Congenital abnormalities of the hair shaft itself lead to weak, thin and broken hairs. In some cases there is a characteristic appearance, for example "spun glass" appearance of pili torti with a twisted hair. In monilethrix there are regular nodes in the hair shaft.
There are other abnormalities of the hair shaft which are not associated with increased fragility, such as the Willi hair syndrome, progressive kinking of the hair and uncombable hair, in which the hair grows in disorderly profusion, completely resistant to combing and brushing. In pili annulati there may be a spangled appearance due to bright bands in the hair shaft.
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