Hair shaft disorders: This group is further divided into two main categories according to the fragility of the hair. For example, argininosuccinic aciduria is an example of a hair shaft disorder with increased fragility. This is an inborn error of urea synthesis due to the deficiency of the argininosuccinic lyase enzyme. Babies are born with normal hair, but untreated cases can develop a condition called trichorrhexis nodosa.
In trichorrhexis nodosa, there is a microscopic appearance of fractures with splaying out of individual cortical cells from the main body of the hair shaft. This absence of the argininosuccinic lyase enzyme causes the build-up of waste that affects different systems causing serious consequences such as mental retardation.
Trichorrhexis nodosa can also be acquired due to heat, chemical or mechanical damage. Hereditary woolly hair is another example of a hair shaft disorder. It involves a mutation in the keratin gene. The hair is short, kinked, and tightly curled.