A comedo naevus is a hair follicle naevus. It is also known as ‘comedone naevus'. This is an unusual type of epidermal naevus, or birthmark, in which there is a localized collection of comedones, dilated follicles filled with keratin. The comedones can arise from normal sized, enlarged or small sebaceous glands or even from sweat ducts. They may be found on any site of the body and are present at birth. Males and females are equally likely to be affected. Lesions may be solitary or multiple.
The comedo naevus is thought to be due to a mosaic genetic abnormality, i.e. 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's syndrome so the comedo naevus may be a ‘mosaic' of Apert's 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's syndrome often suffer from severe acne, which also arises within a comedo naevus.
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