Question
Tinea capitis.
Answer
Tinea capitis is a dermatophyte infection of the scalp and hair, caused predominantly by Trichophyton and Microsporum species, occurring mainly in pre-pubertal children (the changed sebum composition/fatty acid content after puberty is thought to confer relative resistance in adults).
Clinical patterns: grey patch tinea capitis — scaly, well-demarcated patches of hair loss with hairs broken off close to the scalp surface (typically caused by Microsporum species); black dot tinea capitis — hairs broken off at the scalp surface, leaving visible dark dots (typically caused by Trichophyton tonsurans, an endothrix infection where the fungus invades within the hair shaft); kerion — a severe, painful, boggy, inflamed nodular/pustular lesion representing an intense host inflammatory reaction, typically to a zoophilic organism, which can cause scarring alopecia if untreated; favus — a chronic, severe form with characteristic yellowish, cup-shaped crusts (scutula) and a distinctive musty odour, caused by Trichophyton schoenleinii, now rare in most regions.
Hair invasion patterns: ectothrix — fungal spores form a sheath around the outside of the hair shaft (typically Microsporum species, which characteristically fluoresce green under Wood’s lamp); endothrix — fungal elements grow within the hair shaft itself (typically Trichophyton tonsurans, which does not fluoresce under Wood’s lamp).
Laboratory diagnosis: Wood’s lamp examination — green fluorescence for ectothrix (Microsporum) infections, no fluorescence for endothrix (T. tonsurans) infections; direct KOH microscopy of plucked hair (not cut hair) showing spores around (ectothrix) or within (endothrix) the hair shaft; culture on Sabouraud dextrose agar for definitive species identification.
Treatment: oral antifungal therapy (griseofulvin, terbinafine, or itraconazole) is required, since topical therapy alone cannot adequately penetrate the hair follicle; treatment is typically prolonged (weeks); kerion may additionally require a short course of corticosteroids to reduce inflammation and scarring risk.