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|Classification and external resources|
|Classification and external resources|
Alopecia areata (AA) is a condition in which hair is lost from some or all areas of the body, usually from the scalp. Because it causes bald spots on the scalp, especially in the first stages, it is sometimes called spot baldness. In 1–2% of cases, the condition can spread to the entire scalp (alopecia totalis) or to the entire epidermis (alopecia universalis). Conditions resembling AA, and having a similar cause, occur also in other species.
Alopecia areata totalis and universalis are rare.
Typical first symptoms of AA are small bald patches. The underlying skin is unscarred and looks superficially normal. These patches can take many shapes, but are most usually round or oval. AA most often affects the scalp and beard, but may occur on any hair-bearing part of the body. Different skin areas can exhibit hair loss and regrowth at the same time. The disease may also go into remission for a time, or may be permanent. It is common in children.
The area of hair loss may tingle or be painful.
The hair tends to fall out over a short period of time, with the loss commonly occurring more on one side of the scalp than the other.
Exclamation point hairs, narrower along the length of the strand closer to the base, producing a characteristic "exclamation point" appearance, are often present.
When healthy hair is pulled out, at most a few should come out, and ripped hair should not be distributed evenly across the tugged portion of the scalp. In cases of AA, hair will tend to pull out more easily along the edge of the patch where the follicles are already being attacked by the body's immune system than away from the patch where they are still healthy.
Alopecia areata is usually diagnosed based on clinical features.
Trichoscopy may aid differential diagnosis. In AA, trichoscopy shows regularly distributed "yellow dots" (hyperkeratotic plugs), small exclamation-mark hairs, and "black dots" (destroyed hairs in the hair follicle opening).
A biopsy is rarely needed in AA. Histologic findings include peribulbar lymphocytic infiltrate ("swarm of bees"). Occasionally, in inactive AA, no inflammatory infiltrates are found. Other helpful findings include pigment incontinence in the hair bulb and follicular stelae, and a shift in the anagen-to-telogen ratio towards telogen.
Alopecia areata is not contagious. It occurs more frequently in people who have affected family members, suggesting heredity may be a factor. Strong evidence of genetic association with increased risk for AA was found by studying families with two or more affected members. This study identified at least four regions in the genome that are likely to contain these genes. In addition, it is slightly more likely to occur in people who have relatives with autoimmune diseases.
The condition is thought to be a systemic autoimmune disorder in which the body attacks its own anagen hair follicles and suppresses or stops hair growth. For example, T cell lymphocytes cluster around affected follicles, causing inflammation and subsequent hair loss. A few cases of babies being born with congenital AA have been reported, but these are not cases of autoimmune disease, because an infant is born without a definitely developed immune system.
In 2010, a genome-wide association study was completed that identified 129 SNPs (single nucleotide polymorphisms) that were associated with alopecia areata. The genes that were identified include regulatory T cells, cytotoxic T lymphocyte-associated antigen 4, interleukin-2, interleukin-2 receptor A, Eos, cytomegalovirus UL16-binding protein, and the human leukocyte antigen region. The study also identified two genes, PRDX5 and STX17, that are expressed in the hair follicle. 
Also, some evidence indicates AA affects the part of the hair follicle associated with hair color. Hair that has turned gray may not be affected.
If the affected region is small, it is reasonable to only observe the progression of the illness, as the problem often spontaneously regresses and the hair may grow back.
In cases of severe hair loss, limited success has been shown from treating AA with the corticosteroids clobetasol or fluocinonide, corticosteroid injections, or cream. The cream however is not as effective and it takes longer in order to see results. Steroid injections are commonly used in sites where the areas of hair loss on the head are small or especially where eyebrow hair has been lost. Whether they are effective is uncertain. Some other medications used are minoxidil, Elocon (mometasone) ointment (steroid cream), irritants (anthralin or topical coal tar), and topical immunotherapy cyclosporine, sometimes in different combinations. Topical corticosteroids frequently fail to enter the skin deeply enough to affect the hair bulbs, which are the treatment target, and small lesions typically also regrow spontaneously. Oral corticosteroids decrease the hair loss, but only for the period during which they are taken, and these drugs have serious adverse side effects.
In most cases which begin with a small number of patches of hair loss, hair grows back after a few months to a year. In cases with a greater number of patches, hair can either grow back or progress to AA totalis or, in rare cases, AA universalis.
Effects of AA are mainly psychological (loss of self-image due to hair loss). Loss of hair also means the scalp burns more easily in the sun. Patients may also have aberrant nail formation because keratin forms both hair and nails.
Hair may grow back and then fall out again later. This may not indicate a recurrence of the condition, however, but rather a natural cycle of growth-and-shedding from a relatively synchronised start; such a pattern will fade over time. Episodes of AA before puberty predispose one to chronic recurrence of the condition.
The condition affects 0.1%–0.2% of humans, occurring in both males and females. Alopecia areata occurs in people who are apparently healthy and have no skin disorder. Initial presentation most commonly occurs in the late teenage years, early childhood, or young adulthood, but can happen with people of all ages. Patients also tend to have a slightly higher incidence of conditions related to the immune system: asthma, allergies, atopic dermal ailments, and hypothyroidism.
A number of medications are under trial
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