Comedo

Blackhead
Classification and external resources
A male with blackheads on his nose
ICD-9	706.1

A comedo is a clogged hair follicle (pore) in the skin.^[1] Keratin (skin debris) combines with oil to block the follicle.^[2] A comedo can be open (blackhead) or closed by skin (whitehead), and occur with or without acne.^[2] The word comedo comes from Latin to suggest the worm-like look of a blackhead that has been secreted.^[3] The plural of comedo is comedones.^[3]

The chronic inflammatory condition that usually includes both comedones and inflamed papules and pustules (pimples) is called acne.^[2][4] Infection causes inflammation and the development of pus.^[1] Whether or not a skin condition classifies as acne depends on the amount of comedones and infection.^[4] Comedones should not be confused with sebaceous filaments.

Comedo-type ductal carcinoma in situ (DCIS) is not related to the skin conditions discussed here. DCIS is a non-invasive form of breast cancer, but comedo-type DCIS may be more aggressive and so may be more likely to become invasive.^[5]

Causes

Oil production in the sebaceous glands increases during puberty, causing comedones and acne to be common in teenagers.^[2][4] Acne is also found pre-menstrually and in women with polycystic ovarian syndrome.^[2] Smoking may worsen acne.^[2]

Oxidation rather than poor hygiene or dirt causes blackheads to be black.^[1] Washing or scrubbing the skin too much could make it worse, by irritating the skin.^[1] Touching and picking at comedones might cause irritation and spread infection.^[1] It is not clear what effect shaving has on the development of comedones or acne.^[1]

Some, but not all, skin products might increase comedones by blocking pores,^[1] and greasy hair products (like pomades) can worsen acne.^[2] Skin products that claim to not clog pores may be labeled noncomedogenic or non-acnegenic.^[6] Make-up and skin products that are oil-free and water-based may be less likely to cause acne.^[6] It is not known whether dietary factors or sun exposure make comedones better, worse or have no effect.^[2]

A hair that does not emerge normally can also block the pore and cause a bulge or lead to infection (causing inflammation and pus).^[4]

Genes may play a role in the chances of developing acne.^[2] Comedones may be more common in some ethnic groups.^[2][7] Africans and African-Americans may experience more inflammation in comedones, more comedonal acne, and earlier onset of inflammation.^[2][7]

Pathophysiology

Illustration of blackhead

Comedones are associated with the pilosebaceous unit, which includes a hair follicle and sebaceous gland. These units are mostly on the face, neck, upper chest, shoulders and back.^[2] Excess keratin combined with sebum can plug the opening of the follicle.^[2][8] This small plug is called a microcomedo.^[8] Androgens increase sebum (oil) production.^[2] If sebum continues to build up behind plug, it can enlarge and form a visible comedo.^[8]

A comedo may be open to the air ("blackhead") or closed by skin ("whitehead").^[1] Being open to the air causes oxidization, which turns it black.^[1] Propionibacterium acne is the infectious agent in acne.^[2] It can proliferate in sebum and cause inflamed pustules (pimples) characteristic of acne.^[2] Nodules are inflamed, painful deep bumps under the skin.^[2]

Comedones that are 1mm or larger are called macrocomedones.^[9] They are closed comedones and are more frequent on the face than neck.^[10]

Solar comedones (sometimes called senile comedones) are related to many years of exposure to the sun, usually on the cheeks, not to acne-related pathophysiology.^[11]

Management

Using non-oily cleansers or mild soap may not cause as much irritation to the skin as regular soap.^[12][13] Blackheads can be removed across an area with commercially available pore-cleansing strips or the more aggressive cyanoacrylate method used by dermatologists.^[14]

Squeezing blackheads and whiteheads can remove them, but it can also damage the skin.^[1] Doing so increases the risk of causing or transmitting infection and scarring, as well as potentially pushing any infection deeper into the skin.^[1] Comedone extractors are used with careful hygiene in beauty salons and by dermatologists, usually after using steam or warm water to open up the pores.^[1]

Complementary medicine options for acne in general have not been shown to be effective in trials.^[2] These include aloe vera, pyroxidine (vitamin B6), fruit-derived acids, kampo (Japanese herbal medicine), ayurvedic herbal treatments and acupuncture.^[2]

Some acne treatments target infection specifically, but there are treatments that are aimed at the formation of comedones as well.^[15] Others remove the dead layers of the skin and may help clear blocked pores.^[1][2][4]

Dermatologists can often extract open comedones with minimal skin trauma, but closed comedones are more difficult.^[2] Laser treatment for acne might reduce comedones,^[16] but dermabrasion and laser therapy have also been known to cause scarring.^[9]

Macrocomedones (1 mm or larger) can be removed by a dermatologist using surgical instruments or cauterized with a device that uses light.^[9][10] The acne drug isotretinoin can cause severe flare-ups of macrocomedones, so dermatologists recommend removal before starting the drug and during treatment.^[9][10]

Some research suggests that the common acne medications, retinoids and azelaic acid, are beneficial and do not cause hyper-pigmentation in skin of color.^[17]

Rare conditions

Favre–Racouchot syndrome occurs in sun-damaged skin and includes open and closed comedones.^[18]

Nevus comedonicus or comedo nevus is a benign hamartoma (birthmark) of the pilosebaceous unit around the oil-producing gland in the skin.^[19] It has widened open hair follicles with dark keratin plugs that resemble comedones - but they are not actually comedones.^[19][20]

Dowling-Degos disease is a genetic pigment disorder that includes comedo-like lesions and scars.^[21][22]

Familial dyskeratotic comedones is a rare autosomal dominant genetic condition, with keratotic (tough) papules and comedo-like lesions.^[23][24]

References

Wikisource has the text of the 1920 Encyclopedia Americana article Comedones.