Hair shaft disorders associated with hair breakage include trichorrhexis nodosa, trichoschisis, pili torti, trichorrhexis invaginata, and monilethrix.
Hair shaft disorders associated with unruly hair, but none or little hair breakage, include uncombable hair syndrome, wooly hair, Marie Unna hereditary hypotrichosis, and hypotrichosis.
Other hair shaft disorders that don’t distinctly fall into either of the above categories are loose anagen syndrome and pili annulati.
Hair shaft disorders can be classified as primary and secondary. A primary disorder is caused by a gene defect that changes the shape or composition of the hair shaft, and a secondary disorder is caused by external factors applied to the hair leading to weakness and often breakage of the hair shaft. Classically, hair shaft disorders are also divided into those that cause hair breakage and those that cause unruly hair, although there can be overlap between the 2 categories.
HAIR SHAFT DISORDERS ASSOCIATED WITH HAIR BREAKAGE
Trichorrhexis nodosa (TN) is the most common hair shaft disorder associated with breakage.
TN presents as brittle, easily broken, and lusterless hair with white nodular swellings at irregular intervals along the hair shaft.1 Patients with TN often complain of inability to grow hair past a certain length.
TN can either be acquired or inherited. Acquired TN is the more common subtype and is caused by external trauma, which includes mechanical, chemical, and thermal injury. Acquired TN has been divided into 3 groups based on localized, proximal, or distal location.2 Localized TN usually presents as isolated patches that occurs from mechanical trauma secondary to a pruritic dermatosis.1 Proximal TN has primarily been observed in the African American population where the use of strong chemical and heat-straightening treatments is more common. Distal TN is described in white and Asian populations and is associated with frequent shampoo use, brushing, and chemical treatments such as bleaching. Other causes of acquired disease include malnutrition, iron deficiency, and hypothyroidism.
There are also instances where TN is either inherited or congenital. When TN is inherited, it occurs at birth or within a few months of birth and is an isolated autosomal dominant condition. Congenital TN can be associated with ectodermal dysplasias or with metabolic disorders such as arginosuccinic aciduria. Arginosuccinic aciduria leads to a buildup of toxic levels of ammonia and presents within a few days of birth with infants displaying lethargy, temperature instability and ataxia.3 Occasionally, an individual may inherit a milder form of the disorder where ammonia builds up in the bloodstream only during periods of illness. The gene defect is in the ASL gene, which provides instructions for making the protein arginosuccinase lyase. Testing for this disorder is included in many state newborn ...