Myxoid Onychomatricoma: An Unusual Variant of a Rare Nail Unit Tumor

A 57-year-old woman presented with a 3-year history of a thickened, yellowed, over-curved nail plate of the left second toe. The proximal nail fold had an associated nodule. When observed head-on, the distal nail plate demonstrated multiple circular perforations. Avulsion of the nail plate revealed a 0.7 cm pale white mass within the proximal nail matrix, which was excised. Histologic sections of the nail plate showed longitudinal cystic spaces associated with papillary projections of nail matrix epithelium. The parenchyma of the lesion demonstrated bland-appearing spindled cells in a prominent myxoid stroma with abundant mast cells. The spindled cells were focally positive for CD34 and factor XIIIa and negative for S100, pancytokeratin, desmin, smooth muscle actin, epithelial membrane antigen, and MART-1. Onychomatricoma (OM) is a rare nail unit biphasic fibroepithelial tumor with multiple radiating digitations lined by papillomatous matrix epithelium. The epithelial component is consisted of matrix cells that generate the thickened nail plate, and the digitations cause characteristic "wormwood" perforations of the nail plate. A fibrous stroma is characteristic with staining positive for CD34 and negative for CD99, epithelial membrane antigen, and S100. This case is unique with the presence of a prominent myxoid stroma associated with the fibrous component of the OM. Only 1 previous report of OM highlights a related finding of a myxocollagenous stroma. This purely myxoid variant represents a new distinct form of OM. Clinicopathologic correlation is essential to avoid confusion with other myxoid tumors such as a superficial acral myxoma, superficial angiomyxoma, or a digital myxoid cyst.