What is hair loss?
Hair grows in cycles: anagen (living growing hair), catagen (in-between phase) and telogen (resting or falling-out phase). Recently a new phase, ketogen, has been recognised; this is when the follicle is empty, after the telogen hair has fallen out and before the anagen hair is visible on the scalp.
How does hair loss develop?
Anagen normally lasts two to seven years. Hair loss occurs when anagen is interrupted by certain medications such as anti-cancer drugs, or by the ‘autoimmune’ disease, alopecia areata. Anagen hair is tapered or broken-off.
Telogen lasts a few months and is terminated by a new anagen hair. The result is shedding a hair with a bulb at the end. It is normal to lose 50 or more telogen hairs a day, rather more in autumn and winter. Excessive shedding results in telogen effluvium, often a couple of months after an event such as child-bearing, fever, an operation, weight loss or certain medications. Sometimes there appears to be no recognizable cause, and the shortened hair cycle can continue for years (chronic telogen effluvium).
Who gets hair loss?
Genetic and hormonal influences result in gradual thinning of scalp hair with age as male or female pattern (androgenetic) hair loss. In some families this type results in male pattern baldness and considerable thinning in females. It is apparent in about 50% of individuals by the age of 50 years.
What causes hair loss?
Other causes of hair loss are associated with poor quality hair, iron deficiency and deficiency of thyroid hormone.
Replacement of iron or thyroid hormone respectively may result in prompt regrowth. If hair loss first occurs in childhood, it may be due to a genetic hair shaft abnormality. These are diagnosed by microscopic examination of the hair, and sometimes by scanning electron microscopy.
A large number of different types of hair shaft abnormality have been described, including:
- Fractures: trichorrhexis nodosa, trichoschisis, trichoclasis (trichothiodystrophy).
- Irregularities: trichorrhexis invaginata (seen with ichthyosis in Netherton's syndrome), Marie-Unna hypotrichosis (uncombable hair), pili bifurcati, pili annulati, pseudopili annulati, monilethrix (beaded hair), pseudomonilethrix.
- Coiling and twisting: pili torti (twisted hair), wooly hair, trichonodosis (knotted hair).
Anagen hair loss in a child may be due to ‘loose anagen syndrome’. Clumps of hair come out with combing. The hair loss gradually becomes less as the child becomes an adult. Trauma, infection and various skin diseases may injure the hair follicle resulting in localized areas of scarring and bald patches in which there are no visible follicles; this is called ‘cicatricial alopecia’.
Infections that may cause cicatricial alopecia include Staphylococcal folliculitis or boils, and animal ringworm infection). Skin diseases that may cause cicatricial alopecia include folliculitis decalvans, lichen planopilaris, frontal fibrosing alopecia, alopecia mucinosa, discoid lupus erythematosus and scleroderma. Scarring hair loss of unknown cause is known as pseudopelade. Although they may cause permanent balding if neglected, early treatment of scalp infections such as tinea capitis prevents permanent baldness.
Hair loss caused by psoriasis, in which there are thick plaques of scale, recovers once the skin condition is controlled. Seborrhoeic dermatitis or atopic dermatitis can sometimes also cause hair loss temporarily. Hair can be pulled out by tight curlers or certain hair styles, sometimes resulting in permanently thinned areas (traction alopecia). The hair shafts can be broken by heat (hair dryer, straightener), or chemicals (perming solution or bleach) or brushing too often.
Trichotillomania is a form of alopecia resulting from repetitive pulling, plucking and breaking of one's own hair.