Congenital melanocytic naevi and pigmentary birthmarks — branching viva

Branching framework

Open with the one-sentence problem representation. This is a large congenital melanocytic naevus on the posterior axis (the thoracic spine) with multiple satellite nevi, in a well six-week-old girl. The decisive features are the size — the projected adult size falls in the large category — and the posterior-axis location with the satellites, which is the high-risk pattern for the neurocutaneous melanocytosis. State the recognition aloud — the brown, the raised and the hairy, the presence at the birth, the size, and the satellites — before you discuss the management. The examiner is listening for whether you stratify the risk before you reach for the plan. ^{[2] [6]}

Name the classification and the size-risk relationship. The CMN is classified by the projected adult size: the small under 1.5 cm, the medium 1.5 to 19.9 cm, the large 20 cm and over, the giant 50 cm and over. The melanoma risk tracks the size — near the population risk for the small and the medium, and elevated and concentrated in the childhood for the large and the giant. Be ready for the probe on the projected adult size: it is size lesion will reach in the adult, derived from the body-site-specific growth factor applied to the size at the presentation, and it is the correct denominator because the lesion grows with the child. ^{[2] [1]}

Branch to the pathophysiology. The CMN is caused by the postzygotic (mosaic) NRAS codon 61 mutation in the melanocyte lineage. The earlier and the more extensive the mutation, larger lesion, the more numerous the satellites, and greater chance of the leptomeningeal involvement, because the neural crest gives rise to both the skin and the central-nervous-system melanocytes. Be ready for the probe on the surgery: the prophylactic excision does not eliminate the melanoma risk, because the melanoma may arise in the residual cells, the satellites, or the central nervous system, so the observation and the surveillance, not the routine excision, are the mainstay. ^{[6] [3]}