The answer is: HAIR SHAFT ABNORMALITIS WITH INCREASED FRAGILITY
In the last few years, more and more articles have underlined the value of trichoscopy in the diagnosis of alopecia due to hair shaft abnormalities with hair fragility. In these cases, trichoscopy provides an immediate diagnosis and avoids a biopsy for histopathological study.
Trichoscopy in Hair Shaft Disorders, by Rudnicka L, Olszewska M, Waśkiel A, Rakowska A. (Dermatol Clin. 2018) https://pubmed.ncbi.nlm.nih.gov/30201151/
The normal scalp hair shaft is uniform in color and thickness throughout its length. Dark hair shafts are usually better visualized on trichoscopy compared to light-colored hairs. Hair shaft diameter varies from thin to normally thick. Terminal hairs are more than 55 µm wide, but there is variability in thickness among different populations as well as in a single person. Terminal hairs represent about 90% of normal human scalp hairs, the remaining 10% being normally represented by vellus hairs, defined as hypopigmented, nonmedullated hairs less than 30 µm thick and less than 2-3 mm long (Fig.1).
Dr Bianca Maria Piraccini
Fig. 1 Trichoscopy of a normal scalp in the vertex area. Groups of 2-3 terminal hair shafts of regular thickness and shape emerge from the ostia. Note the presence of few intermediate and vellus hair which are a normal finding.
The length, shape and diameter of the hair shaft is modified in the different types of hair loss and the recognition of these morphological changes is often a clue for diagnosis.
This is particularly true in genetic hair shaft disorders, where trichoscopy has substituted hair shaft microscopy and histopathology (Dermatoscopy of hair shaft disorders, by Miteva M, Tosti A. (J Am Acad Dermatol. 2013)https://pubmed.ncbi.nlm.nih.gov/22940404/.
GENETIC HAIR SHAFT ABNORMALITIS WITH INCREASED FRAGILITY
Genetic abnormalities of the hair shaft usually involve scalp hair, but sometimes may involve other terminal hair, seldom all body hair. They are usually evident already at birth or after a few months.
They can be classified into 2 groups:
Two congenital abnormalities of the hair shaft associated with increased fragility can be seen during routine dermatological visit and can be easily diagnosed by a manual dermatoscope, due to the unique and characteristic aspect of the hair shafts: monilethrix and trichorrexis invaginata.
Monilethrix is a rare hereditary condition characterized by fragile hair with typical beaded appearance, like the beads of a necklace. The name monilethrix derives from the Latin word for necklace (monile) and the Greek word for hair (thrix).
The condition is transmitted as an autosomal dominant trait with incomplete penetrance and variable expressiveness. Several families have shown mutation of genes which encode hard keratins KRT81, KRT83 and KRT86, which are primarily expressed in cortical trichocytes. Shaft abnormalities in fact are the consequence of a structural defect of cortical cells that lead to the presence of periodic narrowing of hair shaftalternated with elliptical nodes. The regular alternation (0.7-1 mm) of nodes and narrowing resembles a pearl necklace (Fig.2). Microscopy highlights the absence of the medulla in the internodes: the latter are therefore the abnormal portions as well as those where the shaft breaks.