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Epidermal nevi are benign, well-circumscribed proliferations of the epidermis and papillary dermis appearing in the distribution of Blaschko's lines.
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INSIGHT 
When examining a lesion that appears linear, consider the swirled or undulating pattern of Blaschko's lines; if present, this will immediately focus the differential diagnosis.
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SYNONYMS Nevus verrucosus, nevus unius lateris, ichthyosis hystrix, and linear nevus sebaceous.
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AGE 80% present in the first year of life. Most of them appear from birth to 18 years. May become more prominent during puberty, increasing in thickness and darkening in color.
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PREVALENCE 1 in 1,000 infants.
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ETIOLOGY Most cases sporadic, some cases familial. Mutations in gene for fibroblast growth factor receptor 3 (FGFR3) identified. Some exhibit a chromosomal break at 1q23. Mutations in PIK3CA and HRAS have also been identified in keratinocytic epidermal nevi.
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Epidermal nevi arise from the pluripotent embryonic basal cell layer. There are likely many different candidate gene mutations that result in epidermal nevi, including those mentioned above.
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Epidermal nevi are present at or soon after birth. Solitary small lesions are common. Larger lesions can affect an entire limb or side of the body with associated adnexal tissue proliferations or hypertrophy. Growth or darkening of the lesion during puberty can occur.
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TYPE At birth: macular/velvety. Later: warty/papillomatous plaques (Fig. 9-1).
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NUMBER Solitary or multiple.
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COLOR At birth: white. Later: flesh-colored, light, or dark brown. Rarely hypopigmented.
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SIZE Few millimeters to several centimeters.
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DISTRIBUTION Typically unilateral. Rarely can be bilateral.
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ARRANGEMENT Linear following the lines of Blaschko.
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SITES OF PREDILECTION Trunk or limb > head or neck. Flexural areas are more verrucous.
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DIFFERENTIAL DIAGNOSIS
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The diagnosis of an epidermal nevus is made based upon history and physical examination. The differential diagnosis includes linear and whorled hypermelanosis, nevus sebaceus, seborrheic keratosis, wart, psoriasis, acanthosis nigricans, lichen striatus, incontinentia pigmenti, hypomelanosis of Ito, or an inflammatory linear verrucous epidermal nevus (ILVEN).
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LABORATORY EXAMINATIONS
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DERMATOPATHOLOGY Skin biopsy shows epidermal hyperplasia, hyperkeratosis, acanthosis, papillomatosis, and parakeratosis. There may be increased melanin in the basal layer in places. There may be ballooning of the cells (epidermolytic hyperkeratosis) in places.
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Epidermal nevi are typically asymptomatic and grow proportionately with the child. ...