This page covers the diagnosis of different structural genetic hair shaft defects. These are classified based on their association with hair fragility.
Structural hair shaft defects associated with hair fragility
The term is derived from the Greek meaning “necklace” or “beaded”. Monilethrix is an autosomal dominant disorder that usually affects individuals in the first few months after birth. Affected hairs appear short, fragile, and brittle. The disorder mainly affects the scalp, however, in severe cases it may involve eyebrows, eyelashes, and nails (koilonychia).
- Affected hairs show abnormal constrictions regularly arranged along the hair shaft causing variability in the shaft thickness and its breakage at the constriction sites
- Medulla is present in nodes and absent in internode segments
- Regrowing hair (pigtail hair)
- Perifollicular casts
- Horny follicular papules appear as large yellow dots when examined with immersion fluid
- Follicular plugging, perifollicular erythema and scales observed when examined using dry dermoscopy.
Monilethrix-like hairs show the same constrictions but without the regular distribution characteristic of true monilethrix. It occurs in monilethrix-like congenital hypotrichosis, alopecia areata, lichen planopilaris and patients receiving chemotherapy.
Trichorrhexis invaginata (bamboo hair)
An abnormality of the hair in which the hair shaft invaginates into itself at multiple points along the shaft. The disorder becomes evident at infancy. Affected hairs are short, sparse, and very fragile. It is pathognomonic for Netherton syndrome. Recently, matchstick hairs were also described in Netherton syndrome as short hairs with a bulging end.
- Multiple small nodules along the hair shaft observed with handheld dermoscopy.
- Using higher magnification, affected hair show invagination of the distal portion of the shaft into its proximal portion, forming a “ball-in-cup” appearance
The most common structural hair shaft abnormality caused by damage of the hair shaft cuticle. Causes:
- Tricho-hepato-enteric syndrome
- Ectodermal dysplasias
- Monilethrix-like congenital hypotrichosis
- Mutation in the XPD gene
- Congenital trichorrhexis nodosa without coexisting defects.
The trichoscopic features of trichorrhexis nodosa depend on the magnification used and the presence of immersion fluid (Fig 50).
- Using a handheld dermatoscope, affected hairs show light colored nodular thickenings along the hair shafts.
- Higher magnification devices show splitting of hair shafts into multiple fibers appearing like two brushes pushed into one another.
- When a hair shaft breaks at the site of the nodule, it leaves a slightly thickened, rounded hair shaft end, which may appear darker compared with light-colored hair shaft.
- At higher magnification, trichoscopy allows appreciating numerous brush-like small fibers at the distal end of the hair shaft.
Pili torti is a structural hair shaft defect which is associated with multiple disorders.
Conditions associated with pili torti:
- Pili torti
- Ronchese type
- Beare type
- Autosomal recessive ichthyosis with hypotrichosis (ARIH)
- Basex syndrome
- Björnstad syndrome
- Crandall syndrome
- Hypohidrotic ectodermal dysplasia
- Menkes syndrome
- Hypotrichosis with juvenile macular dystrophy
- Laron syndrome.
Affected hair shafts show flattening with twisting of the hair fiber three to ten twists on its long axis. This abnormality is best observed by dry dermoscopy at high magnification.
It is an autosomal recessive disorder characterised by sulfur-deficient hair. This disorder is mainly diagnosed using polarized light microscopy as it shows alternating bright and dark bands along the hair shaft, resembling a tiger’s tail.
Trichoscopic examination is of limited value in diagnosing the disorder; it shows heterogeneous hair shafts resembling grains of sand with slightly wavy contour.
Structural hair abnormalities not associated with hair fragility
Pili annulati is characterized by multiple air-filled cavities along the hair shaft. The disorder is detected in blonde hair much more easily than in darkly pigmented hair.
Alternating dark and light bands along the hair shaft corresponding to the air-filled cavities.
Hutchinson et al. classified the condition into three variants:
- Woolly hair naevus
- Autosomal dominant
- Autosomal recessive.
This disorder is characterised by tightly curled hair shafts with 180° longitudinal twisting, giving the appearance of a crawling snake. Common associations:
- Cardio-facio-cutaneous syndrome
- Noonan syndrome
- Costello syndrome
- Naxos and Carvajal syndrome.
See Trichoscopy: an overview for more images on trichoscopic findings.