3.1.5 Naevus sebaceous

Postzygotic somatic mutations of the RAS gene family, most commonly HRAS (95% of nevi). KRAS mutations may be found in 5% of patients. In patients with Schimmelpenning syndrome, mosaicism of HRAS and KRAS mutations has been reported.

At birth, nevus sebaceous presents as a slightly raised plaque with a smooth, yellowish surface. It is most commonly present on the scalp. The lesion is usually oval but, in some cases, multiple lesions develop in a linear distribution along the lines of Blaschko. During puberty, due to hormone changes, the lesions become thicker with a more verrucous surface. In adulthood, neoplasms (most commonly benign) may develop within the lesion.

Extensive sebaceous nevi may be the sign of Schimmelpenning syndrome, a neurocutaneous disorder that affects central nervous system, eyes and bones.

Initial lesions are characterized by acanthosis and papillomatosis, and immature pilosebaceous units. At puberty, the epidermis becomes more hyperplastic, papillomatous, and verrucous. Numerous sebaceous glands with an increased number of sebaceous lobules are found in the superficial dermis. The hair follicles contain more vellus hairs than terminal hairs.

In adulthood, secondary neoplasms may develop. Formerly, it has been thought that malignant transformation develops in 10-15% of lesions. Today it is known that malignant tumours are less common. Among malignancies, basal cell carcinoma develops most frequently, but only in less than 1% of lesions. The most common tumour within the naevus sebaceous is trichoblastoma, followed by syringocystadenoma papilliferum. Other secondary tumours include sebaceous adenoma, tricholemomma, desmoplastic tricholemomma, apocrine adenoma, and poroma.

Since the risk of malignancy is low, particularly before adolescence, observation is recommended.

Surgical excision is the treatment of choice. For cosmetic reasons, facial lesions should be considered for surgical excision during childhood since scarring is less intense.