3.1.3 Atypical Melanocytic Naevus and Syndrome

Grading & Level of Importance: B




Atypical navus; Atypical mole syndrome; Dysplastic naevus syndrome; Familial atypical multiple mole melanoma syndrome.


Approximately 5-10% of Caucasian populations show atypical melanocytic naevi, with likely lower rates in populations with darker skin types. Increasing numbers of atypical melanocytic naevi increase the risk of developing malignant melanoma.


Atypical melanocytic naevus syndrome is much rarer: only around 5-12% of melanomas are considered to have an inherited component. In the familial atypical nevus type one or more first-degree or second-degree relatives diagnosed with malignant melanoma are at a young age (< 40 years).They show a large number of naevi (often more than 50), some of which are atypical;


Atypical melanocytic naevi are pigmented lesions similar to ordinary melanocytic naevi but they usually measure more than 0.6cm and have irregular or poorly defined borders and/or irregular pigmentation within the lesion, as well as possibly irregular texture. There  are sporadic and family associated subtypes .They can also arise over the second to third decade of life. On examination this is referred to as a clinically atypical naevus, however, when the abnormalities are microscopically confirmed, the term dysplastic is used. (See ABCDE rules in the melanoma chapter).


Atypical melanocytic naevus syndrome is where there are more than 50 pigmented lesions on the same individual. the lesion is confirmed to be dysplastic on histological analysis and there is a family history of confirmed melanoma in one or more 1st/2nd degree relative under the age of 40.

Aetiology & Pathogenesis

Various genetic mutations have been described reducing the efficacy of tumour-suppressor genes. These mutations may be magnified by repeated sunburn. The most important are primary oncogenic mutations such as BRAF and germline mutations CDKN2A (600160) .

Signs & Symptoms

Apart from the appearance and potential changes in the appearance, atypical naevi are asymptomatic but occasionally may be itchy or bleed.


Sun-exposed sites are the most commonly affected, however any body site may be affected.


A.) Sporadic

B.) Familial. Histological classification is by the subjective degree of change of dysplasis of melanocytes and amount of atypical cells : mild, moderate and severe.

Laboratory & other workups

Additional laboratory tests are not usually required unless a melanoma is diagnosed.


When the clinical abnormalities of an atypical nevus are microscopically confirmed, the term dysplastic is used.


Melanocytic cells may have a different appearance to typical melanocytes (unusually spindle-shaped, or broad) and they may be of an unusual size (small or large) or the cytoplasm shows dot-like pigment distribution. The cells may be arranged in an unusual pattern, such as with bridging between nests of melanocytes. An inflammatory infiltrate, fibrosis in the papillary dermis and increased vascularization may also be present.


The course of atypical naevi is controversial, since the most unusual ones are almost always excised once noticed. Some clinically atypical naevi probably never become malignant, but some may also develop into melanoma. However, there are lesions which represent a genomic profile intermediate between a benign naevus and a melanoma, and their malignant potential is uncertain.


Malignant transformation to a melanoma may occur. As excision is essential for the diagnosis, scarring complications may occur after the procedure including hypertrophic or keloid scars, as well as functional impairment in large procedures.


The diagnosis is clinical for an atypical melanocytic naevus. It follows the ABCD(E) rule . It is mandatory to obtain a complete excision with a  0.5 cm margin for histological analysis if a melanoma is clinically possible. Histological confirmation is necessary to be able to diagnose the atypical melanocytic naevus syndrome. See also  chapter 3.1.5 Melanocytic Nevus.

Differential diagnosis

The differential diagnosis includes seborrhoeic keratoses, benign melanocytic naevi, blue and combined nevi, pigmented basal cell carcinomas and melanomas.

Prevention & Therapy

Avoidance of sun burn is important, however with strict sun avoidance, vitamin D replacement therapy is advisable. Regular dermatological follow-up (including digital photography to track changes in individual lesions) is required for those with the atypical melanocytic naevus syndrome or numerous atypical naevi. The acute eruption of an atypical nevus is also an option for excision. Screening of relatives should be offered. Histological excision of doubtful clinical lesions and those with changes during follow-up to exclude development of a melanoma is essential. Consider genetic testing in appropriate individuals.





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