Congenital melanocytic naevus
CMN; congenital naevus; giant CMN
Congenital melanocytic naevi (CMN) are melanocytic naevi present at birth or appearing in the first months of life ('tardive CMN'). Classified by projected adult size: small <1.5 cm; medium 1.5–20 cm; large 20–40 cm; giant >40 cm. Lifetime melanoma risk varies sharply by size — small / medium CMN have minimal increased risk, while giant CMN carry a ~2–5% lifetime melanoma risk (highest in giant lesions with multiple satellites; historical series quoted higher) and significant neurocutaneous melanosis (NCM) risk.
Classification by projected adult size
| Class | PAS | Melanoma risk |
|---|---|---|
| Small | <1.5 cm | Minimal increase (<1%) |
| Medium | 1.5–20 cm | Slight increase (<1%) |
| Large | 20–40 cm | ~1–2% |
| Giant | >40 cm | ~2–5% (highest in giant lesions with multiple satellites; historical series quoted higher) |
'Projected adult size' (PAS) is calculated by extrapolating the lesion size in infancy to adult body proportions.
Clinical features
- Brown to black pigmented patch / plaque, often with darker speckling and hypertrichosis.
- Surface texture ranges from smooth to verrucous.
- Satellite naevi (small CMN at distance from main lesion) frequent in giant CMN; their number correlates with NCM and melanoma risk.
- Large/multiple CMN are typically NRAS Q61-mutant; a proportion of smaller CMN harbour BRAF.
Neurocutaneous melanosis (NCM)
NCM is melanocytic infiltration of the leptomeninges. Risk factors: large/giant CMN especially in posterior axial location ('bathing trunk'), multiple satellite naevi (≥20). Symptoms range from asymptomatic radiological finding to seizures, hydrocephalus, raised ICP. Baseline brain MRI in infancy (without contrast) is recommended for high-risk CMN.
Management
Small / medium CMN
- Observation with photo documentation.
- Excise if dermoscopic / clinical concern develops, or for cosmetic reasons.
- Patient education on self-examination.
Large / giant CMN
- Multidisciplinary care with paediatric dermatology, plastic surgery, neurology if NCM.
- Staged excision with serial expansion or skin substitutes — major reconstructive undertaking.
- Surgery does not eliminate melanoma risk (residual melanocytes deep in dermis / leptomeninges) — surveillance lifelong.
- Counsel families about modest cosmetic improvements vs surgical morbidity.
Surveillance
- Clinical examination 6–12 monthly.
- Photograph documentation.
- Low threshold for biopsy of any new nodule, ulceration or change.
- Brain MRI baseline for high-risk CMN; repeat if neurological symptoms develop.
References
- Kinsler VA et al. Multiple congenital melanocytic naevi and neurocutaneous melanosis are caused by postzygotic mutations in NRAS. J Invest Dermatol; 2013.
- Krengel S et al. Risk of melanoma in patients with congenital melanocytic naevi: a systematic review. Br J Dermatol; 2006.
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