Last updated: January 2024
What are congenital melanocytic naevi?
Congenital melanocytic naevi (CMN) are skin lesions consisting of nests of melanocytes (cells that produce pigment). They are present at birth or shortly after birth. They occur in approximately 1 in 100 live births. 1
CMN are classified according to their predicted adult size:
- Small <1.5cm in diameter
- Medium between 1.5cm -19.9cm
- Large (giant) ≥ 20cm in diameter
Who get congenital melanocytic naevi?
CMN can affect all races and ethnic groups. Both males and females can develop CMN.
What cause congenital melanocytic naevi?
CMN usually occur sporadically. The condition is generally not inherited but arises from a mutation in the body’s cells that occurs after conception.
NRAS gene mutations cause most cases of giant CMN. Rarely, mutations in the BRAF gene are responsible for this condition. The proteins produced from these genes instruct the cell to grow and divide (proliferate) or to mature and take on specialised functions (differentiate). The NRAS or BRAF gene mutations responsible for giant CMN are somatic, meaning that they are acquired after conception.
A somatic mutation in one copy of the NRAS or BRAF gene is sufficient to cause CMN.
These mutations occur early in embryonic development during the growth and division (proliferation) of cells that develop into melanocytes. The overactive protein may contribute to the development of giant CMN by allowing cells that develop into melanocytes to grow and divide uncontrollably, starting before birth.
What do congenital melanocytic naevi look like?
Compared to common moles, CMN are often larger and more raised above the skin. Many are different shades of brown and black, and some are hairy.
How are congenital melanocytic naevi diagnosed?
CMN is usually diagnosed from a clinical examination of the skin. In some cases, a biopsy may be needed.
Risk of developing melanoma with congenital melanocytic naevi
There are certain characteristics that may make an individual with CMN more likely to develop melanoma. The risk of melanoma is mainly related to the size of the CMN (i.e., large or giant size).
How are congenital melanocytic naevi treated?
Treatment options will vary depending on the individual and their needs.
The method for managing a CMN will depend on the individuals age, the size of the lesion, its location and depth and the risk of malignancy emerging within the lesion.
Giant or large lesions
The primary reason for surgery in a giant or large CMN is if a melanoma develops within it.
Infants with giant or large CMN are usually managed by a multidisciplinary team and have regular follow-up by a dermatologist because of the increased risk of complications.
Complete surgical excisions may reduce the risk of melanomas. However, this is associated with complications, and total removal may be impractical for giant or large lesions.
Small lesions
Unless the small CMN is changing, it is often not necessary for it to be surgically removed. Some reasons why an individual may choose to have the small CMN surgically removed, include unsightly appearance, difficulty in observing the mole (i.e., on back), change in lesion (i.e., colour darkening) or melanoma-like appearance.
Neurocutaneous melanocytosis
Individuals with symptoms suggestive, or at high risk, of neurocutaneous melanocytosis should undergo a magnetic resonance imaging (MRI) to detect the disease. Those at high risk should have an MRI in the first 6 months of life.
Other treatment options
Other treatment options may include dermabrasion, tangential (shave) excision, chemical peels and laser ablation.
What is the likely outcome of congenital melanocytic naevi?
CMN usually grow as the child grows. Some may become lighter with time. However, they generally persist for life.
- Friedman EB, Scolyer RA, Thompson JF. Management of pigmented skin lesions in childhood and adolescence. Aust J Gen Pract. 2019 Aug;48(8):539-544. doi: 10.31128/AJGP-04-19-48951. PMID: 31370129.
| Dr Andrew Chen and Prof Orli Wargon | January 2024 |
| Dr Andrew Chen and Prof Orli Wargon | March 2017 |