Dermatology · PANCE / PANRE

Onychomycosis

Written & clinically reviewed by Ryan Busch, PA-C · Updated August 2026 · Report an error

Fungal infection of the nail unit producing thickened, discolored, dystrophic nails.

Also known as: tinea unguium, fungal nail infection, nail fungus

Overview

Fungal infection of the nail plate, nail bed, or matrix, most commonly by dermatophytes (Trichophyton rubrum, T. interdigitale), and less commonly by non-dermatophyte molds (Scopulariopsis, Aspergillus, Fusarium) and yeasts (Candida). Toenails are far more frequently affected than fingernails.

Epidemiology

Most common nail disorder in adults; prevalence rises with age (up to 20-50% of those >60). More common in men, in patients with peripheral vascular disease, diabetes, immunosuppression, tinea pedis, and those who use occlusive footwear or communal showers.

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Question 1DermatologyMedium
A 64-year-old man presents with a thickened, yellow-brown, crumbly right great toenail that has worsened over 8 months. He has type 2 diabetes and coexisting moccasin-pattern scaling on both soles. Examination shows subungual hyperkeratosis and distal onycholysis. He has no skin plaques, nail pitting, or joint complaints. His clinician plans 12 weeks of oral terbinafine but wants laboratory confirmation of fungal infection first. Which of the following is the most appropriate next diagnostic test?
  • ADermoscopy of the affected nail plate
  • BWood lamp examination of the nail plate
  • CPAS stain of a nail-clipping specimen
  • DPunch biopsy of the nail matrix tissue
Reveal answer & full explanation
Correct answer: C — PAS stain of a nail-clipping specimen
  • ADermoscopy of the affected nail plate
  • BWood lamp examination of the nail plate
  • CPAS stain of a nail-clipping specimen✓
  • DPunch biopsy of the nail matrix tissue

Why PAS stain of a nail-clipping specimen is correct

  • PAS staining of a nail clipping is the highest-sensitivity office test for onychomycosis and is the preferred confirmatory study per the American Academy of Dermatology, outperforming KOH and fungal culture.
  • Because oral terbinafine commits this diabetic patient to ~12 weeks of therapy with LFT monitoring, fungal elements should be confirmed first; clinical appearance overlaps substantially with psoriasis and trauma.
  • It is fast, low cost, and reliably detects hyphae within the keratinized nail plate.

Why the others are wrong

  • Wood lamp examination of the nail plate is useful for some superficial fungal or bacterial skin infections such as tinea capitis and erythrasma, but it does not confirm nail-plate dermatophyte invasion.
  • Punch biopsy of the nail matrix tissue is invasive and is reserved for suspected nail tumors or a longitudinal pigmented band concerning for subungual melanoma, not routine confirmation of fungus.
  • Dermoscopy of the affected nail plate can show suggestive ruin or spiked patterns and helps differentiate from melanonychia, but it is not a confirmatory laboratory test for fungal elements before systemic therapy.
Question 2DermatologyMedium
A 58-year-old man presents with a thickened, yellow-brown right great toenail that has slowly worsened over the past year and now catches on his socks. He also has scaling and maceration between his toes. His medical history includes hypertension treated with amlodipine, and he reports no liver disease or alcohol use. A periodic acid-Schiff stain of a nail clipping shows septate hyphae consistent with a dermatophyte. Baseline AST and ALT are normal. Which of the following is the most appropriate initial management?
  • AOral fluconazole for 6 months
  • BOral itraconazole for 12 weeks
  • CTopical ciclopirox for 48 weeks
  • DOral terbinafine for 12 weeks
Reveal answer & full explanation
Correct answer: D — Oral terbinafine for 12 weeks
  • AOral fluconazole for 6 months
  • BOral itraconazole for 12 weeks
  • CTopical ciclopirox for 48 weeks
  • DOral terbinafine for 12 weeks✓

Why Oral terbinafine for 12 weeks is correct

  • Oral terbinafine 250 mg/day for 12 weeks (toenails) is the guideline-defined first-line therapy for dermatophyte onychomycosis, with the highest cure rates (~70%) of the oral agents.
  • This patient has a PAS-confirmed dermatophyte infection, normal baseline LFTs, and no contraindication, so systemic terbinafine is appropriate.
  • Coexisting tinea pedis should also be treated (topical allylamine) to reduce reinfection, but the nail itself requires a systemic allylamine.

Why the others are wrong

  • Oral itraconazole for 12 weeks — an effective azole reserved for when terbinafine is contraindicated or for Candida/non-dermatophyte molds; as a CYP3A4 inhibitor it interacts with the patient's amlodipine (a CYP3A4-cleared calcium channel blocker), making terbinafine the better fit here.
  • Topical ciclopirox for 48 weeks — a topical lacquer with notably lower cure rates, appropriate for mild disease or when oral therapy is contraindicated, not for confirmed thick subungual disease in a candidate for systemic treatment.
  • Oral fluconazole for 6 months — an off-label alternative most useful for Candida; it is less effective than terbinafine against dermatophytes and is not first-line.
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Risk factors

  • Tinea pedis (a near-universal precursor in distal subungual onychomycosis)
  • Older age
  • Diabetes mellitus
  • Peripheral arterial disease
  • Immunosuppression (HIV, transplant, biologics)
  • Hyperhidrosis, occlusive footwear
  • Repetitive nail trauma (runners, dancers)
  • Communal moisture exposure (locker rooms, pools)

Pathophysiology

Dermatophytes invade the keratinized nail unit, most commonly entering through the hyponychium and spreading proximally beneath the nail plate (distal lateral subungual onychomycosis). White superficial, proximal subungual, and total dystrophic patterns reflect different routes of invasion and host factors (e.g., proximal subungual disease in HIV).

Clinical presentation

Symptoms

  • Most patients asymptomatic or report cosmetic concerns
  • Pain, pressure, or difficulty with footwear in advanced disease
  • Secondary cellulitis or ulceration in diabetic or immunocompromised patients

Signs / physical exam

  • Distal subungual onychomycosis: yellow-brown discoloration starting distally, subungual hyperkeratosis, onycholysis, thickening
  • White superficial onychomycosis: chalky white patches on the dorsal nail plate (T. interdigitale)
  • Proximal subungual onychomycosis: white patch under the proximal nail fold; classically suggests immunosuppression (HIV)
  • Total dystrophic onychomycosis: end-stage involvement of the entire nail unit
  • Coexisting tinea pedis (moccasin scaling, interdigital maceration)

Classic findings

Thickened, yellow, crumbly toenail with subungual hyperkeratosis and concurrent tinea pedis.

Differential diagnosis

  • Nail psoriasis — Pitting, oil-drop sign, salmon patches; concurrent psoriatic skin or joint disease; KOH negative
  • Lichen planus of the nail — Longitudinal ridging, thinning, pterygium formation; oral or skin lichen planus
  • Traumatic onychodystrophy — Single affected nail with history of trauma; KOH negative
  • Yellow nail syndrome — All 20 nails yellow, slow growing, with lymphedema and respiratory disease
  • Onycholysis from contact/irritant exposure — Distal nail separation from the bed without onychocryptosis or fungal hyphae; remove insult
  • Subungual melanoma — Single longitudinal pigmented band with Hutchinson sign (pigment on proximal nail fold) — biopsy mandatory; do not assume fungal

Diagnostic workup

Diagnostic criteria

Confirmation of fungal elements (positive KOH, PAS, culture, or PCR) is recommended before systemic therapy, because clinical appearance overlaps substantially with psoriasis, trauma, and other dystrophies. Treat empirically only in limited circumstances when access to testing is poor and a confident clinical diagnosis is supported.

Labs

  • KOH preparation of subungual debris — quick, low cost, requires experience
  • Periodic acid-Schiff (PAS) staining of nail clipping — most sensitive office test; preferred per AAD
  • Fungal culture of subungual debris — identifies organism; slow (4-6 weeks)
  • PCR for dermatophytes when available — fast, sensitive, increasingly used
  • Baseline ALT/AST before oral antifungal therapy

Imaging

  • Generally not required
  • Imaging of foot/ankle reserved for evaluation of associated osteomyelitis or diabetic complications, not the nail disease itself

Diagnostic algorithm

PatternTypical OrganismClinical ClueImmunosuppression Association
Distal lateral subungualT. rubrumDistal yellow discoloration + hyperkeratosisNo
White superficialT. interdigitaleChalky white patches on nail surfaceNo
Proximal subungualT. rubrumWhite patch under proximal nail foldYes (HIV, organ transplant)
Total dystrophicVariousEntire nail involvedCommon in chronic disease
Candida onychomycosisCandida albicansFingernails > toenails; chronic paronychia commonYes (immunosuppression, wet work)
Clinical patterns of onychomycosis and their organism associations.

Treatment

First-line

  • Oral terbinafine 250 mg/day for 6 weeks (fingernails) or 12 weeks (toenails) — most effective oral agent for dermatophyte onychomycosis; cure rates ~70%
  • Oral itraconazole 200 mg/day continuous × 12 weeks, or pulse dosing 200 mg BID × 1 week per month × 2 (fingernails) or 3 (toenails) cycles — useful when terbinafine contraindicated or for Candida/non-dermatophyte molds
  • Oral fluconazole 150-300 mg weekly × 6-12 months — off-label but useful for Candida and as alternative
  • Treat coexisting tinea pedis with topical terbinafine or other allylamine to reduce recurrence
  • Patient education: rotate footwear, dry feet thoroughly, treat shoes, separate towels

Second-line / adjunct

  • Topical efinaconazole 10% solution daily × 48 weeks — useful for mild-moderate disease or when systemic therapy contraindicated
  • Topical tavaborole 5% solution daily × 48 weeks
  • Topical ciclopirox 8% nail lacquer daily × 48 weeks — lower cure rates than newer topicals
  • Nail debridement and chemical or surgical avulsion as adjuncts
  • Laser and photodynamic therapy — modest evidence; not first-line

Complications

  • Recurrence in 20-50% within several years even after cure
  • Secondary bacterial cellulitis, particularly in diabetics and patients with vascular disease
  • Diabetic foot ulceration originating at dystrophic nail
  • Hepatotoxicity from terbinafine (monitor LFTs at baseline and consider at 6 weeks)
  • Drug interactions: itraconazole (CYP3A4 inhibitor) with statins, benzodiazepines, warfarin, calcium channel blockers — review medications carefully; contraindicated with several CYP3A4-cleared drugs

PANCE pearls

  • Confirm fungal infection before committing a patient to 3 months of oral antifungal therapy — many 'fungal' toenails are psoriasis or trauma.
  • PAS-stained nail clipping is the highest-sensitivity office test and outperforms KOH and culture.
  • Always look for tinea pedis — eradicate it or the nails will reinfect.
  • Proximal subungual onychomycosis is uncommon and should prompt evaluation for HIV or other immunosuppression.
  • Single nail with a longitudinal pigmented band is melanoma until proven otherwise — biopsy, do not treat as fungus.
  • Terbinafine is first-line for dermatophytes; itraconazole is preferred for non-dermatophyte molds and Candida.

References

  • AAD 2019 — Lipner SR, Scher RK. Onychomycosis: clinical overview and diagnosis (JAAD 2019)
  • BAD 2014 — Ameen M et al. British Association of Dermatologists' guidelines for the management of onychomycosis (BJD 2014)
  • Gupta meta-analysis — Gupta AK et al. Treatments for onychomycosis: a systematic review and network meta-analysis (BJD 2020)

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Related Dermatology diagnoses

Acne VulgarisRosaceaAtopic Dermatitis (Eczema)Allergic and Irritant Contact DermatitisSeborrheic DermatitisPsoriasisLichen PlanusPityriasis RoseaTinea Infections (corporis, capitis, pedis, cruris, versicolor)Cutaneous CandidiasisScabiesPediculosisImpetigoHerpes Zoster (Shingles)

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