Epidermal naevus and naevus sebaceous — Viva

Q1: Definition and classification (2 min)

Define an epidermal naevus. How are epidermal naevi classified into keratinocytic and organoid types?

Model answer. An epidermal naevus is a benign, congenital, hamartomatous proliferation of epidermal or adnexal tissue that typically follows Blaschko's lines, reflecting postzygotic somatic mosaicism. Keratinocytic epidermal naevi arise from keratinocytes and include verrucous epidermal naevus (VEN), inflammatory linear verrucous epidermal naevus (ILVEN), epidermolytic epidermal naevus (EEN), and naevus comedonicus. Organoid epidermal naevi arise from adnexal structures and include naevus sebaceous (sebaceous/apocrine/follicular hamartoma) and Becker naevus (smooth muscle hamartoma with epidermal and pigmentary change).

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Q2: Pathophysiology (3 min)

Why do epidermal naevi follow Blaschko's lines? Which genes are commonly mutated, and which phenotype does each produce?

Model answer. Epidermal naevi follow Blaschko's lines because they arise from a postzygotic somatic mutation in a clone of ectodermal cells that migrates along embryonic ectodermal pathways. Key genes: HRAS and KRAS in naevus sebaceous and Schimmelpenning syndrome (mosaic RASopathy); FGFR3 and PIK3CA in keratinocytic epidermal naevi; KRT1 or KRT10 in epidermolytic epidermal naevus; AKT1 in Proteus syndrome. Becker naevus is androgen-dependent with increased androgen receptors in lesional skin.

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Q3: Clinical presentation (3 min)

Describe the presentation of naevus sebaceous at birth, during childhood, and after puberty. How does it differ from Becker naevus?

Model answer. Naevus sebaceous presents at birth as a yellow-orange, velvety, hairless, well-circumscribed plaque on the scalp or face. In childhood it is relatively flat and waxy. After puberty, androgen stimulation causes sebaceous and apocrine hyperplasia, producing a thicker, greasier, more verrucous or nodular plaque. Becker naevus presents in adolescence as a unilateral brown hyperpigmented patch on the shoulder or upper trunk that progressively enlarges and darkens and develops hypertrichosis; it is androgen-dependent. Histologically, naevus sebaceous shows large sebaceous glands draining directly to the surface and immature hair follicles, whereas Becker naevus shows acanthosis, basal hyperpigmentation, and increased smooth muscle bundles.

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Q4: Differential diagnosis and investigations (3 min)

A 7-year-old child has a scaly linear plaque on the leg. How do you distinguish ILVEN from linear psoriasis, lichen striatus, and incontinentia pigmenti? When would you biopsy?

Model answer. ILVEN is congenital or early-onset, intensely pruritic, follows Blaschko's lines, and is refractory to topical corticosteroids. Linear psoriasis usually occurs in a patient with or at risk of psoriasis, may follow trauma (Koebner), responds to steroids and calcipotriol, and may have nail or scalp psoriasis. Lichen striatus is a self-limited eruption in children with flat-topped violaceous or skin-coloured papules in a linear array that resolves spontaneously over months. Incontinentia pigmenti is X-linked dominant, usually female, with a verrucous stage followed by swirled hyperpigmentation and dental, ocular, and CNS anomalies. Biopsy is indicated when the diagnosis is uncertain, when histology is needed to distinguish psoriasis from ILVEN, or when a naevus sebaceous develops a new nodule or ulceration.

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Q5: Complications (2 min)

What secondary tumours can arise in naevus sebaceous, and what is the modern approach to malignancy risk?

Model answer. Secondary tumours in naevus sebaceous include benign trichoblastoma and syringocystadenoma papilliferum, and malignant basal cell carcinoma. Modern data suggest the overall rate of secondary neoplasms is approximately 12.8% and the rate of malignant transformation is around 2.4%, with BCC-specific risk generally cited as 0-1.7%. Many lesions previously called BCC were actually trichoblastomas. Management has shifted from routine prophylactic excision to observation with biopsy of any new nodule, ulceration, or bleeding.

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Q6: Epidermal naevus syndrome (3 min)

What is Schimmelpenning syndrome? Name the extracutaneous associations and the molecular basis.

Model answer. Schimmelpenning syndrome (epidermal naevus syndrome) is a neurocutaneous mosaic disorder in which a naevus sebaceous—usually on the face or scalp—is associated with ipsilateral extracutaneous anomalies. Associations include neurological (seizures, intellectual disability, hemimegalencephaly, cortical dysplasia), ocular (coloboma, choristoma, strabismus), skeletal (limb length discrepancy, scoliosis, hypophosphataemic rickets driven by FGF23), and cardiovascular or genitourinary anomalies. The molecular basis is postzygotic activating mutations in HRAS or KRAS in the developing ectoderm and affected organs.

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Q7: Management (3 min)

Outline a stepwise approach to the management of a symptomatic verrucous epidermal naevus and a naevus sebaceous.

Model answer. For a symptomatic verrucous epidermal naevus or ILVEN: first-line trial of topical calcipotriol 0.005% ointment and/or potent topical corticosteroid; topical retinoids for hyperkeratosis; small lesions can be excised. For extensive or refractory lesions, laser ablation (CO2 or erbium:YAG) or dermabrasion can improve cosmesis but recurrence is common. For naevus sebaceous: modern management is conservative observation with clinical photography and periodic review; biopsy any new nodule, ulceration, or bleeding; excise completely if symptomatic, cosmetically unacceptable, or if malignancy is confirmed. Avoid destructive laser or curettage when malignancy is suspected because they can obscure future diagnosis.

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Q8: Special situations (2 min)

A 20-year-old woman with an epidermolytic epidermal naevus wishes to become pregnant. What do you advise?

Model answer. An epidermolytic epidermal naevus represents cutaneous mosaicism for pathogenic keratin variants. Rarely, individuals with linear epidermal naevi showing epidermolytic hyperkeratosis transmit the inherited form — epidermolytic ichthyosis (generalised erythema, blistering and scaling at birth, evolving to widespread hyperkeratosis) — to their children. Refer for genetics consultation with biopsy confirmation; reproductive risk appears higher when the naevus involves more than one anatomic site. Prenatal counselling and testing options can then be discussed.^[6]

References7ShowHide

1. [1]Khan W, Ibrahim A, Alvaro A, et al. Laser Treatment of Verrucous Epidermal Naevi: A Systematic Review J Cutan Med Surg, 2022.PMID 35603930
2. [2]Zakrzewski JL, Luecke T, Bentele KH, et al. Epidermal naevus and segmental hypermelanosis associated with an intraspinal mass: overlap between different mosaic neuroectodermal syndromes Eur J Pediatr, 2001.PMID 11686504
3. [3]Rosen H, Schmidt B, Lam HP, et al. Management of nevus sebaceous and the risk of Basal cell carcinoma: an 18-year review Pediatr Dermatol, 2009.PMID 19686305
4. [4]Atzmony L, Ugwu N, Hamilton C, et al. Inflammatory linear verrucous epidermal nevus (ILVEN) encompasses a spectrum of inflammatory mosaic disorders Pediatr Dermatol, 2022.PMID 35853659
5. [5]Arjona-Aguilera C, Collantes-Rodríguez C, Villegas-Romero I, et al. Rounded and velvety epidermal naevus: Dermoscopic findings and literature review Australas J Dermatol, 2018.PMID 28736849
6. [6]Nelson JM, Isaac JM, Mervak JE, et al. Epidermal nevi and epidermolytic hyperkeratosis: A review of cases, highlighting indications for biopsy and genetics referral. Pediatr Dermatol, 2024.PMID 38898621
7. [7]Atzmony L, Ugwu N, Hamilton C, et al. Inflammatory linear verrucous epidermal nevus (ILVEN) encompasses a spectrum of inflammatory mosaic disorders Pediatr Dermatol, 2022.PMID 35853659