Monilethrix is a congenital & hereditary malformation of the hair shaft characterized by regular "Nodes" of normal thickness and "Internodes" of abnormal thinning and the defect can infect the nails of the toes and of the fingers as well.
This condition is also known as beaded or knotted hair. as the hair looks like beads on a string.
The hair will also be dry and fragile and only growth a little before it breaks off due to its twisted shape.
This is a very rare occurance and have only seen this once in my 30 years as the Cosmetologist.