What is a comedo naevus?
A comedo naevus (comedo nevus), also known as naevus comedonicus, is a rare, benign, cutaneous anomaly consisting of grouped, dilated follicular openings containing soft, dark keratin that resemble comedones.
Some authorities consider it to be a hamartoma arising from pilosebaceous follicles; others consider it to be an epidermal naevus involving hair follicles. They are usually solitary but may be part of a comedo naevus syndrome with associated skeletal, central nervous system, ocular, and other cutaneous abnormalities.
Who gets comedo naevus?
This epidermal naevus is rare. The true incidence is unknown but there are no sex or ethnic differences. Approximately 50% of cases present at birth and the remainder usually present before 10 years of age.
Adult onset is also described, even as late as the 7th decade, usually following trauma or a dermatosis such as herpes zoster, lichen planus, and pyoderma.
What causes comedo naevus?
Comedo naevus is a mosaic disorder (some cells have the abnormal gene whilst others do not) resulting from somatic mutations in embryonic development. Mutations in FGFR2 are thought to be important, with increased expression of IL-1 alpha.
Recently, somatic mutations in the NEK9 gene have been described. This leads to activation of NEK9 kinase and disruption of normal follicular differentiation. Whilst these DNA changes have been found in follicular and non-follicular tissue, a follicle- specific effect of NEK9 has been hypothesised in the pathogenesis of comedo nevi.
More recently, upregulation of ABCA12 has been described. Germline mosaicism has also been postulated, where the earlier the errors occur in development, the more comedo naevi an individual is likely to develop.
What are the clinical features of comedo naevus?
Comedo naevi usually presents as a single group of dark hyperkeratotic papules and horny plugs with a honeycomb appearance. Comedo naevi are found most commonly on the face, trunk, neck, and upper extremities, but can affect any part of the body.
Multiple comedo naevi may be seen and the distribution pattern may be:
- Blaschkoid ie following Blaschko lines.
Two specific subtypes are seen:
Additional abnormalities associated with comedo naevus syndrome may affect the:
- Skeleton: syndactyly, supernumerary digits, scoliosis, and spina bifida
- Central nervous system: seizures, developmental delay, microcephaly, transverse myelitis, and EEG abnormalities
- Eye: usually cataract
- Dentition: usually oligodontia
- Verrucous epidermal naevus
- Hidradenitis suppurativa
- Basal cell carcinoma
- Squamous cell carcinoma
- Linear morphoea
- Lichen striatus
- Accessory breast tissue
- Epidermolytic hyperkeratosis
- Naevoid hyperkeratosis of areola
- Hidradenoma papilliform
- Syringocystadenoma papilliform
- Epidermoid cysts
- Pilar sheath tumour
- Other epidermal nevi.
How do clinical features vary in differing types of skin?
There are no differences in clinical features seen amongst different ethnic groups.
What are the complications of comedo naevus?
The complications often manifest during adolescence and include:
- Bacterial infection
- Chronic inflammation
- Large cyst formation.
How is comedo naevus diagnosed?
Comedo naevus is usually a clinical diagnosis. Dermoscopy may be helpful to highlight the typical appearance.
Histopathological evaluation is rarely required and will show epidermal invaginations with moderate atrophy resembling enlarged hair follicles. The invaginations contain lobes of sebaceous glands and have associated acanthosis and concentric laminae due to keratin production.
What is the differential diagnosis for comedo naevus?
- Acne neonatorum
- Keratosis pilaris
- Comedonal acne
- Other epidermal naevi
- Familial dyskeratotic comedones
What is the treatment for comedo naevus?
Comedo naevus is benign and asymptomatic so treatment is often unnecessary. Treatment may be considered for cosmetic reasons or for complications such as cysts and abscesses.
Topical therapies may show modest benefit:
- Tretinoin and other topical retinoids +/- topical steroids or calcipotriol
- Salicylic acid
- Ammonium lactate lotions.
- Antibiotics are not effective, but may be required for the treatment of secondary infections
- Isotretinoin may reduce cyst formation, but long-term treatment is needed, thus it is not usually recommended.
- Curettage and dermabrasion may be used for smaller lesions
- Laser therapy may be effective but large defects may result that need skin grafts
- Erbium YAG and CO2 lasers
- 1450nm diode laser
- Microneedling fractional radiofrequency device
- Full thickness surgical excision +/- implantation of tissue expanders for defect closure.
What is the outcome for comedo naevi?
Comedo naevi remain unchanged over a person’s lifetime unless treated. As previously described, they may be subject to complications.