What is CMN


What is Congenital Melanocytic Naevus (CMN)

  • Congenital simply means that it is present at birth.
  • Melanocytic means that it is pigment-based.
  • Naevus (also spelled nevus; plural naevi or nevi) means birthmark.

Though CMN means birthmark or mole present at birth. However, not all of them are present at birth. Around 1% of CMN appear after birth, usually in the first year of life. They look exactly the same as the ones present at birth, but are called CMN “tardive”, which means “late”.

What does it look like?

Every CMN is unique. People of all races and colors of skin can be born with one.
CMN generally grow in proportion to the child. In other words, they will usually continue to cover the same proportional part of the body surface as at birth – for instance, a part of a limb, or extending between the shoulder blades and the upper thigh.
Rare exceptions to this are “tardive” CMN, as these become visible after birth and can therefore appear to grow. Occasionally, where part of a CMN is very pale, it can darken during the first few months and thus appear to grow. The size that a CMN will end up, can therefore be estimated from the size at birth by looking at its size and where it is located.

CMN can be on any part of the skin, including palms, soles, and scalp, and even inside the mouth.

Usually some sort of shade of brown to black, sometimes CMN can be reddish. It is often very mixed inside the CMN, with smaller patches of different colors on a solid background. CMN at birth are often deep black, or even dark purple/red as they have many blood vessels visible through a thin surface. They can change color over time, as the child grows. When they change, they usually become paler, but can also become darker for those that were initially light or speckled. Lightening appears to happen more in children with light or red hair and pale skin than in those with darker hair and skin tone.

CMN usually have hair growing out of them, although this is often not visible at birth and can remain unnoticeable throughout life. The hair color can be darker than the child’s head hair, or the same, or very occasionally lighter. If a CMN is on the scalp, there is usually more luxuriant hair growth over the lesion and the hair grows faster than on the rest of the scalp. Body CMN – be they small or large – can develop terminal hair, which is more visible and often dense. Occasionally, CMN are completely hairless, even ones that occur in the scalp, or can have patchy hair loss or hairs without pigment (gray or white hair).

The texture of large CMN tends to be different from that of normal skin, sometimes being softer, looser and more wrinkled. The CMN can be nearly flat or can be very folded or lumpy.

The total number of visible nevi can continue to increase after birth, although this does not always happen. When it does, they often develop most during the first few years of life, when the rate of appearance of different ones then usually tapers off. However, they are always relatively small. Such “new” nevi, sometimes called “satellites”, are considered by doctors to also be tardive. That is, they may have already been there as scattered, unpigmented CMN at the time of birth. Appearance of these additional nevi is more likely to occur in children with very large and multiple CMN, but in themselves are not a reason for concern.

How often does CMN occur?

They are classified based on the expected size in adulthood (‘projected adult size’, PAS). CMNs can occur according to the following distribution:

  1. A single (usually quite large congenital) birthmark.
  2. A (larger) ‘nevus’, accompanied by smaller nevi around it.
  3. Multiple smaller CMN (more than 3) medium-sized CMNs (1.5 cm ~ 20 cm PAS) spread across the body.
Small CMN <1.5 cm PAS 1 in 100 births 1%
Medium CMN 1.5-20 cm PAS 1 in 1000 births 0,1%
Large CMN 20-40 cm PAS 1 in 20,000 births 0,005%
Giant CMN >40 cm PAS 1 in 500,000 births 0,0002%

What causes CMN?

According to recent insights, precursor cells of pigment cells acquire a mutation early in pregnancy during the formation of the child’s organs. The ‘timing’ of the mutation likely largely determines the extent of CMN: an early mutation results in the widespread dissemination of nevus cells to the skin and central nervous system. A late mutation leads to smaller CMN, confined to a specific area of the skin, without satellites. CMN generally arises sporadically and is not hereditary.

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