Dermatology Made Easy is based on the most popular topics from DermNet NZ's vast array of material. The book combines the essential focus of the ‘Made Easy’ book series with the authority and knowledge base of DermNet NZ's unparalleled resources.
Author: Dr Amanda Oakley, Dermatologist, Hamilton, New Zealand, 2001. Updated by Giri Raj, Dermatologist, New Plymouth; Dr Amanda Oakley, June 2014.
A congenital melanocytic naevus (American spelling nevus) is a proliferation of benign melanocytes (pigment cells) that are present at birth or develop shortly after birth. (1) Naevi (American spelling nevi) are also known as brown birthmarks.
Other melanocytic naevi, or moles, that look like birthmarks, but were not present at birth, are often called ‘congenital melanocytic naevus-like’ naevi, ‘congenital type’ naevi or ‘tardive’ naevi.
Naevi may also form from other skin cells (eg vascular naevi, which are formed from blood vessels). Some of these are also congenital (present at birth) and are described on other pages of DermNet.
Congenital melanocytic naevi are usually classified by size. There are several different classifications.
A modification of the above criteria is used in some centers (2) in an effort to increase the accuracy of classification.
In 2013, a new categorisation of congenital melanocytic naevi using predicted adult size was proposed (4):
Congenital melanocytic naevi should be described according to their body site, colours, surface features and whether or not there is hypertrichosis (hairs).
They occur in all races and ethnic groups, and males and females are at equal risk.
Congenital melanocytic naevi present as single or multi-shaded, round or oval shaped pigmented patches (2). They may have increased hair growth (hypertrichosis). The surface may be slightly rough or bumpy.
Congenital melanocytic naevi usually grow proportionally with the child. As a rough guide, the likely adult size of a congenital naevus can be calculated as follows:
Congenital naevi may become smaller and less obvious with time. Rarely some may even disappear. However they may also become darker, raised, more bumpy and hairy, particularly around the time of puberty.
Congenital melanocytic naevi are usually asymptomatic, however, some may be itchy, particularly larger lesions. It is thought there may be reduced function of sebaceous (oil) and eccrine (sweat) glands, which may result in skin dryness and a heightened sensation of itch.
The overlying skin may become fragile and erode or ulcerate. Deep nests of melanocytes in the dermis may weaken the bonds between the epidermis and the dermis and account for skin fragility. (3)
Congenital melanocytic naevi are often unsightly, especially when extensive, ie large or giant congenital melanocytic naevi. They may therefore result in anxiety and impaired self image, especially when the lesions are in visible areas.
Giant melanocytic naevi, and to a lesser degree small lesions, are associated with increased risk of developing cutaneous melanoma, neurocutaneous melanoma and rarely other tumours (see below).
Congenital melanocytic naevi are caused by localised genetic abnormalities resulting in the proliferation of melanocytes; these are cells in the skin responsible for normal skin colour. This abnormal proliferation is thought to occur between the 5th and 24th weeks of gestation. If proliferation starts early in development, giant and medium sized congenital melanocytic naevi are formed (1). Smaller congenital melanocytic naevi are formed later in development, after the melanoblasts (immature melanocytes) have migrated from the neural crest to the skin (1).
In some cases, there are also overgrowth of hair-forming cells and epidermis, forming an organoid naevus.
Very early onset of congenital naevus before separation of the upper and lower eyelids results in kissing naevi, ie one part of the naevus is on the upper lid and the other part is on the lower eyelid.
Proto–oncogenes c-met and c-kit have important roles in the development of melanocytes. Hepatocyte growth factor, a cytokine (messenger protein) that regulates the proliferation and migration of melanocytes, may also be important in the development of congenital melanocytic naevi (1).
Neurocutaneous melanocytosis is a rare syndrome defined by the proliferation of melanocytes in the central nervous system (brain and spinal cord) and the presence of a congenital melanocytic naevus (1,2). The majority of cases are associated with a giant congenital melanocytic naevus and satellite lesions.
It is estimated neurocutaneous melanosis affects 5–10% of people that have a giant congenital melanocytic naevus. However it is likely that the majority of cases remain asymptomatic, and the true incidence remains unknown (3). The melanocytes in the brain and spinal cord may often be detected by an MRI scan but the use of these scans is controversial, because the condition is not easily treatable.
Neurocutaneous melanocytosis may present with symptoms of raised intracranial pressure (2), such as:
The diagnosis of a congenital melanocytic naevus is usually based on the clinical appearance. If there is any doubt, examining the lesion with dermoscopy or taking a sample of the lesion for histology (biopsy) may be done.
Evaluation of the congenital melanocytic naevus by dermoscopy will reveal the pattern of pigmentation and its symmetry or lack of symmetry. The most common global pattern of congenital or tardive melanocytic naevus is globular, but reticular, structureless and mixed patterns may occur. The naevus may have differing structures across the lesion, sometimes leading to overall asymmetry of structure.
Congenital melanocytic naevi are usually larger than acquired naevi (which are melanocytic naevi that appear after 2 years of age), and the naevus cells often extend deeper into the dermis, fat layer, and deeper structures. The naevus cells characteristically cluster around blood vessels, hair follicles, sebaceous and eccrine glands, and other skin structures. Congenital naevus cells tend to involve collagen bundles in the deeper layers of the skin more than is the case in an acquired naevus (1,2).
The risk of melanoma is mainly related to the size of the congenital melanocytic naevus. Small and medium sized congenital melanocytic naevi have a very small risk, well under 1%. Melanoma is more likely to develop in giant congenital naevi (lifetime estimates are 5-10%), particularly in lesions that lie across the spine or where there are multiple satellite lesions. Melanoma can start deep inside the naevus or within any neuromelanosis found in the brain and spinal cord. Very rarely, other tissues that contain melanocytes may also be a source of melanoma such as the gastrointestinal tract mucosa. In 24% of cases, the origin of the melanoma cannot be identified (2).
Melanoma associated with a giant congenital melanocytic naevus or neuromelanosis can be very difficult to detect and treat.
The risk of development of melanoma is greater in early childhood; 70% of melanomas associated with giant congenital melanocytic naevi are diagnosed by the age of ten years. (1,3)
Rarely, other types of tumour may develop within giant congenital melanocytic naevi including benign tumours (lipomas, schwannomas) and other malignant tumours (including sarcomas).
Melanoma can also develop within a small congenital melanocytic naevus. This is rare, and likely to occur on the periphery of the naevus during adult life.
Unfortunately when melanoma arises within a giant congenital melanocytic naevus, the prognosis is unfavourable . This is due to the deeper origin of the tumour rendering it more difficult to detect on clinical examination, resulting in a later stage at presentation. The deeper location also facilitates earlier spread through blood and lymph vessels. In 24% of cases the melanoma has already spread to other sites (metastases) at the time of first diagnosis.
Management of congenital melanocytic naevi must take into account the age of the subject, the lesion size, the location and depth, and the risk of developing malignant change within the lesion.
The only definite indication for surgery in a giant congenital melanocytic naevus is when a melanoma develops within it.(2)
If a small congenital naevus is growing at the same rate as the child, and is not changing in any other way, the usual practice is not to remove it until the child is old enough to co-operate with a local anaesthetic injection, usually around the age of 10 to 12 years. Even then, removal is not essential.
Reasons to consider surgical removal may include:
Prophylactic surgical removal is performed if it is felt that there is a high risk that a melanoma may arise within the lesion. The following factors should be considered.
Complications that may occur after surgery include:
Dermabrasion can allow partial removal of a large congenital naevus. However, deeper naevus cells may persist. Dermabrasion may lighten the colour of the naevus but may not reduce hair growth within it. It can cause scarring.
Tangential or shave excision uses a blade to remove the top layers of the skin (epidermis and upper dermis). This may reduce the pigmentation but the lesion may not be completely removed. Shave excision may result in significant scarring.
Chemical peels using trichloroacetic acid or phenol may lighten the pigmentation of a superficial (surface) congenital naevus that is located in the upper layers of the skin
Laser treatment be considered if surgical intervention is not possible. They may result in lightening of the lesion. Suitable devices include:
Techniques that result in partial removal of a congenital naevus can make the lesion more difficult to assess during long term surveillance.(2)
See the DermNet NZ bookstore.
© 2018 DermNet New Zealand Trust.
DermNet NZ does not provide an online consultation service. If you have any concerns with your skin or its treatment, see a dermatologist for advice.