Atypical melanocytic naevus

What is a melanocytic naevus?

A melanocytic naevus (Nevus in american spelling) is a mole, a common skin lesion composed of naevus cells or specialised melanocytes. There are various kinds of melanocytic naevi (nevi). 

What is an atypical melanocytic naevus?

An atypical melanocytic naevus is a mole with unusual features. 

• "Atypical melanocytic naevus" is often shortened to "atypical naevus". 
• Older names for atypical melanocytic naevus include Clark naevus and B-K mole. 
• Some people prefer to use the term dysplastic naevus when referring to an atypical naevus, but strictly speaking dysplastic naevus is a histological description of a particular kind of melanocytic naevus.

What does an atypical naevus look like?

The term atypical naevus is sometimes used to mean any funny-looking mole, and sometimes to mean a melanocytic lesion that is suspected of being a malignant melanoma (a cancerous 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 naevus

Atypical naevus

Atypical naevus

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Who gets atypical naevi?

Atypical naevi may occur sporadically or they may be familial (inherited).

Sporadic atypical naevi

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

Atypical naevi may develop at any time but most of them develop within the first 15 years of life. Typically, people with sporadic atypical naevi have one to ten moles that are larger than 6 mm in diameter.

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:

1. One or more first-degree or second-degree relative diagnosed with malignant melanoma at a young age (< 40 years);
2. A large number of naevi (often more than 50), some of which are atypical;
3. Naevi that are dysplastic on histopathology.

FAMM syndrome was previously known as the dysplastic naevus syndrome.

People with FAMM syndrome may have several hundred atypical naevi.

Atypical naevus

Atypical naevi

Atypical naevi

What is a dysplastic naevus?

The term dysplastic naevus is best used for a naevus with a specific microscopic appearance. Only a minority of clinically atypical naevi fulfil microscopic criteria for dysplastic naevus. Many histologically dysplastic naevi are clinically banale (eg, small in size, and uniform in colour and in structure).

Histological dysplasia may be mild, moderate or severe. Distinct pathological criteria of a dysplastic naevus are listed here.

• The dysplastic naevus may be a junctional naevus (when the melanocytes are found at the epidermodermal junction) or a compound naevus (when the melanocytes are found at the epidermodermal junction and within the dermis).
• The naevus cells form a row along the dermoepidermal junction (this is reported as lentiginous proliferation), with or without naevus cells in nests (also called theques).
• These theques are often irregular in size and shape and may 'bridge' or join together.
• The cells may be spindle-shaped (elongated) or epithelioid (broad, resembling epidermal keratinocytes).
• There may be cytologic atypia (cells that are smaller or larger than usual). 
• 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.

Melanoma may arise within a dysplastic naevus, within a non-dysplastic, normal, melanocytic naevus, or more often, in normal-appearing skin.

What is the importance of atypical naevi?

People with 5 or more clinically atypical naevi have a higher risk than the general population of developing melanoma; the relative risk is reported to be six times that of people without atypical naevi. People with FAMM syndrome have an extremely high risk of developing melanoma.

Melanocytic naevi are 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 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. Partial biopsy is best avoided, as the test may miss a small focus of melanoma. A pathologist will usually make the correct diagnosis, although sometimes deeper levels and/or a second opinion may be required.

People diagnosed with atypical naevi should be taught how to self-examine their skin for new skin lesions and for changes to existing moles that may indicate the development of melanoma. People with numerous moles should visit their family doctor or dermatologist regularly for a thorough skin check.

It is often helpful to keep photographic records of melanocytic naevi (preferably with dermatoscopic images); digital archiving at a photographic skin surveillance clinic is convenient for people with many moles or atypical naevi. The close-up photographs with dermatoscopic views should be repeated from time to time, so change can be detected early and its significance determined.

Careful sun protection is recommended for everyone but is especially important for people with many naevi or atypical naevi. Avoid excessive sun exposure and sunburn, dress up, and use a SPF50+ sunscreen when outdoors in the middle of the day or for prolonged periods.