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Comedo
A comedo is a clogged hair follicle (pore) in the skin.[2] Keratin (skin debris) combines with oil to block the follicle.[3] A comedo can be open (blackhead) or closed by skin (whitehead) and occur with or without acne.[3] The word comedo comes from Latin comedere 'to eat up' and was historically used to describe parasitic worms; in modern medical terminology, it is used to suggest the worm-like appearance of the expressed material.[1] The chronic inflammatory condition that usually includes comedones, inflamed papules, and pustules (pimples) is called acne.[3][4] Infection causes inflammation and the development of pus.[2] Whether a skin condition classifies as acne depends on the number 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 noninvasive form of breast cancer, but comedo-type DCIS may be more aggressive, so may be more likely to become invasive.[5] CausesOil production in the sebaceous glands increases during puberty, causing comedones and acne to be common in adolescents.[3][4] Acne is also found premenstrually and in women with polycystic ovarian syndrome.[3] Smoking may worsen acne.[3] Oxidation rather than poor hygiene or dirt causes blackheads to be black.[2] Washing or scrubbing the skin too much could make it worse, by irritating the skin.[2] Touching and picking at comedones might cause irritation and spread infection.[2] What effect shaving has on the development of comedones or acne is unclear.[2] Some skin products might increase comedones by blocking pores,[2] and greasy hair products (such as pomades) can worsen acne.[3] Skin products that claim to not clog pores may be labeled noncomedogenic or nonacnegenic.[6] Make-up and skin products that are oil-free and water-based may be less likely to cause acne.[6] Whether dietary factors or sun exposure make comedones better, worse, or neither is unknown.[3] A hair that does not emerge normally, an ingrown hair, 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.[3] Comedones may be more common in some ethnic groups.[3][7] People of Latino and recent African descent may experience more inflammation in comedones, more comedonal acne, and earlier onset of inflammation.[3][7] Pathophysiology 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.[3] Excess keratin combined with sebum can plug the opening of the follicle.[3][8] This small plug is called a microcomedo.[8] Androgens increase sebum (oil) production.[3] If sebum continues to build up behind the plug, it can enlarge and form a visible comedo.[8] A comedo may be open to the air ("blackhead") or closed by skin ("whitehead").[2] Being open to the air causes oxidation of the melanin pigment, which turns it black.[9][2] Cutibacterium acnes is the suspected infectious agent in acne.[3] It can proliferate in sebum and cause inflamed pustules (pimples) characteristic of acne.[3] Nodules are inflamed, painful, deep bumps under the skin.[3] Comedones that are 1 mm or larger are called macrocomedones.[10] They are closed comedones and are more frequent on the face than neck.[11] 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.[12] ManagementUsing nonoily cleansers and mild soap may not cause as much irritation to the skin as regular soap.[13][14] Blackheads can be removed across an area with commercially available pore-cleansing strips (which can still damage the skin by leaving the pores wide open and ripping excess skin) or the more aggressive cyanoacrylate method used by dermatologists.[15] Squeezing blackheads and whiteheads can remove them, but can also damage the skin.[2] Doing so increases the risk of causing or transmitting infection and scarring, as well as potentially pushing any infection deeper into the skin.[2] Comedo extractors are used with careful hygiene in beauty salons and by dermatologists, usually after using steam or warm water.[2] Complementary medicine options for acne in general have not been shown to be effective in trials.[3] These include aloe vera, pyridoxine (vitamin B6), fruit-derived acids, kampo (Japanese herbal medicine), ayurvedic herbal treatments, and acupuncture.[3] Some acne treatments target infection specifically, but some treatments are aimed at the formation of comedones, as well.[16] Others remove the dead layers of the skin and may help clear blocked pores.[2][3][4] Dermatologists can often extract open comedones with minimal skin trauma, but closed comedones are more difficult.[3] Laser treatment for acne might reduce comedones,[17] but dermabrasion and laser therapy have also been known to cause scarring.[10] Macrocomedones (1 mm or larger) can be removed by a dermatologist using surgical instruments or cauterized with a device that uses light.[10][11] The acne drug isotretinoin can cause severe flare-ups of macrocomedones, so dermatologists recommend removal before starting the drug and during treatment.[10][11] Some research suggests that the common acne medications retinoids and azelaic acid are beneficial and do not cause increased pigmentation of the skin.[18] If using a retinoid, sunscreen is recommended. Rare conditionsFavre–Racouchot syndrome occurs in sun-damaged skin and includes open and closed comedones.[19] Nevus comedonicus or comedo nevus is a benign hamartoma (birthmark) of the pilosebaceous unit around the oil-producing gland in the skin.[20] It has widened open hair follicles with dark keratin plugs that resemble comedones, but they are not actually comedones.[20][21] Dowling–Degos disease is a genetic pigment disorder that includes comedo-like lesions and scars.[22][23] Familial dyskeratotic comedones are a rare autosomal-dominant genetic condition, with keratotic (tough) papules and comedo-like lesions.[24][25] References
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