By Gary M. White, MD
The congenital melanocytic nevus (CMN) is a melanocytic lesion present at or soon after birth.
See also melanoma in small to medium congenital nevi.
|Type||Diameter (in an adult)||Frequency||Lifetime Risk of Melanoma|
|Small||< 1.5 cm||Common||< 1%|
|Medium||1.5-19.9 cm||Uncommon||< 1%|
|Large||>= 20 cm.||Rare||5-6%|
|Hands, feet, torso, forearms, arms, and buttocks||7 cm|
See also Large congenital nevus, satellite nevi and neurocutaneous melanosis.
In the newborn, the congenital nevus is often flat, resembling a cafe au lait macule. Later, it usually thicken slightly and develops a mildly irregular, verrucous surface. Hypertrichosis with terminal hairs occurs in up to 75%. Commonly smaller, darker macules or papules may develop in the center of larger lesions. Sometimes, a pigmented lesion at birth is not diagnosable clinically. Biopsy may show such lesions to be melanocytic nevi, lentigines, cafe au lait macules, or non-specific fibrosis with increased capillaries etc. [BJD 1976;95;389-96].
CMN may occur anywhere on the body, even rarely in the mouth [PD 1995;12;145]. Hypoplasia of the underlying fat may occur [PD 17;387]. CMN of the scalp may be associated with cutis verticis gyrata.
With regard to other congenital birthmarks, approximately 40% of patients with CMN have CALM as well. CMN are also more common in patients with vitiligo (see vitiligo). For associations with the CMN syndrome, see CMN syndrome.
Patients with LCMN may also develop other smaller "satellite nevi" (SatN) scattered on the body. Satellite nevi may appear at birth or months to years later in life. SatN confer a greater risk of neurocutaneous melanosis. Some studies suggest that the presence of SatN with LCMN confers a greater risk of melanoma, but other studies do not confirm this. It is extremely rare for melanomas to develop in the SatN themselves.
The risk of development of melanoma in the small and medium sized nevi is small--less than 1%. For example, one study of 227 medium-sized congenital nevi with an average follow up of 6.7 years did not find any developing melanoma [JAAD 1998;39;428]. Of all melanomas, probably around 20% develop in CMN. See also melanoma in small to medium congenital nevi.
The lifetime risk of melanoma in patients with LCMN is probably 5% or less. One study found that when melanoma did develop, it did so in the LCMN 67% of the time, was metastatic 14%, and extracutaneous sites 8% of the time [BJD 2006;155;1-8].
For large lesions over the scalp, the possibility of associated leptomeningeal melanocytosis should be considered (see neurocutaneous melanosis). MRI is helpful in detecting this feature. Large melanocytic nevi over the spine may be associated with a tethered cord. MRI may be used here as well.
Within LCMN, tumorous nodules may form, called proliferative nodules. These must be distinguished from melanoma, but this can be difficult. Immunohistochemistry and FISH (fluorescence in situ hybridization) do not seem helpful in distinguishing proliferative nodules from childhood melanoma [JAMA Derm 2016;152;1147].
Small and medium congenital nevi have a low risk of melanoma and lifelong medical observation is a reasonable alternative to excision. Parents should be educated that the congenital nevus will in general grow as the child grows. When a melanoma does occur, the lesions develops a change in just one area. Pictures are often helpful. See for example, Wikipedia.
Melanoma developing in a congenital nevus. Although rare, it does happen.
Halo congenital nevi.
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