Cutaneous Nevi

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Chapter 643 Cutaneous Nevi

Nevus skin lesions are characterized histopathologically by collections of well-differentiated cell types normally found in the skin. Vascular nevi are described in Chapter 642. Melanocytic nevi are subdivided into 2 broad categories: those that appear after birth (acquired nevi) and those that are present at birth (congenital nevi).

Acquired Melanocytic Nevus

Melanocytic nevus is a benign cluster of melanocytic nevus cells that arises as a result of alteration and proliferation of melanocytes at the epidermal-dermal junction.

Atypical Melanocytic Nevus

Atypical melanocytic nevi occur both in an autosomal dominant familial melanoma-prone setting (familial mole–melanoma syndrome, dysplastic nevus syndrome, BK mole syndrome) and as a sporadic event. Only 2% of all pediatric melanomas occur in individuals with this familial syndrome; melanoma develops before age 20 yr in 10% of individuals with the syndrome. Malignant melanoma has been reported in children with the dysplastic nevus syndrome as young as 10 yr. Risk for development of melanoma is essentially 100% in individuals with dysplastic nevus syndrome who have 2 family members who have had melanomas. The term atypical mole syndrome describes lesions in those individuals without an autosomal dominant familial history of melanoma but with > 50 nevi, some of which are atypical. The lifetime risk of melanoma associated with dysplastic nevi in this context is estimated to be 5-10%.

Atypical nevi tend to be large (5-15 mm) and round to oval. They have irregular margins and variegated color, and portions of them are elevated. These nevi are most common on the posterior trunk, suggesting that intermittent, intense sun exposure has a role in their genesis. They may also occur in sun-protected areas such as the breasts, buttocks, and scalp. Atypical nevi do not usually develop until puberty, although scalp lesions may be present earlier. Atypical nevi demonstrate disordered proliferation of atypical intraepidermal melanocytes, lymphocytic infiltration, fibroplasia, and angiogenesis. It may be helpful to obtain histopathologic documentation of dysplastic change by biopsy to identify these individuals. It is prudent to excise borderline atypical nevi in immunocompromised children or in those treated with irradiation or chemotherapeutic agents. Although chemotherapy has been associated with the development of a greater number of melanocytic nevi, it has not been directly linked to increased risk for development of melanoma. The threshold for removal of clinically atypical nevi is also lower at sites that are difficult to observe, such as the scalp. Children with atypical nevi should undergo a complete skin examination every 6-12 mo. In these children, photographic mole mapping serves as a useful adjunct in following nevus change. Parents must be counseled about the importance of sun protection and avoidance and should be instructed to look for early signs of melanoma on a regular basis, approximately every 3-4 mo.

Congenital Melanocytic Nevus

Congenital melanocytic nevi are present in ≈1% of newborn infants. These nevi have been categorized by size: giant congenital nevi are >20 cm in diameter (adult size) or >5% of the body surface, small congenital nevi are <2 cm in diameter, and intermediate nevi are in between these dimensions. Congenital nevi are characterized by the presence of nevus cells in the lower reticular dermis; between collagen bundles; surrounding cutaneous appendages, nerves, and vessels in the lower dermis; and occasionally extending to the subcuticular fat. Identification is often uncertain, however, because they may have the histologic features of ordinary junctional, compound, or intradermal nevi. Some nevi that were not present at birth display histopathologic features of congenital nevi; these should not be considered congenital. Furthermore, congenital nevi may be difficult to distinguish clinically from other types of pigmented lesions, adding to the difficulty that parents may have in identifying nevi that were present at birth. The clinical differential diagnosis includes mongolian spots, café-au-lait spots, smooth muscle hamartoma, and dermal melanocytosis (nevi of Ota and Ito).

Sites of predilection for small congenital nevi are the lower trunk, upper back, shoulders, chest, and proximal limbs. The lesions may be flat, elevated, verrucous, or nodular and may be various shades of brown, blue, or black. Given the difficulty in identifying small congenital nevi with certainty, data regarding their malignant potential are controversial and likely overstated. The true incidence of melanoma in congenital nevi, especially small and medium-sized lesions, is unknown. Removal of all small congenital nevi is not warranted because the development of melanoma in a small congenital nevus is an exceedingly rare event before puberty. A number of factors must be weighed in the decision about whether or not to remove a nevus, including its location, the ability to be monitor it clinically, the potential for scarring, the presence of other risk factors for melanoma, and the presence of atypical clinical features.

Giant congenital pigmented nevi (<1/20,000 births) occur most commonly on the posterior trunk (Fig. 643-2

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