Classification of nevi
Melanocytic nevi are commonly classified based on a mix of clinical and histopathological criteria. Clinically nevi have been initially classified as either acquired or congenital, both further subdivided into junctional, compound and dermal naevi based on the location of the nests of melanocytes and nevocytes within the skin (epidermis or dermis). Dermoscopically, seven groups of nevi have been identified.
Globular (congenital) naevus
In the definition of globular congenital naevi we include benign melanocytic lesions that are present at birth (‘true’ congenital naevi) or appear before puberty (‘early’ acquired naevi). In children, congenital naevi are of brown color, flat or slightly elevated, symmetrical, and usually < 15 mm in size. The most striking dermoscopic feature of congenital naevi in children is the globular pattern, in which brown globules are seen throughout the lesion but are especially visible at the periphery. Globular naevi may also show areas of hypopigmentation and/or structureless brown pigmentation that may obscure the globules completely (globular homogenous pattern). In adults these naevi become more elevated, with a papillomatous surface, and are characterized dermoscopically by a cobblestone pattern. If larger than 15 mm, congenital naevi are clinically flat to elevated pigmented plaques often characterized by a verrucous and/or hairy surface. Dermoscopically they are typified by reticular, globular (cobblestone) and/or homogeneous patterns. Multiple small roundish areas of depigmentation, corresponding to hair follicles, can frequently be seen. Histopathologically, globular naevi in children are most frequently compound melanocytic tumours, with discrete junctional nests as ‘shoulders’ of the proliferation (the histopathological counterpart of the globules seen by dermoscopy). Congenital-like features (splaying of naevus cells among the collagen bundles and around the adnexa, large pigmented nests within the dermis, periadnexal extension) can be variably found. We speculate that globular nevi of children become dermal naevi, Unna type, in adults (see chapter on nevogenesis). Congenital-like features can be commonly detected within the dermal component of these naevi, namely, small naevus cells in the reticular dermis (and sometimes in the subcutis) with extension between the collagen bundles and around nerves, vessels and adnexa. Compound lesions may contain larger intraepidermal melanocytes, with pagetoid extension in early lesions.
Reticular (acquired) naevus
Reticular naevi are late-acquired melanocytic naevi, most commonly seen in adults. They can be small (< 6 mm) or large (>6 mm), flat or slightly elevated, brown to black lesions.
Dermoscopically they are characterized by a regular pigment network with or without areas of hypopigmentation and/or structureless brown to black coloration. Histopathologically, reticular naevi are junctional or compound, benign melanocytic proliferations with pigmented, thin, and elongated rete ridges, which are the counterpart of the network seen by dermoscopy. Reticular nevi show a tendency to regress during the patient’s lifetime and, at least in our estimation, they finally disappear by the seventh to ninth decades.
Starburst (Spitz ⁄Reed) nevus
Starburst, Spitz ⁄Reed nevi are acquired melanocytic nevi, that are seen in both children and adults, rarely in elderly patients.
Blue (homogeneous) naevus
Blue nevi can be either congenital or acquired and usually persist throughout the patient’s life. Dermoscopically, they are characterized by a homogeneous pattern of steel blue coloration.
Site-related naevi exhibit peculiar clinical, dermoscopic and histopathological features that are related to the specific anatomical structure where they are located. Table 1 summarizes the morphological features of naevi located on the face or on the palms ⁄soles, the two locations where melanocytic naevi exhibit the most peculiar clinical, dermoscopic and histopathological features. A more extensive list of site-related naevi would include naevi located on the nails, genitalia, umbilicus and nipple. (see chapter on site related aspects of nevi)
Naevi with special features
This group includes combined nevi, halo nevi, irritated nevi, nevi with eczematous halo, and recurrent nevi (see corresponding chapters)
Unclassifiable melanocytic lesions
Melanocytic nevi from all previous categories may sometimes show conflicting diagnostic criteria from a clinical, dermoscopic and ⁄or a histopathological point of view. A grey zone always exists where the borders between benign and malignant are unfocused. There are two categories, namely: ‘superficial atypical melanocytic proliferations of uncertain significance’ (SAMPUS) and ‘melanocytic tumours of uncertain malignant potential’ (MELTUMP).The latter group can be differentiated from the former because they appear as expansile nodules (‘tumours’) within the dermis.
