Naevi - Atypical (dysplastic) melanocytic naevus

Latest update 07/09/10

This chapter is set out as follows:

• Aetiology
• Key diagnostic features
• Risk assessment and referral
• Histology

Aetiology

• Atypical melanocytic naevi (AMN) are skin lesions whose clinical and histologic features sit somewhere on the proliferative continuum from a common mole to a melanoma, although they rarely progress to melanoma. As with other moles the majority disappear in time but some persist
• AMN may be inherited as an autosomal dominant trait or occur sporadically - clinical and histological appearances of AMN occurring in a familial setting appear to overlap with those occurring sporadically
• It is likely that both genetics and UV radiation have a role to play in their development
• The prevalence of AMN in white populations has been reported to be between 2-5%. Celtic patients are much more prone to developing AMN, whereas AMN are rare in black, Asian or Middle Eastern populations
• The term dysplastic naevus is given to the histological appearance of AMN

Key diagnostic features

• AMN can develop throughout a person's lifetime
• Patients with sporadic AMN typically have 1-10 lesions, although they occasionally have many more. Individuals with the Familial Atypical Moles and Melanoma Syndrome (FAMM) may have from one to several hundred AMN
• Distribution - they can arise on any body site although they are most commonly found on the trunk and upper limbs, scalp and buttocks
• Size – tend to be large, commonly more than 7mm
• Colour - some have a variegated appearance showing mixtures of tan, dark brown and pink areas. While others have a normal colour or are erythematous
• Outline - irregular / distinct. Some have a pale edge, while others are flared

Risk assessment and referral

• Level of risk
  • It is very difficult to assess the risk of individual atypical naevi however the risk of solitary lesions transforming to melanoma is very low. There is no place for the prophylactic excision of atypical moles, the main reason for excising such lesions is where their is diagnostic uncertainty
  • As the number of atypical naevi increases so does the level of risk, the presence of five atypical moles represents a relative risk of 6.36 (range 3.8-10.33) for the development of melanoma
  • Irrespective of whether on not a patient is referred, all patients should be given advice on appropriate levels of UV protection and along with this the need to increase dietary intake of vitamin D3 - for more information please refer to the chapter on UV protection. Patients should also be taught self-examination
• Routine referral for risk assessment, advice on self examination, and photography is recommended for patients with:
  • Several atypical moles
  • Very large numbers of moles, some of which are atypical. The term Atypical Mole Syndrome (AMS) is used to define patients who have 100 or more moles, at least 2 of which appear atypical, with the histological confirmation of a dysplastic naevus. Such a definition is very arbitrary
  • The Familial Atypical Mole and Melanoma syndrome (FAMM) – this defined by patients with large numbers of typical and atypical melanocytic lesions AND a family history of melanoma in 1 or more 1st or 2nd degree relatives. These patients usually have an increased number of typical melanocytic naevi appearing in childhood, with AMN first appearing in adolescence. The total number of naevi is relatively stable by the end of puberty and is often more than 50. Such patients have a very high risk of melanoma (RR 100-400) and require long-term follow up by a dermatologist
• Urgent referral to a dermatologist as a 2-week wait is needed for patients with:
  • Any 'atypical mole' which is changing in size, shape or colour
  • Any atypical lesions causing diagnostic uncertainty

Histology

• While the majority of melanocytic lesions have unequivocal histological features, there is a small group of atypical lesions, which even experienced histopathologists have difficulty differentiating between dysplastic naevi and melanoma. As a result all AMN must be excised with a 2 mm clear margin and all AMN reported as being incompletely excised dysplastic naevi must be re-excised

	Figure 1 – Atypical melanocytic naevus The patient had been aware of the lesion for many years. Follow up demonstrated no change
	Figure 2 - Solitary atypical melanocytic naevus of lower abdomen Lesion shows variation in colour
	Figure 3 – Dermoscopic appearance of figure two Given the high degree of asymmetry of the pigment network the lesion was removed as a precaution. Histology revealed a dysplastic naevus
	Figure 4 – Atypical melanocytic naevus Patient was referred was a large number of lesions but this one on his lower back stood out
	Figure 5 – Dermoscopic image of figure 4 The image shows an eccentric peripheral pigment with a particularly dense area at 10 o'clock. A peripheral blotch of pigmentation can be found in melanoma and so this lesion was excised. Histology revealed a dysplastic naevus
	Figure 6 – Atypical mole syndrome
	Figure 7 – Atypical mole syndrome
	Figure 8 – Familial Atypical Moles and Melanoma syndrome Such patients need long-term follow up by dermatologists and assessment of relevant family members