Human hair has been classified into 3 major groups, as determined by ethnic origin. These include Caucasian, Asian, and African hair. Apart from reported differences in the distribution of low- and high-sulfur proteins, the keratin types and amino acid hair composition are similar in these groups.1-6 However, significant structural variations are seen. Most notable of these features are the shape and diameter of the hair fiber.5 While Asian hair is cylindrical in cross-section with a greater diameter, the Caucasian hair shaft may be round to oval.1,7 In contrast, African hair is markedly elliptical, with flattening, twisting, random reversals in direction and variability of the diameter along the hair shaft.1,8-10 This has been described as a â€œtwisted oval rod.â€11 Also, the curly, spiral nature of African hair is attributed to the curved shape of the follicle itself.12
Other significant structural differences include the decreased tensile strength and moisture content of African hair, and fewer elastic fibers anchoring the hair follicles to the dermis.9,10,13 The dry quality of African hair and scalp is also due in part to decreased activity of sebaceous glands in these patients.14
These structural variations manifest clinically as the woollier, drier, and more brittle hair of Africans as compared with other ethnic groups. Because of the tight curl pattern of black hair, individuals may use chemicals relaxers or heat pressing to straighten the hair.15 These processes can result in damage to the cuticle and further loss of moisture from the hair shaft.
It is important that clinicians recognize the unique hair morphology in ethnic populations, as some hair and scalp disorders seen in these populations are directly linked to morphology. Also, clinicians who do not appreciate these differences can suggest treatments that further injure the fragile hair shaft or are ineffective.
Acquired Trichorrhexis Nodosa
Trichorrhexis nodosa (TN), a hair shaft disorder, is characterized by fragility and nodes along the shaft.16 The hair appears lusterless and dry, and patients may complain of â€œwhitish spotsâ€ along the hair.17 On microscopic examination, there is cuticular cell disruption and fraying and breaking of the cortical fibers, with the resulting appearance likened to 2 paintbrushes crushed against each other, hence the description â€œpaintbrush fracture.â€18,19 TN may be congenital or acquired. The rare congenital form may be an isolated finding or may present as part of a syndrome, such as Menkes kinky hair syndrome, trichothiodystrophy, or argininosuccinic aciduria.20-22
Acquired TN, resulting from repeated trauma to the hair shaft, is much more common.17 Specific etiologies include combing practices, chemical trauma, excessive brushing, and application of heat to the hair. Occasionally, acquired TN may result from iron deficiency or hypothyroidism.23
In 2011, Martin and Sugathan reported 3 cases of acquired TN induced by a specific comb and combing habit, often used by men in Kerala, a southern state of India.17 This comb is used 8 to 10 times per day by young men who style their hair soon after getting off their motorcycles. The hair shaft is entangled among the bristles of this comb, resulting in cuticular disruption and breakage of the shaft.17 It has also been reported that regular use of a helmet for extended periods of time can contribute to TN.24
Chemical trauma to the hair includes excessive exposure to perming, bleaching, shampooing, and dyeing.16 This is particularly significant in African American hair, where chemical â€œrelaxersâ€ are used to straighten the tightly curled hair. Not only do these relaxers disrupt the hair cuticle, but they also reduce cysteine levels within the hair shaft.25 Cysteine is a