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DermNet NZ


Facts about the skin from DermNet New Zealand Trust. Topic index: A B C D E F G H I J K L M N O P Q R S T U V W X Y Z


Defects of the hair shaft

Structure of a normal hair follicle

The hair follicle is the structure of skin from which the hair shaft emerges. The hair shaft grows from germinative cells in the hair bulb deep within the dermis.

The follicle is lined by inner and outer sheaths, which protect and shape the growing hair.

A muscle called the arector pili attaches to the outer root sheath below the sebaceous gland. When the erector pili muscle contracts, e.g. when cold, the hair shaft stands up (goose bumps).

A single hair shaft is made up of three parts:

The medulla and cortex have pigment cells responsible for hair colour.

The cuticle is the outer and strongest part of the hair shaft. It is made up of dead flattened cells that overlap each other. The cuticle is keratinised i.e., the cells are largely made of a protein called keratin; keratin is also an important component of skin and nails.

The sebaceous gland produces sebum, which protects the hair shaft and acts as a natural conditioner.

Structure of hair
The hair bulb
Structure of hair
Phases of the hair cycle
Hair structure and growth

Hair grows in cycles so the structure of an individual hair apparatus varies.

Then the new anagen hair shaft starts to germinate within the same follicle, and it pushes out the telogen hair resulting in normal hair shedding or moulting.

What is a hair shaft defect?

A hair shaft defect is any structural abnormality of the hair shaft. Some hair shaft defects are easily diagnosed with the naked eye, others may require microscopic examination.

Hair shaft defects may result in:

Frizzy hair of unknown cause Frizzy hair of unknown cause
Frizzy hair of unknown cause

What causes hair shaft defects?

Hair shaft defects may be due to external injury or genetic abnormality.

External injury to hair shaft.

Repeated physical injury is the commonest cause of increased hair fragility. This may be due to:

Chemical injury may be caused by:

Traction alopecia Traction alopecia
Traction alopecia

Genetic hair shaft abnormalities

Genetic hair shaft abnormalities may be broadly divided into two types: those with increased hair fragility and those without increased hair fragility.

Increased fragility No increased hair fragility

Bubble hair

Bubble hair is characterised by hair shafts with large “bubbles” within them. These thin the hair cortex and break the hair shaft.

There may be a genetic predisposition to bubble hair. However presentation is often precipitated by external injury such as use of a hair dryer, physical or chemical hair straightening or curling irons.

Beaded hair / monilethrix

The name beaded hair reflects hair shafts with narrowed segments between “nodes” of normal thickness. It is also called monilethrix. The scalp is most commonly involved.

The affected hair shafts appear beaded a few months after birth. They break off leaving patches of scalp with abnormally short hairs. The scalp may also have a rough appearance because of a build up of keratin around the hair follicle.

Monilethrix is inherited as an autosomal dominant characteristic. This means an affected individual has a 50% chance of passing it on to each of their male and female children. It is caused by changes in the keratin gene cluster at chromosome 12 (gene map locus 12q13, OMIM ID #158000).

There is no known treatment.

Beaded hair or monilethrix Beaded hair or monilethrix
Beaded hair or monilethrix

Pili torti

Pili torti describes a flattened hair shaft with clusters of narrow twists at irregular intervals. The hairs are abnormally fragile. It may be genetic in origin (inherited) or acquired.

When inherited, pili torti may be present at birth or appear during infancy (OMIM ID %261900).

Pili torti is associated with a number of inherited syndromes.

Acquired pili torti may arise in the following circumstances:

Bamboo hair / trichorrhexis invaginata

Bamboo hair or trichorrhexis invaginata is due to abnormal keratinisation. The hair shaft cortex is structurally weak at specific points along its length. Harder adjacent segments of the hair shaft impact into these softer portions creating a ball and socket appearance. The hair shaft may break with minimal trauma.

The cause is a mutation in the SPINK5 gene (locus 5q32, OMIM ID *605010)

Trichorrhexis invaginata is a classical feature of Netherton syndrome (OMIM ID #256500).

Trichorrhexis nodosa

Trichorrhexis nodosa is the most common abnormality of hair structure. When a weak hair shaft partially fractures and the cortical cells fragment, uneven spurs form at the fracture point.

Trichorrhexis nodosa may be congenital and occurs in children with mental retardation and argininosuccinic aciduria. (OMIM ID #207900).

Trichorrhexis nodosa may also be acquired, due to hair straightening, overheating by a hair dryer, frequent swimming in chlorinated water, excessive traction with tight braids and other forms of hair trauma.

Broken-off hair with trichorrhexis nodosa
Excessive use of hair dryer
Trichorrhexis nodosa

Trichothiodystrophy

In trichothiodystrophy, the hair shafts are deficient in sulphur resulting in transverse fractures. Polarising microscopy reveals intermittent dark bands.

Trichothiodystrophy may occur as an isolated hair shaft abnormality or may be associated with photosensitivity, ichthyosis, brittle hair, mental retardation, short stature, neurologic abnormalities, and nail and teeth dystrophy (OMIM ID %275550).

Acquired progressive kinking of hair

Acquired progressive kinking of hair is also called pubeshaarnaevus because it resembles pubic hair. In this rare disorder, scalp hair becomes progressively curly, frizzy, dry and difficult to comb.

It usually affects the vertex or frontotemporal area of young men and may progress to male pattern balding (androgenetic alopecia).

Acquired progressive kinking of hair may also be a side effect of retinoid therapy.

Loose anagen hair syndrome

Loose anagen syndrome is due to defective keratinisation of the inner root sheath, rather than the hair shaft itself. The loose hair shaft can be easily pulled out of the follicle, leaving localised or diffuse bald areas of the scalp.

On light microscopy the abnormal hairs are seen to be in the anagen phase and often appear twisted with longitudinal grooves and ruffled cuticles.

Loose anagen (hair) syndrome results in short, brittle hair. Although both sexes may be affected, it most often presents in young girls over the age of 2 years. They usually have lighter colour hair types.

It may be diagnosed by the painless extraction of at least 10 hairs on a hair pull test of which over 80% of the hairs are in the anagen phase.

It is inherited as an autosomal dominant condition with variable penetrance (OMIM ID 600628). Unfortunately there is no known treatment.

Loose anagen hair syndrome Loose anagen hair syndrome Loose anagen hair syndrome
Loose anagen hair syndrome

Ringed hair / pili annulati

Pili annulati or ringed hair is used to describe hair in which there are scattered groups of air filled “bubbles” along the hair shaft. These appear as bright bands when seen with reflected light (OMIM ID %180600).

Pili bifurcati

Pili bifurcati describes abnormal branching of the hair shaft. This can occur at multiple points. The branches often fuse together again further up the shaft. Each branch has its own cuticle.

Spun glass hair / pili trianguli et canaliculi

Spun glass hair is also called pili trianguli et canaliculi, uncombable hair syndrome and cheveux incoiffables. The hair shafts have an irregular, triangular shape on cross section and an abnormal longitudinal groove running along the shaft.

Spun glass hair is due to abnormal keratinisation of the inner root sheath. This forms an inflexible tube through which the hair shaft must pass, distorting its shape and rendering it stiff and brittle. Light reflected from the flattened surface gives the hair a “spun glass” appearance.

Spun glass hair is associated with atopy (eczema), alopecia areata and ichthyosis vulgaris. (OMIM ID #191480).

There is no known treatment for spun glass hair. However, the condition may improve with age.

Uncombable, spun glass hair
Uncombable, spun glass hair

Woolly hair

Woolly hair describes unusually frizzy, tightly coiled hair. The hair shafts have elliptical cross sections, axial twisting and scattered areas of splaying.

It may affect the entire scalp or just a small area when it is considered a type of birthmark (naevus), the woolly hair naevus.

Woolly hair may be an inherited as an isolated condition (OMIM ID #194300) or present as part of a syndrome.

Other conditions that may be associated with the presence of woolly hair include keratosis pilaris, ichthyosis, abnormal teeth and nail dystrophies.

Woolly hair naevus Woolly hair naevus Woolly hair naevus
Woolly hair naevus

Tufted hair / pili multigemini

Tufted hair or pili multigemini describes multiple hair shafts bunched together and emerging from one follicle (like a toothbrush or doll's hair). Light microscopy confirms that all the hairs are surrounded by a single outer root sheath. The hair shafts are normal.

Tufted hair is a feature of scarring inflammatory scalp conditions, particularly the severe forms of scalp folliculitis known as folliculitis decalvans and dissecting cellulitis.

Tufted folliculitis Tufted folliculitis Tufted folliculitis
Tufted folliculitis due to folliculitis decalvans

Treatment of hair shaft defects

At present there is no specific treatment for inherited structural hair disorders. Occasionally there is spontaneous improvement as the child ages.

In acquired structural hair disorders, stopping exposure to the causative agent may lead to improvement.

Gentle hair care is recommended. Short hairstyles are advisable. In some cases, a wig or hairpiece may be the most practical approach.

Related information

References:

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Author: Dr Giri Raj, Dermatology Registrar, Liverpool UK, and Dr Amanda Oakley, Dermatologist, Hamilton NZ.

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