Hair Shaft Intro - Hairy Foundations
- Structure: Hair shaft comprises three layers:
- Cuticle: Outermost, protective, overlapping scales.
- Cortex: Main bulk, contains melanin (pigment) and keratin.
- Medulla: Innermost core, may be absent in fine hair.
- Abnormalities Classification:
- Fractures (e.g., Trichorrhexis nodosa)
- Irregularities/Invaginations (e.g., Pili annulati)
- Coiling/Twisting (e.g., Pili torti)
- Extraneous Matter (e.g., Piedra)
⭐ The cuticle is the first layer damaged by external factors like heat and chemicals, leading to dull, brittle hair.
Genetic Fragility - Brittle Breakdowns
Key genetic conditions causing hair fragility and breakage:
| Disorder | Microscopic Appearance & Inheritance | Clinical Features & Associations |
|---|---|---|
| Monilethrix | Elliptical nodes & constrictions ("beaded hair"); AD (KRT genes) | Sparse, fragile, short hair; follicular hyperkeratosis. Improves with age. |
| Trichorrhexis Nodosa | Node-like swellings, brush-like fractures ("two brooms"); Genetic/Acquired | Brittle hair, easy breakage; Argininosuccinic aciduria, Menkes. |
| Pili Torti | Flattened shaft, twisted 180° on its axis; AD/AR | Brittle, coarse, spangled/glistening hair; alopecia. Menkes, Bjornstad. |
| Trichorrhexis Invaginata | "Bamboo hair" (ball-and-socket invagination); AR (SPINK5) | Short, sparse, fragile hair. Pathognomonic for Netherton Syndrome. |
⭐ Trichorrhexis invaginata ("bamboo hair") is pathognomonic for Netherton syndrome.
Genetic Morphology - Curious Coils & Coifs
- Inherited hair shaft structural variants; unique textures, not primarily fragile.
| Abnormality | Inheritance | Features | Microscopic (Shaft) | Associations |
|---|---|---|---|---|
| Pili Annulati | AD | Light/dark bands ("ringed"), normal strength |  Bright (air) & dark bands | Isolated |
| Uncombable Hair Syndrome | AD/AR | Silvery, frizzy, unmanageable; improves with age |  Triangular/grooved cross-section | Isolated |
| Woolly Hair | AD/AR | Tightly coiled, fine, hypopigmented; diffuse/local | Elliptical cross-section, ↓diameter | Naxos/Carvajal (cardiac) |
| Pili Torti | AD/AR/XLR | Flattened, twisted 180°; brittle, glistening | Twists of flattened shaft | Menkes, Björnstad, Crandall |
Acquired Damage - Environmental Endings
- Trichoptilosis (Split Ends): Longitudinal splitting of distal hair shaft.
- Causes: Excessive grooming, weathering, chemical treatments.
- Mgt: Trim ends, gentle care.
- Trichoclasis: Transverse "greenstick" fracture of shaft.
- Cause: Trauma (e.g., vigorous brushing).
- Trichorrhexis Nodosa: Most common defect; nodes where cuticle is lost, fibers fray (like two brooms).
- Causes: Physical (brushing, heat) or chemical trauma.
⭐ Acquired trichorrhexis nodosa is the most common cause of brittle hair.
- Bubble Hair: Air-filled cavities (bubbles) in shaft.
- Cause: Acute localized overheating (e.g., faulty hair dryer). Hair becomes brittle.
- Microscopy: "Swiss cheese" appearance.
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Clinical Approach - Scalp Scope & Strategy
- History: Onset, progression, family history, hair care practices, chemical exposures.
- Examination:
- Scalp: Scaling, inflammation, alopecia pattern.
- Hair pull test: Assesses active shedding (normal < 6-10 hairs).
- Wood's lamp: Fungal infections (e.g., Microsporum fluoresces).
- Trichoscopy: Non-invasive; visualizes hair shafts, follicles, perifollicular skin.
⭐ Trichoscopy significantly aids in differentiating various hair shaft disorders by revealing specific morphological features (e.g., nodes, constrictions, fractures).
- Hair Mount Microscopy: Plucked/cut hairs examined under light microscope; KOH mount for fungi.
High‑Yield Points - ⚡ Biggest Takeaways
- Monilethrix: Beaded hair (AD, Keratin genes), leads to patchy alopecia.
- Pili torti: Twisted hair, associated with Menkes syndrome (copper deficiency).
- Trichorrhexis nodosa: Most common defect; "two brooms" appearance; acquired or congenital.
- Trichorrhexis invaginata (Bamboo hair): Pathognomonic for Netherton syndrome (SPINK5).
- Pili annulati: Ringed hair (alternating light/dark bands); hair glistens.
- Uncombable hair syndrome: Spun glass hair (triangular on cross-section); PADI3, TGM3, TCHH genes.
- Trichothiodystrophy: Sulfur-deficient brittle hair ("tiger tail" banding); associated with PIBIDS.
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