Epidermal Nevi



Epidermal Nevi


Christina Stefanaki

Andreas Katsambas

Dimitrios Rigopoulos

Alexander Stratigos




BACKGROUND

Epidermal nevi (EN) are benign neoplasms comprised of cell types that reside in the epidermis, including keratinocytes, sebaceous glands, hair follicles, apocrine and eccrine glands, and smooth muscle cells. EN comprised primarily of keratinocytes are termed “keratinocytic,” whereas those composed of adnexal differentiation (sebaceous, follicular, or apocrine) are termed “organoid.” The incidence of EN has been reported to range from 1 to 3 per 1000 live births affecting males and females equally.1,2 The percentage of individuals with EN who have extracutaneous abnormalities is not precisely known. The most common extracutaneous associations involve the central nervous system (CNS), ocular, and skeletal systems.3

Keratinocytic epidermal nevi (KEN) are the most common form and include linear EN and linear verrucous EN.

Nevus sebaceous (NS) is relatively common, accounting for approximately one half of all EN.4

Inflammatory linear verrucous epidermal nevus (ILVEN) is a type of epidermal nevus and represents 6% of EN.3

Porokeratotic adnexal ostial nevus (PAON) was coined in 2009 to encompass porokeratotic eccrine ostial and dermal duct nevus, porokeratotic eccrine nevus, and porokeratotic eccrine and hair follicle nevus.5 PAON is rare entity and since the original description, there have been at least 25 reported cases.5

Papular epidermal nevus with “skyline” basal cell layer (PENS) is a newly recognized type of keratinocytic nevus.6


PRESENTATION

EN present as flesh-colored, pink, or lightly pigmented tan-to-brown papules or plaques.

KEN are usually present at birth or during infancy. They are found on trunk and extremities and rarely on the head and neck. They often present as slightly papillated skin-colored to brown growths that follow a linear pattern (Figure 12.3.1).

Nevus sebaceus is always present at birth; however, it may not be noticed until later in childhood or until after puberty, when hormonal influences to sebaceous glands trigger thickening and the development of a papillomatous surface.

Nevus comedonicus (NC) presents at birth or during childhood with a linear array or clusters of dilated follicles containing keratin, resembling comedones (Figure 12.3.2).






FIGURE 12.3.1 Keratinocytic epidermal nevus. These lesions may be flat initially, but they tend to become more elevated, verrucous, and darker over time.

ILVEN usually presents in infancy or early childhood on the lower body as an intensely pruritic, unilateral array of linear erythematous and hyperkeratotic papules that often coalesce into plaques.4,7

PAON is found equally in boys and girls and majority of cases present at birth, occasionally as neonatal erosions, or soon after.5,8,9,10,11 Clinically, it presents as hyperkeratotic papules and plaques with comedo-like punctuate pits, often filled with keratin plugs.8

PENS lesions appear at birth as small, scattered, hyperkeratotic papules; they are rather round, oval, comma-shaped, or polygonal and do not follow the lines of Blaschko.6






FIGURE 12.3.2 Nevus sebaceous. This lesion is a salmon to yellow colored and has a characteristic smooth waxy surface. Typically on the scalp or face but may appear on other parts of the body. (From Stedman’s Medical Dictionary. 28th ed. Baltimore, Maryland: LWW; 2005.)


Becker nevus (BN) appears between the first and second decade of life usually on the trunk or upper extremities as a solitary lesion that is typically hyperpigmented with hypertrichosis. This is usually categorized as a pigmented neoplasm even though it contains an epidermal hamartomatous component (see chapter 12.1iii).




PATHOGENESIS

A variety of cell types may form nevi or nests of cells. Cell types that reside in the epidermis, including epidermal cells or keratinocytes, sebaceous glands, hair follicles, apocrine and eccrine glands, and smooth muscle cells, are all candidates for EN formation. In general, EN are the result of cutaneous mosaicism and multiple epidermal structures may be seen
in an individual epidermal nevus, most exhibiting the predominance of one such structure and named accordingly. EN with prominent adnexal (sebaceous, follicular, and/or apocrine) differentiation are referred to as organoid nevi, and those with primarily epidermal differentiation are known as nonorganoid or keratinocytic EN.

A postzygotic mutation or another genetic alteration in an embryonic cell destined to populate an area in the epidermis is the pathogenetic basis for EN. Genetic mosaicism of gain-of-function mutations in genes encoding fibroblast growth factor receptor 3 (FGFR3) and oncogenes Ras and PIK3CA has been reported. Given that keratinocyte progenitor cells migrate and proliferate along Blaschko lines, linear EN follow those lines. Mutations that arise very early in embryonic development lead to more extensive EN and may affect organ systems other than the skin. When other extracutaneous abnormalities occur in conjunction with EN, specific syndromes are recognized.5


Keratinocytic Epidermal Nevi

In general, KEN are the result of cutaneous mosaicism as mentioned above. Several mutations have been identified in KEN, including mutations in RAS, fibroblast growth factor receptor 3 (FGFR3), phosphatidylinositol-4,5-bisphosphate 3-kinase catalytic subunit alpha (PIK3CA) apart from the keratins39,40,41,42; therefore KEN in terms of genomics are not a single entity.

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Jun 29, 2020 | Posted by in Dermatology | Comments Off on Epidermal Nevi
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