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170 CHAPTER 13 Fig. 13.4 This photograph shows part of a speckled naevus. lar in appearance to true congenital naevi, seem to develop at distant sites, such as on the face, as the child ages. These may be referred to as ‘satellite naevi’. Congenital naevi often have clinically obvious hair at birth but the growth of terminal hair often becomes particularly obvious with time. As the child ages, the surface of the naevus commonly becomes uneven: either mam-millated or nodular, particularly giant naevi. These normal changes under-standably often cause concern in families previously advised to keep the naevus under review in case of malignant change. Generally speaking, as for acquired naevi, the slow almost imperceptible development of nodules is reassuring, whereas rapid growth over weeks is worrying. Rarely, plexiform neurofibroma-like new growths develop [7]. In summary, most congenital naevi become paler in colour with time and, although the skin surface may become more warty and more hairy with time, the majority of congenital naevi become a less serious cosmetic problem than they are at birth. In a proportion, the naevus remains deeply pigmented and may actually grow proportionally in surface area, and these are the most worrying cosmetically and in terms of risk of melanoma. Aetiology Congenital naevi may be more common in black or Asian children [8]. There may be an excess in females [7]. Families with more than one case of large congenital pigmented naevi have rarely been described, suggesting a possible genetic origin. The presence of a large naevus was said to have existed in four second-degree relatives of 80 pa-tients with giant naevi (>20cm in diameter) reported by Ruiz-Maldonado et al. [7]. While it would seem appropriate to acknowledge this to patients seeking genetic counselling advice, the significance of this remains to be established. CONGENITAL MELANOCYTIC NAEVI 171 As a genetic basis of giant CMN, the concept of a lethal mutation surviving by mosacism has been proposed [9]. Cells carrying the mutation would sur-vive only in a mosiac state, in close proximity with normal cells. This would explain why giant CMN exclusively occur sporadically, and why identical twins may be discordant for this trait [10]. If this hypothesis holds true, the ex-ceptional occurrence of a large naevus in relatives of a patient with a giant nae-vus would best be explained by paradominant inheritance [11]. Heterozygous individuals would be, in general, clinically unaffected, which is why the muta-tion would be transmitted, unrecognized, through many generations. The trait would only become manifest when a postzygotic event of allelic loss gave rise to a homozygous or hemizygous cell clone forming a mosiac patch. However, it should be emphasized that this concept cannot be applied to a small or medium sized CMN which most likely have a polygenic basis as similarly proposed for acquired naevi, both of a dysplastic or non-dysplastic type [12]. In normal development of melanocytes in utero, pigment cell precursors known as melanoblasts, derived from the neural crest, appear to populate the dermis [13]. In several case reports in the literature, naevus cell aggregates have been reported in the placenta of mothers giving birth to babies with giant pigmented naevi [14]. Similarly, cellular proliferations of melanocytes may be seen in the central nervous system in babies, usually presenting with hydro-cephalus: a proportion of whom also have giant congenital naevi [15]. It has been suggested that these melanocytic proliferations in skin, central nervous system and placenta may result from aberrant migration of neural crest cells to primitive mesoderm early in embryogensis [14]. The presumption is that in such an aberrant site, differentiation of cells would be abnormal. Congenital naevi may therefore represent a developmental anomaly in which excessive melanocyte proliferation occurs as a result of aberrant migration of cells from the neural crest during embryogenesis. Histological characteristics Congenital naevi tend to be larger and thicker than acquired naevi and more commonly exhibit naevus cells in or around skin appendages and vasculature [16]. Such may be the depth of large naevi that cells may be seen extending well into fat and skeletal muscle. Some authors have described the infiltration of single naevus cells between collagen bundles and a naevus-cell-poor subepi-dermal zone. Everett [17] reviewed all of these characteristics in 39 congenital naevi and concluded that the differences between congenital and acquired naevi were confirmed for large but not for small naevi. This would parallel our observations of the clinical appearance of the vast majority of small naevi which appear to behave like acquired naevi in many ways. 172 CHAPTER 13 The giant naevi may well have the more or less ‘characteristic’ histological appearances listed above, but they may also have complex and variegated mixtures of tissues histologically consistent with their origin as hamartomas of the ectoderm. There may be cells exhibiting neural differentiation. The melanocytic structures may have the appearance of Spitz tumours or blue naevi. The benign nodules which develop within giant naevi and malignant tumours may all exhibit pleomorphic patterns which causes diagnostic diffi-culty for pathologists. Bizarre tumours with a malignant-looking histology may behave in a benign fashion so that the morphological distinction between benign and malignant may be blurred [18]. The term nodular proliferative neurocristic hamartoma may be used to describe massive rapidly enlarging ulcerative masses present at birth in which the histological appearances are of diverse tissues of neural or mesoenchymal appearance, but which still behave in a benign fashion. In normal individuals melanomas nearly always arise from junctional cells. True origin from dermal cells is excessively rare if it happens at all. In giant congenital naevi, however, origin from dermal cells has been reported [19,20]. The interpretation of the histological appearance of giant naevi demands close correlation with the clinical behaviour of the naevus. If the histology of a nodule suggests malignancy but it has been clinically stable, then the prognosis may be less somber, although wider excision should be carried out. The reporting of histological appearances of these naevi is the province of an expert. Complications, or sequelae Psychological problems are common in patients with large naevi because of the cosmetic deficit suffered. Attempts to improve cosmesis will be discussed subsequently. When large naevi occur on a limb there is, not uncommonly, demonstrably reduced growth in that limb. Less significantly, there may be an absence of sub-cutaneous fat underlying truncal naevi. Proliferation of melanocytes within the central nervous system may lead to hydrocephalus, developmental delay or even central nervous system melanoma [15] but these are uncommon. In a series of 80 Mexican patients with giant naevi, only one case of hydrocephalus was reported [7]. In patients with giant naevi, central nervous system involvement should nevertheless be sought for prognostic purposes clinically and with ultrasound scan when the naevus lies in the midline. The possibility of using a magnetic resonance image (MRI) scan and lumbar puncture should be considered. There is undoubtedly an increased risk of melanoma arising in congenital CONGENITAL MELANOCYTIC NAEVI 173 naevi; however, the controversy rages as to the definite risk. More data are available for the giant naevi. The lifetime risk in these patients has been esti-mated at between 4 and 14%: most frequently at around 5% [7,21]. It would appear that the risk of malignancy in this group of patients is highest in the first 10 years of life [7,22,23], but malignancy can occur at any time subsequently [24]. In one series both melanomas reported occurred in the patients’ early 20s [21]. One of the difficulties in assessing the suggestion that malignancies occur predominantly in early life is in differentiating melanoma from the simulants of melanoma, which may occur in childhood, as discussed above. Rarely, malignant tumours of neural origin or mesenchymanl origin, such as rhab-domyosarcomas and liposarcomas, develop in giant naevi [7,18]. For the clinician managing these patients, the great difficulty is that the melanoma may be difficult to diagnose clinically and indeed may present as nodal disease or even widespread metastases. The risk to patients with smaller naevi is unclear. Certainly, melanomas do arise in congenital naevi as they do in acquired naevi. The public health issues concern the level of risk for naevi which are present in 1% of the population. There are no means of estimating the lifetime risk from these naevi but the risk is likely to be very small. In these patients, whole-scale removal prophylacti-cally would be costly both in terms of health service costs and cosmesis to the patient. In the UK the perception is that the data do not support the prophy-lactic excision of such naevi [25]. It is the authors’ view that the risk of melanoma in all individuals is pro-portional to the melanocyte mass. In patients with atypical naevi, this may present clinically as multiple and atypical naevi. In patients with congenital naevi, the risk is likely to be further determined by the proportion of melanocytes which remain proliferative. We think it likely therefore that risk is proportional to the volume of junctional melanocytes. If surgery is to be considered for prophylactic excision in order to reduce the risk of melanoma, rather than for cosmesis, which naevi should be removed? · Small naevi which remain pigmented and are in sites which are difficult to monitor, such as the scalp. · Medium or large-sized naevi in which excision with primary closure may be accomplished with good cosmetic results. · Partial excision may be considered in giant naevi which remain pigmented in such a way as to improve the cosmetic appearance of the naevus. Surgical treatment The purpose of treatment of these naevi is to reduce the risk of malignancy and to improve cosmesis. In terms of reduction of risk, any reduction in the volume of proliferative melanocytes is likely to be of benefit. For patients with large or 174 CHAPTER 13 giant naevi then there may be some benefit from most surgical treatments of all naevi. However, any procedure must be balanced against the cosmetic result of the operation. Many naevi fade progressively with time, and teenage and adult patients then commonly feel that the most cosmetically troublesome areas are the scars from early reconstructive attempts. For this reason our view of when surgery should be attempted, in an attempt to prevent melanoma, is presented above. The need to improve the cosmetic appearance of large naevi is usually the preoccupation of both patient and doctor and the literature is full of differing surgical approaches to the problem. If considering surgery, however, we must always remember that patients often do well with no intervention at all from a cosmetic point of view. If surgery is planned for large or giant naevi then the aims must be to reduce melanoma risk andimprove the cosmesis. There are numerous approaches in the literature and indeed one of the problems is that the rarity of giant naevi is such that few clinicians have a large experience of their management. An attempt has been made to summarize the approaches usually taken to the surgery of giant naevi by reconstructive surgeons. The techniques used de-pend on the site. Tissue expansion is viewed as superior cosmetically than ser-ial excision or grafting [26] but although it is the preferred option on the head and neck, it is unsuitable for the extremities and buttocks. On the trunk in an infant it may be possible to perform abdominoplasty: the naevus is widely excised down to fatty abdominal muscle and the wound closed by primary intention by greatly undermining the adjacent normal skin and stretching the expansile infantile skin [27]. Some authors have reported the use of cryopre-served or cultured epithelium to cover cutaneous defects [28] but others found this to be disappointing cosmetically [26]. Many authors advocate early intervention, as soon as infants can safely tolerate general anaesthesia, to take advantage of the relative excess of skin early in infancy and the excellent elasticity and healing of infantile skin [27]. A large series of 78 patients with giant naevi treated surgically at the Childrens’ Hospital in Chicago was reported by Bauer & Vicari [29]. Their approach to surgery is summarized in Table 13.1. A number of centres have developed a different approach to surgical treat-ment: the very early removal of naevus cells from the epidermis and the epi-dermodermal junction using a technique either of curettage or dermabrasion [30,31]. It has been found that by removing the tissue in this way in the newborn period, or certainly within the first year of life, reasonable cosmetic results may be obtained without grafting or the use of tissue expanders (Fig. 13.5a–c). The rationale was the probably mistaken view that melanocytes are more superficial at birth and then descend. The fact that newborns do better with these techniques than adults may in fact reflect the nature of their skin: ... - tailieumienphi.vn
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