Naevi
A naevus is a benign proliferation of one or more of the normal constituent cells of the skin. Naevi may be present at birth or may develop later. The commonest naevi are those containing benign collections of melanocytic naevus cells, but other types of naevus are found (Table 1). Vascular naevi are dealt with on page 117.
Table 1 A classification of naevi
| Group | Example |
|---|---|
| Melanocytic | |
| Vascular | |
| Epidermal | |
| Connective tissue |
Melanocytic naevi
Aetiopathogenesis and pathology
The naevus cells in melanocytic naevi are thought to be derived from melanocytes that migrate to the epidermis from the neural crest during embryonic development (p. 3). The reason for the development of naevi is unknown, but they seem to be an inherited trait in many families.
The position of the naevus cells within the dermis determines the type of naevus (Fig. 1). The junctional type has clusters of naevus cells at the dermoepidermal junction, the intradermal type has nests of naevus cells in the dermis and the compound naevus shows both components.
Clinical presentation
A congenital naevus, that is one present at or soon after birth, is seen in about 1–3% of infants, but most naevi develop during childhood or adolescence. Their number reaches a peak in the third decade, and they tend to become less numerous thereafter. However, it is not unusual to see a few new naevi appear after the third decade, especially if provoked by excessive sun exposure or pregnancy. The average young white adult has between 20 and 50 melanocytic naevi. Dermoscopy is helpful in assessment (p. 20). The clinical features of different types of naevus are as follows:
Congenital naevi. Present at or shortly after birth, they are usually more than 1 cm in size, vary in colour from light brown to black and often become protuberant and hairy. They can be disfiguring, as in the rare bathing trunk naevus, and carry a lifetime risk of up to 5% for the development of malignant melanoma (p. 102).
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