Sebaceous naevi are also called
organoid naevi because they may include components of the entire skin
organ. They consist of overgrown epidermis (upper layers of the skin), sebaceous glands, hair follicles, apocrine glands and connective tissue. They are a variant of epidermal naevi and are also classified as benign hair follicle tumours.
Sebaceous naevi most often appear on the scalp, but they may also arise on the face, neck or forehead. Lesions are always present at birth and do not spread during childhood. However, they become more raised at puberty or earlier.
The abnormality arises from a defect in the ectoderm. This is the outer layer of the embryo that gives rise to epidermis and neural tissue.
They 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.
Features of sebaceous naevus
A sebaceous naevus is at first a solitary smooth yellow-orange hairless patch. Sebaceous naevi become more pronounced around adolescence, often appearing bumpy, warty or scaly. Neurological disorders are rare, but occasionally occur in children with very large sebaceous naevi. Epileptic seizures are the most common problem.
Sebaceous naevus syndrome refers to the association of a large sebaceous naevus with disorders of the eye, brain and skeleton, but this is extremely rare (several types of epidermal naevus syndrome). There may be eye tumours and/or the skull may be asymmetrical. Characteristic associated neurological features may include:
- Developmental delay
- 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
Phakomatosis pigmentokeratotica is the association of a sebaceous naevus and a speckled lentiginous naevus. This is a type of congenital melanocytic naevus (pigmented birthmark). Neurological defects seen in this syndrome may include hemiatrophy (one side of the body is under-developed), muscle weakness, sensory nerve abnormalities and hyperhidrosis (exessive sweating).
Complications & treatment
Most sebaceous naevi remain unchanged in adulthood and do not cause any problems. However, another tumour can arise within the lesion. This may be harmless: syringocystadenoma papilliferum (sweat gland naevus), trichoblastoma, trichilemmoma. Skin cancers may also arise: basal cell carcinoma, squamous cell carcinoma, sebaceous carcinoma (oil gland tumour), apocrine carcinoma or eccrine carcinoma (sweat duct or gland tumours). Therefore, most dermatologists recommend sebaceous naevi be excised when the patient is a teenager or in early adult life.
If a lump or sore appears within a sebaceous naevus, arrange for it to be reviewed by your dermatologist. It may require a biopsy or cutting out.
Book: Textbook of Dermatology. Ed Rook A, Wilkinson DS, Ebling FJB, Champion RH, Burton JL. Fourth edition. Blackwell Scientific Publications.
On DermNet NZ:
- Epidermal naevi
- Hair follicle tumours
- Becker naevus
- Comedone naevus
- Linear porokeratosis
- Sebaceous gland carcinoma
- Nonmelanocytic congenital naevi – common skin lesions course
- Nevus Sebaceus – Medscape Reference