What are melanocytic Naevus ?

Melanocytic (or melanocytic) nevi derive from the proliferation of melanocytes and constitute, from a clinical point of view, a group of conditions with various characteristics: for example, many Naevus are easy to diagnose, others require meticulous observation and great experience to be recognized, still others need a histological examination to be identified with certainty; all individuals, during their life, can develop melanocytic nevi on the skin. The natural history of melanocytic nevi includes an initial growth phase (by 30 years of age), followed by a stabilization phase and a partial or total regression phase.

From a clinical point of view, nine types of melanocytic nevi can be distinguished:

  • Freckle : Freckle (or lentigo simplex) is a small flat pigmented macula, acquired or congenital, which can take the form of single isolated elements, of variable number, or within a framework of freckles; generally the first form occurs, with multiple isolated elements of about 1-3 mm in diameter, light brown or dark, round, oval or polygonal shape and well-defined edges.Freckles can appear in any skin area (photoexposed or not), on the mucocutaneous passage areas and on the mucous membranes, including conjunctiva; they can remain unchanged over time or represent the initial phase of a melanocytic nevus acquired slowly; in old age freckles tend to gradually disappear.These lesions must be differentiated from the freckles because they have a darker colour, less dissemination and above all do not darken or increase in number with exposure to the sun; since they are benign lesions, they do not need any treatment.
  • Melanocytic Naevus acquired slowly : The melanocytic nevus acquired slowly (or Clark’s Naevus) is very common in Caucasian subjects; it appears on the trunk and on the root of the limbs and has a flat or slightly raised profile at the centre. When the diameter is less than 6 mm (melanocytic nevus acquired on a common plane) the lesion is generally symmetrical; when it is superior, the shape becomes asymmetrical, the edges irregular and the pigmentation uneven: we speak then of melanocytic nevus acquired on an atypical plane. Clark’s Naevus appears in adolescence or adulthood, isolated or more frequently in multiple elements: the majority of Caucasian subjects of adulthood have from 15 to 30. The colour is brown, darker in the centre and shaded at the periphery (“atoll” appearance), the shape is round or oval, the surface is smooth or slightly mammillated, while the superficial skin pattern often appears amplified. This Naevus may experience spontaneous regression, generally in late age but sometimes even earlier (around 20-30 years old).
  • Miescher’s Naevus : It is an acquired skin lesion, dome-shaped, of a colour varying from the pale brown to that of the normal skin, placed almost exclusively on the face (chin, nose, cheeks, forehead and perioral region); it appears after puberty or later, especially in female subjects, in isolated form or in groups of few units, with a diameter from a few mm up to 1 cm, round shape and smooth surface. Since the area of the face is rich in pilosebaceous structures, episodes of folliculitis are quite frequent in the context of nevus, with a consequent increase in volume and reddening of the lesion, pain and possible release of purulent material.
  • Naevus of Unna; It is an acquired skin lesion, pedunculate or sessile, often papillomatous, of soft consistency, colour varying from brown to that of normal skin, present in a few isolated units on the trunk or neck. The nevus usually appears after the 30 years of age, more often in the female sex; it can be easily traumatized, with consequent bleeding and formation of crusts, and at times it can detach partially or twist on its vertical axis, thus becoming bigger, more painful and of black-red colour.
  • Congenital melanocytic Naevus : This type of nevus is present from birth; similar lesions both clinically and histologically may appear weeks or months after birth (late congenital melanocytic nevi). In terms of size, there are small (diameter less than 1.5 cm), medium (between 1.5 and 2 cm) and large (larger than 2 cm) Naevus. The colour, usually brown with blackish or bluish shades, intensifies during the maturity and reduces in old age. The small forms, very common, are round or oval, macular or papular, in patch or plate or in small nodules, sessile or pedunculate, with smooth or warty surface, sometimes with big terminal hairs. The medium and large forms are, on the contrary, patches or plates of extremely variable and bizarre shape, with mammillated or warty surface and frequent terminal hairs; they may affect vast body areas and have several satellite lesions in their immediate periphery. In the context of congenital melanocytic nevi, more frequently than in other nevi, a melanoma may develop: the risk of such an event is higher in the larger forms.
  • Naevus of Spitz and Reed : Spitz’s Naevus and Reed’s nevus are melanocytic lesions that are almost always acquired and growing rapidly. The former typically occurs in childhood, is pinkish red and mainly localized to the face, tends to regress spontaneously, completely or almost, over the years, the nevus Reed instead appears more often around 20-30 years of age, has a dark brown or black and prevails in women on the lower limbs. Both snows have dimensions less than one centimetre of diameter, roundish papular shape, sometimes nodular, rarely papillomatous, with smooth or keratosic or warty surface.
  • Mongolian bush : The Mongolian spot is a bluish-grey pigmented lesion present at birth or appearing within the first year of life in more than 90% of Asian and Native American subjects, less frequently in blacks and in about 1-2% of white subjects; it is mainly located in the lumbo-sacral region. The diameter varies from less than 1 cm to many centimetres, the shape may be round or oval, in a single or multiple element, and the colour may have shades of green or brown. In addition to the lumbosacral region, this nevus can also be located on buttocks, hips, back, abdomen, legs and arms, while the manifestation to the face is called aberrant shape. Usually the nevus spontaneously regresses within the first 10 years of life; when this does not happen we speak of persistent Mongolian spot.
  • Naevus of Ota and Ito : Ota’s Naevus is a pigmented lesion, usually unilateral, of the skin and mucous membranes, located in the territory of innervation of the first and second branch of the trigeminal nerve (forehead, temples, cheeks, auricles, pre and retro-auricular regions, ipsilateral eyelids and scleres). Ito’s nevus is similar to the previous one, but it is located in the acromio-deltoid region, crossed by lateral supraclavicular and lateral brachial nerves. Both nevi have a bluish colour with brownish or blackish slate shades; the pigment is evenly distributed, the diameter varies from a few mm to many cm and also the shape is quite varied. Both snows also persist for the whole life, even if with fluctuations of size and colour.
  • Blue Naevus : This is a blue or blue-black pigmented lesion, acquired or sometimes congenital, mainly located at the extremities (particularly on the back of the hands and feet). It appears as a papule or plaque or roundish nodule, with a smooth surface and a diameter usually less than one cm. The blue Naevus remains stable for years and often gradually undergoes spontaneous regression.