Genetics is thought to be the primary cause of acne in 80% of cases.[2] The role of diet and cigarette smoking is unclear, and neither cleanliness nor exposure to sunlight appear to play a part.[2][13][14] In both sexes, hormones called androgens appear to be part of the underlying mechanism, by causing increased production of sebum.[5] Another frequent factor is excessive growth of the bacterium Propionibacterium acnes, which is normally present on the skin.[5]
Many treatment options for acne are available, including lifestyle changes, medications, and medical procedures. Eating fewer simple carbohydrates such as sugar may help.[7] Treatments applied directly to the affected skin, such as azelaic acid, benzoyl peroxide, and salicylic acid, are commonly used.[8] Antibiotics and retinoids are available in formulations that are applied to the skin and taken by mouth for the treatment of acne.[8] However, resistance to antibiotics may develop as a result of antibiotic therapy.[15] Several types of birth control pills help against acne in women.[8] Isotretinoin pills are usually reserved for severe acne due to greater potential side effects.[8][16] Early and aggressive treatment of acne is advocated by some in the medical community to decrease the overall long-term impact to individuals.[4]
In 2015, acne was estimated to affect 633 million people globally, making it the 8th most common disease worldwide.[9][17] Acne commonly occurs in adolescence and affects an estimated 80–90% of teenagers in the Western world.[18][19][20] Lower rates are reported in some rural societies.[20][21] Children and adults may also be affected before and after puberty.[22] Although acne becomes less common in adulthood, it persists in nearly half of affected people into their twenties and thirties and a smaller group continue to have difficulties into their forties
Classification
The severity of acne vulgaris (Gr. ἀκµή, “point” + L. vulgaris, “common”)[23] can be classified as mild, moderate, or severe as this helps to determine an appropriate treatment regimen.[19] Mild acne is classically defined by the presence of clogged skin follicles (known as comedones) limited to the face with occasional inflammatory lesions.[19] Moderate severity acne is said to occur when a higher number of inflammatory papules and pustules occur on the face compared to mild cases of acne and are found on the trunk of the body.[19] Severe acne is said to occur when nodules (the painful ‘bumps’ lying under the skin) are the characteristic facial lesions and involvement of the trunk is extensive.[19][24]
Large nodules were previously referred to as cysts, and the term nodulocystic has been used in the medical literature to describe severe cases of inflammatory acne.[24] True cysts are in fact rare in those with acne and the term severe nodular acne is now the preferred terminology.[24]
Acne inversa (L. invertō, “upside down”) and acne rosacea (rosa, “rose-colored” + -āceus, “forming”) are not true forms of acne and respectively refer to the skin conditions hidradenitis suppurativa (HS) and rosacea.[25][26][27] Although HS shares certain common features with acne vulgaris, such as a tendency to clog skin follicles with skin cell debris, the condition otherwise lacks the defining features of acne and is therefore considered a distinct skin disorder.
Typical features of acne include increased secretion of oily sebum by the skin, microcomedones, comedones, papules, nodules (large papules), pustules, and often results in scarring.[28][29] The appearance of acne varies with skin color. It may result in psychological and social problems.[19]
Scars
Acne scars are caused by inflammation within the dermal layer of skin and are estimated to affect 95% of people with acne vulgaris.[30] The scar is created by abnormal healing following this dermal inflammation.[31] Scarring is most likely to take place with severe acne, but may occur with any form of acne vulgaris.[30] Acne scars are classified based on whether the abnormal healing response following dermal inflammation leads to excess collagen deposition or loss at the site of the acne lesion.[32]
Atrophic acne scars have lost collagen from the healing response and are the most common type of acne scar (account for approximately 75% of all acne scars).[31][32] They may be further classified as ice-pick scars, boxcar scars, and rolling scars.[30] Ice-pick scars are narrow (less than 2 mm across), deep scars that extend into the dermis.[31] Boxcar scars are round or ovoid indented scars with sharp borders and vary in size from 1.5–4 mm across.[31] Rolling scars are wider than icepick and boxcar scars (4–5 mm across) and have a wave-like pattern of depth in the skin.[31]
Hypertrophic scars are uncommon, and are characterized by increased collagen content after the abnormal healing response.[31] They are described as firm and raised from the skin.[31][33] Hypertrophic scars remain within the original margins of the wound, whereas keloid scars can form scar tissue outside of these borders.[31] Keloid scars from acne occur more often in men and people with darker skin, and usually occur on the trunk of the body.[31]
Pigmentation
Postinflammatory hyperpigmentation (PIH) is usually the result of nodular acne lesions. These lesions often leave behind an inflamed darkened mark after the original acne lesion has resolved. This inflammation stimulates specialized pigment-producing skin cells (known as melanocytes) to produce more melanin pigment which leads to the skin’s darkened appearance.[34] People with darker skin color are more frequently affected by this condition.[35] Pigmented scar is a common term used for PIH, but is misleading as it suggests the color change is permanent. Often, PIH can be prevented by avoiding any aggravation of the nodule, and can fade with time. However, untreated PIH can last for months, years, or even be permanent if deeper layers of skin are affected.[36] Even minimal skin exposure to the sun’s ultraviolet rays can sustain hyperpigmentation.[34] Daily use of SPF 15 or higher sunscreen can minimize such a risk.[36]
Causes
Risk factors for the development of acne, other than genetics, have not been conclusively identified. Possible secondary contributors include hormones, infections, diet and stress. Studies investigating the impact of smoking on the incidence and severity of acne have been inconclusive.[2][37][38] Sunlight and cleanliness are not associated with acne.[14]
Genes
The predisposition to acne for specific individuals is likely explained by a genetic component, a theory which is supported by studies examining the rates of acne among twins and first-degree relatives.[2] Severe acne may be associated with XYY syndrome.[39] Acne susceptibility is likely due to the influence of multiple genes, as the disease does not follow a classic (Mendelian) inheritance pattern. Multiple gene candidates have been proposed including certain variations in tumor necrosis factor-alpha (TNF-alpha), IL-1 alpha, and CYP1A1 genes, among others.[18] Increased risk is associated with the 308 G/A single nucleotide polymorphism variation in the gene for TNF.[40]
Hormones
Hormonal activity, such as occurs during menstrual cycles and puberty, may contribute to the formation of acne. During puberty, an increase in sex hormones called androgens causes the skin follicle glands to grow larger and make more oily sebum.[12] Several hormones have been linked to acne, including the androgens testosterone, dihydrotestosterone (DHT), and dehydroepiandrosterone (DHEA); high levels of growth hormone (GH) and insulin-like growth factor 1 (IGF-1) have also been associated with worsened acne.[41] Both androgens and IGF-1 seem to be essential for acne to occur, as acne does not develop in individuals with complete androgen insensitivity syndrome (CAIS) or Laron syndrome (insensitivity to GH, resulting in very low IGF-1 levels).[42][43]
Medical conditions that commonly cause a high-androgen state, such as polycystic ovary syndrome, congenital adrenal hyperplasia, and androgen-secreting tumors, can cause acne in affected individuals.[44][45] Conversely, people who lack androgenic hormones or are insensitive to the effects of androgens rarely have acne.[44] An increase in androgen and oily sebum synthesis may be seen during pregnancy.[45][46] Acne can be a side effect of testosterone replacement therapy or of anabolic steroid use.[1][47] Over-the-counter bodybuilding and dietary supplements are commonly found to contain illegally added anabolic steroids
