proliferation and differentiation of mesoderm-derived tissue, leading to formation of hamartoma.