A young woman comes to the office because her husband has noted that the moles on her back are changing (Figure 162-1). A few have white halos around the brown pigmentation and some have lost their pigment completely, with a light area remaining. She 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 is reassured.
A. Multiple halo nevi on the back. B. Close-up of a halo nevus in transition. (Courtesy of 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% of neonates increasing through childhood and peaking at puberty; new ones may continue to appear in adulthood. In a population study of children (N = 180, ages 1 to 15 years) in Barcelona, the mean number of nevi was 17.5.1
- Adults typically have 10 to 40 nevi scattered over the body. In a population study in Germany, 60.3% of 2823 adults (mean age: 49 years; 50% women) exhibited 11 to 50 common nevi and 5.2% had at least 1 atypical nevus.2
- The peak incidence of melanocytic nevi (MN) is in the fourth to fifth decades of life; the incidence decreases with each successive decade.3
- 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.4
- Sun (UV) exposure, skin-blistering events (e.g., sunburn), and genetics play a role in the formation of new nevi.3
- Nevi commonly darken and/or enlarge during pregnancy. Melanocytes have receptors for estrogens and androgens and melanogenesis is responsive to these hormones.3
- Three broad categories of MN are based on location of nevus cells2:
- 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 162-2).
- Compound nevi—A nevus in which a portion of nevus cells have migrated into the dermis (Figure 162-3).
- Dermal nevi—Composed of nevus cells located within the dermis (usually found ...