Congenital melanocytic nevus (CMN) is a benign nevus present at birth or within the first few weeks of life. CMNs are often flat and tan in color, initially resembling an irregular café au lait macule, but may change during the first few years of life and can vary tremendously in size. They are mostly on the trunk (~38%) or extremities (~48%) and less commonly on the head and neck (~14%). Most are elevated lesions that are primarily brown but may have variation in pigmentation including black, gray, and blue. Approximately 3% of patients have multiple CMNs. Compared with acquired nevi, CMNs have an earlier onset and are often larger with a more heterogeneous morphology.
One of the principal categorizations of CMNs is by size based on the projected maximal diameter of the CMN in adulthood. Different size cutoffs have been used with a recent proposed categorization including the categories of: small (< 1.5 cm), medium (M1: 1.5-10 cm, M2: > 10-20 cm), large (L1: > 20-30 cm, L2: > 30-40 cm), and giant (G1: > 40-60 cm, G2: > 60 cm). Most CMNs are less than 3-4 cm, with larger lesions being less common. CMNs have a slight female predominance (3:2) and are more common among patients of African and Japanese descent compared with individuals of Hispanic or Northern European descent.
CMNs have been associated with many benign lesions, the most common of which include café au lait macules and mucosal nevi.
CMNs have also been reported to have an increased risk for transformation to melanoma. The risk of melanoma is believed to correlate with CMN size. The incident rate of melanoma arising in small or medium CMNs is reported to be less than 1%. In one meta-analysis, melanoma was estimated to develop in approximately 2% of large CMNs (> 20 cm). Melanomas were diagnosed at a mean age of 12 years (range: birth to 58 years old), with 82% cutaneous and 13% visceral. Most cutaneous melanomas were located on the trunk (68%), and most of the melanoma-associated CMNs exceeded 40 cm in diameter (74%), with 94% of patients also having satellite nevi.
Large CMNs overlying limbs can be associated with atrophy of the subcutaneous tissue. Large CMNs over the head or axial skeleton should alert the physician to assess for neurocutaneous involvement (developmental delays, seizures, hydrocephalous, increased intracranial pressure).
Neurocutaneous melanosis is when a CMN is associated with neuromelanosis, a congenital error in ectodermal morphogenesis resulting in melanocytic proliferation within the leptomeninges and brain parenchyma. This most commonly occurs in larger CMNs. While not all patients are symptomatic, there can be diverse neurologic findings that typically present by 2 years of age.
CMN may also be associated with other syndromes such as Carney (pigmented skin lesions and atrial myxomas), LAMB (lentigines, atrial myxoma, mucocutaneous myxoma, blue nevi), NAME (nevi, atrial myxoma, myxoid neurofibromata, ephelides), neurocutaneous melanosis, premature aging syndrome, occult spinal dysraphism, and possibly neurofibromatosis type 1, and other malformation syndromes.
A birthmark (congenital melanocytic nevus, CMN) is a mole that is present at birth or shortly thereafter. A congenital melanocytic nevus is one common type of birthmark, caused by a cluster of color (pigment) cells in the skin and sometimes in deeper tissues.
All birthmarks have a 2-5% lifetime risk of turning into a cancerous (malignant) mole, which is called melanoma. This risk is higher in children who have a giant (larger than 20 cm, or about 8 inches) congenital melanocytic nevus.
Large moles on the head or spine may rarely have associated nervous system problems.
Who’s At Risk
Most babies are born without moles but will gradually acquire them with age and sun exposure. Congenital, meaning "born with," moles are seen in about 1-2% of births. There is no racial or sex difference in occurrence.
Signs & Symptoms
Congenital melanocytic nevi are brown or black, usually raised spots or patches, that are present at birth or shortly after (before 1 year). They are usually single but can be multiple and range in size greater than 1 cm to less than 20 cm (bathing suit or garment lesions cover large sections of the body). Smaller, so-called, satellite lesions are sometimes associated with large lesions.
The most common locations for these birthmarks are the buttocks, thighs, and trunk, but lesions may occur on the face and extremities, and, least commonly, on the palms, soles, and scalp.
Changes in thickness, color, and hair content occur through childhood and adolescence. In later adult years, they may actually fade in color to a lighter brown.
No activity or other restrictions are needed. However, use sun protection clothing (and sunblock after age 1) and make-up coverage, if desired for cosmetic reasons.
When to Seek Medical Care
Any baby with a birthmark should have it checked by at least one doctor. If the doctor recommends only observation, watch it lifelong for any change in size (other than slow growth with the child), shape, or color and for any change in the surface (flat areas that become more raised).
A skin biopsy may be done if there is suspicion of malignant change to the birthmark.
Small- to medium-size congenital melanocytic nevi (less than 20 cm, or about 8 inches) are often watched rather than removed surgically. In addition to observing the mole for any changes, photographs may be taken to document and follow potential changes in the mole.
Consultation and management by a dermatologist with further consultation to the plastic surgeon may be necessary. Removal may be desired for cosmetic reasons.
Ideally, giant congenital melanocytic nevi (greater than 20 cm) are removed surgically to reduce the risk of malignant change, although this is not always possible. If the moles are numerous or involve the head or spine, consultation with a neurologist may be suggested.
Bolognia, Jean L., ed. Dermatology, pp.1782-1785. New York: Mosby, 2003.