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 the Latincomedere, meaning "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]
Causes
Oil 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
Multiple open comedones at the nasolabial fold and the alar of the nose
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]
Management
Using 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.
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]
^ abBritish Association of Dermatologists. "Acne". Patient information leaflet. British Association of Dermatologists. Archived from the original on 2013-10-04. Retrieved 12 June 2013.
^ abcBurkhart, CG; Burkhart, CN (October 2007). "Expanding the microcomedone theory and acne therapeutics: Propionibacterium acnes biofilm produces biological glue that holds corneocytes together to form plug". Journal of the American Academy of Dermatology. 57 (4): 722–4. doi:10.1016/j.jaad.2007.05.013. PMID17870436.
^Kumar, Vinay; Abbas, Abul K.; Aster, Jon C.; Turner, Jerrold R.; Perkins, James A.; Robbins, Stanley L.; Cotran, Ramzi S., eds. (2021). Robbins & Cotran Pathologic Basis of Disease (10th ed.). Philadelphia, PA: Elsevier. p. 1165. ISBN978-0-323-53113-9.
^ abcPrimary Care Dermatology Society. "Acne: macrocomedones". Clinical Guidance. Primary Care Dermatology Society. Retrieved 12 June 2013.
^DermNetNZ. "Solar comedones". New Zealand Dermatological Society. Retrieved 16 June 2013.
^Poli, F (Apr 15, 2002). "[Cosmetic treatments and acne]". La Revue du Praticien. 52 (8): 859–62. PMID12053795.
^Korting, HC; Ponce-Pöschl, E; Klövekorn, W; Schmötzer, G; Arens-Corell, M; Braun-Falco, O (Mar–Apr 1995). "The influence of the regular use of a soap or an acidic syndet bar on pre-acne". Infection. 23 (2): 89–93. doi:10.1007/bf01833872. PMID7622270. S2CID39430391.
^Pagnoni, A; Kligman, AM; Stoudemayer, T (1999). "Extraction of follicular horny impactions the face by polymers. Efficacy and safety of a cosmetic pore-cleansing strip (Bioré)". Journal of Dermatological Treatment. 10 (1): 47–52. doi:10.3109/09546639909055910.
^Khaddar, RK; Mahjoub, WK; Zaraa, I; Sassi, MB; Osman, AB; Debbiche, AC; Mokni, M (January 2012). "[Extensive Dowling-Degos disease following long term PUVA therapy]". Annales de Dermatologie et de Vénéréologie. 139 (1): 54–7. doi:10.1016/j.annder.2011.10.403. PMID22225744.
^Hallermann, C; Bertsch, HP (Jul–Aug 2004). "Two sisters with familial dyskeratotic comedones". European Journal of Dermatology. 14 (4): 214–5. PMID15319152.
