Introduction

• Alternate names: naevus sebaceous of Jadassohn, organoid naevus
• Congenital hamartoma of skin comprised of:
  • Immature hair follicles
  • Epidermal hyperplasia
  • Sebaceous and apocrine glands
• 0.3% of newborns; no sex predilection

Pathogenesis

• Caused by post-zygotic somatic mutations in HRAS (95%) or KRAS (5%) genes
• Mutations lead to activation of MAPK and PIK3-Akt pathways

Clinical manifestations

• Usually apparent at birth, but subtle lesions may be missed until later in childhood
• Well-circumscribed, yellow-orange, hairless plaque
• Usually located on the scalp, face, neck; trunk is rarely affected
  • Scalp lesions are most common and cause alopecia
• Linear lesions are distributed along lines of Blaschko
• Grows proportionally with age
• At puberty becomes thicker, with verrucous/nodular surface

Diagnosis

• Usually clinical
• Histopathology can confirm the diagnosis

Histopathology

• Acanthosis, papillomatosis
• Presence of immature hair follicles is diagnostic
• After puberty: abundant sebaceous glands
• Apocrine glands usually present in specimens deep enough

Complications

• Multiple types of neoplasms have been reported to arise within sebaceous naevus:
  • Trichoblastoma is the most common
  • Other tumours: tricholemmoma, syringocystadenoma pappiliferum
• Malignant transformation is rare: basal cell carcinoma (< 1%)

Management

• Excision in late childhood is commonly utilised for facial lesions before verrucous surface changes emerge
• Excision for the purpose of preventing future malignancy is debated
• Observation is reasonable for lesions not situated in cosmetically sensitive areas