Onychomycosis
Tinea unguium; fungal nail infection; nail mycosis
Onychomycosis is the commonest cause of chronic nail dystrophy — accounting for up to 50% of all nail-disease referrals. The principal pathogens are dermatophytes (Trichophyton rubrum, T. interdigitale), yeasts (Candida) and non-dermatophyte moulds (Scopulariopsis, Aspergillus, Fusarium). Onychomycosis is the single most important differential diagnosis for chronic nail change when subungual SCC, subungual melanoma, or onychomatricoma are being considered — and indeed many subungual cancers are mistreated as onychomycosis for years before correct diagnosis. The rule is therefore: confirm onychomycosis mycologically before treating, and biopsy chronic nail change that fails to resolve.
Clinical features
- Yellow-brown thickening and crumbling of the nail plate; onycholysis (separation from nail bed).
- Subungual hyperkeratosis with debris under the nail plate.
- Patterns:
- Distal lateral subungual onychomycosis (DLSO) — commonest; T. rubrum / T. interdigitale; starts distally and progresses proximally.
- White superficial onychomycosis (WSO) — chalky white patches on nail surface; T. interdigitale.
- Proximal subungual onychomycosis (PSO) — starts proximally; often immunosuppressed (HIV).
- Total dystrophic onychomycosis (TDO) — end-stage; whole nail involved.
- Candidal onychomycosis — paronychia + nail involvement; chronic mucocutaneous candidiasis.
- Toenails affected in ~ 75% of cases (great toe most often); fingernails less common.
- Risk factors — older age, diabetes, peripheral vascular disease, tinea pedis, occlusive footwear, immunosuppression.
Critical differential — chronic nail dystrophy
- Confirm onychomycosis mycologically before starting antifungal therapy:
- Nail clippings and subungual debris for KOH preparation, dermatophyte culture and PAS-stained histology.
- PCR for dermatophyte DNA — faster, more sensitive.
- Refractory or atypical nail dystrophy warrants nail-matrix biopsy to exclude:
- Subungual SCC — HPV-16 driven; chronic paronychia / onycholysis.
- Subungual melanoma — pigmented; Hutchinson sign.
- Onychomatricoma — yellow nail with longitudinal cavitations.
- Psoriasis — nail pitting; oil drop sign.
- Lichen planus — pterygium, ridging.
- Trauma-induced dystrophy.
- The biggest skin-oncology trap is the patient mistreated for > 1 year as onychomycosis when underlying disease is SCC or melanoma.
Management
- First confirm diagnosis — mycology before treatment.
- Topical — amorolfine 5% lacquer, ciclopirox 8% lacquer; limited efficacy as monotherapy (~ 30% cure); used for distal disease < 50% of nail.
- Systemic — terbinafine 250 mg daily for 6 weeks (fingernails) or 12 weeks (toenails) is first-line. Cure rates 70–80%.
- Alternatives — itraconazole pulse dosing; fluconazole for Candida.
- Monitor LFTs during oral therapy.
- Combination topical + oral — improved cure rates.
- Laser, photodynamic therapy — adjunctive; modest evidence.
- Nail removal occasionally — for total dystrophy or severe symptoms.
- Recurrence rates 20–40% — emphasis on shoe / sock hygiene, tinea pedis treatment, drying after washing.
- Diabetic / immunosuppressed — lower threshold for treatment to prevent secondary infection.
References
- Ameen M et al. British Association of Dermatologists guidelines for the management of onychomycosis. Br J Dermatol; 2014.
- Lipner SR, Scher RK. Onychomycosis — clinical overview and diagnosis. J Am Acad Dermatol; 2019.
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