Trichoblastomas with Merkel cell proliferation in nevi sebacei in Schimmelpenning-Feuerstein-Mims syndrome--histological differentiation between trichoblastomas and basal cell carcinomas.

The hallmark of Schimmelpenning-Feuerstein-Mims syndrome (SFMS) is a systematized nevus sebaceous that follows Blaschko lines and usually involves the face. It represents a rare congenital nevus syndrome with alterations of skin, bones, CNS, eyes and heart. Nevi sebacei can proliferate and develop into epithelial tumors like trichoblastoma, syringocystadenoma and basal cell carcinoma. The histological differentiation between basal cell carcinoma and trichoblastoma is difficult. We present an adult woman with SFMS who was followed by multiple specialties since birth without the correct diagnosis being made. She was referred to us with the diagnosis of multiple basal cell carcinomas of head and face. Our diagnosis of systematized nevus sebaceus was crucial for the correct classification of SFMS. We identified multiple trichoblastomas in the nevi sebacei and could exclude basal cell carcinomas. The essential clue was the detection of multiple Merkel cells within the epidermal layer by cytokeratin 20 staining.