Epidermal nevi are a subset of cutaneous hamartomas resulting from somatic mutations of epidermal cells, presenting as keratinocyte or epidermal appendage overgrowths. The most common type appear in a linear distribution and are termed linear epidermal nevi or linear verrucous epidermal nevi.
There are variations of epidermal nevi (EN) that can be composed of superficial epidermal keratinocytes, sebaceous glands, apocrine or eccrine glands, hair follicles, or smooth muscle. For example, many consider a nevus sebaceous to be a type of epidermal nevus as well. The incidence of EN is approximately 1 in 1,000 newborns. Postzygotic cell mutations result in a mosaic distribution that follows embryonic migration patterns, appearing in a Blaschkoid distribution.
EN present most frequently as unilateral linear or whorled hyperpigmented coalescing papules. The lesions can be present at birth or during childhood, and after appearing, grow with the patient. Typically the lesions become more raised and verrucous around puberty. The differential diagnosis of linear EN include lichen striatus, warts, and incontinentia pigmenti. Lichen striatus can be differentiated because it presents later in life and self-resolves. Verrucae are the most commonly mistaken diagnosis for EN; warts do not usually persist in the same pattern over time with proportionate growth and typically respond to locally destructive treatments such as liquid nitrogen, unlike EN. Incontinentia pigmenti presents as vesicles initially and shows a quick evolution, differentiating it from EN. Inflammatory linear verrucous epidermal nevus (ILVEN) is a variant of linear EN that has associated chronic and intermittent erythema, scale, and pruritus. Lichen nitidus often has a pruritic presentation; however, it is flat topped and skin colored, helping differentiate it from linear EN.
There has been recent research advancing gene associations for linear EN displaying many lesions associated with mosaic mutations in oncogenes. Multiple genes have been identified with EN including RAS, FGFR3, and PIK3CA1. FGFR3 and PIK3CA mutations are associated with 50% of keratinocytic nevi. Of the RAS family, the HRAS pathway has been most closely associated with nevus sebaceous. While KRAS and NRAS genes have been associated with EN, it is to a lesser degree. However, there are multiple recent case reports demonstrating a potential association of G12D mosaicism of the KRAS gene in EN with rhabdomyosarcoma and bladder cancers2.
The diagnosis of epidermal nevus syndrome should be considered when there is a nevus with associated developmental abnormality of the central nervous system, eyes, or musculoskeletal systems. The most common systemic symptoms include delays in developmental milestones, seizure disorders, coloboma, strabismus, muscle weakness, and hemihypertrophy. To date, there are six specific epidermal nevus syndromes identified: sebaceous nevus syndrome, nevus comedonicus syndrome, Becker nevus syndrome, phakomatosis pigmentokeratotica, Proteus syndrome, congenital hemidysplasia with ichthyosiform nevus and limb defects, and cutaneous-skeletal hypophosphatemia syndrome3. In addition to the syndromes described, there are reports of associations between keratinocytic nevi and ILVEN with hypophosphatemic rickets and precocious puberty.
Linear EN are rarely associated with malignant transformation to basal cell carcinoma or squamous cell carcinoma, depending on the cell type involved. Given the low risk of malignancy, the lesions do not need to be removed routinely. For small lesions, monitoring often is the preferred management. However, lesions with functional significance, or causing strangulation or deformity, can be treated with surgical excision, curettage, or laser destruction
Dr. Kaushik is with the division of pediatric and adolescent dermatology at Rady Children’s Hospital–San Diego, and Dr. Eichenfield is chief of pediatric and adolescent dermatology at Rady Children’s Hospital–San Diego. He is vice chair of the department of dermatology and professor of dermatology and pediatrics at the University of California, San Diego. There are no conflicts of interest or financial disclosures for Dr. Kaushik or Dr. Eichenfield. Email them at firstname.lastname@example.org.