Congenital melanocytic naevi

Congenital melanocytic naevi (nevi) are are brown or black moles which are present at birth, or which develop in the first year or so of life (brown birthmarks). Moles that look like birthmarks but were not present at birth may be called ‘congenital naevus-like’ naevi or ‘congenital-type’ naevi.

About one baby in 100 has a small or medium sized congenital naevus, so they are quite common. Very large, giant or bathing trunk naevi are very rare.

What do they look like?

Congenital melanocytic naevi present as multi-shaded pigmented patches, oval-shaped and fairly uniform in appearance. Those with large naevi often have numerous smaller satellite lesions.

Most congenital naevi grow with the child, and 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. Rarely they fade away and may even disappear altogether.

• Congenital melanocytic naevi in adults which are smaller than 1.5cm diameter are called ‘small’
• Those that are up to 10cm in diameter are called ‘medium-sized’
• Naevi larger than 10cm in diameter are called ‘large’ congenital naevi.
• ‘Giant’ congenital naevi are greater than 20cm 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 melanocytic naevi

Congenital naevus (small)	Congenital naevus (medium-sized)	Congenital naevus (giant)
Congenital naevus (giant)	Café-au-lait macule	Speckled lentiginous naevus

What causes congenital melanocytic naevi?

The cause of congenital melanocytic naevi is unknown but they are probably caused by localised genetic abnormalities. They are formed by overgrowths of the 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.

Neurocutaneous melanocytosis

Patients with neurocutaneous melanocytosis have many congenital melanocytic naevi and abnormal melanocytes clustered in the brain and spinal cord (the central nervous system). Neurocutaneous melanocytosis affects about 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

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 (perhaps about 5% over a lifetime) especially in those that lie across the spine; the cancer can start in the skin or within the central nervous system. It is then very difficult to detect and treat.

Treatment

Most congenital 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.

Congenital 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.

If small congenital naevi are just growing with 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.

Further reading

On DermNet NZ:

• Moles
• Melanoma
• Mole mapping
• Benign melanocytic lesions – common skin lesions course

Other websites:

• Nevus Outreach, Inc.
• Nevus Network
• Melanocytic nevi – emedicine dermatology, the online textbook