Asian Journal of Research in Dermatological Science

Hair shaft disorders comprise congenital and acquired structural abnormalities of the keratinised hair fibre. Although individually rare, they matter clinically because they cause fragility and cosmetic and psychosocial burden and, in syndromic forms, may provide the earliest visible sign of serious neurodevelopmental, metabolic, immunological or cardiac disease. Existing reviews have catalogued morphological entities, but few have appraised the strength of the diagnostic, genetic and therapeutic evidence or examined where the literature conflicts. This critical narrative review synthesises literature published between January 1990 and August 2026, identified through searches of biomedical and multidisciplinary databases, genetic and trial registries, and citation tracking, with foundational earlier studies retained where conceptually necessary. Evidence was appraised qualitatively for design, diagnostic verification, outcome measurement and replication. Four principal conclusions emerge. First, the long-standing division into disorders with and without increased fragility remains clinically useful but conflates primary keratinisation defects with secondary weathering, and signs such as pili torti and trichorrhexis nodosa are non-specific end points rather than diagnoses. Second, trichoscopy has largely replaced routine light microscopy at the bedside, yet its accuracy for congenital shaft disorders rests mainly on case series without reference-standard comparison, so that polarised light microscopy, scanning electron microscopy and genetic testing retain decisive roles. Third, molecular studies have mapped shaft disorders onto discrete pathways, including hair keratins, desmosomal adhesion, inner root sheath cornification enzymes, lipid signalling, protease inhibition, transcription and translation, and copper transport, but genotype–phenotype correlations remain weak and pili annulati still lacks a causal gene. Fourth, therapeutic evidence is dominated by uncontrolled reports; the most persuasive data concern neonatal copper replacement in Menkes disease, whereas responses to minoxidil, biotin and biologic agents in other disorders remain provisional. Priorities include validated diagnostic criteria, standardised hair-specific outcome measures, natural-history registries, ancestry-diverse genetic cohorts and controlled trials. Recognition of systemic warning patterns, such as woolly hair accompanied by palmoplantar keratoderma, should prompt targeted evaluation. Hair shaft disorders are therefore best approached as phenotypes of follicular biology requiring integrated morphological, molecular and clinical interpretation rather than as isolated cosmetic curiosities.