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.
A. Multiple halo nevi on the back. B. Close-up of a halo nevus in transition. (Used with permission from Richard P. Usatine, MD.)
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.
Acquired nevi are common lesions, forming during early childhood; few adults have none.
Prevalence appears to be lower in dark-skinned individuals.
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
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
Adults typically have 10 to 40 nevi scattered over the body.
The peak incidence of melanocytic nevi (MN) is in the fourth to fifth decades of life; the incidence decreases with each successive decade.3
Etiology and Pathophysiology
Benign tumors composed of nevus cells derived from melanocytes, pigment-producing cells that colonize the epidermis.
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....