Author: Vanessa Ngan, Staff Writer; Amanda Oakley, Dermatologist, Hamilton, New Zealand, 2003.
An atypical naevus is a mole or melanocytic naevus (nevus in American spelling) with unusual features. It may resemble or be confused with malignant melanoma (cancerous mole). Older names for atypical naevus include Clark naevus and B-K mole.
Atypical naevi may occur sporadically or may be familial (inherited).
The term atypical naevus is sometimes used to mean any funny-looking mole. One definition of an atypical naevus is a mole with at least 3 of the following features.
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.
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 have the following:
FAMM syndrome was previously known as dysplastic naevus syndrome.
People with FAMM syndrome may have several hundred atypical naevi.
Clinically an atypical naevus is sometimes called a dysplastic naevus, but this term is best used for a specific microscopic appearance. Only a minority of clinically atypical naevi fulfil microscopic criteria for dysplastic naevus.
Dysplasia may be mild, moderate or severe. Distinct pathological criteria of a dysplastic naevus are listed here.
Malignant melanoma may arise within a dysplastic naevus, or within a non-dysplastic, otherwise harmless-looking naevus.
People with 5 or more clinically atypical naevi have a slightly higher risk than the general population of developing melanoma (relative risk is six times that of people without atypical naevi). People with FAMM syndrome have an extremely high risk of developing melanoma.
Naevi are harmless (benign) and do not need to be removed. It is not always easy even for an experienced dermatologist to tell whether a lesion is a naevus or a melanoma, especially if there are atypical features. Dermatoscopy 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 (preferably with dermatoscopic images); digital archiving at a photographic skin surveillance clinic is convenient. The close-up photographs should be repeated from time to time. Dermatoscopic views enable your dermatologist to detect change early and to determine the significance of any changes.
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