Alopecia

Alopecia means baldness. Usually we have about 300,000 hairs on our heads, and we lose anything up to 300 each day. Each hair grows at a rate of around about a centimetre a month - a phase in the life of the hair called anagen. After about three years of life the hair falls out - a phase called catagen. The hair follicle then goes into a resting phase called telogen before starting a new three-year cycle. There are many conditions that can affect the growth and life of a hair, the most common being so-called male pattern baldness where there is gradual loss of hair from the front and sides of the scalp, causing a receding hairline, and thinning of the hair on the top of the head. This process is dependent upon the male hormone testosterone: the more testosterone you have the faster it happens. Over time most of us are affected to a greater or lesser extent.

Some conditions cause a large number of hairs to enter the resting phase altogether, so they are shed a few weeks later. This can happen following any acute severe illness, as part of pregnancy, after surgery, in association with Iron deficiency, as a result of a shock or severe stress, or as a consequence of severe dieting in anorexia or bulimia. Once the cause has been remedied or has faded into the past regrowth of the hair is normal.

Scalp disease causing scarring will result in baldness because the hair follicles arise in the dermis, which is destroyed by any process involving scarring and the hair follicles do not regenerate. Scarring may follow a direct injury to the scalp, any prolonged severe scalp infection or conditions such as lichen planus (see skin disorders).
Alopecia areata is a condition causing patchy areas of hair loss which can vary in size and from less than a centimetre to large areas of the scalp. In men in may also affect the beard area. While the cause is unknown it is thought possibly to be due to an autoimmune phenomenon involving both genetic and environmental factors. There is no evidence of underlying scalp disease and there is no scarring, and examination of the bald patch shows hairs which are short, thin near the scalp and thicker at the end (and so-called exclamation mark hairs). Usually the hair regrows, but there may be recurrent attacks, while in some cases the baldness is permanent.

Complete hair loss is a side-effect of many drugs used in the treatment in of malignant disease, when the hair loss occurs during the growing phase - since these drugs are designed to stop cell division this is an understandable side-effect. Hair regrowth usually occurs when the treatment is completed.
 
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