**4. Clinical features**

**2. Epidemiology**

106 Hair and Scalp Disorders

treatment response [5].

**3. Pathogenesis**

autoantibodies [2].

does not interfere with the hair follicle integrity [6].

suppressive treatment also support autoimmune etiology [2].

Alopecia areata is the most prevalent autoimmune disorder and the second most frequent disease causing hair loss after androgenetic alopecia [3]. The prevalence rate of the disease is approximately 1 in 1000 people worldwide; in the United States, the lifetime risk is estimated to be 1.7% [2, 4]. Alopecia areata is mainly a disease of young adults; as many as 60% of patients are under the age of 20 at first presentation. Pediatric cases constitute approximately 20% of alopecia areata patients [2]. However, patients of any age can be affected. There is no gender predilection [4]. Reported cases in elderly were of milder severity and had a better

Alopecia areata is an organ-specific T cell mediated autoimmune disease targeting hair follicles. Peribulbar lymphocytic infiltration impairing the normal hair cycle is considered to be the main pathophysiologic mechanism responsible for the disease process. Normal hair cycle is disrupted in alopecia areata; dystrophic changes of anagen follicles along with rapid progression of hair follicles from anagen to catagen and telogen phases are observed. In alopecia areata, perifollicular inflammatory infiltrate spares the bulge region of the follicle where follicular epithelial stem cells reside. Thus, in contrast to cicatricial alopecias, the inflammation

The complex pathophysiology of alopecia areata involves an autoimmune basis. Association of alopecia areata with other autoimmune diseases such as thyroiditis and vitiligo is reported. Presence of lymphocytes around hair follicles and the good response of patients to immuno-

Etiopathogenetic theories are based upon the loss of immune-privileged status of hair follicles leading to an immune response against follicular antigens. In fact, normal proximal epithelium of anagen hair follicles have a very low expression of MHC class I antigens and no MHC class II antigen expression, along with a potent expression of immunosuppressive cytokines, such as TGF-β1 and α-melanocyte–stimulating hormone (α-MSH) [6]. According to the recent evidence, mechanism leading to hair loss involves the following steps: firstly, hair follicles must enter an anagen phase without the immune privilege described above, making those vulnerable to immune reactions. Subsequently, perifollicular CD8+ T cell infiltration of the anagen hair bulb epithelium ensues, along with a significant increase in interferon-gamma (IFN-γ) and various other cytokines. This milieu further damages the immune-privileged status of hair follicles, autoreactive CD8+ T lymphocytes, and IFN-γ and causes hair follicle dystrophy and premature catagen induction, leading to clinical hair loss [7]. Patients with alopecia areata have also been found to have an increased frequency of hair follicle-specific

Genetic basis is implicated in disease pathogenesis. Many patients report a family history of alopecia areata with a frequency ranging from 10% to 42% of cases [8]. Studies show a higher Alopecia areata patches are mostly asymptomatic and are discovered incidentally. Rarely, patients may complain of burning or itching sensation preceding hair loss. Typically, hair loss presents as one or more well-demarcated round to oval skin-colored patches [1]. The affected skin has a normal appearance with visible follicular orifices. Rarely, a soft edematous infiltration can be felt upon palpation and a peachy or reddened coloration can be observed [12]. Any hair-bearing site may be affected [1]. The scalp is the most common site of involvement, with or without the involvement of other body sites, such as the eyebrows, eyelashes, and beard [3].

According to the extent of involvement, alopecia areata can be classified into alopecia circumscripta presenting with limited hair loss, alopecia totalis involving the entire scalp, and alopecia universalis involving whole body (**Figures 1** and **2**).

A clinical classification regarding the pattern of hair loss is also frequently used. Patchy alopecia is the most common type seen in up to 75% of patients. Reticular type has a net-like pattern with multiple active and regressing patches (**Figure 3**). A band-like pattern involving occipital scalp is called ophiasis type of alopecia areata. The very rare ophiasis inversus, also called sisaipho type, presents with hair loss in the central scalp, resembling androgenetic alopecia [12]. Another unusual variant, perinevoid alopecia, is reported as presenting with alopecia patches around the nevi [13]. Diffuse alopecia areata presents with widespread thinning of the scalp hair. A recently defined variant, acute diffuse and total alopecia is characterized with rapid progression, female preponderance, and a favorable prognosis [14].

Initially, white hairs may be spared in patients with graying hair. As disease progresses, the white hair will also be lost. Initial hair regrowth, spontaneous or therapy-induced may be depigmented or hypopigmented, but the color usually returns with time [2].

**Figure 1.** Circular area of nonscarring alopecia incidentally found in an adult patient.

Nail changes are observed in up to 7–66% of patients [15]. The nail matrix may be affected, resulting in pitting of the nail plate, which is the most common nail involvement in alopecia areata [6]. Other nail features found in alopecia areata are trachyonychia, Beau's line, onychorrhexis, onychomadesis, koilonychias, punctuate or transverse leukonychia, and red-spotted lunula [16]. Nail disease may precede, follow, or coexist with active hair loss [2].
