Throughout life, hair follicles undergo normal cycling characterized by periods of growth (anagen), involution (catagen), and rest (telogen). At any given time, 90 percent of hair follicles on the scalp are in the anagen phase. The rate of hair growth and duration of anagen vary with type and location of hair. For example, scalp growth is 0.3mm/ day and lasting two – six years; whereas eyebrow growth is 0.1 mm/day and lasting 2–3 months. The extent of normal daily hair loss is from 50–150 hair follicles per day, with marked increases in hair loss on shampoo days for those who shampoo hair infrequently.
Hair loss can be due to:
- Disorders of hair cycling, such as cicatricial (scarring) alopecia characterized by irreversible damage to the hair follicle and non-scarring alopecia, characterized by hair follicle not being destroyed, and thus spontaneous or treatment-induced hair regrowth occurs
- Inflammatory conditions such as psoriasis, contact dermatitis, and lupus erythematosus
- Structural hair disorders inherited or acquired from harmful hair care practices such as chemical processing, excessive brushing or teasing or excessive application of heat, leading to fragility of hair
Assessment of hair loss begins with its description and medical and family history, including current medical illness and medications and hair care products being used. Physical examination should involve inspection of the scalp and other body sites, looking for a pattern, distribution (focal vs. diffuse), density and degree of hair loss and areas of inflammation.
Diagnostic techniques such as microscopic examination of hair and scalp biopsies may provide information as to etiology of hair loss.