Congenital melanocytic naevi
What are congenital melanocytic naevi?
Moles that look like birthmarks but were not present at birth may be called ‘congenital naevus-like’ naevi, ‘congenital-type’ naevi, or ‘tardive’ naevi.
Naevi may form from other skin cells (e.g. vascular naevi are formed from blood vessels), and may be present at birth, when they are called congenital naevi. Only those derived from melanocytes are known as moles.
Who gets congenital melanocytic naevi?
About one baby in 100 has a congenital melanocytic naevus, so they are not uncommon. Very large, giant or bathing trunk naevi are very rare.
There are a variety of types of congenital melanocytic naevi.
What do congenital melanocytic naevi look like?
Congenital melanocytic naevi present as single or multi-shaded pigmented patches. They are often oval-shaped and fairly uniform in appearance, but they may also have unusual shapes with thickened irregular surface. People with large naevi often have numerous smaller satellite lesions.
Congenital melanocytic naevi grow with the child. Generally they are proportionally smaller and less obvious with time. However, sometimes the lesion becomes darker especially at puberty. They may develop a bumpy surface and/or become more hairy. Some congenital naevi fade and they may even disappear altogether.
- Congenital melanocytic naevi in adults which are smaller than 1.5cm diameter are called ‘small’.
- Those that are up to 20cm in diameter are called ‘medium-sized’.
- Large congenital naevi are greater than 20cm in diameter, and giant naevi are greater than 40cm in diameter. They are often found on the buttocks, when they are known as ‘bathing trunk’ naevi.
- A café-au-lait macule is a flat tan mark, usually oval in shape: the tendency for these is inherited. Multiple café-au-lait macules may be a sign of neurofibromatosis.
- A speckled lentiginous naevus (also known as naevus spilus) has dark spots scattered on a flat tan background.
Congenital naevus (small)
Congenital naevus (medium-sized)
Congenital naevus (giant)
Congenital naevus (giant)
Speckled lentiginous naevus
What causes congenital melanocytic naevi?
Congenital melanocytic naevi are caused by localised genetic abnormalities resulting in proliferation of melanocytes; these are the cells in the skin responsible for normal skin colour. In some cases, there is also overgrowth of the hair-forming cells and epidermis forming an organoid naevus. Specific mutations in the melanocyte genes seem to be responsible for the different clinical patterns seen. They may start to grow early in embryonic development accounting for some moles crossing boundaries such as eyelids.
Patients with neurocutaneous melanocytosis usually have a giant congenital melanocytic naevus and satellite naevi on the skin. Abnormal melanocytes are clustered in the brain and spinal cord (the central nervous system). Neurocutaneous melanocytosis affects about 5 to 10% of those with giant melanocytic naevi. The melanocytes in the central nervous system can be detected by MRI scan (magnetic resonance imaging). They may increase the pressure inside the brain, causing symptoms such as seizures, hydrocephalus (water on the brain) and delayed development.
Risk of melanoma within congenital melanocytic naevi
Malignant melanoma, a type of cancer, sometimes develops within congenital melanocytic naevi. The risk of this happening in a small or medium-sized mole is very small (under 1%) and melanoma never arises from café-au-lait macules.
Melanoma is more likely in the giant naevi (in about 5% of individuals over a lifetime) especially in those that lie across the spine or have neurocutaneous melanosis; the cancer can start deep inside the naevus or within neuromelanosis in the central nervous system. It is then very difficult to detect and treat.
Rarely, other types of tumour may develop within giant congenital melanocytic naevi.
How is the diagnosis made?
Congenital melanocytic naevi are usually diagnosed clinically by their typical appearance.
Treatment of congenital melanocytic naevi
Most congenital melanocytic naevi do not need specific treatment. However it can be useful for follow-up to have taken a close-up photograph of the mole with a ruler beside it. This makes it easier to see if there has been growth or change in it some time later. Digital surveillance using dermatoscopic images (mole mapping) are also recommended.
Congenital melanocytic naevi are sometimes surgically removed. Reasons include:
- Unsightly appearance
- Worry about risk of melanoma
- Difficulty in keeping an eye on the mole (e.g. scalp, back or sole)
- Change (darkening, lumpiness etc.) within the lesion or unexpected growth (this is not always due to melanoma)
- Melanoma-like appearance (i.e. irregular shape, variegated colour).
However, because they are large and often in awkward sites, surgical removal can be difficult (or impossible) and always results in scarring. Laser surgery is sometimes helpful, but does not always offer advantages over conventional surgery. In selected lesions, pigment lasers can be used to fade flat naevi but this is not always successful. The subsequent changes in appearance may also make the mole more difficult to assess during longterm surveillance.
If small congenital melanocytic naevi are just growing at the same rate as the child, and not changing in any other way, the usual practice at present is not to remove them until the child is old enough to co-operate with a local anaesthetic injection, usually aged around ten or twelve years. Even then, removal is not essential.
Show brown birthmarks to your doctor who can help you decide whether they should be removed or refer you to an expert. Dermoscopic examination is often performed to evaluate the structure of the lesion.
On DermNet NZ:
- Congenital melanocytic naevi – pathology
- Mole mapping
- Benign melanocytic lesions – common skin lesions course
- Nevus Support Australia Inc
- Nevus Outreach, Inc.
- Nevus Network
- Melanocytic nevi – Medscape Reference
See the DermNet NZ bookstore