GIANT CONGENITAL MELANOCYTIC NEVUS
One percent of newborn infants has small congenital melanocytic nevi, but fortunately giant nevi (defined as 20 cm in diameter or larger) are very rare, estimated in one study to be one in 20,455 live births (Castilla et al. 1981). Most cases are sporadic and are thought to arise as a result of a post-zygotic mutation, possibly in the NRAS gene (Phadke et al. 2011). Very rare cases have been reported of more than one case in the same extended family where the mode of inheritance was postulated to be a polygenic paradominant pattern of inheritance (de Wijn et al. 2010). The risk of melanoma in congenital nevi is unclear due to its rarity, and publication bias and the published estimates of this risk for giant nevi range from 2 to 14 % (Shah 2010; Yun et al. 2012), with a significant proportion of melanomas occurring in the first 10 years of life (Shah 2010; Araim et al. 2004; Angelo et al. 2001; Makkar and Frieden 2002; Ruiz-Maldonado et al. 1992; Hoanq et al. 2002).
Imaging using MRI may identify anomalies in the CNS: both intracranial melanosis and others (Kinsler et al. 2001). Neuromelanosis may be complicated by hydrocephalus or other neurological symptoms (Kinsler et al. 2001). A very small proportion of neuromelanosis eventuates in intracranial melanoma, but the incidence is not established.