Author: Dr Amanda Oakley MBChB FRACP, Dept of Dermatology Waikato Hospital, Hamilton, New Zealand, 1999.
A comedo naevus is a hair follicle naevus. It is also known as ‘comedone naevus’ and ‘nevus comedonicus’.
A comedo naevus is an unusual type of epidermal naevus, or birthmark (a pilosebaceous hamartoma), in which there is a localized collection of dilated follicles filled with keratin. It is more accurately termed follicular keratotic naevus, as there are no true comedones.
Comedo naevus is usually present at birth or develops in early childhood. Males and females are equally likely to be affected. Lesions often grow more quickly at puberty. They can appear anywhere on the body but are most commonly found on the face, trunk, neck and upper extremeties.
The lesions appear as closely grouped slightly elevated papules with central keratinous plugs so they look like comedones. They may be distributed in a linear, interrupted, unilateral or bilateral pattern and sometimes follow the lines of Blaschko.
The exact cause of comedo naevus is unknown, but it is thought to be due to cutaneous mosaicism; that is, a line of cells with a genetic error. If this error occurs early in development of the embryo, the cells may spread out to cause multiple comedo naevi.
Mutations in FGFR2 have been detected in the naevi of some patients. This is the same gene that has found to be abnormal in a more serious generalised genetic condition, Apert syndrome, so the comedo naevus may be a ‘mosaic’ of Apert syndrome (some cells have the abnormal gene, and other cells have the normal gene). Apert syndrome causes craniofacial dysostosis (abnormalities of the bone structure of skull and face) and other skeletal abnormalities. Patients with Apert syndrome often suffer from severe acne, which also arises within a comedo naevus.
Comedo naevus syndrome is when comedo naevus is associated with other medical conditions. Like other epidermal naevi, comedo naevus is rarely associated with other abnormalities of the cell of origin, the embryonic ectoderm. These may include:
There is no specific treatment for comedo naevus. Lesions are usually symptomless and treatment is usually sought for cosmetic purposes. Some improvement of lesions may be seen with the use of topical tretinoin, salicylic acid or ammonium lactate lotion.
Rarely, at puberty or later, a comedo naevus may develop inflammatory acne-like lesions within it. These can lead to cysts, recurrent bacterial infections, abscesses, and scarring. These should be treated with appropriate antibiotics or surgical drainage. Topical and oral treatment for acne may improve the appearance.
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