What are atypical naevi?
Atypical naevi are moles or melanocytic naevi (nevi in American spelling) with unusual features. They may resemble malignant melanoma (cancerous mole), but are actually harmless. Other names for atypical naevi include Clark naevi and B-K moles.
There are basically two types of atypical naevi, sporadically occurring atypical naevi and familial (inherited) atypical naevi.
What does an atypical naevus look like?
The term atypical naevus is sometimes used to mean any funny-looking mole. However, strictly speaking, an atypical naevus is defined as a mole with at least 3 of the following features.
- Size >5 mm diameter
- Ill-defined or blurred borders
- Irregular margin resulting in an unusual shape
- Varying shades of colour (mostly pink, tan, brown, black)
- Flat and bumpy components
Sporadic atypical naevi
Atypical naevi may develop at any time throughout life but most of them develop during childhood, usually within the first 15 years of life. Typically, people with sporadic atypical naevi have one to ten lesions.
Familial atypical naevi
Atypical naevi that run in families may be part of the FAMM syndrome. FAMM is an abbreviation for Familial Atypical Mole and Melanoma. People with FAMM syndrome must have the following:
- One or more first-degree or second-degree relative with malignant melanoma;
- A large number of naevi (often more than 50), some of which are atypical naevi;
- Naevi that are dysplastic on histopathology.
FAMM syndrome was previously known as dysplastic naevus syndrome.
People with FAMM syndrome may have several hundred atypical naevi.
What are dysplastic naevi?
Clinically atypical naevi are sometimes called dysplastic naevi, but this is term is best used for a specific microscopic appearance. Only a minority of clinically atypical naevi fulfill microscopic criteria for dysplastic naevus. Dysplasia may be mild, moderate or severe. Distinct pathological criteria of a dysplastic naevus are listed here.
- The lesion may be a junctional naevus or more frequently a compound naevus (the cells are found at the epidermodermal junction and within the dermis).
- The naevus cells form a row along the dermoepidermal junction (called lentiginous proliferation), with or without naevus cells in nests (called theques).
- These theques are often irregular in size and shape and may 'bridge' or join together.
- The cells may be odd-looking i.e. they have cytologic atypia, and they may be spindle-shaped (elongated) or epithelioid (resembling epidermal keratinocytes i.e., broad).
- There may be fibrosis or scarring in the dermis.
- Inflammatory cells may infiltrate the lesion.
- Associated blood vessels may be increased in number or enlarged.
Malignant melanoma may sometimes arise within a dysplastic naevus.
What is the importance of atypical naevi?
People with atypical naevi have a slightly higher risk than the general population of developing melanoma, particularly if they have five or more atypical naevi (relative risk is six times that of people without atypical naevi). People with FAMM syndrome have an extremely high risk of developing melanoma.
Atypical naevi are actually harmless (benign) and do not need to be removed. However, it is not always easy even for an experienced dermatologist to tell whether a lesion is an atypical naevus or a melanoma. Dermoscopy in trained hands may help. If in doubt, a suspicious or changing atypical naevus should be removed by excision biopsy. A pathologist will usually make the correct diagnosis, although sometimes a second opinion may be required.
People diagnosed with atypical naevi should be taught how to self-examine their skin for new moles or for changes to existing moles that may indicate melanoma development. If you have numerous moles you should visit your family doctor or dermatologist regularly for a thorough skin check.
It is often helpful to keep photographic records of the naevi; digital archiving at a mole mapping clinic is convenient. The close-up photographs should be repeated from time to time. Dermoscopic views enable your dermatologist to detect change early.