Merkel cell carcinoma of the eyelid and periocular region: A review

Merkel cell carcinoma (MCC) is a rare primary cutaneous neuroendocrine carcinoma with a high mortality rate. It typically affects elderly Caucasians, with a slight predilection for males. It is associated with chronic sun exposure and/or immunosuppression. Almost half of all cases occur on the head or neck and an estimated 2.5%-10% arise on the eyelids or periocular skin. It ranks as the 5^th most common malignant tumor at these sites, preceded in frequency by basal cell, squamous cell and sebaceous carcinoma, as well as melanoma. Its clinical presentation as a violaceous nodule/plaque lacks specificity, and it can be mistaken for cysts, chalazia or basal cell carcinomas. Sub-specialized histopathological and immunohistochemical evaluations are required for diagnosis. Clinical staging defines the extent of disease and governs management. This includes surgery and adjuvant radiotherapy for localized tumors and of late, immunotherapy for metastatic disease. Significant advances in our understanding of the dual etiopathogenesis (Merkel cell polyomavirus- and Ultraviolet radiation-induced) and the biology of the neoplasm have been achieved in recent years. Issuing from the tumor's known susceptibility to host immunity, a recent therapeutic breakthrough has occurred whereby immune checkpoint inhibition has been shown to mitigate advanced disease. These factors and the increased global incidence of the tumor have brought it to the forefront of medical attention. This review provides a clinically relevant update on MCC, with special reference to cases arising on the eyelid/periocular region.