A teenager is brought to the office by her mother who has noted that the moles on her daughter’s back are changing (Figure 143-1). A few have white halos around the brown pigmentation and some have lost their pigment completely, with a light area remaining. The teenager has no symptoms but wants to make sure these are not skin cancers. Halo nevi are an uncommon variation of common nevi. These appear benign and the patient and her mother are reassured.

Most nevi are benign tumors caused by the aggregation of melanocytic cells in the skin. However, nevi can occur on the conjunctiva, sclera and other structures of the eye. There are also nonmelanocytic nevi that are produced by other cells as seen in Becker nevi and comedonal nevi. Although most nevi are acquired, many nevi are present at birth.

Moles.

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  Acquired nevi are common lesions, forming during early childhood; few adults have none.

  
  
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  Prevalence appears to be lower in dark-skinned individuals.

  
  
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  Present in 1 percent of neonates, increasing through childhood, and peaking at puberty; new ones may continue to appear in adulthood. In a convenience sample of children in Colorado, non-Hispanic white children had the highest number of nevi compared to other racial/ethnic groups. Beginning at age 6 years, non-Hispanic white boys had significantly more nevi than non-Hispanic white girls (median, 21 versus 17), Hispanic white children (median 11), black children (median 7), and Asian/Pacific Islander children (median 6).¹ This number is similar to a study of children (N = 180, ages 1 to 15 years) in Barcelona where the mean number of nevi was 17.5.²

  
  
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  In the Colorado study previously cited, non-Hispanic white children developed an average of 4 to 6 new nevi per year from 3 to 8 years of age. Development of new nevi leveled off in chronically exposed body sites at 7 years of age and at a higher level for boys than girls.¹

  
  
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  Adults typically have 10 to 40 nevi scattered over the body.

  
  
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  The peak incidence of melanocytic nevi (MN) is in the fourth to fifth decades of life; the incidence decreases with each successive decade.³

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  Benign tumors composed of nevus cells derived from melanocytes, pigment-producing cells that colonize the epidermis.

  
  
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  MN represent proliferations of melanocytes that are in contact with each other, forming small collections of cells known as nests. Genetic mutations present in common nevi as well as in melanomas include BRAF, NRAS, and c-kit.⁴

  
  
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  Sun (UV) exposure, skin-blistering events (e.g., sunburn), and genetics play a role in the formation of new nevi.³

  
  
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  Three broad categories of MN are based on location of nevus cells:⁵

  
  
  
  
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    Junctional nevi—Composed of nevus cells located in the dermoepidermal junction; may change into compound nevi after childhood (except when located on the palms, soles, or genitalia; Figure 143-2).

    
    
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    Compound nevi—A nevus in which a portion of nevus cells have migrated into the dermis (Figure 143-3).

    
    
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    Dermal nevi—Composed of nevus cells located within the dermis (usually found only in adults). These are usually raised and have little to no visible hyperpigmentation (Figures 143-4 and 143-5).

  
  
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  Special categories of nevi:

  
  
  
  
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    Halo nevus—Compound or dermal nevus that develops a symmetric, sharply demarcated, depigmented border (Figure 143-1). Most commonly occurs on the trunk and develops during adolescence. Repigmentation may occur.

    
    
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    Blue nevus—A dermal nevus that contains large amounts of pigment so that the brown pigment absorbs the longer wavelengths of light and scatters blue light (Tyndall effect; Figure 143-6). Blue nevi are not always blue and color varies from tan to blue, black, and gray. The nodules are firm because of associated stromal sclerosis. Usually appears in childhood on the extremities, dorsum of the hands and face. A rare variant, the cellular blue nevus is large (>1 cm), frequently located on the buttocks, and may undergo malignant degeneration.

    
    
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    Nevus spilus—Hairless, oval, or irregularly shaped brown lesion with darker brown to black dots containing nevus cells (Figure 143-7). May appear at any age or be present at birth; unrelated to sun exposure.

    
    
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    Spitz nevus (formerly called benign juvenile melanoma because of its clinical and histologic similarity to melanoma)—Hairless, red, or reddish brown dome-shaped papules generally appearing suddenly in children, sometimes following trauma (Figures 143-8 and 143-9). The pink color is caused by increased vascularity. Most importantly, these should be fully excised with clear margins.

    
    
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    Nevus of Ota—Dark brown nevus that occurs most commonly around the eye and can involve the sclera (Figure 143-10).

  
  
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  Both acquired and congenital MN hold some risk for the development of melanoma; the number of MN, especially more than 100, is an important independent risk factor for cutaneous melanoma.⁶