Subungual leiomyoma in the left thumb of a 16-year-old female.

Cutaneous leiomyomata, which are benign smooth muscle neoplasms, commonly present as dermal-based nodules or papules with smooth borders and firm consistency. Digital, particularly subungual leiomyomata are quite rare. A 16-year-old female presented to nail clinic complaining of discoloration of the lunula of the left thumbnail for 2.5 months. On initial examination, a pink longitudinal band was present in the center of the nail plate, with yellow discoloration and distal onycholysis. The patient had only mild tenderness with firm palpation, and did not recall trauma of the area. A nail matrix biopsy was performed to determine the etiology of the lesion. Microscopic examination demonstrated a well-demarcated dermal-based spindle-cell fascicular proliferation. Bland cells exhibited eosinophilic cytoplasm and elongate nuclei with blunt ends and minimal cytologic atypia. Prominent nucleoli, mitoses or necrosis were not appreciated. Immunohistochemical stains for smooth muscle actin and caldesmon highlighted the cells. Contrarily, S-100, epithelial membrane antigen, p63, factor XIIIa, CD34, CD68 and p75 were all negative. Ki-67 showed a low proliferative index. The immunoprofile combined with the morphologic features were interpreted as subungual leiomyoma. Subungual leiomyoma is a very rare diagnosis. We seek to bring awareness and expedite the diagnosis in patients with this lesion.