Congenital melanocytic naevus

Congenital melanocytic nevus
Synonyms	CMN
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A congenital melanocytic nevus (CMN) is a type of melanocytic nevus (or mole) found in infants at birth. These nevi are caused by a proliferation of melanocytes, the cells responsible for producing melanin, the pigment that gives skin its color.

Classification

Congenital melanocytic nevi are classified based on their size:

• Small – less than 1.5 cm in diameter
• Medium – 1.5 to 20 cm in diameter
• Large or giant – greater than 20 cm in diameter

Epidemiology

CMN occur in approximately 1% of newborns. The incidence of large or giant CMN is much rarer, occurring in about 1 in 20,000 newborns.

Pathophysiology

The development of CMN is due to a mutation in the NRAS or BRAF genes, which are involved in the MAPK/ERK pathway, a signaling pathway that regulates cell division and differentiation.

Clinical Features

CMN can vary in color from light brown to black and may have an irregular surface. They can be flat or raised and may have hair growing from them. Large or giant CMN can cover significant portions of the body and may be associated with neurocutaneous melanosis, a condition where melanocytes are present in the central nervous system.

Diagnosis

Diagnosis of CMN is primarily clinical, based on the appearance of the nevus. Dermatoscopy can be used to examine the nevus in more detail. In some cases, a biopsy may be performed to rule out malignant transformation.

Management

The management of CMN depends on the size and location of the nevus, as well as the risk of malignant melanoma development. Options include:

• Observation – Regular monitoring for changes in size, color, or texture.
• Surgical excision – Removal of the nevus, particularly if there is a suspicion of malignancy.
• Laser therapy – Used to reduce pigmentation or hair growth.

Prognosis

The prognosis for individuals with CMN varies. Small and medium-sized nevi have a low risk of malignant transformation, while large or giant nevi have a higher risk. Regular follow-up with a dermatologist is recommended.

See also

• Melanoma
• Nevus
• Neurocutaneous melanosis

External links

• DermNet NZ: Congenital Melanocytic Naevus
• American Academy of Dermatology: Congenital Melanocytic Nevus