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 STRUCTURAL ABNORMALITIES OF THE HAIR SHAFT

Uncombable hair syndrome

Etiology and pathogenesis

In the uncombable hair syndrome, the hair has longitudinal grooves visible by electron microscopy, making it abnormally rigid. These changes have been called pili trianguli et canaliculi. Onset may be in infancy or delayed until puberty. Some people consider this as a diffuse form of a straight hair nevus. Familial cases have been described with autosomal dominant inheritance and variable penetrance. Occasionally, other defects have been described.

Fig. 28.43 Uncombable hair syndrome. The child’s hair is dry, coarse, and fuzzy. Combing is difficult, if not impossible.

Fig. 28.44 Monilethrix in a newborn. Monilethrix is an inherited condition where beading of the hair causes hair fragility and resultant breakage. Significant loss of hair at the nape of a newborn, where friction is the greatest, is the first manifestation of monilethrix.

Clinical

Hair that sticks out from the scalp and cannot be combed (or only with great difficulty) occurs in uncombable hair syndrome (Fig.28.43). Often, the hair itself is described as dry, coarse, or fuzzy. It may or may not be brittle. Sometimes the hair takes on a spangled or glittering appearance. The hair is usually straw-colored or silvery blonde. The clinical picture may be highly suggestive, and documenting a triangular cross-section of hairs embedded in paraffin is supportive. However, definitive proof requires scanning electron microscopy. Longitudinal, deep grooves running along the shaft are seen. The cuticle is intact.

Differential diagnosis

As with the other hair shaft disorders, simple weathering or frictional change may need to be considered, especially in young children and in milder cases. Generally, the differential diagnosis of hair shaft disorders includes other hair shaft disorders, so microscopic examination of the hair shaft is the critical investigation.

Treatment

No treatment has been found effective, although a response to biotin (0.3mg three times a day) has been reported in one case. The disease usually improves with age.

PRACTICE POINTS

Fig. 28.45 Patients with monilethrix have sparse, short hair, often with keratotic papules on the occiput (a). Closer examination reveals beading of the hair shafts (b), more easily seen on microscopy (c).

Monilethrix

Monilethrix is an autosomal dominantly inherited disorder of the hair shaft that results in periodic beading and hair fragility. Various causative genetic mutations have been reported, including mutations in the hair keratins hHb1 and hHb6. Short sparse hair occurs most commonly on the scalp but may affect the eyebrows or elsewhere (Fig.28.44). Onset may be from birth to late teenage years, or it may be brought out by rough hair care (e.g. vigorous brushing). The changes are most commonly seen over the nape and occiput (Fig.28.45). Follicular keratosis over the nape, associated with broken hairs, is characteristic. The hair looks beaded under the microscope (see Fig. 28.45). Some families have noted associated nail changes. Hair beads can be induced by the application of excessive amounts of hair lacquer and hair gel.

    There is no cure. Gentle hair care is essential. There is usually little change throughout life, but spontaneous recovery has occurred. Acitretin may have some benefit. Minoxidil topically has been used, as has oral l -cystine (500 mg twice daily for 3 months).

Pili torti

The hair is brittle and short in the affected child (Fig.28.46). Pili torti (twisted hairs) is a malformation of the hair shaft, which is twisted about its axis. It may occur as an isolated finding, usually with autosomal dominant inheritance, or it may occur with other features in various syndromes, including Menkes kinky hair syndrome, Björnstad syndrome (hearing loss), Crandall syndrome (hearing loss and hypogonadism), pseudomonilethrix, Bazex syndrome, and several other ectodermal abnormalities.

Pili annulati

The hair has a scintillating appearance (Fig.28.47). It may be normal in strength or somewhat fragile. Onset of the condition is usually at birth or in infancy. Autosomal dominant inheritance is common, but sporadic and autosomal recessive cases have been seen. The axillary and pubic hair may be similarly affected. Microscopic examination of the hair shows alternating areas of light (normal) and dark (gas-filled) hair. If the hair is viewed with reflected light, the normal hair is dark and the gas-filled hair light. Electron microscopy shows both large and small gas-filled cavities. The appearance is attractive to most, and no treatment is needed.

Fig. 28.46 Pili torti. The hair has a tangled appearance (a), with irregular spangling on closer examination (b).

Trichorrhexis nodosa

Trichorrhexis nodosa represents structural damage to the hair, wherein the outer surface is damaged to such an extent that the cortical fibers splay out at both ends, forming a node. In the most common form, distal trichorrhexis nodosa, small, white ‘beads' are seen near the ends of hairs as a result of some sort of trauma (e.g. excessive combing or brushing, and scratching; Fig.28.48). A more severe, but rarer, form occurs in darker-skinned women and is called proximal trichorrhexis nodosa. It may also occur in association with several inherited conditions involving the hair (e.g. in association with Netherton syndrome and pili annulati). No treatment is needed. In the case of distal trichorrhexis nodosa, eliminating the cause will allow the hairs to grow out normally.

White/Cox: Diseases of the Skin, 2ed.(c) 2006, Elsevier Inc. All rights reserved.