A case of cutaneous symplastic leiomyoma - a rare variant of cutaneous pilar leiomyoma

We describe the case of a cutaneous symplastic leiomyoma in a 37-year-old woman who presented with a 4-year history of a painful slow growing lesion on the left upper arm. The lesion was excised and subjected to histological examination. A poorly circumscribed lesion was seen in the dermis composed of spindle shaped cells with marked nuclear pleomorphism. No mitotic figures or necrosis were seen. The cells stained strongly positive with desmin and smooth muscle actin, and negative with S100, melan A, MNF116 a mouse monoclonal antibody to cytokeratin and CK5/6. The diagnosis was felt to be in keeping with a cutaneous symplastic leiomyoma, a rarely reported variant of the pilar leiomyoma. Histologically, it shows features similar to the symplastic variant of uterine leiomyoma with cytological atypia, nuclear pleomorphism and minimal mitotic activity. Although the long-term outlook is probably benign, the presence of cytological atypia and mitoses in any spindle cell tumor is generally a concerning feature and warrants long-term follow up.