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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). It has been shown that one of the keys to recognize melanoma is knowledge of the many faces of benign lesions.
Globular (congenital) naevus
In the definition of globular congenital naevi we include benign melanocytic lesions that are present at birth (traditionally known as ‘true’ congenital naevi) or appear before puberty (traditionally described as ‘early’ acquired naevi). From a clinical, dermoscopic or histopathological perspective, distinguishing between true small congenital naevi and earlyacquired naevi is usually impossible. This is not a problem because small congenital naevi and early-acquired naevi often share the same features. They show most frequently a globular pattern, whereas acquired naevi in adults are most commonly reticular. In children congenital naevi are of browncolor, 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. This has been described as globular homogenous pattern. In adults these naevi become more elevated, with a papillomatous surface, and are characterized dermoscopically by a cobblestone pattern. The latter is a variation of the globular pattern in which the globules are larger and more angulated, resembling a cobblestone. Another pattern often seen in congenital naevi in adults is the fried-egg pattern. This consists of a somewhat elevated section in the centre, with globules/cobblestones or structureless brown to pale pigmentation, and a flat portion at the periphery, usually typified by a regular pigment network by dermoscopy. This pattern can be easily identified by the wobble test. We speculate that the peripheral network is an acquired portion of the lesion within an otherwise stereotypical congenital naevus. 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 or more), but are usually < 15 mm, flat or slightly elevated, brown to black lesions. Small lesions are usually symmetrical and monomorphous. Dermoscopically they are characterized by a regular pigment network with or without areas of hypopigmentation and/or structureless brown to black coloration. Large lesions typically have a multifocal pattern, consisting of a patchy distribution of multiple hypo- and hyperpigmented areas, but occasionally exhibit atypical features. 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. In these lesions, histopathological features of the so-called ‘dysplastic’ nevi can be sometimes observed, but most of these features – lentiginous melanocytic hyperplasia, ‘random’ cytological atypia, lamellar ⁄concentric fibroplasia – have no dermoscopic counterpart. For the histopathological diagnosis of dysplastic nevi some authors emphasize the importance of architectural features of atypia (uneven distribution of nests with some ‘bridging’; ‘shoulder phenomenon’). These features can give a pronounced irregularity of the pigment network. 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. The latter concept has already been suggested by other authors who noticed that multiple nevi are rarely seen in individuals over 70 years of age.
Starburst (Spitz ⁄Reed) nevus Starburst, Spitz ⁄Reed nevi are acquired melanocytic nevi, that are seen in both children and adults. Like reticular nevi, starburst nevi are rarely seen in elderly patients, suggesting that they tend to disappear over the patient’s lifetime. Although initial descriptions defined Spitz/Reed nevi as nonpigmented lesions, recent data showed that Spitz ⁄Reed nevi were brown to black. Such a high frequency of pigmented variants in surgical series could be the result of a better clinical recognition due to dermoscopy (see below) coupled with low excision rates of classical ‘pink’ Spitz nevi in children. Clinically, pigmented Spitz/Reed nevi are brown to black, flat to slightly elevated, symmetrical lesions showing a relative preference for certain locations, including face, limbs and buttocks. The most relevant and peculiar feature is the starburst pattern seen by dermoscopy. This is typified by multiple streaks of pigmentation or large globules arranged symmetrically at the periphery of the lesion in a radiating pattern like that of a star. Dotted vessels and reticular depigmentation are seen in nonpigmented lesions. Histopathologically, starburst nevi are junctional or compound neoplasms composed of heavily pigmented, highly cohesive spindle and ⁄or epithelioid melanocytes, parallel and ⁄or perpendicular to the skin surface with variable epidermal hyperplasia. The demarcation between the epidermis and the junctional nests of melanocytes is either sharp (with half-moon shaped clefts: ‘capping’) or barely discernible. A periadnexal junctional component is often evident, and a band-like dermal infiltration of melanophages is a common ancillary feature. Classical Spitz nevi are plaque- or dome-shaped lesions that are typically symmetrical and sharply demarcated. Classical Spitz nevi are made up of large spindle and ⁄or epithelioid cells that are mainly arranged in nests. Pigmentation is usually sparse or may even be absent. The dermal component often has a desmoplasia encircling single melanocytes (‘desmoplastic Spitz nevus’). It should be noted that melanoma might rarely exhibit a type of starburst pattern, especially in lesions undergoing change in adults. In these circumstances, excision of lesions with a starburst pattern is recommended.
Blue (homogeneous) naevus
Blue nevi can be either congenital or acquired and usually persist throughout the patient’s life. By naked eye examination they are flat to elevated, blue to black papules, plaques, or nodules of variable size. Dermoscopically blue nevi are typified by a homogeneous pattern of structureless blue coloration. The histopathological pattern of common blue nevus is defined as dendritic-sclerotic. This pattern is typified by the presence of heavily pigmented dendritic melanocytes interspersed with some melanophages among thickened bundles of collagen in the mid- and the upper dermis. A thick Grenz zone usually separates the lesion from the unaffected epidermis. Not uncommonly, some areas of otherwise typical blue nevi are populated by oval melanocytes almost devoid of any pigment (‘hypochromic’ blue nevi, either ‘hypomelanotic’ or ‘amelanotic’). On dermoscopy most of these ‘hypochromic’ lesions appear as ‘white’ blue nevi. Pigment loss in blue nevi is most commonly observed in blue nevi of the limbs and is likely to be a site-related phenomenon, rather than a consequence of ageing.
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. The three levels clinical, dermoscopic and histopathological) of the morphological diagnosis of melanocytic proliferations exhibit a different extent of the grey zone, being larger with the naked eye examination and smaller with the histopathological examination. 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.