Collision of Subungual Neurofibroma and Onychomatricoma: S100 Positivity as a Clue.

We present a 41-year-old man with a hemionychodystrophy of the first toe, appearing as a longitudinal thickening of the nail plate, overcurved and with holes in its thickened free margin, thus leading to the clinical diagnosis of onychomatricoma. Complete excision showed typical nail plate of onychomatricoma and, underlying it, curvy disorganized neural-looking fascicles without atypia and with diffuse positivity for S100, interpreted as subungual neurofibroma (NF). Subungual NF is a very rare tumor, with only 12 previous cases reported. Its diagnosis is based on histopathology, as the tumor presents waves or whorls of disorganized neural-looking cells positive for S100. Regarding onychomatricoma, it is characterized by typical glove finger digitations (which were present in our case) and an underlying stroma composed by a cellular superficial layer (this layer expresses CD34 but not CD99) and a more sclerotic and deeper area. As we did not find information on S100 expression in the stroma of onychomatricoma, we have stained 4 typical cases, and all were negative with S100 and positive with CD34, as expected. In conclusion, as "subungual NF" is so rare and, in our case, seems to collide with a typical onychomatricoma, we recommend adding S100 staining to properly characterize tumors involving nail plate, to detect underlying neural tumors, as has happened in our case.