A congenital melanocytic naevus (CMN) is one of many different types of birthmark that may be found in newborn babies:

• congenital indicates that the abnormality is present at birth;
• melanocytic is the adjective derived from the word melanocyte, a type of cell present in normal skin and in certain other organs, whose function is to produce melanin - a brown pigment;
• naevus is the technical (Greek) word that dermatologists (skin specialists) and paediatricians use to describe any type of birthmark that occurs in the skin, the plural is naevi.

Birthmarks are the visible effect of errors that have occurred during a child's development before birth. The causes of these errors are not known for certain, but may include exposure to radiation during pregnancy, infections during pregnancy, and exposure to certain drugs and, perhaps, certain chemicals including some that may sometimes be present in food.

A CMN is a type of benign tumour composed of a large collection of melanocytes. Why such a collection develops is still unknown. Melanocytes originate near the developing spine, and migrate along nerves that connect the spinal cord with the skin. As they arrive in the skin, they are normally evenly and thinly distributed among the other skin cells. Their function is to produce pigment, which protects the skin from damage by ultraviolet rays in sunlight. The amount of pigment produced depends on skin colour and the degree of exposure to sunlight. It seems likely that a CMN reflects a failure of the normal process and many melanocytes gather at the same spot. This might happen because their progress is somehow impeded, or because they are actually attracted to this site and collect there voluntarily.

CMNs are classified according to size. A 'small' CMN measures less than 2.5cm across; a 'medium'-sized CMN measures 2.5-20cm across, and a 'large' (sometimes called 'giant') CMN will measure over 20cm across. Small CMNs are found in about 1 in 100 newborn babies: it is one of the most common types of birthmark. Medium CMNs occur in approximately 1 in 1,000 newborns; whereas large CMNs only occur in about 1 in 10,000 newborns.

Characteristics of CMNs

Although a CMN is a primarily an abnormally large accumulation of melanocytes and melanocyte precursor cells in the skin, CMNs show a number of characteristics which vary considerably, and which may change somewhat in any CMN over a period of time. These include:

• Size. CMNs vary greatly in their size (see above);
• Site. CMNs occur most commonly on the head and neck followed by the trunk, although they can occur at any site on the skin;
• Pigmentation. The colour of a CMN depends on the skin colour of the child, lighter brown in fair-skinned children and almost black in dark-skinned children. Colour is often uneven and can vary over the years, generally becoming lighter in the first two years. Very rarely they can disappear altogether. Although darker areas may appear, they are of little significance;
• Texture. The texture of large CMNs tends to be different from that of normal skin, being softer, looser and more wrinkled. The skin tends to be more fragile than normal, tearing rather easily if traumatised;
• Hairiness. CMNs are usually hairier than normal areas of skin, but this is very variable. The colour of the hair over a CMN is generally relatively dark. They can occasionally be completely hairless, even ones on the scalp;
• Lumpiness. In large CMNs, quite often there are raised or lumpy areas. This doesn't imply any special medical problem. Some lumps are paler and softer than the rest of the CMN, and some are firmer and darker;
• Eczema. The skin overlying a CMN is often rather dry and itchy, and may sometimes develop eczema. If eczema becomes a problem, you may need to see your GP or speak to a dermatologist;
• Underlying absence of fat. The presence of many melanocytes interferes with the development of the layer of fat which is normally present between the skin and underlying muscle and bone. In large CMNs, this can result in the CMN actually appearing to be depressed below the general skin surface, or it may cause a noticeable thinness in an affected leg, for example. This has no special significance other than its aesthetic effect;
• 'Satellites'. Often children with an extensive CMN will have a scattering of smaller pigmented spots elsewhere on the skin. These are known as 'satellites.' While these usually look quite similar to ordinary 'moles', they tend to be bigger. Although most frequently about 0.5-1cm across they become progressively, but proportionately, larger as the child grows: some ending up at 10cm across. The number of satellites can increase over the years, but it is impossible to predict how many are likely to appear. Their final number may be as many as a few hundred. Sunlight may increase the rate of their appearance locally, and this is one of the reasons why anyone with a large CMN should minimise sun exposure.

There is a malignancy risk involved but the scale is at present unknown.

Similar accumulations of melanocytes can be present on the surface of the brain. This is called intracranial melanosis and will generally occur in combination with CMNs in the skin. This combination is called neurocutaneous melanosis (NCM).

Intracranial melanosis may interfere with the brain's function, and the principal problems that may result are fits and developmental delay. The fits may have their onset at any age, but is most likely to happen during the first few years of life. In a young child, slow attainment of developmental milestones might provide an important clue, or educational difficulties in an older child. Other symptoms that might be provoked by intracranial melanosis include unsteadiness or clumsiness, but the wide variety of possible locations of the accumulations of melanocytes is reflected in an equally wide variety of possible consequences.

When a CMN is very close to an eye, there is a small risk of an abnormality in the eye - glaucoma. It is most likely that the glaucoma occasionally seen in this situation is due to interference with the drainage process, but the exact cause is unknown.

There are surgical treatment options available which should be discussed with a paediatric dermatologist.

Inheritance patterns
CMNs are sporadic events and there appears to be very little risk for further children.

Prenatal diagnosis
It is extremely unlikely that the presence of a CMN could be identified currently by ultra sound scanning or any other method.

Medical text written October 2001 by Dr D Atherton. Last updated November 2006 by Dr D Atherton, Consultant in Paediatric Dermatology, Great Ormond Street Hospital, London, UK.

Support and information for Congenital Melanocytic Naevi is provided by the Birthmark Support Group (see entry, Vascular Birthmarks)