Orthopedics

Hair Shaft Disorders

Hair shaft defects or hair shaft disorders are common and can indicate an underlying pathology.  Evaluation of hair samples under light microscopy can reveal the distinct pattern of injury and thereby facilitate an accurate diagnosis.  Hair shaft abnormalities must be evaluated from different regions of scalp to increase accuracy in diagnosis, as there can be wide structural variability even in normal hair.

In abnormal and in normal hair, a condition known as weathering can occur in which the hair subjected to normal wear and tear shows signs of damage to the cuticle and cortex.  Weathering can be exacerbated by excessive hairstyling, washing, and environmental exposure (swimmin g, sun, etc.).  The proximal root is usually spared or less marked in damage with increase toward the distal end where longitudinal fractures, commonly known as “split ends” can become more evident.

To cover the entire spectrum of hair shaft disorders in this brief introduction to the topic lies beyond the scope of what is desired.  Instead, a description of the major types of defects will be outlined to introduce the reader to the various types of hair shaft problems that can be encountered.

Fractures of the Hair Shaft

Transverse Fractures-Trichorrhexis Nodosa
Trichorrhexis nodosa is the most common defect of the hair shaft, which appears as beaded swelling associated with loss of the cuticle.  The basic cause of trichorrhexis nodosa is mechanical and chemical trauma, which can be exacerbated by the inherent weakness of the hair shaft.  Excessive hairstyling and tight braiding/weaving/ponytails can create this problem.  The absence of a cuticle can also cause a straight break through the hair shaft (trichoschisis) in addition to the nodular elements of trichorrhexis nodosa.

Although trichorrhexis nodosa can occur in normal hair exposed to repeated or severe trauma, it can be associated with conditions that predispose toward a weak hair shaft like alopecia areata.  The pathogenesis of trichorrhexis nodosa lies in the localized loss of the cuticle cells.  The exposed cortical fibers then separate and fray causing the distinctive nodular swelling of the hair shaft that look like two paintbrushes stuck against each other at the point of the nodular swelling.  Depending on the extent of the injury, fractures can occur at the nodular swelling leading to patchy or diffuse hair loss.

Besides acquired conditions of trichorrhexis nodosa, congenital conditions can also give rise to this problem.  Metabolic deficiencies like argininosuccinicaciduria (say that 3 times fast) can cause trichorrhexis nodosa, as arginine deficiency is postulated to cause abnormal keratin formation and therefore structural hair shaft weakness.  In addition, Menkes’ kinky hair syndrome which is a defective copper metabolism predisposes toward trichorrhexis nodosa, as copper is also required for proper keratin formation.  Menkes’ can also cause pili torti defect (to be discussed).

Acquired trichorrhexis nodosa can be seen in Africans who after years of hair straightening and hot combs can create these nodular fractures.  Cessation of these treatments may require 2 to 4 years for the hair to resume to normal.  A special condition known as acquired distal trichorrhexis nodosa is seen in Caucasians and Asians in which nodes appear only in the last few inches of the hair shaft due to excessive brushing or back combing.

Trichoclasis

Trichoclasis refers to the common “greenstick” fracture of the hair shaft, consisting of a transverse fracture splinted by an intact cuticle.  There is no constant abnormality of the cuticle or cortex in this condition.  Trichoclasis can be found in different hair shaft abnormalities or simply following excessive hair care and trauma.  Usually when physical trauma accounts for trichoclasis, the affected area shows sporadic and patchy damage, which can be reversed with gentle hair care.

 

Trichoschisis including trichothiodystrophy

As mentioned above, trichoschisis refers to a clean transver