Congenital melanocytic nevi

Congenital melanocytic nevi (CMN) are classified primarily by their projected adult size (PAS) with small nevi measuring <1.5 cm, medium between 1.5 and 19.9 cm (M1:1.5-10 cm, M2:>10-19.9 cm), large between 20 and 40 cm (L1:>20-30, L2:>3040 cm) and giant measuring >40 cm (G1:>4060 cm, G2:>60 cm). The vast majority of CMN are small.1-3 The PAS of CMN is correlated with the risk for developing melanoma. Small and medium-sized CMN are at low risk with less than 1% of lesions developing melanoma.3 In contrast, for large and giant CMN the risk for developing either cutaneous or extracutaneous melanoma is between 5% and 10%.4-7 It has been shown that small CMN often harbor BRAF mutations and larger CMN are mosaic RASopathies resulting from intrauterine postzygotic mutations in codon 61 of NRAS.8 It is interesting to note that these same somatic mutations are found in 15%-50% (NRAS and BRAF, respectively) of superficial spreading melanoma in adults, and in individuals with CMN these mutations may act as the first step in a multiple hit model toward malignant progression.8,9 Clinical presentation The clinical morphology of CMN is quite variable. Their size can range from tiny subclinical incipient nests of nevo-melanocytes in the dermis to nevi covering over 50% of the body surface area. They can also manifest irregular shapes, rugous to nodular surface, hypertrichosis, and variegation in color. For the smaller CMN, their clinical and dermoscopic morphology can be indistinguishable from acquired nevi. In general, the nevo-melanocytes in smaller CMN tend not to penetrate as deeply into the dermis as compared to the larger CMN-a characteristic that has important implications for selection of CMN amenable to reflectance confocal microscopy (RCM) imaging.