This is thought to be a variant form of sebaceous naevus (naevus sebaceous) and is a hamartomatous tumour characterised by hyperplasia of the epidermis and/or its associated components. They may present in a variety of forms – Becker's naevus, verrucous epidermal naevus, inflammatory linear verrucous epidermal naevus, naevus comedonicus, eccrine naevus, apocrine naevus and white sponge naevus. They are classified according to clinical morphology, site and extent of involvement as well as the predominating epidermal structure within the individual lesion.¹

Their most common forms are probably the sebaceous naevus affecting the scalp, and the verrucous epidermal naevus, which has the appearance of verrucous papules that coalesce to form well-demarcated papillomata, ranging in colour from that of the surrounding skin to much more deeply pigmented. They may be congenital lesions or develop during the early years of life. They tend to grow during childhood and then stabilise during the teenage years.¹ They may be localised to a small area or occur in more diffuse forms. They are often arranged in a linear fashion along skin tension lines or the lines of Blashcko (purported lines of epidermal growth derived from clonal tissue patterns thought to be formed in embryo).² If there are extensive epidermal naevi then they can be associated with extra-dermal abnormalities as part of the epidermal naevus syndrome.

They have the potential to have benign appendigeal tumours arise within them (often syringocystadenoma papilliferum), or to transform into malignant tumours such as basal cell carcinoma or squamous cell carcinoma. The lifetime risk of malignant transformation in sebaceous naevi is quoted at 5–22%,³ but more recent prospective analyses suggest this figure may be significantly lower.⁴ Verrucous epidermal naevi seem to have a much lower chance of forming tumours.¹ Malignant transformation of the lesion is rare before adolescence/adulthood.

Visual Appearance

Epidemiology

Estimated prevalence in newborns for sebaceous naevus is 0.3%. Epidermal naevus probably has a similar or lower prevalence.⁴

Presentation

• The lesions are usually noted at birth or shortly thereafter.
• They are often pigmented but can be the same as surrounding skin.
• They are often unilateral, linear and tend to follow Blaschko's lines.²
• They may grow and become wart-like during childhood and tend to stabilise during adolescence (in contrast to sebaceous naevi that tend to become hyperplastic in response to sex hormones during adolescence).

Differential Diagnosis

• Sebaceous naevus
• Nevus syringocystadenomatosus papilliferus
• Juvenile xanthogranulomata
• Solitary mastocytoma
• Multiple melanocytic naevi

Associated Diseases

Epidermal naevus syndrome (Jadassohn naevus phacomatosis). This is the presence of multiple or large epidermal naevi associated with disorders affecting the central nervous system, bones and the eye. There are multiple variant syndromes.⁵ Problems that may arise include:

• Epilepsy
• Mental retardation
• Neurological deficits
• Vitamin-D-resistant rickets
• Spina bifida
• Hyperplastic bone growth
• Ptosis
• Nystagmus
• Optic nerve defects
• Occulomotor dysfunction

Investigations

• None is required if the diagnosis is certain and the lesion has not changed appreciably.
• If there is doubt as to the nature of the lesion, or it has recently developed changes that may signify the presence of benign or malignant tumours, then biopsy should be carried out.
• If there are extensive epidermal naevi then referral/investigations such as neuroimaging, to detect the presence of an epidermal naevus syndrome, should be considered.

Management

• There is no treatment required as such, other than reassurance that they are, on the whole, a benign problem.
• If there is poor cosmetic appearance then consider dermatological/plastic surgical referral for advice on surgical excision or laser ablation.
• If the naevus is extremely warty or chronically inflamed (as some forms are) then topical immunomodulatory agents such as calcipotriol, tacrolimus and fluocinonide may be helpful.⁶

Complications

• Poor cosmetic appearance
• Inflammation of the lesion in certain forms (inflammatory linear verrucous epidermal naevus)
• Benign or malignant tumours arising within the lesion

Prognosis

• Very good
• Low risk of benign or malignant tumours arising from the lesion