Large congenital melanocytic nevus. Light and electron microscopic findings

The family of a 10-month-old black girl with a large periocular congenital melanocytic nevus (CMN) was given markedly differing advice from experienced physicians regarding how the lesion should be managed. The nevus was eventually removed surgically because of concern about the risk of malignant transformation and because of its cosmetic appearance. Histologically, the lesion was a dermal nevus, but extended deeply into subcutaneous tissue and had neuroid features. The ultrastructural characteristics of melanocytes varied according to the depth at which they resided, but cells did not differ basically from melanocytes found in smaller acquired nevi. Given the limited understanding of the biologic basis for malignant transformation, large size remains the most important risk factor in CMN.