Introduction
Alopecia may be localized (patchy) or generalized. It may also be scarring or non-scarring. Scarring alopecia is caused by severe inflammation of the hair follicle resulting in irreversible damage. While non-scarring alopecia is reversible. It may be abrupt or gradual in onset.
Epidemiology
Androgenetic alopecia is the most common nonscarring alopecia, affecting both males and females. Its prevalence is greatly influenced by age, gender, and race; Caucasian individuals are notably the most affected, followed by Asians, African Americans, Native Americans, and Inuit populations. Additionally, its incidence among Caucasian males is closely tied with age, with incidence increasing with age. Androgenetic alopecia is also common in females though most commonly occurring following menopause.
In the case of alopecia areata, it is noted that the lifetime risk of alopecia areata in the general population is estimated to be around 2 percent, with one in 50 developing the disease at some time in their lives). The incidence and prevalence of alopecia areata is notably greater in non-White populations (ie Asian, African American, Hispanic) than White populations. The incidence is equal among males and females.
Pathophysiology
In the case of androgenetic alopecia, activation of the androgen receptor leads to shortening of the anagen phase, the growth phase, of the normal hair growth cycle, thus leading to the thinning and shortening of the hair follicles. Individuals with androgenetic alopecia were noted to exhibit elevated dihydrotestosterone (DHT) production, heightened levels of 5 alpha-reductase, and an abundance of androgen receptors in the areas of the scalp affected by balding.
Based on studies, alopecia areata is an autoimmune disease, wherein hair follicles in the anagen phase prematurely transitions into the nonproliferative involution (catagen) and resting (telogen) phases, thus leading to sudden hair shedding and inhibition of hair regrowth. Triggers such as emotional or physical stress, vaccinations, infections, and even medications lead to the inhibition of anti-inflammatory cytokines and the expression of major histocompatibility complex I (MHC-1) polypeptide-related sequence A (MICA) on the hair follicles. At the same time, there is increased secretion of interferon-γ (IFN-γ) and interleukin-15 (IL-15). In turn, IL-15 and IFN- γ activate target immune cells; these immune cells then target the hair follicle matrix, prematurely pushing them into the catagen or telogen phase.
Etiology
The most common causes of alopecia include androgenic or androgenetic alopecia (male and female pattern baldness) and alopecia areata.
Classification
Alopecia Areata
Alopecia areata is an autoimmune non-scarring hair follicle disease characterized by patches of significant hair loss.
Androgenic Alopecia
Androgenic alopecia is a chronic follicular disorder characterized by progressive hair loss with a patterned distribution. Notably, alopecia totalis mimics androgenic alopecia.