CMN are common neoplasm of the newborn skin; the estimated prevalence of CMN varies widely depending on the study, ranging from 0.5 to 31.7 %. GCMN are uncommon among them [3, 5, 15–18]. Most of them are 3–4 cm in diameter; the incidence of lesions greater than 10 cm in diameter has been calculated as only 1 in 20,000 newborn. Larger ones are less common. GCMN have an estimated incidence of 1 in 500,000 live births (Table 27.1) [4, 9, 14]. Kanada et al. [19] in a prospective study enrolled 594 infants in San Diego, California, USA, seen in the first 48 h of life, to provide data based on ethnicity, prematurity, and body site for vascular, pigmented, and other common cutaneous findings and compared their results with previous international prospective studies. In their study CMN was defined as flat or raised, tan brown or black, usually sharp regular symmetrical borders. CMN were found in 2.4 % of the studied newborns, among them 2.6 % were Caucasian, 17.9 % were African-American, 1.9 % Asian, none lesions were found among Hispanic patients, and 1.3 among other races (Table 27.3). Comparing their study with the incidence of CMN in previous international studies is shown in Table 27.4. Using the sliding scale metric defined by Marghoob et al. [10] for infants, all CMN reported in Kanada et al. [19] study qualified as small or medium. No neonate has more than one CMN, and none had large or giant CMN. People of African and Japanese decent appear to have higher incidences of CMN than Hispanics or whites [15–18, 20–28].

Although patients with darker skin types usually develop malignant melanoma (MM) in nonglabrous skin, as in patients with lighter skin types, there are multiple reports of MM associated with GCMN in patients with darker skin [4, 17].

The classical presentation of CMN is a brown-black elevated plaque studded with small nodules and coarse hair, with a regular smooth and well demarcated border [5, 29, 30]. Compared to benign acquired nevi, CMN are larger in size and display a mottled heterogeneous morphology (Fig. 27.1). They typically occur on trunk (38 %) and extremities (38 %). Head and neck lesions are infrequent (14 %) but quite disfiguring [4–7, 31]. The clinical appearance may change with age. In neonates, they may be lighter in color and relatively hairless, with flat poorly marginated border, or raised darkly pigmented, well marginated lesion with an irregular rugged surface, associated with dense, coarse, darkly pigmented hairs. In childhood, there is a progressive darkening of lightly colored lesions, and may acquire long, coarse, darkly pigmented terminal hairs in 75 % of them. Less commonly lanugo hairs overlie the lesion (Figs. 27.2 and 27.3) [4, 5, 30].

The color is generally uniform, but sometimes they exhibit considerably pigment color variation. Nearly all CMN have brown as their primary color, although it may be admixed with black, blue, and gray (Fig. 27.4). With the passage of time CMN may get darker, lighter, loose pigmentation, become more heterogeneous, or, rarely, regress (Fig. 27.5) [4, 11, 31].

The surface of CMN may be smooth, but frequently as the patient ages, a papular, pebbly, verrucous, or even cerebriform appearance became evident. Proliferative nodules may appear within the nevus, which usually represent benign neurotization of the nevus [4–6, 11, 32].

GCMN are often associated with satellite melanocytic nevi; these are smaller CMN that are present at birth or arise months to years later (Fig. 27.6) [11, 32].

CMN have a dynamic course and may change over time. They can increase in size during childhood, become darker in color, become hairy, or show pigmentary regression. Spontaneous involution of CMN is rare and is usually associated with hypopigmented halo phenomenon or vitiligo (Fig. 27.7) [33–35]. Cusack et al. [36] described a case of complete regression over a 4-month period with the halo phenomenon of a medium-sized CMN. A control biopsy showed prominent lymphocytic infiltrate in the papillary dermis and aggregates of lymphocytes adjacent to the nevus in the deeper dermis, supporting an autoimmune mediated phenomenon which involves T-cell immunity and the presence of IgM autoantibodies [37, 38].

Spontaneous regression without the halo phenomenon has also been described. Despite the clinical involution, pigment regression may be accompanied by histological persistence of nevus cells. Halo nevi usually occur during childhood or adolescence [3]. The depigmentation may occur within the lesion, around it or at a distant site (Fig. 27.8a–d) [37]. Spontaneous regression of a CMN starts with loss of terminal hairs and epidermal pigments resulting in bluish color of the plaque, which is followed by flattening and loss of dermal pigments resulting in vitiligo-like leukoderma. Dermal nevus cells, however, do not disappear; they just loose the ability to produce melanin pigments. The process of regression is associated with lymphocytic infiltration [37].

Regression of CMN without the halo phenomenon has been described [33]. Vilarrasa et al. [39] reported the clinical cases of medium-to-large CMN that clinically disappeared without the halo phenomenon. In both cases, skin biopsies showed that a high proportion of amelanotic nevus cells were still present in the dermis and even extended to the subcutaneous fat and pilosebaceous unit, indicating a decrease in melanin production by dermal nevus cells, rather than a reduction in their number [11, 33, 40–43].

Desmoplastic hairless hypopigmented nevus (DHHN) was first described by Ruiz-Maldonado et al. [44] as a hard, ligneous, progressively hypopigmented and alopecic GCMN. In such cases they noted progression of the induration in three cases and regression in the other patient. Histopathological (HP) appearance is characterized by intense dermal fibrosis and nevus cell depletion without evidence of malignant transformation. Boente and Asial [45] described a case of DHHN in which a well demarcated tumor appeared during the follow-up. Histology of such tumor showed nevus cells of normal morphology between thick collagen bundles; immunostaining revealed that such nevus cells have S100+, Vim+, HMB45 staining. An immune response against the melanocyte of the nevus may explain this type of evolution. Persistence of nevus cells was described in a case of involution of a neonatal eroded giant CMN with desmoplastic reaction (Fig. 27.9a–c) [46].

CMN rarely affect the scalp in the form of a giant cerebriform nevus; the skin is thrown into folds resembling the undulation of the cortical surface of the brain [30].

Kissing nevus or divided nevus of the eyelid occurs in adjacent parts of the upper and lower eyelids. When the eyelid is closed it appears as a single lesion. This implies that CMN develop between the 9th and 20th week of gestation, when the eyelids are fused (Fig. 27.10) [5, 47].