Plaque-Type Syringoma Mimicking Microcystic Adnexal Carcinoma in a Patient With Generalized Syringomas: Case Report and Review of Diagnostic Pitfalls in the Assessment of Unusual Syringomatous Proliferations.

We report a 70-year-old man with a long-standing history of eruptive syringomas, including scattered plaque-type syringomas that raised clinical concern for basal cell carcinoma. A biopsy of a plaque-type lesion on the left clavicle revealed a syringoma with superficial follicular infundibular cysts resembling those of microcystic adnexal carcinoma (MAC). The lesion also exhibited cords and trabeculae streaming from the epidermis, and solid nests of variable sizes extending to the deep dermis, reminiscent of the solid variant of MAC. Excision revealed a fairly circumscribed tumor within a sclerotic stroma that did not invade nerve or infiltrate fat, supporting a diagnosis of plaque-type syringoma. Additional lesions have since been biopsied, revealing similar findings, but none of them have been re-excised nor have they recurred. The patient had facial milia, but was not thought to have Nicolau-Balus syndrome given the absence of atrophoderma vermiculata and a lack of a pertinent family history. Herein, we discuss the histomorphologic diversity of syringomas and helpful findings that distinguish them from carcinoma. An unequivocal distinction between syringoma and MAC is not always feasible on a partial sample, but awareness of rare syringoma variants can sometimes prevent unnecessary surgery.