dermatologic disorders
Alopecia
Pathophysiology of Alopecia
Hair grows in cycles. Each cycle consists of phases: Anagen: A long (2- to 6-year) growing phase Catagen: A brief (3-week) transitional apoptotic phase Telogen: A short (2- to 3-month) resting phase At the end of the resting phase, the hair falls out (exogen). Normally, about 50 to 100 scalp hairs reach the end of resting phase each day and fall out. When a new hair starts growing in the follicle, the cycle begins again. Disorders of the growth cycle include Anagen effluvium—a disruption of the growing phase causing abnormal loss of anagen hairs Telogen effluvium—significantly more than 100 hairs/day going into resting phase Alopecia can be classified as focal or diffuse and by the presence or absence of scarring. is the result of active destruction of the hair follicle. The follicle is irreparably damaged and replaced by fibrotic tissue. Several hair disorders show a biphasic pattern in which nonscarring alopecia occurs early in the course of the disease, and then scarring alopecia and permanent hair loss occurs as the disease progresses. Scarring alopecias can be subdivided further into primary forms, where the target of inflammation is the follicle itself, and secondary forms, where the follicle is destroyed as a result of nonspecific inflammation (see table Some Causes of Alopecia ). Nonscarring alopeciaresults from processes that reduce or slow hair growth without irreparably damaging the hair follicle. Disorders that primarily affect the hair shaft (trichodystrophies) also are considered nonscarring alopecia. 