What is a sebaceous naevus?
A sebaceous naevus is an uncommon type of birthmark. Present at birth, it is most often found on the scalp, but sebaceous naevi may also arise on the face, neck or forehead. It consists of overgrown epidermis (upper layers of the skin), sebaceous glands, hair follicles, apocrine glands and connective tissue. It is a type of epidermal naevus and is classified as a benign hair follicle tumour. A sebaceous naevus is also called an organoid naevus because it may include components of the entire skin.
What does sebaceous naevus look like?
In an infant or young child, sebaceous naevus presents as a solitary, smooth, yellow-orange hairless patch, often oval or linear in shape.
Sebaceous naevi become more pronounced around adolescence, often appearing bumpy, warty or scaly.
What is the cause of sebaceous naevus?
The abnormality resulting in sebaceous naevus arises from a defect in the ectoderm. This is the outer layer of the embryo that gives rise to epidermis and neural tissue.
Sebaceous naevi are thought to be due to a mosaic genetic abnormality, i.e. a line of cells with a genetic error. In some patients, an abnormality of the PTCH gene has been detected.
How is sebaceous naevus diagnosed?
Sebaceous naevus has a characteristic clinical appearance and is therefore often diagnosed in childhood or adolescence. Sometimes the diagnosis is made by a pathologist after the lesion has been surgically removed.
A biopsy undertaken during childhood will show a hairless lesion with immature hair follicles and sebaceous glands. In adults, there is thickening of the epidermis and mature hair follicles and prominent sebaceous glands. Sometimes a second tumour is found in the specimen (see below).
Sebaceous naevus syndrome refers to the rare association of a large sebaceous naevus with disorders of the eye, brain and skeleton.
Sebaceous naevus syndrome may result in eye tumours and the skull may be asymmetrical. Characteristic associated neurological features may include:
- Developmental delay
- Epileptic seizures, especially infantile spasms
- Hemiparesis (paralysis of half the body) or cranial nerve palsies (paralysis of individual nerves)
- X-ray images and ultrasound evaluation may be quite normal
- Various structural abnormalities may be found within the brain
For further information, see DermNet NZ page on epidermal naevus syndrome.
Phakomatosis pigmentokeratotica is the association of a sebaceous naevus and a speckled lentiginous naevus. Speckled lentiginous naevus is a flat, light-brown birthmark with darker spots within it, and is classified as a type of congenital melanocytic naevus.
Neurological defects in phakomatosis pigmentokeratotica may include hemiatrophy (in which one side of the body under-developed), muscle weakness, sensory nerve abnormalities and hyperhidrosis (exessive sweating).
Tumours arising within sebaceous naevus
Most sebaceous naevi remain unchanged throughout life and do not cause any problems. However, another tumour may grow within the lesion. This is most often benign (in 0-50% of cases), most often trichoblastoma. Other hair follicle unit lesions (trichilemmoma, infundibular cyst and sebaceoma) and sweat gland tumours (eccrine poroma, syringocystadenoma papilliferum) may also arise.
Less commonly (in 0-22% of cases), a skin cancer may grow within a sebaceous naevus: basal cell carcinoma, squamous cell carcinoma, sebaceous carcinoma (oil gland tumour), or an adnexal carcinoma (apocrine or eccrine carcinoma).
How should sebaceous naevus be treated?
A sebaceous naevus should be monitored. If a lump or sore appears within a sebaceous naevus, arrange for it to be reviewed by a dermatologist. It may require a biopsy or the whole sebaceous naevus may be excised (cut out).
Full skin thickness excision may also be arranged for cosmetic reasons but a scar is inevitable.
Lasers have been used to treat sebaceous naevus with varying success.