Histological features of value in differentiating small congenital melanocytic naevi from acquired naevi.

One hundred and twelve melanocytic naevi measuring less than 5 cm in largest diameter were excised from 85 patients referred because of minor concern about possible growth or malignant potential of these naevi. Sections from all 112 naevi were examined histologically, without access to the clinical history, and the following features noted: type of melanocytic proliferation in the basal layer area, contiguity of epidermal and dermal components, total width and depth of naevus, involvement of subcutaneous fat and of skin appendages, the presence or absence of an 'Indian file' type of naevus cell infiltration, cellular atypia and evidence of maturation of these cells. On the basis of these features, naevi were assigned to either a 'congenital type' or 'acquired type' classification, and their distribution in these two classifications then compared with the clinical history. It was found that in 107 of 112 lesions correct assignation had been made on histological grounds alone, 43 as congenital and 64 as acquired. A histological pattern not previously reported to be associated with congenital naevi was seen in 17 (34%) of the congenital lesions. This study suggests that it is possible to differentiate the majority of small congenital naevi from acquired naevi on histopathological grounds alone. This observation could be of considerable value in the examination of excised primary malignant melanomas for evidence of a pre-existing naevus of either type.