Comedo naevus (syn. comedone naevus; naevus comedonicus; follicular naevus)

A comedo naevus is a rare benign cutaneous anomaly first described in 1895. It is characterised by grouped, dilated follicular openings with dark keratin plugs, which are not true comedones.

This chapter is set out as follows:

• A comedo naevus is a mosaic disorder resulting from somatic mutations in embryonic development
• Some authorities consider it to be a hamartoma arising from pilosebaceous follicles; others consider it to be an epidermal naevus involving hair follicles

• Males and females are equally affected
• Approximately 50% of lesions are evident at birth, most of the rest develop during childhood, usually before the age of 10 years
• Inflammatory (acneiform) comedo naevi tend to arise during childhood or adolescence
• A few case reports describe onset later in life; usually following trauma, infection, or other inflammatory skin change

Distribution

• A comedo naevus can affect any body site, although are most commonly found on the face, neck, trunk, and upper arm

Morphology

• The usual presentation is in the form of a solitary group of dilated and plugged follicular ostia in a honeycomb pattern. Other patterns seen include linear, segmental, or Blaschkoid. The lesions may rarely be bilateral
• Acneiform lesions with pustules and cysts can be associated with chronic inflammation and scarring

Non-cutaneous features (rare)

• Ipsilateral cataracts
• Transverse myelitis
• Skeletal abnormalities, possibly to include Apert's syndrome, which is characterised by craniosynostosis, mid-face hypoplasia, syndactyly and a broad distal phalanx of the thumb

• Lesions are usually asymptomatic and safe to leave. However, treatment may be sought for cosmetic purposes
   
• Some improvement has been noted with topical retinoids, and also gentle hyfriecation (using a topical anaesthetic) of the individual comedones
   
• Acneiform lesions should be managed like acne. Troublesome cases not responding to appropriate topical / systemic treatment should be referred