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Pili annulati
Pili annulati
Clinical features Pili annulati (ringed hair) usually presents as a rare autosomal dominant disorder with high penetrance, although very occasionally sporadic cases are seen. Alternate light and dark bands are present along the length of the hair shaft, giving it a spangled and sandy appearance best observed under a bright light and microscopic examination. It is most obvious in patients with light-colored hair. However, it has also been reported in patients of African heritage.1 The condition usually presents in children, and mild fragility of the hair shaft is sometimes a feature. Other hair-bearing areas are occasionally affected. The growth of the scalp hair is usually normal.2
Pili annulati has been associated with alopecia areata and, interestingly, the clinical manifestations sometimes disappear when the hair regrows.3โ6 Microscopic examination, however, shows that the banding persists although it is less frequent.5 A simultaneous association with alopecia areata, autoimmune thyroid disease, and primary IgA deficiency simultaneously has also been reported. However, some authors have regarded this association as a coincidence. 7,8
Pathogenesis and histologic features The pathogenesis of pili annulati is unknown. It has been suggested that the alterations result from a disorder of protein metabolism in which malfunction of cytoplasmic ribosomes results in formation of defective cortical keratins, not yet identified.9,10 Another hypothesis states that the genetic defect may be a mutation in proteins involved in the signaling and regulation of formation and degradation of the lamina densa and sublamina densa region, resulting in abnormal assembly or remodeling of the basement membrane zone.11 Recently, a locus for pili annulati was mapped to chromosome 12q24.32โ24.33, but as yet the gene or genes responsible have not been completely identified.12โ14
1117 Hair shaft coiling and twisting
Pathogenesis and histologic features Monilethrix is a disease of the hair cortex. The mutation has been mapped to chromosome 12q11-q13, which contains the genes that encode three type II hair keratins expressed in the hair cortex: HB1 (KRT81), HB6 (KRT86), and HB3 (KRT83). The main defect is in HB6.1,8โ12 The mutations in HB1 produce a less severe phenotype and have been associated with nail dystrophy.13
Histologically, the nodal areas represent the normal hair and the narrowed areas represent affected hair. In the latter areas, there is a reduction in the thickness of the cortex and an absence of the medulla. The fractures occur within these foci, and in some patients, other hair shaft abnormalities are present.
Light microscopic examination reveals alternating dark and light bands that become less frequent in the distal part of the hair shaft. The light bands represent air-filled spaces between the macrofibrillar units within the cortex of the hair shaft.15 An unknown defect in the formation of the micro-/macrofibrillar matrix complex is considered to be the cause. The medullary portion of the hair shaft is not affected.10,16 It has also been observed that the cystine content of hair is lower than normal, while the level of lysine is elevated compared to normal controls.17
Electron microscopy studies have confirmed that the internodal region is the site of the pathology.14 The appearances of the nodes and the distance between them vary considerably, not only in different hairs but also within the same hair.15
Differential diagnosis Pseudomonilethrix has been classified into three types1: familiar pseudomonilethrix of Bentley-Phillips (autosomal dominant inheritance), acquired pseudomonilethrix in dysplastic disorders with hair fragility, and iatrogenic pseudominolethrix that represents an artifact, rather than a true abnormality of the hair shaft, induced by compression of hairs between glass slides during preparation of the specimen.2,16,17
With scanning electron microscopy, the air-filled cavities are seen within cortical cells or in larger cavities replacing these cells. Occasional hair shafts show an unusual weathering pattern. This may consist of minor surface abnormalities at regular intervals in areas overlying the abnormal spaces or else it presents as marked damage to the cuticle with exposure and even cracking of the underlying cortex.18
Differential diagnosis Pseudopili annulati has been described when normal hairs show a similar banding pattern to that seen in pili annulati as a result of flattening or twisting of the hair.19,20 This has no clinical implications.
Histologically, there are rosary-like nodular thickenings of the hair shaft alternating with thin areas, in a more irregular fashion than in monilethrix.2,17