Atypical naevus

Author: Vanessa Ngan, Staff Writer; Amanda Oakley, Dermatologist, Hamilton, New Zealand, 2003.

What is an atypical naevus?

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).

What does an atypical naevus look like?

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.

  • 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
Atypical naevi

See more images of atypical naevi ...

Sporadic atypical naevi

Sporadic atypical naevi mainly affect fair-skinned individuals with light coloured hair and freckles (phototype 1-2), especially if they have been frequently exposed to the sun.

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 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.

Atypical naevi

What is a dysplastic naevus?

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.

  • 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 arise within a dysplastic naevus, or within a non-dysplastic, otherwise harmless-looking naevus.

What is the importance of atypical naevi?

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.

Careful sun protection is recommended: avoid excessive sun exposure and use a SPF15+ sunscreen when outdoors in the middle of the day or for prolonged periods.


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