Traction alopecia and trichotillomania
In terms of the mechanical action behind hair loss, traction alopecia and trichotillomania are exactly the same. The hair is plucked out of the skin leaving clear bald patches or diffuse, thin hair.
Traction alopecia can be caused by tight hat bands, pulling the hair into a tight pony tail, cornrow hair styles, and anything else that pulls on the roots of the hair. If traction alopecia continues for a long time and the same hair is repeatedly pulled out, then the hair follicles in the skin can become so damaged that they stop growing hair permanently.
Trichotillomania occurs when an individual plucks out their own hair. Often the hair on the scalp is plucked to leave bald patches, but the individual may focus on the eyelashes, eyebrows, pubic hair, or any other hair-bearing region. There is much argument about whether trichotillomania is a habit like nail biting, or a more psychological problem. Either way, affected individuals are usually not aware that they are plucking their hair, and when they are made aware of it they often find it very hard to stop.
Some individuals who pluck their hair also then eat it, a condition called trichophagia. This is a very dangerous condition that needs to be treated with some urgency. Hair is not digestible in the stomach and can build up into a hair ball. This can severely irritate the stomach lining, leading to severe ulceration. It is possible to die from trichophagia. Treating trichotillomania is difficult; therapists can probably help more than dermatologists.
The condition monilethrix makes hair fiber look like a string of beads. Along the length of a hair fiber there are nodes and constrictions making the edge of the fiber undulate. The beading that happens with monilethrix weakens the fiber.
Seen under a microscope, the hair fibers have lost their cuticle covering over the nodes while the constrictions keep their cuticle. The brittle hair easily breaks once it is exposed above the skin and the fibers rarely grow very long as a result. Breakage occurs in the weak constriction points along the fiber.