Moles & Atypical Nevi

What are moles? Moles, or nevi, are skin growths made up of cells called melanocytes or nevus cells. The average person has fewer than 40 moles. Moles can be flat or raised and can range in color from skin colored to light, medium, or dark brown (no particular type is "better" than another). Moles are primarily genetically determined, although ultraviolet (UV) exposure can increase the number of moles a person might otherwise develop. Moles can be acquired from early childhood through early adulthood. The average person stops growing new moles by their early thirties.

The ABCDE rules are guidelines to help determine if a mole is normal or abnormal. A is for asymmetry, B is for Border, C is for Color, D is for Diameter, and E is for Elevation and/or Evolving. Normal moles generally follow the ABCDE rules: they are symmetrical (i.e. one half looks like the other half), have smooth borders, have uniform color that is not too dark, are smaller than a pencil eraser in diameter, and are uniformly elevated above the skin surface and not evolving (changing) over time. A normal mole that suddenly breaks the ABCDE rules might be in the process of becoming cancerous – a malignant melanoma.

A small number of moles, called congenital nevi, are present at birth or appear shortly thereafter. Congenital nevi tend to be larger than moles acquired later in life and may have a higher risk of cancerous change over the lifetime of the patient when compared to acquired moles. The risk of cancerous change tends to be proportional to the size of the congenital nevus at birth: low if less than or equal to 1cm, intermediate if between 1 and 20 cm, and high if >20 cm. Some congenital nevi may slowly change in appearance over time, especially in early childhood, developing slightly uneven color, a thickened "pebbly" surface, and increased amounts of hair. These changes in congenital nevi are felt to be part of the maturation of these types of moles. They are not necessarily cause for concern, but such changes, as well as whether it is necessary to remove a congenital nevus, should be discussed with a dermatology health care provider.

What are atypical nevi? Atypical nevi, also called "dysplastic nevi" or Clark's nevi, are acquired moles that have unusual features both in terms of how they look clinically and how they look when biopsied (removed and examined under a microscope). They tend to break the ABCDE rules from the time they first appear: they may be asymmetrical, have irregular borders, be unevenly and/or very darkly colored, be larger than a pencil eraser, be unevenly elevated above the skin surface, or some combination of these findings. In other words, to the naked eye, they can mimic a malignant melanoma. Microscopically, atypical nevi have a characteristic appearance in terms of their architecture (how the cells are arranged relative to each other in the skin) and their cytology (how each individual cell looks) that is different from those of "typical" moles. The degree to which atypical moles are unusual microscopically can be graded as mild, moderate, or severe.

What is the significance of having atypical nevi? One mildly atypical nevus is generally not cause for great concern in and of itself, although people who have a lot of these moles tend to be at statistically higher risk for developing a melanoma over their lifetime compared to people who do not. Having a lot of unusual looking moles is an external marker of genetically increased risk of melanoma, especially if there is a family history of melanoma; this is true regardless of the patient's history of tanning or sunburns. (UV exposure is a separate risk factor.) Being "moley" and having a lot of unusual looking moles – particularly when there is a family history of the same – is referred to as "dysplastic nevus syndrome" or "familial atypical mole syndrome". Mildly atypical nevi are not necessarily any more likely to become cancerous than any "typical" mole the patient might have. However, nevi with moderate or severe atypia may be more likely to become cancerous over time, so it is advised that they be removed completely. The difficulty lies in determining which moles need to be removed and how much surgery is needed to remove them. Both are topics of ongoing debate among dermatologists that providers should discuss with their patients.

How are atypical nevi managed? It is very important for patients with unusual looking moles to have a thorough baseline examination by a dermatology health care provider in order to determine which moles, if any, are abnormal enough to remove. The results of that baseline exam will determine the frequency of subsequent follow-up exams. There are several ways a provider can determine which moles should be biopsied and which ones can be monitored. In general, even if a patient has a lot of unusual looking moles, those moles will usually follow one or more "themes". Looking for the "ugly duckling" that doesn't follow the theme(s) is often a good indicator as to which mole should be removed. If there is any history of a mole changing or being symptomatic, that mole should also be evaluated. In addition to having regular monitoring by a dermatology health care provider, it is very important for patients with atypical nevi to monitor their moles at home regularly. Finally, sun protection measures are also important for these patients. Having atypical nevi is a genetically determined trait, but excessive exposure to UV light over time increases a patient's risk not only of malignant melanoma but of other types of skin cancer as well. A combination of regularly scheduled in-office exams, frequent self-monitoring at home, targeted biopsy of selected nevi, and sun protection measures is a very effective strategy in managing patients with atypical nevi.

Elizabeth S. Miller, M.D., FAAD, FAAP