What is it?

Onychomycosis is a fungal infection of the nail unit, representing the most common nail disorder encountered in clinical practice. The condition is caused predominantly by dermatophytes (fungi that feed on keratin), with Trichophyton rubrum responsible for the majority of cases. Onychomycosis leads to progressive nail plate changes including discoloration, thickening, and structural distortion that may cause discomfort and significant cosmetic concern.

Who is affected?

Onychomycosis affects approximately 5.5% of the global population, with prevalence increasing substantially with age.

Prevalence: Affects approximately 1 in 18 people (5.5%) worldwide; accounts for nearly 50% of all nail disorders
Age demographics: Prevalence rises sharply with age—affects over 20% of adults above 60 years and over 50% of those above 70 years; rare in children (0.16%)
Gender distribution: More common in men than women; approximately 2:1 ratio
Risk factors: Advancing age; diabetes mellitus (up to one-third of diabetic patients affected); peripheral vascular disease; immunosuppression (HIV, organ transplant recipients); concomitant tinea pedis (athlete's foot); family history; occlusive footwear; communal bathing facilities; nail trauma; psoriasis

What causes it?

Onychomycosis results from fungal invasion of the nail unit:

Dermatophytes: Responsible for approximately 90% of toenail and 50% of fingernail infections; Trichophyton rubrum is the predominant species, followed by T. mentagrophytes and Epidermophyton floccosum
Yeasts: Candida species account for approximately 2% of cases, primarily affecting fingernails in individuals with chronic paronychia or frequent water exposure
Non-dermatophyte molds: Fusarium, Aspergillus, and Scopulariopsis species represent approximately 8% of infections
Predisposing factors: Impaired peripheral circulation reduces the delivery of immune defenses to the nail; diabetes alters immune function and promotes fungal growth; nail trauma creates entry points for fungal organisms; warm, moist environments (occlusive footwear) favor fungal proliferation

What are the clinical features?

Onychomycosis predominantly affects toenails (particularly the great toe), with fingernail involvement being less frequent. Toenails are affected approximately 10 times more often than fingernails.

Clinical subtypes:

Distal lateral subungual onychomycosis (DLSO): The most common form (75%); begins at the distal (free) edge or lateral nail fold, progressing proximally with yellowish-brown discoloration, subungual hyperkeratosis (thickening beneath the nail), and onycholysis (nail plate separation)
White superficial onychomycosis (WSO): White, chalky deposits on the nail surface that can be easily scraped away; accounts for approximately 10% of cases
Proximal subungual onychomycosis (PSO): The rarest form; infection begins at the proximal nail fold and progresses distally; frequently associated with immunosuppression (HIV indicator)
Total dystrophic onychomycosis (TDO): The most severe form, representing end-stage disease; complete nail destruction after years of untreated infection

Common features:

Yellow, brown, or white nail discoloration
Nail plate thickening and brittleness
Subungual debris (accumulation beneath the nail)
Onycholysis (separation of the nail from the nail bed)
Nail distortion and crumbling

How is it diagnosed?

Laboratory confirmation is recommended before initiating treatment, as approximately 50% of dystrophic nails are caused by non-fungal conditions:

KOH (potassium hydroxide) preparation: Microscopic examination of nail clippings dissolved in KOH to visualize fungal elements (hyphae); sensitivity approximately 60%
Fungal culture: Identifies the specific organism and species; highly specific but sensitivity is approximately 60%; results may take 3-6 weeks
PAS (periodic acid-Schiff) staining: Histological examination of nail clippings; sensitivity approximately 95%, considered the most sensitive method
PCR (polymerase chain reaction): Molecular testing that improves detection by approximately 20% compared to culture alone; rapid results
Dermoscopy: Non-invasive clinical tool revealing characteristic patterns including jagged proximal edge with spikes (sharp projections at the border of the onycholytic area), longitudinal striae, yellowish-brown discoloration, and ruin appearance in advanced forms (irregular destruction of the distal nail portion)
Differential diagnosis: Must distinguish from nail psoriasis, lichen planus nail involvement, traumatic nail dystrophy, yellow nail syndrome, and nail changes associated with systemic disease

What treatment options are available?

Treatment selection depends on the extent of nail involvement, causative organism, and patient comorbidities:

Oral antifungal therapy (moderate to severe disease):

Terbinafine: First-line systemic agent; mycological cure rate approximately 70-76%; 250 mg daily for 6 weeks (fingernails) or 12 weeks (toenails)
Itraconazole: Alternative first-line agent; mycological cure approximately 61%; continuous or pulse dosing regimens available
Fluconazole: Second-line option; 150 mg weekly for 6-12 months (off-label use)

Topical antifungal therapy (mild to moderate disease):

Efinaconazole 10% solution: Highest efficacy among topicals; mycological cure 53-62%; applied daily for 48 weeks
Amorolfine 5% nail lacquer: Weekly application; mycological cure 70-89% (approved in Europe)
Ciclopirox 8% nail lacquer: Applied daily; complete cure approximately 5.5-8.5%
Tavaborole 5% solution: Applied daily for 48 weeks; mycological cure 31-36%

Combination therapy:

Oral terbinafine combined with topical agents yields the highest cure rates for moderate to severe infections

Adjunctive measures:

Mechanical or chemical nail debridement to reduce fungal load
Treatment of concurrent tinea pedis to prevent reinfection
Antifungal powder in footwear for prophylaxis

Prognosis:

Complete cure rates range from 38% (terbinafine) to under 10% for topical monotherapy
Treatment requires patience; visible improvement takes months as the nail grows out (toenails: 12-18 months)
Recurrence rates are high (5-50%), particularly in patients with predisposing risk factors
Older age, severe nail involvement, and peripheral vascular disease predict poorer treatment outcomes

Key points for patients

Onychomycosis is a common, chronic infection that does not resolve spontaneously and tends to worsen without treatment
Laboratory confirmation is important before starting treatment—not all thickened or discolored nails are fungal
Oral medication is more effective than topical treatment for most cases, particularly when multiple nails or more than 50% of the nail is affected
When to seek medical attention: Progressive nail changes, pain or discomfort, signs of secondary bacterial infection (redness, swelling, pus), particularly if you have diabetes or impaired circulation
Prevention strategies: Keep feet clean and dry; wear breathable footwear; use antifungal powder in shoes; wear protective footwear in communal areas (pools, showers, gyms); treat tinea pedis promptly; avoid sharing nail clippers or files; replace old footwear that may harbor fungi

Onychomycosis